File: //opt/netdata/share/info/bash.info
This is bash.info, produced by makeinfo version 7.2 from bashref.texi.
This text is a brief description of the features that are present in the
Bash shell (version 5.3, 18 May 2025).
This is Edition 5.3, last updated 18 May 2025, of ‘The GNU Bash
Reference Manual’, for ‘Bash’, Version 5.3.
Copyright © 1988-2025 Free Software Foundation, Inc.
Permission is granted to copy, distribute and/or modify this
document under the terms of the GNU Free Documentation License,
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no Back-Cover Texts. A copy of the license is included in the
section entitled "GNU Free Documentation License".
INFO-DIR-SECTION Basics
START-INFO-DIR-ENTRY
* Bash: (bash). The GNU Bourne-Again SHell.
END-INFO-DIR-ENTRY
File: bash.info, Node: Top, Next: Introduction, Prev: (dir), Up: (dir)
Bash Features
*************
This text is a brief description of the features that are present in the
Bash shell (version 5.3, 18 May 2025). The Bash home page is
<http://www.gnu.org/software/bash/>.
This is Edition 5.3, last updated 18 May 2025, of ‘The GNU Bash
Reference Manual’, for ‘Bash’, Version 5.3.
Bash contains features that appear in other popular shells, and some
features that only appear in Bash. Some of the shells that Bash has
borrowed concepts from are the Bourne Shell (‘sh’), the Korn Shell
(‘ksh’), and the C-shell (‘csh’ and its successor, ‘tcsh’). The
following menu breaks the features up into categories, noting which
features were inspired by other shells and which are specific to Bash.
This manual is meant as a brief introduction to features found in
Bash. The Bash manual page should be used as the definitive reference
on shell behavior.
* Menu:
* Introduction:: An introduction to the shell.
* Definitions:: Some definitions used in the rest of this
manual.
* Basic Shell Features:: The shell "building blocks".
* Shell Builtin Commands:: Commands that are a part of the shell.
* Shell Variables:: Variables used or set by Bash.
* Bash Features:: Features found only in Bash.
* Job Control:: What job control is and how Bash allows you
to use it.
* Command Line Editing:: Chapter describing the command line
editing features.
* Using History Interactively:: Command History Expansion
* Installing Bash:: How to build and install Bash on your system.
* Reporting Bugs:: How to report bugs in Bash.
* Major Differences From The Bourne Shell:: A terse list of the differences
between Bash and historical
versions of /bin/sh.
* GNU Free Documentation License:: Copying and sharing this documentation.
* Indexes:: Various indexes for this manual.
File: bash.info, Node: Introduction, Next: Definitions, Up: Top
1 Introduction
**************
* Menu:
* What is Bash?:: A short description of Bash.
* What is a shell?:: A brief introduction to shells.
File: bash.info, Node: What is Bash?, Next: What is a shell?, Up: Introduction
1.1 What is Bash?
=================
Bash is the shell, or command language interpreter, for the GNU
operating system. The name is an acronym for the ‘Bourne-Again SHell’,
a pun on Stephen Bourne, the author of the direct ancestor of the
current Unix shell ‘sh’, which appeared in the Seventh Edition Bell Labs
Research version of Unix.
Bash is largely compatible with ‘sh’ and incorporates useful features
from the Korn shell ‘ksh’ and the C shell ‘csh’. It is intended to be a
conformant implementation of the IEEE POSIX Shell and Tools portion of
the IEEE POSIX specification (IEEE Standard 1003.1). It offers
functional improvements over ‘sh’ for both interactive and programming
use.
While the GNU operating system provides other shells, including a
version of ‘csh’, Bash is the default shell. Like other GNU software,
Bash is quite portable. It currently runs on nearly every version of
Unix and a few other operating systems − independently-supported ports
exist for Windows and other platforms.
File: bash.info, Node: What is a shell?, Prev: What is Bash?, Up: Introduction
1.2 What is a shell?
====================
At its base, a shell is simply a macro processor that executes commands.
The term macro processor means functionality where text and symbols are
expanded to create larger expressions.
A Unix shell is both a command interpreter and a programming
language. As a command interpreter, the shell provides the user
interface to the rich set of GNU utilities. The programming language
features allow these utilities to be combined. Users can create files
containing commands, and these become commands themselves. These new
commands have the same status as system commands in directories such as
‘/bin’, allowing users or groups to establish custom environments to
automate their common tasks.
Shells may be used interactively or non-interactively. In
interactive mode, they accept input typed from the keyboard. When
executing non-interactively, shells execute commands read from a file or
a string.
A shell allows execution of GNU commands, both synchronously and
asynchronously. The shell waits for synchronous commands to complete
before accepting more input; asynchronous commands continue to execute
in parallel with the shell while it reads and executes additional
commands. The “redirection” constructs permit fine-grained control of
the input and output of those commands. Moreover, the shell allows
control over the contents of commands' environments.
Shells also provide a small set of built-in commands (“builtins”)
implementing functionality impossible or inconvenient to obtain via
separate utilities. For example, ‘cd’, ‘break’, ‘continue’, and ‘exec’
cannot be implemented outside of the shell because they directly
manipulate the shell itself. The ‘history’, ‘getopts’, ‘kill’, or ‘pwd’
builtins, among others, could be implemented in separate utilities, but
they are more convenient to use as builtin commands. All of the shell
builtins are described in subsequent sections.
While executing commands is essential, most of the power (and
complexity) of shells is due to their embedded programming languages.
Like any high-level language, the shell provides variables, flow control
constructs, quoting, and functions.
Shells offer features geared specifically for interactive use rather
than to augment the programming language. These interactive features
include job control, command line editing, command history and aliases.
This manual describes how Bash provides all of these features.
File: bash.info, Node: Definitions, Next: Basic Shell Features, Prev: Introduction, Up: Top
2 Definitions
*************
These definitions are used throughout the remainder of this manual.
‘POSIX’
A family of open system standards based on Unix. Bash is primarily
concerned with the Shell and Utilities portion of the POSIX 1003.1
standard.
‘blank’
A space or tab character.
‘whitespace’
A character belonging to the ‘space’ character class in the current
locale, or for which ‘isspace()’ returns true.
‘builtin’
A command that is implemented internally by the shell itself,
rather than by an executable program somewhere in the file system.
‘control operator’
A ‘token’ that performs a control function. It is a ‘newline’ or
one of the following: ‘||’, ‘&&’, ‘&’, ‘;’, ‘;;’, ‘;&’, ‘;;&’, ‘|’,
‘|&’, ‘(’, or ‘)’.
‘exit status’
The value returned by a command to its caller. The value is
restricted to eight bits, so the maximum value is 255.
‘field’
A unit of text that is the result of one of the shell expansions.
After expansion, when executing a command, the resulting fields are
used as the command name and arguments.
‘filename’
A string of characters used to identify a file.
‘job’
A set of processes comprising a pipeline, and any processes
descended from it, that are all in the same process group.
‘job control’
A mechanism by which users can selectively stop (suspend) and
restart (resume) execution of processes.
‘metacharacter’
A character that, when unquoted, separates words. A metacharacter
is a ‘space’, ‘tab’, ‘newline’, or one of the following characters:
‘|’, ‘&’, ‘;’, ‘(’, ‘)’, ‘<’, or ‘>’.
‘name’
A ‘word’ consisting solely of letters, numbers, and underscores,
and beginning with a letter or underscore. ‘Name’s are used as
shell variable and function names. Also referred to as an
‘identifier’.
‘operator’
A ‘control operator’ or a ‘redirection operator’. *Note
Redirections::, for a list of redirection operators. Operators
contain at least one unquoted ‘metacharacter’.
‘process group’
A collection of related processes each having the same process
group ID.
‘process group ID’
A unique identifier that represents a ‘process group’ during its
lifetime.
‘reserved word’
A ‘word’ that has a special meaning to the shell. Most reserved
words introduce shell flow control constructs, such as ‘for’ and
‘while’.
‘return status’
A synonym for ‘exit status’.
‘signal’
A mechanism by which a process may be notified by the kernel of an
event occurring in the system.
‘special builtin’
A shell builtin command that has been classified as special by the
POSIX standard.
‘token’
A sequence of characters considered a single unit by the shell. It
is either a ‘word’ or an ‘operator’.
‘word’
A sequence of characters treated as a unit by the shell. Words may
not include unquoted ‘metacharacters’.
File: bash.info, Node: Basic Shell Features, Next: Shell Builtin Commands, Prev: Definitions, Up: Top
3 Basic Shell Features
**********************
Bash is an acronym for ‘Bourne-Again SHell’. The Bourne shell is the
traditional Unix shell originally written by Stephen Bourne. All of the
Bourne shell builtin commands are available in Bash, and the rules for
evaluation and quoting are taken from the POSIX specification for the
'standard' Unix shell.
This chapter briefly summarizes the shell's 'building blocks':
commands, control structures, shell functions, shell parameters, shell
expansions, redirections, which are a way to direct input and output
from and to named files, and how the shell executes commands.
* Menu:
* Shell Syntax:: What your input means to the shell.
* Shell Commands:: The types of commands you can use.
* Shell Functions:: Grouping commands by name.
* Shell Parameters:: How the shell stores values.
* Shell Expansions:: How Bash expands parameters and the various
expansions available.
* Redirections:: A way to control where input and output go.
* Executing Commands:: What happens when you run a command.
* Shell Scripts:: Executing files of shell commands.
File: bash.info, Node: Shell Syntax, Next: Shell Commands, Up: Basic Shell Features
3.1 Shell Syntax
================
* Menu:
* Shell Operation:: The basic operation of the shell.
* Quoting:: How to remove the special meaning from characters.
* Comments:: How to specify comments.
When the shell reads input, it proceeds through a sequence of
operations. If the input indicates the beginning of a comment, the
shell ignores the comment symbol (‘#’), and the rest of that line.
Otherwise, roughly speaking, the shell reads its input and divides
the input into words and operators, employing the quoting rules to
select which meanings to assign various words and characters.
The shell then parses these tokens into commands and other
constructs, removes the special meaning of certain words or characters,
expands others, redirects input and output as needed, executes the
specified command, waits for the command's exit status, and makes that
exit status available for further inspection or processing.
File: bash.info, Node: Shell Operation, Next: Quoting, Up: Shell Syntax
3.1.1 Shell Operation
---------------------
The following is a brief description of the shell's operation when it
reads and executes a command. Basically, the shell does the following:
1. Reads its input from a file (*note Shell Scripts::), from a string
supplied as an argument to the ‘-c’ invocation option (*note
Invoking Bash::), or from the user's terminal.
2. Breaks the input into words and operators, obeying the quoting
rules described in *note Quoting::. These tokens are separated by
‘metacharacters’. This step performs alias expansion (*note
Aliases::).
3. Parses the tokens into simple and compound commands (*note Shell
Commands::).
4. Performs the various shell expansions (*note Shell Expansions::),
breaking the expanded tokens into lists of filenames (*note
Filename Expansion::) and commands and arguments.
5. Performs any necessary redirections (*note Redirections::) and
removes the redirection operators and their operands from the
argument list.
6. Executes the command (*note Executing Commands::).
7. Optionally waits for the command to complete and collects its exit
status (*note Exit Status::).
File: bash.info, Node: Quoting, Next: Comments, Prev: Shell Operation, Up: Shell Syntax
3.1.2 Quoting
-------------
* Menu:
* Escape Character:: How to remove the special meaning from a single
character.
* Single Quotes:: How to inhibit all interpretation of a sequence
of characters.
* Double Quotes:: How to suppress most of the interpretation of a
sequence of characters.
* ANSI-C Quoting:: How to expand ANSI-C sequences in quoted strings.
* Locale Translation:: How to translate strings into different languages.
Quoting is used to remove the special meaning of certain characters or
words to the shell. Quoting can be used to disable special treatment
for special characters, to prevent reserved words from being recognized
as such, and to prevent parameter expansion.
Each of the shell metacharacters (*note Definitions::) has special
meaning to the shell and must be quoted if it is to represent itself.
When the command history expansion facilities are being used (*note
History Interaction::), the “history expansion” character, usually ‘!’,
must be quoted to prevent history expansion. *Note Bash History
Facilities::, for more details concerning history expansion.
There are four quoting mechanisms: the “escape character”, single
quotes, double quotes, and dollar-single quotes.
File: bash.info, Node: Escape Character, Next: Single Quotes, Up: Quoting
3.1.2.1 Escape Character
........................
A non-quoted backslash ‘\’ is the Bash escape character. It preserves
the literal value of the next character that follows, removing any
special meaning it has, with the exception of ‘newline’. If a
‘\newline’ pair appears, and the backslash itself is not quoted, the
‘\newline’ is treated as a line continuation (that is, it is removed
from the input stream and effectively ignored).
File: bash.info, Node: Single Quotes, Next: Double Quotes, Prev: Escape Character, Up: Quoting
3.1.2.2 Single Quotes
.....................
Enclosing characters in single quotes (‘'’) preserves the literal value
of each character within the quotes. A single quote may not occur
between single quotes, even when preceded by a backslash.
File: bash.info, Node: Double Quotes, Next: ANSI-C Quoting, Prev: Single Quotes, Up: Quoting
3.1.2.3 Double Quotes
.....................
Enclosing characters in double quotes (‘"’) preserves the literal value
of all characters within the quotes, with the exception of ‘$’, ‘`’,
‘\’, and, when history expansion is enabled, ‘!’. When the shell is in
POSIX mode (*note Bash POSIX Mode::), the ‘!’ has no special meaning
within double quotes, even when history expansion is enabled. The
characters ‘$’ and ‘`’ retain their special meaning within double quotes
(*note Shell Expansions::). The backslash retains its special meaning
only when followed by one of the following characters: ‘$’, ‘`’, ‘"’,
‘\’, or ‘newline’. Within double quotes, backslashes that are followed
by one of these characters are removed. Backslashes preceding
characters without a special meaning are left unmodified.
A double quote may be quoted within double quotes by preceding it
with a backslash. If enabled, history expansion will be performed
unless an ‘!’ appearing in double quotes is escaped using a backslash.
The backslash preceding the ‘!’ is not removed.
The special parameters ‘*’ and ‘@’ have special meaning when in
double quotes (*note Shell Parameter Expansion::).
File: bash.info, Node: ANSI-C Quoting, Next: Locale Translation, Prev: Double Quotes, Up: Quoting
3.1.2.4 ANSI-C Quoting
......................
Character sequences of the form ‘$'STRING'’ are treated as a special
kind of single quotes. The sequence expands to STRING, with
backslash-escaped characters in STRING replaced as specified by the ANSI
C standard. Backslash escape sequences, if present, are decoded as
follows:
‘\a’
alert (bell)
‘\b’
backspace
‘\e’
‘\E’
An escape character (not in ANSI C).
‘\f’
form feed
‘\n’
newline
‘\r’
carriage return
‘\t’
horizontal tab
‘\v’
vertical tab
‘\\’
backslash
‘\'’
single quote
‘\"’
double quote
‘\?’
question mark
‘\NNN’
The eight-bit character whose value is the octal value NNN (one to
three octal digits).
‘\xHH’
The eight-bit character whose value is the hexadecimal value HH
(one or two hex digits).
‘\uHHHH’
The Unicode (ISO/IEC 10646) character whose value is the
hexadecimal value HHHH (one to four hex digits).
‘\UHHHHHHHH’
The Unicode (ISO/IEC 10646) character whose value is the
hexadecimal value HHHHHHHH (one to eight hex digits).
‘\cX’
A control-X character.
The expanded result is single-quoted, as if the dollar sign had not been
present.
File: bash.info, Node: Locale Translation, Prev: ANSI-C Quoting, Up: Quoting
3.1.2.5 Locale-Specific Translation
...................................
* Menu:
* Creating Internationalized Scripts:: How to use translations and different
languages in your scripts.
Prefixing a double-quoted string with a dollar sign (‘$’), such as
$"hello, world", causes the string to be translated according to the
current locale. The ‘gettext’ infrastructure performs the lookup and
translation, using the ‘LC_MESSAGES’, ‘TEXTDOMAINDIR’, and ‘TEXTDOMAIN’
shell variables, as explained below. See the gettext documentation for
additional details not covered here. If the current locale is ‘C’ or
‘POSIX’, if there are no translations available, or if the string is not
translated, the dollar sign is ignored, and the string is treated as
double-quoted as described above. Since this is a form of double
quoting, the string remains double-quoted by default, whether or not it
is translated and replaced. If the ‘noexpand_translation’ option is
enabled using the ‘shopt’ builtin (*note The Shopt Builtin::),
translated strings are single-quoted instead of double-quoted.
The rest of this section is a brief overview of how you use gettext
to create translations for strings in a shell script named SCRIPTNAME.
There are more details in the gettext documentation.
File: bash.info, Node: Creating Internationalized Scripts, Up: Locale Translation
Once you've marked the strings in your script that you want to translate
using $"...", you create a gettext "template" file using the command
bash --dump-po-strings SCRIPTNAME > DOMAIN.pot
The DOMAIN is your “message domain”. It's just an arbitrary string
that's used to identify the files gettext needs, like a package or
script name. It needs to be unique among all the message domains on
systems where you install the translations, so gettext knows which
translations correspond to your script. You'll use the template file to
create translations for each target language. The template file
conventionally has the suffix ‘.pot’.
You copy this template file to a separate file for each target
language you want to support (called "PO" files, which use the suffix
‘.po’). PO files use various naming conventions, but when you are
working to translate a template file into a particular language, you
first copy the template file to a file whose name is the language you
want to target, with the ‘.po’ suffix. For instance, the Spanish
translations of your strings would be in a file named ‘es.po’, and to
get started using a message domain named "example," you would run
cp example.pot es.po
Ultimately, PO files are often named DOMAIN.po and installed in
directories that contain multiple translation files for a particular
language.
Whichever naming convention you choose, you will need to translate
the strings in the PO files into the appropriate languages. This has to
be done manually.
When you have the translations and PO files complete, you'll use the
gettext tools to produce what are called "MO" files, which are compiled
versions of the PO files the gettext tools use to look up translations
efficiently. MO files are also called "message catalog" files. You use
the ‘msgfmt’ program to do this. For instance, if you had a file with
Spanish translations, you could run
msgfmt -o es.mo es.po
to produce the corresponding MO file.
Once you have the MO files, you decide where to install them and use
the ‘TEXTDOMAINDIR’ shell variable to tell the gettext tools where they
are. Make sure to use the same message domain to name the MO files as
you did for the PO files when you install them.
Your users will use the ‘LANG’ or ‘LC_MESSAGES’ shell variables to
select the desired language.
You set the ‘TEXTDOMAIN’ variable to the script's message domain. As
above, you use the message domain to name your translation files.
You, or possibly your users, set the ‘TEXTDOMAINDIR’ variable to the
name of a directory where the message catalog files are stored. If you
install the message files into the system's standard message catalog
directory, you don't need to worry about this variable.
The directory where the message catalog files are stored varies
between systems. Some use the message catalog selected by the
‘LC_MESSAGES’ shell variable. Others create the name of the message
catalog from the value of the ‘TEXTDOMAIN’ shell variable, possibly
adding the ‘.mo’ suffix. If you use the ‘TEXTDOMAIN’ variable, you may
need to set the ‘TEXTDOMAINDIR’ variable to the location of the message
catalog files, as above. It's common to use both variables in this
fashion: ‘$TEXTDOMAINDIR’/‘$LC_MESSAGES’/LC_MESSAGES/‘$TEXTDOMAIN’.mo.
If you used that last convention, and you wanted to store the message
catalog files with Spanish (es) and Esperanto (eo) translations into a
local directory you use for custom translation files, you could run
TEXTDOMAIN=example
TEXTDOMAINDIR=/usr/local/share/locale
cp es.mo ${TEXTDOMAINDIR}/es/LC_MESSAGES/${TEXTDOMAIN}.mo
cp eo.mo ${TEXTDOMAINDIR}/eo/LC_MESSAGES/${TEXTDOMAIN}.mo
When all of this is done, and the message catalog files containing
the compiled translations are installed in the correct location, your
users will be able to see translated strings in any of the supported
languages by setting the ‘LANG’ or ‘LC_MESSAGES’ environment variables
before running your script.
File: bash.info, Node: Comments, Prev: Quoting, Up: Shell Syntax
3.1.3 Comments
--------------
In a non-interactive shell, or an interactive shell in which the
‘interactive_comments’ option to the ‘shopt’ builtin is enabled (*note
The Shopt Builtin::), a word beginning with ‘#’ introduces a comment. A
word begins at the beginning of a line, after unquoted whitespace, or
after an operator. The comment causes that word and all remaining
characters on that line to be ignored. An interactive shell without the
‘interactive_comments’ option enabled does not allow comments. The
‘interactive_comments’ option is enabled by default in interactive
shells. *Note Interactive Shells::, for a description of what makes a
shell interactive.
File: bash.info, Node: Shell Commands, Next: Shell Functions, Prev: Shell Syntax, Up: Basic Shell Features
3.2 Shell Commands
==================
A simple shell command such as ‘echo a b c’ consists of the command
itself followed by arguments, separated by spaces.
More complex shell commands are composed of simple commands arranged
together in a variety of ways: in a pipeline in which the output of one
command becomes the input of a second, in a loop or conditional
construct, or in some other grouping.
* Menu:
* Reserved Words:: Words that have special meaning to the shell.
* Simple Commands:: The most common type of command.
* Pipelines:: Connecting the input and output of several
commands.
* Lists:: How to execute commands sequentially.
* Compound Commands:: Shell commands for control flow.
* Coprocesses:: Two-way communication between commands.
* GNU Parallel:: Running commands in parallel.
File: bash.info, Node: Reserved Words, Next: Simple Commands, Up: Shell Commands
3.2.1 Reserved Words
--------------------
Reserved words are words that have special meaning to the shell. They
are used to begin and end the shell's compound commands.
The following words are recognized as reserved when unquoted and the
first word of a command (see below for exceptions):
‘if’ ‘then’ ‘elif’ ‘else’ ‘fi’ ‘time’
‘for’ ‘in’ ‘until’ ‘while’ ‘do’ ‘done’
‘case’ ‘esac’ ‘coproc’‘select’‘function’
‘{’ ‘}’ ‘[[’ ‘]]’ ‘!’
‘in’ is recognized as a reserved word if it is the third word of a
‘case’ or ‘select’ command. ‘in’ and ‘do’ are recognized as reserved
words if they are the third word in a ‘for’ command.
File: bash.info, Node: Simple Commands, Next: Pipelines, Prev: Reserved Words, Up: Shell Commands
3.2.2 Simple Commands
---------------------
A simple command is the kind of command that's executed most often.
It's just a sequence of words separated by ‘blank’s, terminated by one
of the shell's control operators (*note Definitions::). The first word
generally specifies a command to be executed, with the rest of the words
being that command's arguments.
The return status (*note Exit Status::) of a simple command is its
exit status as provided by the POSIX 1003.1 ‘waitpid’ function, or 128+N
if the command was terminated by signal N.
File: bash.info, Node: Pipelines, Next: Lists, Prev: Simple Commands, Up: Shell Commands
3.2.3 Pipelines
---------------
A ‘pipeline’ is a sequence of one or more commands separated by one of
the control operators ‘|’ or ‘|&’.
The format for a pipeline is
[time [-p]] [!] COMMAND1 [ | or |& COMMAND2 ] ...
The output of each command in the pipeline is connected via a pipe to
the input of the next command. That is, each command reads the previous
command's output. This connection is performed before any redirections
specified by COMMAND1.
If ‘|&’ is the pipeline operator, COMMAND1's standard error, in
addition to its standard output, is connected to COMMAND2's standard
input through the pipe; it is shorthand for ‘2>&1 |’. This implicit
redirection of the standard error to the standard output is performed
after any redirections specified by COMMAND1, consistent with that
shorthand.
If the reserved word ‘time’ precedes the pipeline, Bash prints timing
statistics for the pipeline once it finishes. The statistics currently
consist of elapsed (wall-clock) time and user and system time consumed
by the command's execution. The ‘-p’ option changes the output format
to that specified by POSIX. When the shell is in POSIX mode (*note Bash
POSIX Mode::), it does not recognize ‘time’ as a reserved word if the
next token begins with a ‘-’. The value of the ‘TIMEFORMAT’ variable is
a format string that specifies how the timing information should be
displayed. *Note Bash Variables::, for a description of the available
formats. Providing ‘time’ as a reserved word permits the timing of
shell builtins, shell functions, and pipelines. An external ‘time’
command cannot time these easily.
When the shell is in POSIX mode (*note Bash POSIX Mode::), you can
use ‘time’ by itself as a simple command. In this case, the shell
displays the total user and system time consumed by the shell and its
children. The ‘TIMEFORMAT’ variable specifies the format of the time
information.
If a pipeline is not executed asynchronously (*note Lists::), the
shell waits for all commands in the pipeline to complete.
Each command in a multi-command pipeline, where pipes are created, is
executed in its own “subshell”, which is a separate process (*note
Command Execution Environment::). If the ‘lastpipe’ option is enabled
using the ‘shopt’ builtin (*note The Shopt Builtin::), and job control
is not active, the last element of a pipeline may be run by the shell
process.
The exit status of a pipeline is the exit status of the last command
in the pipeline, unless the ‘pipefail’ option is enabled (*note The Set
Builtin::). If ‘pipefail’ is enabled, the pipeline's return status is
the value of the last (rightmost) command to exit with a non-zero
status, or zero if all commands exit successfully. If the reserved word
‘!’ precedes the pipeline, the exit status is the logical negation of
the exit status as described above. If a pipeline is not executed
asynchronously (*note Lists::), the shell waits for all commands in the
pipeline to terminate before returning a value. The return status of an
asynchronous pipeline is 0.
File: bash.info, Node: Lists, Next: Compound Commands, Prev: Pipelines, Up: Shell Commands
3.2.4 Lists of Commands
-----------------------
A ‘list’ is a sequence of one or more pipelines separated by one of the
operators ‘;’, ‘&’, ‘&&’, or ‘||’, and optionally terminated by one of
‘;’, ‘&’, or a ‘newline’.
Of these list operators, ‘&&’ and ‘||’ have equal precedence,
followed by ‘;’ and ‘&’, which have equal precedence.
A sequence of one or more newlines may appear in a ‘list’ to delimit
commands, equivalent to a semicolon.
If a command is terminated by the control operator ‘&’, the shell
executes the command asynchronously in a subshell. This is known as
executing the command in the “background”, and these are referred to as
“asynchronous” commands. The shell does not wait for the command to
finish, and the return status is 0 (true). When job control is not
active (*note Job Control::), the standard input for asynchronous
commands, in the absence of any explicit redirections, is redirected
from ‘/dev/null’.
Commands separated by a ‘;’ are executed sequentially; the shell
waits for each command to terminate in turn. The return status is the
exit status of the last command executed.
AND and OR lists are sequences of one or more pipelines separated by
the control operators ‘&&’ and ‘||’, respectively. AND and OR lists are
executed with left associativity.
An AND list has the form
COMMAND1 && COMMAND2
COMMAND2 is executed if, and only if, COMMAND1 returns an exit status of
zero (success).
An OR list has the form
COMMAND1 || COMMAND2
COMMAND2 is executed if, and only if, COMMAND1 returns a non-zero exit
status.
The return status of AND and OR lists is the exit status of the last
command executed in the list.
File: bash.info, Node: Compound Commands, Next: Coprocesses, Prev: Lists, Up: Shell Commands
3.2.5 Compound Commands
-----------------------
* Menu:
* Looping Constructs:: Shell commands for iterative action.
* Conditional Constructs:: Shell commands for conditional execution.
* Command Grouping:: Ways to group commands.
Compound commands are the shell programming language constructs. Each
construct begins with a reserved word or control operator and is
terminated by a corresponding reserved word or operator. Any
redirections (*note Redirections::) associated with a compound command
apply to all commands within that compound command unless explicitly
overridden.
In most cases a list of commands in a compound command's description
may be separated from the rest of the command by one or more newlines,
and may be followed by a newline in place of a semicolon.
Bash provides looping constructs, conditional commands, and
mechanisms to group commands and execute them as a unit.
File: bash.info, Node: Looping Constructs, Next: Conditional Constructs, Up: Compound Commands
3.2.5.1 Looping Constructs
..........................
Bash supports the following looping constructs.
Note that wherever a ‘;’ appears in the description of a command's
syntax, it may be replaced with one or more newlines.
‘until’
The syntax of the ‘until’ command is:
until TEST-COMMANDS; do CONSEQUENT-COMMANDS; done
Execute CONSEQUENT-COMMANDS as long as TEST-COMMANDS has an exit
status which is not zero. The return status is the exit status of
the last command executed in CONSEQUENT-COMMANDS, or zero if none
was executed.
‘while’
The syntax of the ‘while’ command is:
while TEST-COMMANDS; do CONSEQUENT-COMMANDS; done
Execute CONSEQUENT-COMMANDS as long as TEST-COMMANDS has an exit
status of zero. The return status is the exit status of the last
command executed in CONSEQUENT-COMMANDS, or zero if none was
executed.
‘for’
The syntax of the ‘for’ command is:
for NAME [ [in WORDS ...] ; ] do COMMANDS; done
Expand WORDS (*note Shell Expansions::), and then execute COMMANDS
once for each word in the resultant list, with NAME bound to the
current word. If ‘in WORDS’ is not present, the ‘for’ command
executes the COMMANDS once for each positional parameter that is
set, as if ‘in "$@"’ had been specified (*note Special
Parameters::).
The return status is the exit status of the last command that
executes. If there are no items in the expansion of WORDS, no
commands are executed, and the return status is zero.
There is an alternate form of the ‘for’ command which is similar to
the C language:
for (( EXPR1 ; EXPR2 ; EXPR3 )) [;] do COMMANDS ; done
First, evaluate the arithmetic expression EXPR1 according to the
rules described below (*note Shell Arithmetic::). Then, repeatedly
evaluate the arithmetic expression EXPR2 until it evaluates to
zero. Each time EXPR2 evaluates to a non-zero value, execute
COMMANDS and evaluate the arithmetic expression EXPR3. If any
expression is omitted, it behaves as if it evaluates to 1. The
return value is the exit status of the last command in COMMANDS
that is executed, or non-zero if any of the expressions is invalid.
Use the ‘break’ and ‘continue’ builtins (*note Bourne Shell
Builtins::) to control loop execution.
File: bash.info, Node: Conditional Constructs, Next: Command Grouping, Prev: Looping Constructs, Up: Compound Commands
3.2.5.2 Conditional Constructs
..............................
‘if’
The syntax of the ‘if’ command is:
if TEST-COMMANDS; then
CONSEQUENT-COMMANDS;
[elif MORE-TEST-COMMANDS; then
MORE-CONSEQUENTS;]
[else ALTERNATE-CONSEQUENTS;]
fi
The TEST-COMMANDS list is executed, and if its return status is
zero, the CONSEQUENT-COMMANDS list is executed. If TEST-COMMANDS
returns a non-zero status, each ‘elif’ list is executed in turn,
and if its exit status is zero, the corresponding MORE-CONSEQUENTS
is executed and the command completes. If ‘else
ALTERNATE-CONSEQUENTS’ is present, and the final command in the
final ‘if’ or ‘elif’ clause has a non-zero exit status, then
ALTERNATE-CONSEQUENTS is executed. The return status is the exit
status of the last command executed, or zero if no condition tested
true.
‘case’
The syntax of the ‘case’ command is:
case WORD in
[ [(] PATTERN [| PATTERN]...) COMMAND-LIST ;;]...
esac
‘case’ will selectively execute the COMMAND-LIST corresponding to
the first PATTERN that matches WORD, proceeding from the first
pattern to the last. The match is performed according to the rules
described below in *note Pattern Matching::. If the ‘nocasematch’
shell option (see the description of ‘shopt’ in *note The Shopt
Builtin::) is enabled, the match is performed without regard to the
case of alphabetic characters. The ‘|’ is used to separate
multiple patterns in a pattern list, and the ‘)’ operator
terminates the pattern list. A pattern list and an associated
COMMAND-LIST is known as a CLAUSE.
Each clause must be terminated with ‘;;’, ‘;&’, or ‘;;&’. The WORD
undergoes tilde expansion, parameter expansion, command
substitution, process substitution, arithmetic expansion, and quote
removal (*note Shell Parameter Expansion::) before the shell
attempts to match the pattern. Each PATTERN undergoes tilde
expansion, parameter expansion, command substitution, arithmetic
expansion, process substitution, and quote removal.
There may be an arbitrary number of ‘case’ clauses, each terminated
by a ‘;;’, ‘;&’, or ‘;;&’. The first pattern that matches
determines the command-list that is executed. It's a common idiom
to use ‘*’ as the final pattern to define the default case, since
that pattern will always match.
Here is an example using ‘case’ in a script that could be used to
describe one interesting feature of an animal:
echo -n "Enter the name of an animal: "
read ANIMAL
echo -n "The $ANIMAL has "
case $ANIMAL in
horse | dog | cat) echo -n "four";;
man | kangaroo ) echo -n "two";;
*) echo -n "an unknown number of";;
esac
echo " legs."
If the ‘;;’ operator is used, the ‘case’ command completes after
the first pattern match. Using ‘;&’ in place of ‘;;’ causes
execution to continue with the COMMAND-LIST associated with the
next clause, if any. Using ‘;;&’ in place of ‘;;’ causes the shell
to test the patterns in the next clause, if any, and execute any
associated COMMAND-LIST if the match succeeds, continuing the case
statement execution as if the pattern list had not matched.
The return status is zero if no PATTERN matches. Otherwise, the
return status is the exit status of the last COMMAND-LIST executed.
‘select’
The ‘select’ construct allows the easy generation of menus. It has
almost the same syntax as the ‘for’ command:
select NAME [in WORDS ...]; do COMMANDS; done
First, expand the list of words following ‘in’, generating a list
of items, and print the set of expanded words on the standard error
stream, each preceded by a number. If the ‘in WORDS’ is omitted,
print the positional parameters, as if ‘in "$@"’ had been
specified. ‘select’ then displays the ‘PS3’ prompt and reads a
line from the standard input. If the line consists of a number
corresponding to one of the displayed words, then ‘select’ sets the
value of NAME to that word. If the line is empty, ‘select’
displays the words and prompt again. If ‘EOF’ is read, ‘select’
completes and returns 1. Any other value read causes NAME to be
set to null. The line read is saved in the variable ‘REPLY’.
The COMMANDS are executed after each selection until a ‘break’
command is executed, at which point the ‘select’ command completes.
Here is an example that allows the user to pick a filename from the
current directory, and displays the name and index of the file
selected.
select fname in *;
do
echo you picked $fname \($REPLY\)
break;
done
‘((...))’
(( EXPRESSION ))
The arithmetic EXPRESSION is evaluated according to the rules
described below (*note Shell Arithmetic::). The EXPRESSION
undergoes the same expansions as if it were within double quotes,
but unescaped double quote characters in EXPRESSION are not treated
specially and are removed. Since this can potentially result in
empty strings, this command treats those as expressions that
evaluate to 0. If the value of the expression is non-zero, the
return status is 0; otherwise the return status is 1.
‘[[...]]’
[[ EXPRESSION ]]
Evaluate the conditional expression EXPRESSION and return a status
of zero (true) or non-zero (false). Expressions are composed of
the primaries described below in *note Bash Conditional
Expressions::. The words between the ‘[[’ and ‘]]’ do not undergo
word splitting and filename expansion. The shell performs tilde
expansion, parameter and variable expansion, arithmetic expansion,
command substitution, process substitution, and quote removal on
those words. Conditional operators such as ‘-f’ must be unquoted
to be recognized as primaries.
When used with ‘[[’, the ‘<’ and ‘>’ operators sort
lexicographically using the current locale.
When the ‘==’ and ‘!=’ operators are used, the string to the right
of the operator is considered a pattern and matched according to
the rules described below in *note Pattern Matching::, as if the
‘extglob’ shell option were enabled. The ‘=’ operator is identical
to ‘==’. If the ‘nocasematch’ shell option (see the description of
‘shopt’ in *note The Shopt Builtin::) is enabled, the match is
performed without regard to the case of alphabetic characters. The
return value is 0 if the string matches (‘==’) or does not match
(‘!=’) the pattern, and 1 otherwise.
If you quote any part of the pattern, using any of the shell's
quoting mechanisms, the quoted portion is matched literally. This
means every character in the quoted portion matches itself, instead
of having any special pattern matching meaning.
An additional binary operator, ‘=~’, is available, with the same
precedence as ‘==’ and ‘!=’. When you use ‘=~’, the string to the
right of the operator is considered a POSIX extended regular
expression pattern and matched accordingly (using the POSIX
‘regcomp’ and ‘regexec’ interfaces usually described in regex(3)).
The return value is 0 if the string matches the pattern, and 1 if
it does not. If the regular expression is syntactically incorrect,
the conditional expression returns 2. If the ‘nocasematch’ shell
option (see the description of ‘shopt’ in *note The Shopt
Builtin::) is enabled, the match is performed without regard to the
case of alphabetic characters.
You can quote any part of the pattern to force the quoted portion
to be matched literally instead of as a regular expression (see
above). If the pattern is stored in a shell variable, quoting the
variable expansion forces the entire pattern to be matched
literally.
The match succeeds if the pattern matches any part of the string.
If you want to force the pattern to match the entire string, anchor
the pattern using the ‘^’ and ‘$’ regular expression operators.
For example, the following will match a line (stored in the shell
variable ‘line’) if there is a sequence of characters anywhere in
the value consisting of any number, including zero, of characters
in the ‘space’ character class, immediately followed by zero or one
instances of ‘a’, then a ‘b’:
[[ $line =~ [[:space:]]*(a)?b ]]
That means values for ‘line’ like ‘aab’, ‘ aaaaaab’, ‘xaby’, and ‘
ab’ will all match, as will a line containing a ‘b’ anywhere in its
value.
If you want to match a character that's special to the regular
expression grammar (‘^$|[]()\.*+?’), it has to be quoted to remove
its special meaning. This means that in the pattern ‘xxx.txt’, the
‘.’ matches any character in the string (its usual regular
expression meaning), but in the pattern ‘"xxx.txt"’, it can only
match a literal ‘.’.
Likewise, if you want to include a character in your pattern that
has a special meaning to the regular expression grammar, you must
make sure it's not quoted. If you want to anchor a pattern at the
beginning or end of the string, for instance, you cannot quote the
‘^’ or ‘$’ characters using any form of shell quoting.
If you want to match ‘initial string’ at the start of a line, the
following will work:
[[ $line =~ ^"initial string" ]]
but this will not:
[[ $line =~ "^initial string" ]]
because in the second example the ‘^’ is quoted and doesn't have
its usual special meaning.
It is sometimes difficult to specify a regular expression properly
without using quotes, or to keep track of the quoting used by
regular expressions while paying attention to shell quoting and the
shell's quote removal. Storing the regular expression in a shell
variable is often a useful way to avoid problems with quoting
characters that are special to the shell. For example, the
following is equivalent to the pattern used above:
pattern='[[:space:]]*(a)?b'
[[ $line =~ $pattern ]]
Shell programmers should take special care with backslashes, since
backslashes are used by both the shell and regular expressions to
remove the special meaning from the following character. This
means that after the shell's word expansions complete (*note Shell
Expansions::), any backslashes remaining in parts of the pattern
that were originally not quoted can remove the special meaning of
pattern characters. If any part of the pattern is quoted, the
shell does its best to ensure that the regular expression treats
those remaining backslashes as literal, if they appeared in a
quoted portion.
The following two sets of commands are _not_ equivalent:
pattern='\.'
[[ . =~ $pattern ]]
[[ . =~ \. ]]
[[ . =~ "$pattern" ]]
[[ . =~ '\.' ]]
The first two matches will succeed, but the second two will not,
because in the second two the backslash will be part of the pattern
to be matched. In the first two examples, the pattern passed to
the regular expression parser is ‘\.’. The backslash removes the
special meaning from ‘.’, so the literal ‘.’ matches. In the
second two examples, the pattern passed to the regular expression
parser has the backslash quoted (e.g., ‘\\\.’), which will not
match the string, since it does not contain a backslash. If the
string in the first examples were anything other than ‘.’, say ‘a’,
the pattern would not match, because the quoted ‘.’ in the pattern
loses its special meaning of matching any single character.
Bracket expressions in regular expressions can be sources of errors
as well, since characters that are normally special in regular
expressions lose their special meanings between brackets. However,
you can use bracket expressions to match special pattern characters
without quoting them, so they are sometimes useful for this
purpose.
Though it might seem like a strange way to write it, the following
pattern will match a ‘.’ in the string:
[[ . =~ [.] ]]
The shell performs any word expansions before passing the pattern
to the regular expression functions, so you can assume that the
shell's quoting takes precedence. As noted above, the regular
expression parser will interpret any unquoted backslashes remaining
in the pattern after shell expansion according to its own rules.
The intention is to avoid making shell programmers quote things
twice as much as possible, so shell quoting should be sufficient to
quote special pattern characters where that's necessary.
The array variable ‘BASH_REMATCH’ records which parts of the string
matched the pattern. The element of ‘BASH_REMATCH’ with index 0
contains the portion of the string matching the entire regular
expression. Substrings matched by parenthesized subexpressions
within the regular expression are saved in the remaining
‘BASH_REMATCH’ indices. The element of ‘BASH_REMATCH’ with index N
is the portion of the string matching the Nth parenthesized
subexpression.
Bash sets ‘BASH_REMATCH’ in the global scope; declaring it as a
local variable will lead to unexpected results.
Expressions may be combined using the following operators, listed
in decreasing order of precedence:
‘( EXPRESSION )’
Returns the value of EXPRESSION. This may be used to override
the normal precedence of operators.
‘! EXPRESSION’
True if EXPRESSION is false.
‘EXPRESSION1 && EXPRESSION2’
True if both EXPRESSION1 and EXPRESSION2 are true.
‘EXPRESSION1 || EXPRESSION2’
True if either EXPRESSION1 or EXPRESSION2 is true.
The ‘&&’ and ‘||’ operators do not evaluate EXPRESSION2 if the
value of EXPRESSION1 is sufficient to determine the return value of
the entire conditional expression.
File: bash.info, Node: Command Grouping, Prev: Conditional Constructs, Up: Compound Commands
3.2.5.3 Grouping Commands
.........................
Bash provides two ways to group a list of commands to be executed as a
unit. When commands are grouped, redirections may be applied to the
entire command list. For example, the output of all the commands in the
list may be redirected to a single stream.
‘()’
( LIST )
Placing a list of commands between parentheses forces the shell to
create a subshell (*note Command Execution Environment::), and each
of the commands in LIST is executed in that subshell environment.
Since the LIST is executed in a subshell, variable assignments do
not remain in effect after the subshell completes.
‘{}’
{ LIST; }
Placing a list of commands between curly braces causes the list to
be executed in the current shell environment. No subshell is
created. The semicolon (or newline) following LIST is required.
In addition to the creation of a subshell, there is a subtle
difference between these two constructs due to historical reasons. The
braces are reserved words, so they must be separated from the LIST by
‘blank’s or other shell metacharacters. The parentheses are operators,
and are recognized as separate tokens by the shell even if they are not
separated from the LIST by whitespace.
The exit status of both of these constructs is the exit status of
LIST.
File: bash.info, Node: Coprocesses, Next: GNU Parallel, Prev: Compound Commands, Up: Shell Commands
3.2.6 Coprocesses
-----------------
A ‘coprocess’ is a shell command preceded by the ‘coproc’ reserved word.
A coprocess is executed asynchronously in a subshell, as if the command
had been terminated with the ‘&’ control operator, with a two-way pipe
established between the executing shell and the coprocess.
The syntax for a coprocess is:
coproc [NAME] COMMAND [REDIRECTIONS]
This creates a coprocess named NAME. COMMAND may be either a simple
command (*note Simple Commands::) or a compound command (*note Compound
Commands::). NAME is a shell variable name. If NAME is not supplied,
the default name is ‘COPROC’.
The recommended form to use for a coprocess is
coproc NAME { COMMAND; }
This form is preferred because simple commands result in the coprocess
always being named ‘COPROC’, and it is simpler to use and more complete
than the other compound commands.
There are other forms of coprocesses:
coproc NAME COMPOUND-COMMAND
coproc COMPOUND-COMMAND
coproc SIMPLE-COMMAND
If COMMAND is a compound command, NAME is optional. The word following
‘coproc’ determines whether that word is interpreted as a variable name:
it is interpreted as NAME if it is not a reserved word that introduces a
compound command. If COMMAND is a simple command, NAME is not allowed;
this is to avoid confusion between NAME and the first word of the simple
command.
When the coprocess is executed, the shell creates an array variable
(*note Arrays::) named NAME in the context of the executing shell. The
standard output of COMMAND is connected via a pipe to a file descriptor
in the executing shell, and that file descriptor is assigned to NAME[0].
The standard input of COMMAND is connected via a pipe to a file
descriptor in the executing shell, and that file descriptor is assigned
to NAME[1]. This pipe is established before any redirections specified
by the command (*note Redirections::). The file descriptors can be
utilized as arguments to shell commands and redirections using standard
word expansions. Other than those created to execute command and
process substitutions, the file descriptors are not available in
subshells.
The process ID of the shell spawned to execute the coprocess is
available as the value of the variable ‘NAME_PID’. The ‘wait’ builtin
may be used to wait for the coprocess to terminate.
Since the coprocess is created as an asynchronous command, the
‘coproc’ command always returns success. The return status of a
coprocess is the exit status of COMMAND.
File: bash.info, Node: GNU Parallel, Prev: Coprocesses, Up: Shell Commands
3.2.7 GNU Parallel
------------------
There are ways to run commands in parallel that are not built into Bash.
GNU Parallel is a tool to do just that.
GNU Parallel, as its name suggests, can be used to build and run
commands in parallel. You may run the same command with different
arguments, whether they are filenames, usernames, hostnames, or lines
read from files. GNU Parallel provides shorthand references to many of
the most common operations (input lines, various portions of the input
line, different ways to specify the input source, and so on). Parallel
can replace ‘xargs’ or feed commands from its input sources to several
different instances of Bash.
For a complete description, refer to the GNU Parallel documentation,
which is available at
<https://www.gnu.org/software/parallel/parallel_tutorial.html>.
File: bash.info, Node: Shell Functions, Next: Shell Parameters, Prev: Shell Commands, Up: Basic Shell Features
3.3 Shell Functions
===================
Shell functions are a way to group commands for later execution using a
single name for the group. They are executed just like a "regular"
simple command. When the name of a shell function is used as a simple
command name, the shell executes the list of commands associated with
that function name. Shell functions are executed in the current shell
context; there is no new process created to interpret them.
Functions are declared using this syntax:
FNAME () COMPOUND-COMMAND [ REDIRECTIONS ]
or
function FNAME [()] COMPOUND-COMMAND [ REDIRECTIONS ]
This defines a shell function named FNAME. The reserved word
‘function’ is optional. If the ‘function’ reserved word is supplied,
the parentheses are optional. The “body” of the function is the
compound command COMPOUND-COMMAND (*note Compound Commands::). That
command is usually a LIST enclosed between { and }, but may be any
compound command listed above. If the ‘function’ reserved word is used,
but the parentheses are not supplied, the braces are recommended. When
the shell is in POSIX mode (*note Bash POSIX Mode::), FNAME must be a
valid shell name and may not be the same as one of the special builtins
(*note Special Builtins::). When not in POSIX mode, a function name can
be any unquoted shell word that does not contain ‘$’.
Any redirections (*note Redirections::) associated with the shell
function are performed when the function is executed. Function
definitions are deleted using the ‘-f’ option to the ‘unset’ builtin
(*note Bourne Shell Builtins::).
The exit status of a function definition is zero unless a syntax
error occurs or a readonly function with the same name already exists.
When executed, the exit status of a function is the exit status of the
last command executed in the body.
Note that for historical reasons, in the most common usage the curly
braces that surround the body of the function must be separated from the
body by ‘blank’s or newlines. This is because the braces are reserved
words and are only recognized as such when they are separated from the
command list by whitespace or another shell metacharacter. When using
the braces, the LIST must be terminated by a semicolon, a ‘&’, or a
newline.
COMPOUND-COMMAND is executed whenever FNAME is specified as the name
of a simple command. Functions are executed in the context of the
calling shell; there is no new process created to interpret them
(contrast this with the execution of a shell script).
When a function is executed, the arguments to the function become the
positional parameters during its execution (*note Positional
Parameters::). The special parameter ‘#’ that expands to the number of
positional parameters is updated to reflect the new set of positional
parameters. Special parameter ‘0’ is unchanged. The first element of
the ‘FUNCNAME’ variable is set to the name of the function while the
function is executing.
All other aspects of the shell execution environment are identical
between a function and its caller with these exceptions: the ‘DEBUG’ and
‘RETURN’ traps are not inherited unless the function has been given the
‘trace’ attribute using the ‘declare’ builtin or the ‘-o functrace’
option has been enabled with the ‘set’ builtin, (in which case all
functions inherit the ‘DEBUG’ and ‘RETURN’ traps), and the ‘ERR’ trap is
not inherited unless the ‘-o errtrace’ shell option has been enabled.
*Note Bourne Shell Builtins::, for the description of the ‘trap’
builtin.
The ‘FUNCNEST’ variable, if set to a numeric value greater than 0,
defines a maximum function nesting level. Function invocations that
exceed the limit cause the entire command to abort.
If the builtin command ‘return’ is executed in a function, the
function completes and execution resumes with the next command after the
function call. Any command associated with the ‘RETURN’ trap is
executed before execution resumes. When a function completes, the
values of the positional parameters and the special parameter ‘#’ are
restored to the values they had prior to the function's execution. If
‘return’ is supplied a numeric argument, that is the function's return
status; otherwise the function's return status is the exit status of the
last command executed before the ‘return’.
Variables local to the function are declared with the ‘local’ builtin
(“local variables”). Ordinarily, variables and their values are shared
between a function and its caller. These variables are visible only to
the function and the commands it invokes. This is particularly
important when a shell function calls other functions.
In the following description, the “current scope” is a currently-
executing function. Previous scopes consist of that function's caller
and so on, back to the "global" scope, where the shell is not executing
any shell function. A local variable at the current local scope is a
variable declared using the ‘local’ or ‘declare’ builtins in the
function that is currently executing.
Local variables "shadow" variables with the same name declared at
previous scopes. For instance, a local variable declared in a function
hides variables with the same name declared at previous scopes,
including global variables: references and assignments refer to the
local variable, leaving the variables at previous scopes unmodified.
When the function returns, the global variable is once again visible.
The shell uses “dynamic scoping” to control a variable's visibility
within functions. With dynamic scoping, visible variables and their
values are a result of the sequence of function calls that caused
execution to reach the current function. The value of a variable that a
function sees depends on its value within its caller, if any, whether
that caller is the global scope or another shell function. This is also
the value that a local variable declaration shadows, and the value that
is restored when the function returns.
For example, if a variable ‘var’ is declared as local in function
‘func1’, and ‘func1’ calls another function ‘func2’, references to ‘var’
made from within ‘func2’ resolve to the local variable ‘var’ from
‘func1’, shadowing any global variable named ‘var’.
The following script demonstrates this behavior. When executed, the
script displays
In func2, var = func1 local
func1()
{
local var='func1 local'
func2
}
func2()
{
echo "In func2, var = $var"
}
var=global
func1
The ‘unset’ builtin also acts using the same dynamic scope: if a
variable is local to the current scope, ‘unset’ unsets it; otherwise the
unset will refer to the variable found in any calling scope as described
above. If a variable at the current local scope is unset, it remains so
(appearing as unset) until it is reset in that scope or until the
function returns. Once the function returns, any instance of the
variable at a previous scope becomes visible. If the unset acts on a
variable at a previous scope, any instance of a variable with that name
that had been shadowed becomes visible (see below how the
‘localvar_unset’ shell option changes this behavior).
The ‘-f’ option to the ‘declare’ (‘typeset’) builtin command (*note
Bash Builtins::) lists function names and definitions. The ‘-F’ option
to ‘declare’ or ‘typeset’ lists the function names only (and optionally
the source file and line number, if the ‘extdebug’ shell option is
enabled). Functions may be exported so that child shell processes
(those created when executing a separate shell invocation) automatically
have them defined with the ‘-f’ option to the ‘export’ builtin (*note
Bourne Shell Builtins::). The ‘-f’ option to the ‘unset’ builtin (*note
Bourne Shell Builtins::) deletes a function definition.
Functions may be recursive. The ‘FUNCNEST’ variable may be used to
limit the depth of the function call stack and restrict the number of
function invocations. By default, Bash places no limit on the number of
recursive calls.
File: bash.info, Node: Shell Parameters, Next: Shell Expansions, Prev: Shell Functions, Up: Basic Shell Features
3.4 Shell Parameters
====================
* Menu:
* Positional Parameters:: The shell's command-line arguments.
* Special Parameters:: Parameters denoted by special characters.
A “parameter” is an entity that stores values. It can be a ‘name’, a
number, or one of the special characters listed below. A “variable” is
a parameter denoted by a ‘name’. A variable has a ‘value’ and zero or
more ‘attributes’. Attributes are assigned using the ‘declare’ builtin
command (see the description of the ‘declare’ builtin in *note Bash
Builtins::). The ‘export’ and ‘readonly’ builtins assign specific
attributes.
A parameter is set if it has been assigned a value. The null string
is a valid value. Once a variable is set, it may be unset only by using
the ‘unset’ builtin command.
A variable is assigned to using a statement of the form
NAME=[VALUE]
If VALUE is not given, the variable is assigned the null string. All
VALUEs undergo tilde expansion, parameter and variable expansion,
command substitution, arithmetic expansion, and quote removal (*note
Shell Parameter Expansion::). If the variable has its ‘integer’
attribute set, then VALUE is evaluated as an arithmetic expression even
if the ‘$((...))’ expansion is not used (*note Arithmetic Expansion::).
Word splitting and filename expansion are not performed. Assignment
statements may also appear as arguments to the ‘alias’, ‘declare’,
‘typeset’, ‘export’, ‘readonly’, and ‘local’ builtin commands
(“declaration commands”). When in POSIX mode (*note Bash POSIX Mode::),
these builtins may appear in a command after one or more instances of
the ‘command’ builtin and retain these assignment statement properties.
For example,
command export var=value
In the context where an assignment statement is assigning a value to
a shell variable or array index (*note Arrays::), the ‘+=’ operator
appends to or adds to the variable's previous value. This includes
arguments to declaration commands such as ‘declare’ that accept
assignment statements. When ‘+=’ is applied to a variable for which the
‘integer’ attribute has been set, the variable's current value and VALUE
are each evaluated as arithmetic expressions, and the sum of the results
is assigned as the variable's value. The current value is usually an
integer constant, but may be an expression. When ‘+=’ is applied to an
array variable using compound assignment (*note Arrays::), the
variable's value is not unset (as it is when using ‘=’), and new values
are appended to the array beginning at one greater than the array's
maximum index (for indexed arrays), or added as additional key-value
pairs in an associative array. When applied to a string-valued
variable, VALUE is expanded and appended to the variable's value.
A variable can be assigned the ‘nameref’ attribute using the ‘-n’
option to the ‘declare’ or ‘local’ builtin commands (*note Bash
Builtins::) to create a “nameref”, or a reference to another variable.
This allows variables to be manipulated indirectly. Whenever the
nameref variable is referenced, assigned to, unset, or has its
attributes modified (other than using or changing the nameref attribute
itself), the operation is actually performed on the variable specified
by the nameref variable's value. A nameref is commonly used within
shell functions to refer to a variable whose name is passed as an
argument to the function. For instance, if a variable name is passed to
a shell function as its first argument, running
declare -n ref=$1
inside the function creates a local nameref variable ‘ref’ whose value
is the variable name passed as the first argument. References and
assignments to ‘ref’, and changes to its attributes, are treated as
references, assignments, and attribute modifications to the variable
whose name was passed as ‘$1’.
If the control variable in a ‘for’ loop has the nameref attribute,
the list of words can be a list of shell variables, and a name reference
is established for each word in the list, in turn, when the loop is
executed. Array variables cannot be given the nameref attribute.
However, nameref variables can reference array variables and subscripted
array variables. Namerefs can be unset using the ‘-n’ option to the
‘unset’ builtin (*note Bourne Shell Builtins::). Otherwise, if ‘unset’
is executed with the name of a nameref variable as an argument, the
variable referenced by the nameref variable is unset.
When the shell starts, it reads its environment and creates a shell
variable from each environment variable that has a valid name, as
described below (*note Environment::).
File: bash.info, Node: Positional Parameters, Next: Special Parameters, Up: Shell Parameters
3.4.1 Positional Parameters
---------------------------
A “positional parameter” is a parameter denoted by one or more digits,
other than the single digit ‘0’. Positional parameters are assigned
from the shell's arguments when it is invoked, and may be reassigned
using the ‘set’ builtin command. Positional parameter ‘N’ may be
referenced as ‘${N}’, or as ‘$N’ when ‘N’ consists of a single digit.
Positional parameters may not be assigned to with assignment statements.
The ‘set’ and ‘shift’ builtins are used to set and unset them (*note
Shell Builtin Commands::). The positional parameters are temporarily
replaced when a shell function is executed (*note Shell Functions::).
When a positional parameter consisting of more than a single digit is
expanded, it must be enclosed in braces. Without braces, a digit
following ‘$’ can only refer to one of the first nine positional
parameters ($1\-$9) or the special parameter $0 (see below).
File: bash.info, Node: Special Parameters, Prev: Positional Parameters, Up: Shell Parameters
3.4.2 Special Parameters
------------------------
The shell treats several parameters specially. These parameters may
only be referenced; assignment to them is not allowed. Special
parameters are denoted by one of the following characters.
‘*’
($*) Expands to the positional parameters, starting from one. When
the expansion is not within double quotes, each positional
parameter expands to a separate word. In contexts where word
expansions are performed, those words are subject to further word
splitting and filename expansion. When the expansion occurs within
double quotes, it expands to a single word with the value of each
parameter separated by the first character of the ‘IFS’ variable.
That is, ‘"$*"’ is equivalent to ‘"$1C$2C..."’, where C is the
first character of the value of the ‘IFS’ variable. If ‘IFS’ is
unset, the parameters are separated by spaces. If ‘IFS’ is null,
the parameters are joined without intervening separators.
‘@’
($@) Expands to the positional parameters, starting from one. In
contexts where word splitting is performed, this expands each
positional parameter to a separate word; if not within double
quotes, these words are subject to word splitting. In contexts
where word splitting is not performed, such as the value portion of
an assignment statement, this expands to a single word with each
positional parameter separated by a space. When the expansion
occurs within double quotes, and word splitting is performed, each
parameter expands to a separate word. That is, ‘"$@"’ is
equivalent to ‘"$1" "$2" ...’. If the double-quoted expansion
occurs within a word, the expansion of the first parameter is
joined with the expansion of the beginning part of the original
word, and the expansion of the last parameter is joined with the
expansion of the last part of the original word. When there are no
positional parameters, ‘"$@"’ and ‘$@’ expand to nothing (i.e.,
they are removed).
‘#’
($#) Expands to the number of positional parameters in decimal.
‘?’
($?) Expands to the exit status of the most recently executed
command.
‘-’
($-, a hyphen.) Expands to the current option flags as specified
upon invocation, by the ‘set’ builtin command, or those set by the
shell itself (such as the ‘-i’ option).
‘$’
($$) Expands to the process ID of the shell. In a subshell, it
expands to the process ID of the invoking shell, not the subshell.
‘!’
($!) Expands to the process ID of the job most recently placed
into the background, whether executed as an asynchronous command or
using the ‘bg’ builtin (*note Job Control Builtins::).
‘0’
($0) Expands to the name of the shell or shell script. This is set
at shell initialization. If Bash is invoked with a file of
commands (*note Shell Scripts::), ‘$0’ is set to the name of that
file. If Bash is started with the ‘-c’ option (*note Invoking
Bash::), then ‘$0’ is set to the first argument after the string to
be executed, if one is present. Otherwise, it is set to the
filename used to invoke Bash, as given by argument zero.
File: bash.info, Node: Shell Expansions, Next: Redirections, Prev: Shell Parameters, Up: Basic Shell Features
3.5 Shell Expansions
====================
Expansion is performed on the command line after it has been split into
‘token’s. Bash performs these expansions:
• brace expansion
• tilde expansion
• parameter and variable expansion
• command substitution
• arithmetic expansion
• word splitting
• filename expansion
• quote removal
* Menu:
* Brace Expansion:: Expansion of expressions within braces.
* Tilde Expansion:: Expansion of the ~ character.
* Shell Parameter Expansion:: How Bash expands variables to their values.
* Command Substitution:: Using the output of a command as an argument.
* Arithmetic Expansion:: How to use arithmetic in shell expansions.
* Process Substitution:: A way to write and read to and from a
command.
* Word Splitting:: How the results of expansion are split into separate
arguments.
* Filename Expansion:: A shorthand for specifying filenames matching patterns.
* Quote Removal:: How and when quote characters are removed from
words.
The order of expansions is: brace expansion; tilde expansion,
parameter and variable expansion, arithmetic expansion, and command
substitution (done in a left-to-right fashion); word splitting; filename
expansion; and quote removal.
On systems that can support it, there is an additional expansion
available: “process substitution”. This is performed at the same time
as tilde, parameter, variable, and arithmetic expansion and command
substitution.
“Quote removal” is always performed last. It removes quote
characters present in the original word, not ones resulting from one of
the other expansions, unless they have been quoted themselves. *Note
Quote Removal:: for more details.
Only brace expansion, word splitting, and filename expansion can
increase the number of words of the expansion; other expansions expand a
single word to a single word. The only exceptions to this are the
expansions of ‘"$@"’ and ‘$*’ (*note Special Parameters::), and
‘"${NAME[@]}"’ and ‘${NAME[*]}’ (*note Arrays::).
File: bash.info, Node: Brace Expansion, Next: Tilde Expansion, Up: Shell Expansions
3.5.1 Brace Expansion
---------------------
Brace expansion is a mechanism to generate arbitrary strings sharing a
common prefix and suffix, either of which can be empty. This mechanism
is similar to “filename expansion” (*note Filename Expansion::), but the
filenames generated need not exist. Patterns to be brace expanded are
formed from an optional PREAMBLE, followed by either a series of
comma-separated strings or a sequence expression between a pair of
braces, followed by an optional POSTSCRIPT. The preamble is prefixed to
each string contained within the braces, and the postscript is then
appended to each resulting string, expanding left to right.
Brace expansions may be nested. The results of each expanded string
are not sorted; brace expansion preserves left to right order. For
example,
bash$ echo a{d,c,b}e
ade ace abe
A sequence expression takes the form ‘X..Y[..INCR]’, where X and Y
are either integers or letters, and INCR, an optional increment, is an
integer. When integers are supplied, the expression expands to each
number between X and Y, inclusive. If either X or Y begins with a zero,
each generated term will contain the same number of digits, zero-padding
where necessary. When letters are supplied, the expression expands to
each character lexicographically between X and Y, inclusive, using the C
locale. Note that both X and Y must be of the same type (integer or
letter). When the increment is supplied, it is used as the difference
between each term. The default increment is 1 or -1 as appropriate.
Brace expansion is performed before any other expansions, and any
characters special to other expansions are preserved in the result. It
is strictly textual. Bash does not apply any syntactic interpretation
to the context of the expansion or the text between the braces.
A correctly-formed brace expansion must contain unquoted opening and
closing braces, and at least one unquoted comma or a valid sequence
expression. Any incorrectly formed brace expansion is left unchanged.
A ‘{’ or ‘,’ may be quoted with a backslash to prevent its being
considered part of a brace expression. To avoid conflicts with
parameter expansion, the string ‘${’ is not considered eligible for
brace expansion, and inhibits brace expansion until the closing ‘}’.
This construct is typically used as shorthand when the common prefix
of the strings to be generated is longer than in the above example:
mkdir /usr/local/src/bash/{old,new,dist,bugs}
or
chown root /usr/{ucb/{ex,edit},lib/{ex?.?*,how_ex}}
Brace expansion introduces a slight incompatibility with historical
versions of ‘sh’. ‘sh’ does not treat opening or closing braces
specially when they appear as part of a word, and preserves them in the
output. Bash removes braces from words as a consequence of brace
expansion. For example, a word entered to ‘sh’ as ‘file{1,2}’ appears
identically in the output. Bash outputs that word as ‘file1 file2’
after brace expansion. Start Bash with the ‘+B’ option or disable brace
expansion with the ‘+B’ option to the ‘set’ command (*note Shell Builtin
Commands::) for strict ‘sh’ compatibility.
File: bash.info, Node: Tilde Expansion, Next: Shell Parameter Expansion, Prev: Brace Expansion, Up: Shell Expansions
3.5.2 Tilde Expansion
---------------------
If a word begins with an unquoted tilde character (‘~’), all of the
characters up to the first unquoted slash (or all characters, if there
is no unquoted slash) are considered a “tilde-prefix”. If none of the
characters in the tilde-prefix are quoted, the characters in the
tilde-prefix following the tilde are treated as a possible “login name”.
If this login name is the null string, the tilde is replaced with the
value of the ‘HOME’ shell variable. If ‘HOME’ is unset, the tilde
expands to the home directory of the user executing the shell instead.
Otherwise, the tilde-prefix is replaced with the home directory
associated with the specified login name.
If the tilde-prefix is ‘~+’, the value of the shell variable ‘PWD’
replaces the tilde-prefix. If the tilde-prefix is ‘~-’, the shell
substitutes the value of the shell variable ‘OLDPWD’, if it is set.
If the characters following the tilde in the tilde-prefix consist of
a number N, optionally prefixed by a ‘+’ or a ‘-’, the tilde-prefix is
replaced with the corresponding element from the directory stack, as it
would be displayed by the ‘dirs’ builtin invoked with the characters
following tilde in the tilde-prefix as an argument (*note The Directory
Stack::). If the tilde-prefix, sans the tilde, consists of a number
without a leading ‘+’ or ‘-’, tilde expansion assumes ‘+’.
The results of tilde expansion are treated as if they were quoted, so
the replacement is not subject to word splitting and filename expansion.
If the login name is invalid, or the tilde expansion fails, the
tilde-prefix is left unchanged.
Bash checks each variable assignment for unquoted tilde-prefixes
immediately following a ‘:’ or the first ‘=’, and performs tilde
expansion in these cases. Consequently, one may use filenames with
tildes in assignments to ‘PATH’, ‘MAILPATH’, and ‘CDPATH’, and the shell
assigns the expanded value.
The following table shows how Bash treats unquoted tilde-prefixes:
‘~’
The value of ‘$HOME’.
‘~/foo’
‘$HOME/foo’
‘~fred/foo’
The directory or file ‘foo’ in the home directory of the user
‘fred’.
‘~+/foo’
‘$PWD/foo’
‘~-/foo’
‘${OLDPWD-'~-'}/foo’
‘~N’
The string that would be displayed by ‘dirs +N’.
‘~+N’
The string that would be displayed by ‘dirs +N’.
‘~-N’
The string that would be displayed by ‘dirs -N’.
Bash also performs tilde expansion on words satisfying the conditions
of variable assignments (*note Shell Parameters::) when they appear as
arguments to simple commands. Bash does not do this, except for the
declaration commands listed above, when in POSIX mode.
File: bash.info, Node: Shell Parameter Expansion, Next: Command Substitution, Prev: Tilde Expansion, Up: Shell Expansions
3.5.3 Shell Parameter Expansion
-------------------------------
The ‘$’ character introduces parameter expansion, command substitution,
or arithmetic expansion. The parameter name or symbol to be expanded
may be enclosed in braces, which are optional but serve to protect the
variable to be expanded from characters immediately following it which
could be interpreted as part of the name. For example, if the first
positional parameter has the value ‘a’, then ‘${11}’ expands to the
value of the eleventh positional parameter, while ‘$11’ expands to ‘a1’.
When braces are used, the matching ending brace is the first ‘}’ not
escaped by a backslash or within a quoted string, and not within an
embedded arithmetic expansion, command substitution, or parameter
expansion.
The basic form of parameter expansion is ${PARAMETER}, which
substitutes the value of PARAMETER. The PARAMETER is a shell parameter
as described above (*note Shell Parameters::) or an array reference
(*note Arrays::). The braces are required when PARAMETER is a
positional parameter with more than one digit, or when PARAMETER is
followed by a character that is not to be interpreted as part of its
name.
If the first character of PARAMETER is an exclamation point (!), and
PARAMETER is not a nameref, it introduces a level of indirection. Bash
uses the value formed by expanding the rest of PARAMETER as the new
PARAMETER; this new parameter is then expanded and that value is used in
the rest of the expansion, rather than the expansion of the original
PARAMETER. This is known as ‘indirect expansion’. The value is subject
to tilde expansion, parameter expansion, command substitution, and
arithmetic expansion. If PARAMETER is a nameref, this expands to the
name of the variable referenced by PARAMETER instead of performing the
complete indirect expansion, for compatibility. The exceptions to this
are the expansions of ${!PREFIX*} and ${!NAME[@]} described below. The
exclamation point must immediately follow the left brace in order to
introduce indirection.
In each of the cases below, WORD is subject to tilde expansion,
parameter expansion, command substitution, and arithmetic expansion.
When not performing substring expansion, using the forms described
below (e.g., ‘:-’), Bash tests for a parameter that is unset or null.
Omitting the colon results in a test only for a parameter that is unset.
Put another way, if the colon is included, the operator tests for both
PARAMETER's existence and that its value is not null; if the colon is
omitted, the operator tests only for existence.
‘${PARAMETER:−WORD}’
If PARAMETER is unset or null, the expansion of WORD is
substituted. Otherwise, the value of PARAMETER is substituted.
$ v=123
$ echo ${v-unset}
123
$ echo ${v:-unset-or-null}
123
$ unset v
$ echo ${v-unset}
unset
$ v=
$ echo ${v-unset}
$ echo ${v:-unset-or-null}
unset-or-null
‘${PARAMETER:=WORD}’
If PARAMETER is unset or null, the expansion of WORD is assigned to
PARAMETER, and the result of the expansion is the final value of
PARAMETER. Positional parameters and special parameters may not be
assigned in this way.
$ unset var
$ : ${var=DEFAULT}
$ echo $var
DEFAULT
$ var=
$ : ${var=DEFAULT}
$ echo $var
$ var=
$ : ${var:=DEFAULT}
$ echo $var
DEFAULT
$ unset var
$ : ${var:=DEFAULT}
$ echo $var
DEFAULT
‘${PARAMETER:?WORD}’
If PARAMETER is null or unset, the shell writes the expansion of
WORD (or a message to that effect if WORD is not present) to the
standard error and, if it is not interactive, exits with a non-zero
status. An interactive shell does not exit, but does not execute
the command associated with the expansion. Otherwise, the value of
PARAMETER is substituted.
$ var=
$ : ${var:?var is unset or null}
bash: var: var is unset or null
$ echo ${var?var is unset}
$ unset var
$ : ${var?var is unset}
bash: var: var is unset
$ : ${var:?var is unset or null}
bash: var: var is unset or null
$ var=123
$ echo ${var:?var is unset or null}
123
‘${PARAMETER:+WORD}’
If PARAMETER is null or unset, nothing is substituted, otherwise
the expansion of WORD is substituted. The value of PARAMETER is
not used.
$ var=123
$ echo ${var:+var is set and not null}
var is set and not null
$ echo ${var+var is set}
var is set
$ var=
$ echo ${var:+var is set and not null}
$ echo ${var+var is set}
var is set
$ unset var
$ echo ${var+var is set}
$ echo ${var:+var is set and not null}
$
‘${PARAMETER:OFFSET}’
‘${PARAMETER:OFFSET:LENGTH}’
This is referred to as Substring Expansion. It expands to up to
LENGTH characters of the value of PARAMETER starting at the
character specified by OFFSET. If PARAMETER is ‘@’ or ‘*’, an
indexed array subscripted by ‘@’ or ‘*’, or an associative array
name, the results differ as described below. If :LENGTH is omitted
(the first form above), this expands to the substring of the value
of PARAMETER starting at the character specified by OFFSET and
extending to the end of the value. If OFFSET is omitted, it is
treated as 0. If LENGTH is omitted, but the colon after OFFSET is
present, it is treated as 0. LENGTH and OFFSET are arithmetic
expressions (*note Shell Arithmetic::).
If OFFSET evaluates to a number less than zero, the value is used
as an offset in characters from the end of the value of PARAMETER.
If LENGTH evaluates to a number less than zero, it is interpreted
as an offset in characters from the end of the value of PARAMETER
rather than a number of characters, and the expansion is the
characters between OFFSET and that result.
Note that a negative offset must be separated from the colon by at
least one space to avoid being confused with the ‘:-’ expansion.
Here are some examples illustrating substring expansion on
parameters and subscripted arrays:
$ string=01234567890abcdefgh
$ echo ${string:7}
7890abcdefgh
$ echo ${string:7:0}
$ echo ${string:7:2}
78
$ echo ${string:7:-2}
7890abcdef
$ echo ${string: -7}
bcdefgh
$ echo ${string: -7:0}
$ echo ${string: -7:2}
bc
$ echo ${string: -7:-2}
bcdef
$ set -- 01234567890abcdefgh
$ echo ${1:7}
7890abcdefgh
$ echo ${1:7:0}
$ echo ${1:7:2}
78
$ echo ${1:7:-2}
7890abcdef
$ echo ${1: -7}
bcdefgh
$ echo ${1: -7:0}
$ echo ${1: -7:2}
bc
$ echo ${1: -7:-2}
bcdef
$ array[0]=01234567890abcdefgh
$ echo ${array[0]:7}
7890abcdefgh
$ echo ${array[0]:7:0}
$ echo ${array[0]:7:2}
78
$ echo ${array[0]:7:-2}
7890abcdef
$ echo ${array[0]: -7}
bcdefgh
$ echo ${array[0]: -7:0}
$ echo ${array[0]: -7:2}
bc
$ echo ${array[0]: -7:-2}
bcdef
If PARAMETER is ‘@’ or ‘*’, the result is LENGTH positional
parameters beginning at OFFSET. A negative OFFSET is taken
relative to one greater than the greatest positional parameter, so
an offset of -1 evaluates to the last positional parameter (or 0 if
there are no positional parameters). It is an expansion error if
LENGTH evaluates to a number less than zero.
The following examples illustrate substring expansion using
positional parameters:
$ set -- 1 2 3 4 5 6 7 8 9 0 a b c d e f g h
$ echo ${@:7}
7 8 9 0 a b c d e f g h
$ echo ${@:7:0}
$ echo ${@:7:2}
7 8
$ echo ${@:7:-2}
bash: -2: substring expression < 0
$ echo ${@: -7:2}
b c
$ echo ${@:0}
./bash 1 2 3 4 5 6 7 8 9 0 a b c d e f g h
$ echo ${@:0:2}
./bash 1
$ echo ${@: -7:0}
If PARAMETER is an indexed array name subscripted by ‘@’ or ‘*’,
the result is the LENGTH members of the array beginning with
‘${PARAMETER[OFFSET]}’. A negative OFFSET is taken relative to one
greater than the maximum index of the specified array. It is an
expansion error if LENGTH evaluates to a number less than zero.
These examples show how you can use substring expansion with
indexed arrays:
$ array=(0 1 2 3 4 5 6 7 8 9 0 a b c d e f g h)
$ echo ${array[@]:7}
7 8 9 0 a b c d e f g h
$ echo ${array[@]:7:2}
7 8
$ echo ${array[@]: -7:2}
b c
$ echo ${array[@]: -7:-2}
bash: -2: substring expression < 0
$ echo ${array[@]:0}
0 1 2 3 4 5 6 7 8 9 0 a b c d e f g h
$ echo ${array[@]:0:2}
0 1
$ echo ${array[@]: -7:0}
Substring expansion applied to an associative array produces
undefined results.
Substring indexing is zero-based unless the positional parameters
are used, in which case the indexing starts at 1 by default. If
OFFSET is 0, and the positional parameters are used, ‘$0’ is
prefixed to the list.
‘${!PREFIX*}’
‘${!PREFIX@}’
Expands to the names of variables whose names begin with PREFIX,
separated by the first character of the ‘IFS’ special variable.
When ‘@’ is used and the expansion appears within double quotes,
each variable name expands to a separate word.
‘${!NAME[@]}’
‘${!NAME[*]}’
If NAME is an array variable, expands to the list of array indices
(keys) assigned in NAME. If NAME is not an array, expands to 0 if
NAME is set and null otherwise. When ‘@’ is used and the expansion
appears within double quotes, each key expands to a separate word.
‘${#PARAMETER}’
Substitutes the length in characters of the value of PARAMETER. If
PARAMETER is ‘*’ or ‘@’, the value substituted is the number of
positional parameters. If PARAMETER is an array name subscripted
by ‘*’ or ‘@’, the value substituted is the number of elements in
the array. If PARAMETER is an indexed array name subscripted by a
negative number, that number is interpreted as relative to one
greater than the maximum index of PARAMETER, so negative indices
count back from the end of the array, and an index of -1 references
the last element.
‘${PARAMETER#WORD}’
‘${PARAMETER##WORD}’
The WORD is expanded to produce a pattern and matched against the
expanded value of PARAMETER according to the rules described below
(*note Pattern Matching::). If the pattern matches the beginning
of the expanded value of PARAMETER, then the result of the
expansion is the expanded value of PARAMETER with the shortest
matching pattern (the ‘#’ case) or the longest matching pattern
(the ‘##’ case) deleted. If PARAMETER is ‘@’ or ‘*’, the pattern
removal operation is applied to each positional parameter in turn,
and the expansion is the resultant list. If PARAMETER is an array
variable subscripted with ‘@’ or ‘*’, the pattern removal operation
is applied to each member of the array in turn, and the expansion
is the resultant list.
‘${PARAMETER%WORD}’
‘${PARAMETER%%WORD}’
The WORD is expanded to produce a pattern and matched against the
expanded value of PARAMETER according to the rules described below
(*note Pattern Matching::). If the pattern matches a trailing
portion of the expanded value of PARAMETER, then the result of the
expansion is the value of PARAMETER with the shortest matching
pattern (the ‘%’ case) or the longest matching pattern (the ‘%%’
case) deleted. If PARAMETER is ‘@’ or ‘*’, the pattern removal
operation is applied to each positional parameter in turn, and the
expansion is the resultant list. If PARAMETER is an array variable
subscripted with ‘@’ or ‘*’, the pattern removal operation is
applied to each member of the array in turn, and the expansion is
the resultant list.
‘${PARAMETER/PATTERN/STRING}’
‘${PARAMETER//PATTERN/STRING}’
‘${PARAMETER/#PATTERN/STRING}’
‘${PARAMETER/%PATTERN/STRING}’
The PATTERN is expanded to produce a pattern and matched against
the expanded value of PARAMETER as described below (*note Pattern
Matching::). The longest match of PATTERN in the expanded value is
replaced with STRING. STRING undergoes tilde expansion, parameter
and variable expansion, arithmetic expansion, command and process
substitution, and quote removal.
In the first form above, only the first match is replaced. If
there are two slashes separating PARAMETER and PATTERN (the second
form above), all matches of PATTERN are replaced with STRING. If
PATTERN is preceded by ‘#’ (the third form above), it must match at
the beginning of the expanded value of PARAMETER. If PATTERN is
preceded by ‘%’ (the fourth form above), it must match at the end
of the expanded value of PARAMETER.
If the expansion of STRING is null, matches of PATTERN are deleted
and the ‘/’ following PATTERN may be omitted.
If the ‘patsub_replacement’ shell option is enabled using ‘shopt’
(*note The Shopt Builtin::), any unquoted instances of ‘&’ in
STRING are replaced with the matching portion of PATTERN. This is
intended to duplicate a common ‘sed’ idiom.
Quoting any part of STRING inhibits replacement in the expansion of
the quoted portion, including replacement strings stored in shell
variables. Backslash escapes ‘&’ in STRING; the backslash is
removed in order to permit a literal ‘&’ in the replacement string.
Users should take care if STRING is double-quoted to avoid unwanted
interactions between the backslash and double-quoting, since
backslash has special meaning within double quotes. Pattern
substitution performs the check for unquoted ‘&’ after expanding
STRING, so users should ensure to properly quote any occurrences of
‘&’ they want to be taken literally in the replacement and ensure
any instances of ‘&’ they want to be replaced are unquoted.
For instance,
var=abcdef
rep='& '
echo ${var/abc/& }
echo "${var/abc/& }"
echo ${var/abc/$rep}
echo "${var/abc/$rep}"
will display four lines of "abc def", while
var=abcdef
rep='& '
echo ${var/abc/\& }
echo "${var/abc/\& }"
echo ${var/abc/"& "}
echo ${var/abc/"$rep"}
will display four lines of "& def". Like the pattern removal
operators, double quotes surrounding the replacement string quote
the expanded characters, while double quotes enclosing the entire
parameter substitution do not, since the expansion is performed in
a context that doesn't take any enclosing double quotes into
account.
Since backslash can escape ‘&’, it can also escape a backslash in
the replacement string. This means that ‘\\’ will insert a literal
backslash into the replacement, so these two ‘echo’ commands
var=abcdef
rep='\\&xyz'
echo ${var/abc/\\&xyz}
echo ${var/abc/$rep}
will both output ‘\abcxyzdef’.
It should rarely be necessary to enclose only STRING in double
quotes.
If the ‘nocasematch’ shell option (see the description of ‘shopt’
in *note The Shopt Builtin::) is enabled, the match is performed
without regard to the case of alphabetic characters.
If PARAMETER is ‘@’ or ‘*’, the substitution operation is applied
to each positional parameter in turn, and the expansion is the
resultant list. If PARAMETER is an array variable subscripted with
‘@’ or ‘*’, the substitution operation is applied to each member of
the array in turn, and the expansion is the resultant list.
‘${PARAMETER^PATTERN}’
‘${PARAMETER^^PATTERN}’
‘${PARAMETER,PATTERN}’
‘${PARAMETER,,PATTERN}’
This expansion modifies the case of alphabetic characters in
PARAMETER. First, the PATTERN is expanded to produce a pattern as
described below in *note Pattern Matching::.
‘Bash’ then examines characters in the expanded value of PARAMETER
against PATTERN as described below. If a character matches the
pattern, its case is converted. The pattern should not attempt to
match more than one character.
Using ‘^’ converts lowercase letters matching PATTERN to uppercase;
‘,’ converts matching uppercase letters to lowercase. The ‘^’ and
‘,’ variants examine the first character in the expanded value and
convert its case if it matches PATTERN; the ‘^^’ and ‘,,’ variants
examine all characters in the expanded value and convert each one
that matches PATTERN. If PATTERN is omitted, it is treated like a
‘?’, which matches every character.
If PARAMETER is ‘@’ or ‘*’, the case modification operation is
applied to each positional parameter in turn, and the expansion is
the resultant list. If PARAMETER is an array variable subscripted
with ‘@’ or ‘*’, the case modification operation is applied to each
member of the array in turn, and the expansion is the resultant
list.
‘${PARAMETER@OPERATOR}’
The expansion is either a transformation of the value of PARAMETER
or information about PARAMETER itself, depending on the value of
OPERATOR. Each OPERATOR is a single letter:
‘U’
The expansion is a string that is the value of PARAMETER with
lowercase alphabetic characters converted to uppercase.
‘u’
The expansion is a string that is the value of PARAMETER with
the first character converted to uppercase, if it is
alphabetic.
‘L’
The expansion is a string that is the value of PARAMETER with
uppercase alphabetic characters converted to lowercase.
‘Q’
The expansion is a string that is the value of PARAMETER
quoted in a format that can be reused as input.
‘E’
The expansion is a string that is the value of PARAMETER with
backslash escape sequences expanded as with the ‘$'...'’
quoting mechanism.
‘P’
The expansion is a string that is the result of expanding the
value of PARAMETER as if it were a prompt string (*note
Controlling the Prompt::).
‘A’
The expansion is a string in the form of an assignment
statement or ‘declare’ command that, if evaluated, recreates
PARAMETER with its attributes and value.
‘K’
Produces a possibly-quoted version of the value of PARAMETER,
except that it prints the values of indexed and associative
arrays as a sequence of quoted key-value pairs (*note
Arrays::). The keys and values are quoted in a format that
can be reused as input.
‘a’
The expansion is a string consisting of flag values
representing PARAMETER's attributes.
‘k’
Like the ‘K’ transformation, but expands the keys and values
of indexed and associative arrays to separate words after word
splitting.
If PARAMETER is ‘@’ or ‘*’, the operation is applied to each
positional parameter in turn, and the expansion is the resultant
list. If PARAMETER is an array variable subscripted with ‘@’ or
‘*’, the operation is applied to each member of the array in turn,
and the expansion is the resultant list.
The result of the expansion is subject to word splitting and
filename expansion as described below.
File: bash.info, Node: Command Substitution, Next: Arithmetic Expansion, Prev: Shell Parameter Expansion, Up: Shell Expansions
3.5.4 Command Substitution
--------------------------
Command substitution allows the output of a command to replace the
command itself. The standard form of command substitution occurs when a
command is enclosed as follows:
$(COMMAND)
or (deprecated)
`COMMAND`.
Bash performs command substitution by executing COMMAND in a subshell
environment and replacing the command substitution with the standard
output of the command, with any trailing newlines deleted. Embedded
newlines are not deleted, but they may be removed during word splitting.
The command substitution ‘$(cat FILE)’ can be replaced by the equivalent
but faster ‘$(< FILE)’.
With the old-style backquote form of substitution, backslash retains
its literal meaning except when followed by ‘$’, ‘`’, or ‘\’. The first
backquote not preceded by a backslash terminates the command
substitution. When using the ‘$(COMMAND)’ form, all characters between
the parentheses make up the command; none are treated specially.
There is an alternate form of command substitution:
${C COMMAND; }
which executes COMMAND in the current execution environment and captures
its output, again with trailing newlines removed.
The character C following the open brace must be a space, tab,
newline, or ‘|’, and the close brace must be in a position where a
reserved word may appear (i.e., preceded by a command terminator such as
semicolon). Bash allows the close brace to be joined to the remaining
characters in the word without being followed by a shell metacharacter
as a reserved word would usually require.
Any side effects of COMMAND take effect immediately in the current
execution environment and persist in the current environment after the
command completes (e.g., the ‘exit’ builtin exits the shell).
This type of command substitution superficially resembles executing
an unnamed shell function: local variables are created as when a shell
function is executing, and the ‘return’ builtin forces COMMAND to
complete; however, the rest of the execution environment, including the
positional parameters, is shared with the caller.
If the first character following the open brace is a ‘|’, the
construct expands to the value of the ‘REPLY’ shell variable after
COMMAND executes, without removing any trailing newlines, and the
standard output of COMMAND remains the same as in the calling shell.
Bash creates ‘REPLY’ as an initially-unset local variable when COMMAND
executes, and restores ‘REPLY’ to the value it had before the command
substitution after COMMAND completes, as with any local variable.
For example, this construct expands to ‘12345’, and leaves the shell
variable ‘X’ unchanged in the current execution environment:
${ local X=12345 ; echo $X; }
(not declaring ‘X’ as local would modify its value in the current
environment, as with normal shell function execution), while this
construct does not require any output to expand to ‘12345’:
${| REPLY=12345; }
and restores ‘REPLY’ to the value it had before the command
substitution.
Command substitutions may be nested. To nest when using the
backquoted form, escape the inner backquotes with backslashes.
If the substitution appears within double quotes, Bash does not
perform word splitting and filename expansion on the results.
File: bash.info, Node: Arithmetic Expansion, Next: Process Substitution, Prev: Command Substitution, Up: Shell Expansions
3.5.5 Arithmetic Expansion
--------------------------
Arithmetic expansion evaluates an arithmetic expression and substitutes
the result. The format for arithmetic expansion is:
$(( EXPRESSION ))
The EXPRESSION undergoes the same expansions as if it were within
double quotes, but unescaped double quote characters in EXPRESSION are
not treated specially and are removed. All tokens in the expression
undergo parameter and variable expansion, command substitution, and
quote removal. The result is treated as the arithmetic expression to be
evaluated. Since the way Bash handles double quotes can potentially
result in empty strings, arithmetic expansion treats those as
expressions that evaluate to 0. Arithmetic expansions may be nested.
The evaluation is performed according to the rules listed below
(*note Shell Arithmetic::). If the expression is invalid, Bash prints a
message indicating failure to the standard error, does not perform the
substitution, and does not execute the command associated with the
expansion.
File: bash.info, Node: Process Substitution, Next: Word Splitting, Prev: Arithmetic Expansion, Up: Shell Expansions
3.5.6 Process Substitution
--------------------------
Process substitution allows a process's input or output to be referred
to using a filename. It takes the form of
<(LIST)
or
>(LIST)
The process LIST is run asynchronously, and its input or output appears
as a filename. This filename is passed as an argument to the current
command as the result of the expansion.
If the ‘>(LIST)’ form is used, writing to the file provides input for
LIST. If the ‘<(LIST)’ form is used, reading the file obtains the
output of LIST. Note that no space may appear between the ‘<’ or ‘>’
and the left parenthesis, otherwise the construct would be interpreted
as a redirection.
Process substitution is supported on systems that support named pipes
(FIFOs) or the ‘/dev/fd’ method of naming open files.
When available, process substitution is performed simultaneously with
parameter and variable expansion, command substitution, and arithmetic
expansion.
File: bash.info, Node: Word Splitting, Next: Filename Expansion, Prev: Process Substitution, Up: Shell Expansions
3.5.7 Word Splitting
--------------------
The shell scans the results of parameter expansion, command
substitution, and arithmetic expansion that did not occur within double
quotes for word splitting. Words that were not expanded are not split.
The shell treats each character of ‘$IFS’ as a delimiter, and splits
the results of the other expansions into fields using these characters
as field terminators.
An “IFS whitespace” character is whitespace as defined above (*note
Definitions::) that appears in the value of ‘IFS’. Space, tab, and
newline are always considered IFS whitespace, even if they don't appear
in the locale's ‘space’ category.
If ‘IFS’ is unset, word splitting behaves as if its value were
‘<space><tab><newline>’, and treats these characters as IFS whitespace.
If the value of ‘IFS’ is null, no word splitting occurs, but implicit
null arguments (see below) are still removed.
Word splitting begins by removing sequences of IFS whitespace
characters from the beginning and end of the results of the previous
expansions, then splits the remaining words.
If the value of ‘IFS’ consists solely of IFS whitespace, any sequence
of IFS whitespace characters delimits a field, so a field consists of
characters that are not unquoted IFS whitespace, and null fields result
only from quoting.
If ‘IFS’ contains a non-whitespace character, then any character in
the value of ‘IFS’ that is not IFS whitespace, along with any adjacent
IFS whitespace characters, delimits a field. This means that adjacent
non-IFS-whitespace delimiters produce a null field. A sequence of IFS
whitespace characters also delimits a field.
Explicit null arguments (‘""’ or ‘''’) are retained and passed to
commands as empty strings. Unquoted implicit null arguments, resulting
from the expansion of parameters that have no values, are removed.
Expanding a parameter with no value within double quotes produces a null
field, which is retained and passed to a command as an empty string.
When a quoted null argument appears as part of a word whose expansion
is non-null, word splitting removes the null argument portion, leaving
the non-null expansion. That is, the word ‘-d''’ becomes ‘-d’ after
word splitting and null argument removal.
File: bash.info, Node: Filename Expansion, Next: Quote Removal, Prev: Word Splitting, Up: Shell Expansions
3.5.8 Filename Expansion
------------------------
* Menu:
* Pattern Matching:: How the shell matches patterns.
After word splitting, unless the ‘-f’ option has been set (*note The Set
Builtin::), Bash scans each word for the characters ‘*’, ‘?’, and ‘[’.
If one of these characters appears, and is not quoted, then the word is
regarded as a PATTERN, and replaced with a sorted list of filenames
matching the pattern (*note Pattern Matching::), subject to the value of
the ‘GLOBSORT’ shell variable (*note Bash Variables::).
If no matching filenames are found, and the shell option ‘nullglob’
is disabled, the word is left unchanged. If the ‘nullglob’ option is
set, and no matches are found, the word is removed. If the ‘failglob’
shell option is set, and no matches are found, Bash prints an error
message and does not execute the command. If the shell option
‘nocaseglob’ is enabled, the match is performed without regard to the
case of alphabetic characters.
When a pattern is used for filename expansion, the character ‘.’ at
the start of a filename or immediately following a slash must be matched
explicitly, unless the shell option ‘dotglob’ is set. In order to match
the filenames ‘.’ and ‘..’, the pattern must begin with ‘.’ (for
example, ‘.?’), even if ‘dotglob’ is set. If the ‘globskipdots’ shell
option is enabled, the filenames ‘.’ and ‘..’ never match, even if the
pattern begins with a ‘.’. When not matching filenames, the ‘.’
character is not treated specially.
When matching a filename, the slash character must always be matched
explicitly by a slash in the pattern, but in other matching contexts it
can be matched by a special pattern character as described below (*note
Pattern Matching::).
See the description of ‘shopt’ in *note The Shopt Builtin::, for a
description of the ‘nocaseglob’, ‘nullglob’, ‘globskipdots’, ‘failglob’,
and ‘dotglob’ options.
The ‘GLOBIGNORE’ shell variable may be used to restrict the set of
file names matching a pattern. If ‘GLOBIGNORE’ is set, each matching
file name that also matches one of the patterns in ‘GLOBIGNORE’ is
removed from the list of matches. If the ‘nocaseglob’ option is set,
the matching against the patterns in ‘GLOBIGNORE’ is performed without
regard to case. The filenames ‘.’ and ‘..’ are always ignored when
‘GLOBIGNORE’ is set and not null. However, setting ‘GLOBIGNORE’ to a
non-null value has the effect of enabling the ‘dotglob’ shell option, so
all other filenames beginning with a ‘.’ match. To get the old behavior
of ignoring filenames beginning with a ‘.’, make ‘.*’ one of the
patterns in ‘GLOBIGNORE’. The ‘dotglob’ option is disabled when
‘GLOBIGNORE’ is unset. The ‘GLOBIGNORE’ pattern matching honors the
setting of the ‘extglob’ shell option.
The value of the ‘GLOBSORT’ shell variable controls how the results
of pathname expansion are sorted, as described below (*note Bash
Variables::).
File: bash.info, Node: Pattern Matching, Up: Filename Expansion
3.5.8.1 Pattern Matching
........................
Any character that appears in a pattern, other than the special pattern
characters described below, matches itself. The NUL character may not
occur in a pattern. A backslash escapes the following character; the
escaping backslash is discarded when matching. The special pattern
characters must be quoted if they are to be matched literally.
The special pattern characters have the following meanings:
‘*’
Matches any string, including the null string. When the ‘globstar’
shell option is enabled, and ‘*’ is used in a filename expansion
context, two adjacent ‘*’s used as a single pattern match all files
and zero or more directories and subdirectories. If followed by a
‘/’, two adjacent ‘*’s match only directories and subdirectories.
‘?’
Matches any single character.
‘[...]’
Matches any one of the characters enclosed between the brackets.
This is known as a “bracket expression” and matches a single
character. A pair of characters separated by a hyphen denotes a
“range expression”; any character that falls between those two
characters, inclusive, using the current locale's collating
sequence and character set, matches. If the first character
following the ‘[’ is a ‘!’ or a ‘^’ then any character not within
the range matches. To match a ‘−’, include it as the first or last
character in the set. To match a ‘]’, include it as the first
character in the set.
The sorting order of characters in range expressions, and the
characters included in the range, are determined by the current
locale and the values of the ‘LC_COLLATE’ and ‘LC_ALL’ shell
variables, if set.
For example, in the default C locale, ‘[a-dx-z]’ is equivalent to
‘[abcdxyz]’. Many locales sort characters in dictionary order, and
in these locales ‘[a-dx-z]’ is typically not equivalent to
‘[abcdxyz]’; it might be equivalent to ‘[aBbCcDdxYyZz]’, for
example. To obtain the traditional interpretation of ranges in
bracket expressions, you can force the use of the C locale by
setting the ‘LC_COLLATE’ or ‘LC_ALL’ environment variable to the
value ‘C’, or enable the ‘globasciiranges’ shell option.
Within a bracket expression, “character classes” can be specified
using the syntax ‘[:’CLASS‘:]’, where CLASS is one of the following
classes defined in the POSIX standard:
alnum alpha ascii blank cntrl digit graph lower
print punct space upper word xdigit
A character class matches any character belonging to that class.
The ‘word’ character class matches letters, digits, and the
character ‘_’.
For instance, the following pattern will match any character
belonging to the ‘space’ character class in the current locale,
then any upper case letter or ‘!’, a dot, and finally any lower
case letter or a hyphen.
[[:space:]][[:upper:]!].[-[:lower:]]
Within a bracket expression, an “equivalence class” can be
specified using the syntax ‘[=’C‘=]’, which matches all characters
with the same collation weight (as defined by the current locale)
as the character C.
Within a bracket expression, the syntax ‘[.’SYMBOL‘.]’ matches the
collating symbol SYMBOL.
If the ‘extglob’ shell option is enabled using the ‘shopt’ builtin,
the shell recognizes several extended pattern matching operators. In
the following description, a PATTERN-LIST is a list of one or more
patterns separated by a ‘|’. When matching filenames, the ‘dotglob’
shell option determines the set of filenames that are tested, as
described above. Composite patterns may be formed using one or more of
the following sub-patterns:
‘?(PATTERN-LIST)’
Matches zero or one occurrence of the given patterns.
‘*(PATTERN-LIST)’
Matches zero or more occurrences of the given patterns.
‘+(PATTERN-LIST)’
Matches one or more occurrences of the given patterns.
‘@(PATTERN-LIST)’
Matches one of the given patterns.
‘!(PATTERN-LIST)’
Matches anything except one of the given patterns.
The ‘extglob’ option changes the behavior of the parser, since the
parentheses are normally treated as operators with syntactic meaning.
To ensure that extended matching patterns are parsed correctly, make
sure that ‘extglob’ is enabled before parsing constructs containing the
patterns, including shell functions and command substitutions.
When matching filenames, the ‘dotglob’ shell option determines the
set of filenames that are tested: when ‘dotglob’ is enabled, the set of
filenames includes all files beginning with ‘.’, but the filenames ‘.’
and ‘..’ must be matched by a pattern or sub-pattern that begins with a
dot; when it is disabled, the set does not include any filenames
beginning with ‘.’ unless the pattern or sub-pattern begins with a ‘.’.
If the ‘globskipdots’ shell option is enabled, the filenames ‘.’ and
‘..’ never appear in the set. As above, ‘.’ only has a special meaning
when matching filenames.
Complicated extended pattern matching against long strings is slow,
especially when the patterns contain alternations and the strings
contain multiple matches. Using separate matches against shorter
strings, or using arrays of strings instead of a single long string, may
be faster.
File: bash.info, Node: Quote Removal, Prev: Filename Expansion, Up: Shell Expansions
3.5.9 Quote Removal
-------------------
After the preceding expansions, all unquoted occurrences of the
characters ‘\’, ‘'’, and ‘"’ that did not result from one of the above
expansions are removed.
File: bash.info, Node: Redirections, Next: Executing Commands, Prev: Shell Expansions, Up: Basic Shell Features
3.6 Redirections
================
Before a command is executed, its input and output may be “redirected”
using a special notation interpreted by the shell. “Redirection” allows
commands' file handles to be duplicated, opened, closed, made to refer
to different files, and can change the files the command reads from and
writes to. When used with the ‘exec’ builtin, redirections modify file
handles in the current shell execution environment. The following
redirection operators may precede or appear anywhere within a simple
command or may follow a command. Redirections are processed in the
order they appear, from left to right.
Each redirection that may be preceded by a file descriptor number may
instead be preceded by a word of the form {VARNAME}. In this case, for
each redirection operator except ‘>&-’ and ‘<&-’, the shell allocates a
file descriptor greater than or equal to 10 and assigns it to {VARNAME}.
If {VARNAME} precedes ‘>&-’ or ‘<&-’, the value of VARNAME defines the
file descriptor to close. If {VARNAME} is supplied, the redirection
persists beyond the scope of the command, which allows the shell
programmer to manage the file descriptor's lifetime manually without
using the ‘exec’ builtin. The ‘varredir_close’ shell option manages
this behavior (*note The Shopt Builtin::).
In the following descriptions, if the file descriptor number is
omitted, and the first character of the redirection operator is ‘<’, the
redirection refers to the standard input (file descriptor 0). If the
first character of the redirection operator is ‘>’, the redirection
refers to the standard output (file descriptor 1).
The WORD following the redirection operator in the following
descriptions, unless otherwise noted, is subjected to brace expansion,
tilde expansion, parameter and variable expansion, command substitution,
arithmetic expansion, quote removal, filename expansion, and word
splitting. If it expands to more than one word, Bash reports an error.
The order of redirections is significant. For example, the command
ls > DIRLIST 2>&1
directs both standard output (file descriptor 1) and standard error
(file descriptor 2) to the file DIRLIST, while the command
ls 2>&1 > DIRLIST
directs only the standard output to file DIRLIST, because the standard
error was made a copy of the standard output before the standard output
was redirected to DIRLIST.
Bash handles several filenames specially when they are used in
redirections, as described in the following table. If the operating
system on which Bash is running provides these special files, Bash uses
them; otherwise it emulates them internally with the behavior described
below.
‘/dev/fd/FD’
If FD is a valid integer, duplicate file descriptor FD.
‘/dev/stdin’
File descriptor 0 is duplicated.
‘/dev/stdout’
File descriptor 1 is duplicated.
‘/dev/stderr’
File descriptor 2 is duplicated.
‘/dev/tcp/HOST/PORT’
If HOST is a valid hostname or Internet address, and PORT is an
integer port number or service name, Bash attempts to open the
corresponding TCP socket.
‘/dev/udp/HOST/PORT’
If HOST is a valid hostname or Internet address, and PORT is an
integer port number or service name, Bash attempts to open the
corresponding UDP socket.
A failure to open or create a file causes the redirection to fail.
Redirections using file descriptors greater than 9 should be used
with care, as they may conflict with file descriptors the shell uses
internally.
3.6.1 Redirecting Input
-----------------------
Redirecting input opens the file whose name results from the expansion
of WORD for reading on file descriptor ‘n’, or the standard input (file
descriptor 0) if ‘n’ is not specified.
The general format for redirecting input is:
[N]<WORD
3.6.2 Redirecting Output
------------------------
Redirecting output opens the file whose name results from the expansion
of WORD for writing on file descriptor N, or the standard output (file
descriptor 1) if N is not specified. If the file does not exist it is
created; if it does exist it is truncated to zero size.
The general format for redirecting output is:
[N]>[|]WORD
If the redirection operator is ‘>’, and the ‘noclobber’ option to the
‘set’ builtin command has been enabled, the redirection fails if the
file whose name results from the expansion of WORD exists and is a
regular file. If the redirection operator is ‘>|’, or the redirection
operator is ‘>’ and the ‘noclobber’ option to the ‘set’ builtin is not
enabled, Bash attempts the redirection even if the file named by WORD
exists.
3.6.3 Appending Redirected Output
---------------------------------
Redirecting output in this fashion opens the file whose name results
from the expansion of WORD for appending on file descriptor N, or the
standard output (file descriptor 1) if N is not specified. If the file
does not exist it is created.
The general format for appending output is:
[N]>>WORD
3.6.4 Redirecting Standard Output and Standard Error
----------------------------------------------------
This construct redirects both the standard output (file descriptor 1)
and the standard error output (file descriptor 2) to the file whose name
is the expansion of WORD.
There are two formats for redirecting standard output and standard
error:
&>WORD
and
>&WORD
Of the two forms, the first is preferred. This is semantically
equivalent to
>WORD 2>&1
When using the second form, WORD may not expand to a number or ‘-’.
If it does, other redirection operators apply (see Duplicating File
Descriptors below) for compatibility reasons.
3.6.5 Appending Standard Output and Standard Error
--------------------------------------------------
This construct appends both the standard output (file descriptor 1) and
the standard error output (file descriptor 2) to the file whose name is
the expansion of WORD.
The format for appending standard output and standard error is:
&>>WORD
This is semantically equivalent to
>>WORD 2>&1
(see Duplicating File Descriptors below).
3.6.6 Here Documents
--------------------
This type of redirection instructs the shell to read input from the
current source until it reads a line containing only DELIMITER (with no
trailing blanks). All of the lines read up to that point then become
the standard input (or file descriptor N if N is specified) for a
command.
The format of here-documents is:
[N]<<[−]WORD
HERE-DOCUMENT
DELIMITER
The shell does not perform parameter and variable expansion, command
substitution, arithmetic expansion, or filename expansion on WORD.
If any part of WORD is quoted, the DELIMITER is the result of quote
removal on WORD, and the lines in the here-document are not expanded.
If WORD is unquoted, DELIMITER is WORD itself, and the here-document
text is treated similarly to a double-quoted string: all lines of the
here-document are subjected to parameter expansion, command
substitution, and arithmetic expansion, the character sequence
‘\newline’ is treated literally, and ‘\’ must be used to quote the
characters ‘\’, ‘$’, and ‘`’; however, double quote characters have no
special meaning.
If the redirection operator is ‘<<-’, the shell strips leading tab
characters are stripped from input lines and the line containing
DELIMITER. This allows here-documents within shell scripts to be
indented in a natural fashion.
If the delimiter is not quoted, the ‘\<newline>’ sequence is treated
as a line continuation: the two lines are joined and the
backslash-newline is removed. This happens while reading the
here-document, before the check for the ending delimiter, so joined
lines can form the end delimiter.
3.6.7 Here Strings
------------------
A variant of here documents, the format is:
[N]<<< WORD
The WORD undergoes tilde expansion, parameter and variable expansion,
command substitution, arithmetic expansion, and quote removal. Filename
expansion and word splitting are not performed. The result is supplied
as a single string, with a newline appended, to the command on its
standard input (or file descriptor N if N is specified).
3.6.8 Duplicating File Descriptors
----------------------------------
The redirection operator
[N]<&WORD
is used to duplicate input file descriptors. If WORD expands to one or
more digits, file descriptor N is made to be a copy of that file
descriptor. It is a redirection error if the digits in WORD do not
specify a file descriptor open for input. If WORD evaluates to ‘-’,
file descriptor N is closed. If N is not specified, this uses the
standard input (file descriptor 0).
The operator
[N]>&WORD
is used similarly to duplicate output file descriptors. If N is not
specified, this uses the standard output (file descriptor 1). It is a
redirection error if the digits in WORD do not specify a file descriptor
open for output. If WORD evaluates to ‘-’, file descriptor N is closed.
As a special case, if N is omitted, and WORD does not expand to one or
more digits or ‘-’, this redirects the standard output and standard
error as described previously.
3.6.9 Moving File Descriptors
-----------------------------
The redirection operator
[N]<&DIGIT-
moves the file descriptor DIGIT to file descriptor N, or the standard
input (file descriptor 0) if N is not specified. DIGIT is closed after
being duplicated to N.
Similarly, the redirection operator
[N]>&DIGIT-
moves the file descriptor DIGIT to file descriptor N, or the standard
output (file descriptor 1) if N is not specified.
3.6.10 Opening File Descriptors for Reading and Writing
-------------------------------------------------------
The redirection operator
[N]<>WORD
opens the file whose name is the expansion of WORD for both reading and
writing on file descriptor N, or on file descriptor 0 if N is not
specified. If the file does not exist, it is created.
File: bash.info, Node: Executing Commands, Next: Shell Scripts, Prev: Redirections, Up: Basic Shell Features
3.7 Executing Commands
======================
* Menu:
* Simple Command Expansion:: How Bash expands simple commands before
executing them.
* Command Search and Execution:: How Bash finds commands and runs them.
* Command Execution Environment:: The environment in which Bash
executes commands that are not
shell builtins.
* Environment:: The environment given to a command.
* Exit Status:: The status returned by commands and how Bash
interprets it.
* Signals:: What happens when Bash or a command it runs
receives a signal.
File: bash.info, Node: Simple Command Expansion, Next: Command Search and Execution, Up: Executing Commands
3.7.1 Simple Command Expansion
------------------------------
When the shell executes a simple command, it performs the following
expansions, assignments, and redirections, from left to right, in the
following order.
1. The words that the parser has marked as variable assignments (those
preceding the command name) and redirections are saved for later
processing.
2. The words that are not variable assignments or redirections are
expanded (*note Shell Expansions::). If any words remain after
expansion, the first word is taken to be the name of the command
and the remaining words are the arguments.
3. Redirections are performed as described above (*note
Redirections::).
4. The text after the ‘=’ in each variable assignment undergoes tilde
expansion, parameter expansion, command substitution, arithmetic
expansion, and quote removal before being assigned to the variable.
If no command name results, the variable assignments affect the
current shell environment. In the case of such a command (one that
consists only of assignment statements and redirections), assignment
statements are performed before redirections. Otherwise, the variables
are added to the environment of the executed command and do not affect
the current shell environment. If any of the assignments attempts to
assign a value to a readonly variable, an error occurs, and the command
exits with a non-zero status.
If no command name results, redirections are performed, but do not
affect the current shell environment. A redirection error causes the
command to exit with a non-zero status.
If there is a command name left after expansion, execution proceeds
as described below. Otherwise, the command exits. If one of the
expansions contained a command substitution, the exit status of the
command is the exit status of the last command substitution performed.
If there were no command substitutions, the command exits with a zero
status.
File: bash.info, Node: Command Search and Execution, Next: Command Execution Environment, Prev: Simple Command Expansion, Up: Executing Commands
3.7.2 Command Search and Execution
----------------------------------
After a command has been split into words, if it results in a simple
command and an optional list of arguments, the shell performs the
following actions.
1. If the command name contains no slashes, the shell attempts to
locate it. If there exists a shell function by that name, that
function is invoked as described in *note Shell Functions::.
2. If the name does not match a function, the shell searches for it in
the list of shell builtins. If a match is found, that builtin is
invoked.
3. If the name is neither a shell function nor a builtin, and contains
no slashes, Bash searches each element of ‘$PATH’ for a directory
containing an executable file by that name. Bash uses a hash table
to remember the full pathnames of executable files to avoid
multiple ‘PATH’ searches (see the description of ‘hash’ in *note
Bourne Shell Builtins::). Bash performs a full search of the
directories in ‘$PATH’ only if the command is not found in the hash
table. If the search is unsuccessful, the shell searches for a
defined shell function named ‘command_not_found_handle’. If that
function exists, it is invoked in a separate execution environment
with the original command and the original command's arguments as
its arguments, and the function's exit status becomes the exit
status of that subshell. If that function is not defined, the
shell prints an error message and returns an exit status of 127.
4. If the search is successful, or if the command name contains one or
more slashes, the shell executes the named program in a separate
execution environment. Argument 0 is set to the name given, and
the remaining arguments to the command are set to the arguments
supplied, if any.
5. If this execution fails because the file is not in executable
format, and the file is not a directory, it is assumed to be a
“shell script”, a file containing shell commands, and the shell
executes it as described in *note Shell Scripts::.
6. If the command was not begun asynchronously, the shell waits for
the command to complete and collects its exit status.
File: bash.info, Node: Command Execution Environment, Next: Environment, Prev: Command Search and Execution, Up: Executing Commands
3.7.3 Command Execution Environment
-----------------------------------
The shell has an “execution environment”, which consists of the
following:
• Open files inherited by the shell at invocation, as modified by
redirections supplied to the ‘exec’ builtin.
• The current working directory as set by ‘cd’, ‘pushd’, or ‘popd’,
or inherited by the shell at invocation.
• The file creation mode mask as set by ‘umask’ or inherited from the
shell's parent.
• Current traps set by ‘trap’.
• Shell parameters that are set by variable assignment or with ‘set’
or inherited from the shell's parent in the environment.
• Shell functions defined during execution or inherited from the
shell's parent in the environment.
• Options enabled at invocation (either by default or with
command-line arguments) or by ‘set’.
• Options enabled by ‘shopt’ (*note The Shopt Builtin::).
• Shell aliases defined with ‘alias’ (*note Aliases::).
• Various process IDs, including those of background jobs (*note
Lists::), the value of ‘$$’, and the value of ‘$PPID’.
When a simple command other than a builtin or shell function is to be
executed, it is invoked in a separate execution environment that
consists of the following. Unless otherwise noted, the values are
inherited from the shell.
• The shell's open files, plus any modifications and additions
specified by redirections to the command.
• The current working directory.
• The file creation mode mask.
• Shell variables and functions marked for export, along with
variables exported for the command, passed in the environment
(*note Environment::).
• Traps caught by the shell are reset to the values inherited from
the shell's parent, and traps ignored by the shell are ignored.
A command invoked in this separate environment cannot affect the
shell's execution environment.
A “subshell” is a copy of the shell process.
Command substitution, commands grouped with parentheses, and
asynchronous commands are invoked in a subshell environment that is a
duplicate of the shell environment, except that traps caught by the
shell are reset to the values that the shell inherited from its parent
at invocation. Builtin commands that are invoked as part of a pipeline,
except possibly in the last element depending on the value of the
‘lastpipe’ shell option (*note The Shopt Builtin::), are also executed
in a subshell environment. Changes made to the subshell environment
cannot affect the shell's execution environment.
When the shell is in POSIX mode, subshells spawned to execute command
substitutions inherit the value of the ‘-e’ option from the parent
shell. When not in POSIX mode, Bash clears the ‘-e’ option in such
subshells See the description of the ‘inherit_errexit’ shell option
(*note Bash Builtins::) for how to control this behavior when not in
POSIX mode.
If a command is followed by a ‘&’ and job control is not active, the
default standard input for the command is the empty file ‘/dev/null’.
Otherwise, the invoked command inherits the file descriptors of the
calling shell as modified by redirections.
File: bash.info, Node: Environment, Next: Exit Status, Prev: Command Execution Environment, Up: Executing Commands
3.7.4 Environment
-----------------
When a program is invoked it is given an array of strings called the
“environment”. This is a list of name-value pairs, of the form
‘name=value’.
Bash provides several ways to manipulate the environment. On
invocation, the shell scans its own environment and creates a parameter
for each name found, automatically marking it for ‘export’ to child
processes. Executed commands inherit the environment. The ‘export’,
‘declare -x’, and ‘unset’ commands modify the environment by adding and
deleting parameters and functions. If the value of a parameter in the
environment is modified, the new value automatically becomes part of the
environment, replacing the old. The environment inherited by any
executed command consists of the shell's initial environment, whose
values may be modified in the shell, less any pairs removed by the
‘unset’ and ‘export -n’ commands, plus any additions via the ‘export’
and ‘declare -x’ commands.
If any parameter assignment statements, as described in *note Shell
Parameters::, appear before a simple command, the variable assignments
are part of that command's environment for as long as it executes.
These assignment statements affect only the environment seen by that
command. If these assignments precede a call to a shell function, the
variables are local to the function and exported to that function's
children.
If the ‘-k’ option is set (*note The Set Builtin::), then all
parameter assignments are placed in the environment for a command, not
just those that precede the command name.
When Bash invokes an external command, the variable ‘$_’ is set to
the full pathname of the command and passed to that command in its
environment.
File: bash.info, Node: Exit Status, Next: Signals, Prev: Environment, Up: Executing Commands
3.7.5 Exit Status
-----------------
The exit status of an executed command is the value returned by the
‘waitpid’ system call or equivalent function. Exit statuses fall
between 0 and 255, though, as explained below, the shell may use values
above 125 specially. Exit statuses from shell builtins and compound
commands are also limited to this range. Under certain circumstances,
the shell will use special values to indicate specific failure modes.
For the shell's purposes, a command which exits with a zero exit
status has succeeded. So while an exit status of zero indicates
success, a non-zero exit status indicates failure. This seemingly
counter-intuitive scheme is used so there is one well-defined way to
indicate success and a variety of ways to indicate various failure
modes.
When a command terminates on a fatal signal whose number is N, Bash
uses the value 128+N as the exit status.
If a command is not found, the child process created to execute it
returns a status of 127. If a command is found but is not executable,
the return status is 126.
If a command fails because of an error during expansion or
redirection, the exit status is greater than zero.
The exit status is used by the Bash conditional commands (*note
Conditional Constructs::) and some of the list constructs (*note
Lists::).
All of the Bash builtins return an exit status of zero if they
succeed and a non-zero status on failure, so they may be used by the
conditional and list constructs. All builtins return an exit status of
2 to indicate incorrect usage, generally invalid options or missing
arguments.
The exit status of the last command is available in the special
parameter $? (*note Special Parameters::).
Bash itself returns the exit status of the last command executed,
unless a syntax error occurs, in which case it exits with a non-zero
value. See also the ‘exit’ builtin command (*note Bourne Shell
Builtins::.
File: bash.info, Node: Signals, Prev: Exit Status, Up: Executing Commands
3.7.6 Signals
-------------
When Bash is interactive, in the absence of any traps, it ignores
‘SIGTERM’ (so that ‘kill 0’ does not kill an interactive shell), and
catches and handles ‘SIGINT’ (so that the ‘wait’ builtin is
interruptible). When Bash receives a ‘SIGINT’, it breaks out of any
executing loops. In all cases, Bash ignores ‘SIGQUIT’. If job control
is in effect (*note Job Control::), Bash ignores ‘SIGTTIN’, ‘SIGTTOU’,
and ‘SIGTSTP’.
The ‘trap’ builtin modifies the shell's signal handling, as described
below (*note Bourne Shell Builtins::.
Non-builtin commands Bash executes have signal handlers set to the
values inherited by the shell from its parent, unless ‘trap’ sets them
to be ignored, in which case the child process will ignore them as well.
When job control is not in effect, asynchronous commands ignore ‘SIGINT’
and ‘SIGQUIT’ in addition to these inherited handlers. Commands run as
a result of command substitution ignore the keyboard-generated job
control signals ‘SIGTTIN’, ‘SIGTTOU’, and ‘SIGTSTP’.
The shell exits by default upon receipt of a ‘SIGHUP’. Before
exiting, an interactive shell resends the ‘SIGHUP’ to all jobs, running
or stopped. The shell sends ‘SIGCONT’ to stopped jobs to ensure that
they receive the ‘SIGHUP’ (*Note Job Control::, for more information
about running and stopped jobs). To prevent the shell from sending the
‘SIGHUP’ signal to a particular job, remove it from the jobs table with
the ‘disown’ builtin (*note Job Control Builtins::) or mark it not to
receive ‘SIGHUP’ using ‘disown -h’.
If the ‘huponexit’ shell option has been set using ‘shopt’ (*note The
Shopt Builtin::), Bash sends a ‘SIGHUP’ to all jobs when an interactive
login shell exits.
If Bash is waiting for a command to complete and receives a signal
for which a trap has been set, it will not execute the trap until the
command completes. If Bash is waiting for an asynchronous command via
the ‘wait’ builtin, and it receives a signal for which a trap has been
set, the ‘wait’ builtin will return immediately with an exit status
greater than 128, immediately after which the shell executes the trap.
When job control is not enabled, and Bash is waiting for a foreground
command to complete, the shell receives keyboard-generated signals such
as ‘SIGINT’ (usually generated by ‘^C’) that users commonly intend to
send to that command. This happens because the shell and the command
are in the same process group as the terminal, and ‘^C’ sends ‘SIGINT’
to all processes in that process group. Since Bash does not enable job
control by default when the shell is not interactive, this scenario is
most common in non-interactive shells.
When job control is enabled, and Bash is waiting for a foreground
command to complete, the shell does not receive keyboard-generated
signals, because it is not in the same process group as the terminal.
This scenario is most common in interactive shells, where Bash attempts
to enable job control by default. See *note Job Control::, for a more
in-depth discussion of process groups.
When job control is not enabled, and Bash receives ‘SIGINT’ while
waiting for a foreground command, it waits until that foreground command
terminates and then decides what to do about the ‘SIGINT’:
1. If the command terminates due to the ‘SIGINT’, Bash concludes that
the user meant to send the ‘SIGINT’ to the shell as well, and acts
on the ‘SIGINT’ (e.g., by running a ‘SIGINT’ trap, exiting a
non-interactive shell, or returning to the top level to read a new
command).
2. If the command does not terminate due to ‘SIGINT’, the program
handled the ‘SIGINT’ itself and did not treat it as a fatal signal.
In that case, Bash does not treat ‘SIGINT’ as a fatal signal,
either, instead assuming that the ‘SIGINT’ was used as part of the
program's normal operation (e.g., ‘emacs’ uses it to abort editing
commands) or deliberately discarded. However, Bash will run any
trap set on ‘SIGINT’, as it does with any other trapped signal it
receives while it is waiting for the foreground command to
complete, for compatibility.
When job control is enabled, Bash does not receive keyboard-generated
signals such as ‘SIGINT’ while it is waiting for a foreground command.
An interactive shell does not pay attention to the ‘SIGINT’, even if the
foreground command terminates as a result, other than noting its exit
status. If the shell is not interactive, and the foreground command
terminates due to the ‘SIGINT’, Bash pretends it received the ‘SIGINT’
itself (scenario 1 above), for compatibility.
File: bash.info, Node: Shell Scripts, Prev: Executing Commands, Up: Basic Shell Features
3.8 Shell Scripts
=================
A shell script is a text file containing shell commands. When such a
file is used as the first non-option argument when invoking Bash, and
neither the ‘-c’ nor ‘-s’ option is supplied (*note Invoking Bash::),
Bash reads and executes commands from the file, then exits. This mode
of operation creates a non-interactive shell. If the filename does not
contain any slashes, the shell first searches for the file in the
current directory, and looks in the directories in ‘$PATH’ if not found
there.
Bash tries to determine whether the file is a text file or a binary,
and will not execute files it determines to be binaries.
When Bash runs a shell script, it sets the special parameter ‘0’ to
the name of the file, rather than the name of the shell, and the
positional parameters are set to the remaining arguments, if any are
given. If no additional arguments are supplied, the positional
parameters are unset.
A shell script may be made executable by using the ‘chmod’ command to
turn on the execute bit. When Bash finds such a file while searching
the ‘$PATH’ for a command, it creates a new instance of itself to
execute it. In other words, executing
filename ARGUMENTS
is equivalent to executing
bash filename ARGUMENTS
if ‘filename’ is an executable shell script. This subshell
reinitializes itself, so that the effect is as if a new shell had been
invoked to interpret the script, with the exception that the locations
of commands remembered by the parent (see the description of ‘hash’ in
*note Bourne Shell Builtins::) are retained by the child.
The GNU operating system, and most versions of Unix, make this a part
of the operating system's command execution mechanism. If the first
line of a script begins with the two characters ‘#!’, the remainder of
the line specifies an interpreter for the program and, depending on the
operating system, one or more optional arguments for that interpreter.
Thus, you can specify Bash, ‘awk’, Perl, or some other interpreter and
write the rest of the script file in that language.
The arguments to the interpreter consist of one or more optional
arguments following the interpreter name on the first line of the script
file, followed by the name of the script file, followed by the rest of
the arguments supplied to the script. The details of how the
interpreter line is split into an interpreter name and a set of
arguments vary across systems. Bash will perform this action on
operating systems that do not handle it themselves. Note that some
older versions of Unix limit the interpreter name and a single argument
to a maximum of 32 characters, so it's not portable to assume that using
more than one argument will work.
Bash scripts often begin with ‘#! /bin/bash’ (assuming that Bash has
been installed in ‘/bin’), since this ensures that Bash will be used to
interpret the script, even if it is executed under another shell. It's
a common idiom to use ‘env’ to find ‘bash’ even if it's been installed
in another directory: ‘#!/usr/bin/env bash’ will find the first
occurrence of ‘bash’ in ‘$PATH’.
File: bash.info, Node: Shell Builtin Commands, Next: Shell Variables, Prev: Basic Shell Features, Up: Top
4 Shell Builtin Commands
************************
* Menu:
* Bourne Shell Builtins:: Builtin commands inherited from the Bourne
Shell.
* Bash Builtins:: Table of builtins specific to Bash.
* Modifying Shell Behavior:: Builtins to modify shell attributes and
optional behavior.
* Special Builtins:: Builtin commands classified specially by
POSIX.
Builtin commands are contained within the shell itself. When the name
of a builtin command is used as the first word of a simple command
(*note Simple Commands::), the shell executes the command directly,
without invoking another program. Builtin commands are necessary to
implement functionality impossible or inconvenient to obtain with
separate utilities.
This section briefly describes the builtins which Bash inherits from
the Bourne Shell, as well as the builtin commands which are unique to or
have been extended in Bash.
Several builtin commands are described in other chapters: builtin
commands which provide the Bash interface to the job control facilities
(*note Job Control Builtins::), the directory stack (*note Directory
Stack Builtins::), the command history (*note Bash History Builtins::),
and the programmable completion facilities (*note Programmable
Completion Builtins::).
Many of the builtins have been extended by POSIX or Bash.
Unless otherwise noted, each builtin command documented as accepting
options preceded by ‘-’ accepts ‘--’ to signify the end of the options.
The ‘:’, ‘true’, ‘false’, and ‘test’/‘[’ builtins do not accept options
and do not treat ‘--’ specially. The ‘exit’, ‘logout’, ‘return’,
‘break’, ‘continue’, ‘let’, and ‘shift’ builtins accept and process
arguments beginning with ‘-’ without requiring ‘--’. Other builtins
that accept arguments but are not specified as accepting options
interpret arguments beginning with ‘-’ as invalid options and require
‘--’ to prevent this interpretation.
File: bash.info, Node: Bourne Shell Builtins, Next: Bash Builtins, Up: Shell Builtin Commands
4.1 Bourne Shell Builtins
=========================
The following shell builtin commands are inherited from the Bourne
Shell. These commands are implemented as specified by the POSIX
standard.
‘: (a colon)’
: [ARGUMENTS]
Do nothing beyond expanding ARGUMENTS and performing redirections.
The return status is zero.
‘. (a period)’
. [-p PATH] FILENAME [ARGUMENTS]
The ‘.’ command reads and execute commands from the FILENAME
argument in the current shell context.
If FILENAME does not contain a slash, ‘.’ searches for it. If ‘-p’
is supplied, ‘.’ treats PATH as a colon-separated list of
directories in which to find FILENAME; otherwise, ‘.’ uses the
directories in ‘PATH’ to find FILENAME. FILENAME does not need to
be executable. When Bash is not in POSIX mode, it searches the
current directory if FILENAME is not found in ‘$PATH’, but does not
search the current directory if ‘-p’ is supplied. If the
‘sourcepath’ option (*note The Shopt Builtin::) is turned off, ‘.’
does not search ‘PATH’.
If any ARGUMENTS are supplied, they become the positional
parameters when FILENAME is executed. Otherwise the positional
parameters are unchanged.
If the ‘-T’ option is enabled, ‘.’ inherits any trap on ‘DEBUG’; if
it is not, any ‘DEBUG’ trap string is saved and restored around the
call to ‘.’, and ‘.’ unsets the ‘DEBUG’ trap while it executes. If
‘-T’ is not set, and the sourced file changes the ‘DEBUG’ trap, the
new value persists after ‘.’ completes. The return status is the
exit status of the last command executed from FILENAME, or zero if
no commands are executed. If FILENAME is not found, or cannot be
read, the return status is non-zero. This builtin is equivalent to
‘source’.
‘break’
break [N]
Exit from a ‘for’, ‘while’, ‘until’, or ‘select’ loop. If N is
supplied, ‘break’ exits the Nth enclosing loop. N must be greater
than or equal to 1. The return status is zero unless N is not
greater than or equal to 1.
‘cd’
cd [-L] [-@] [DIRECTORY]
cd -P [-e] [-@] [DIRECTORY]
Change the current working directory to DIRECTORY. If DIRECTORY is
not supplied, the value of the ‘HOME’ shell variable is used as
DIRECTORY. If the shell variable ‘CDPATH’ exists, and DIRECTORY
does not begin with a slash, ‘cd’ uses it as a search path: ‘cd’
searches each directory name in ‘CDPATH’ for DIRECTORY, with
alternative directory names in ‘CDPATH’ separated by a colon (‘:’).
A null directory name in ‘CDPATH’ means the same thing as the
current directory.
The ‘-P’ option means not to follow symbolic links: symbolic links
are resolved while ‘cd’ is traversing DIRECTORY and before
processing an instance of ‘..’ in DIRECTORY.
By default, or when the ‘-L’ option is supplied, symbolic links in
DIRECTORY are resolved after ‘cd’ processes an instance of ‘..’ in
DIRECTORY.
If ‘..’ appears in DIRECTORY, ‘cd’ processes it by removing the
immediately preceding pathname component, back to a slash or the
beginning of DIRECTORY, and verifying that the portion of DIRECTORY
it has processed to that point is still a valid directory name
after removing the pathname component. If it is not a valid
directory name, ‘cd’ returns a non-zero status.
If the ‘-e’ option is supplied with ‘-P’ and ‘cd’ cannot
successfully determine the current working directory after a
successful directory change, it returns a non-zero status.
On systems that support it, the ‘-@’ option presents the extended
attributes associated with a file as a directory.
If DIRECTORY is ‘-’, it is converted to ‘$OLDPWD’ before attempting
the directory change.
If ‘cd’ uses a non-empty directory name from ‘CDPATH’, or if ‘-’ is
the first argument, and the directory change is successful, ‘cd’
writes the absolute pathname of the new working directory to the
standard output.
If the directory change is successful, ‘cd’ sets the value of the
‘PWD’ environment variable to the new directory name, and sets the
‘OLDPWD’ environment variable to the value of the current working
directory before the change.
The return status is zero if the directory is successfully changed,
non-zero otherwise.
‘continue’
continue [N]
‘continue’ resumes the next iteration of an enclosing ‘for’,
‘while’, ‘until’, or ‘select’ loop. If N is supplied, Bash resumes
the execution of the Nth enclosing loop. N must be greater than or
equal to 1. The return status is zero unless N is not greater than
or equal to 1.
‘eval’
eval [ARGUMENTS]
The ARGUMENTS are concatenated together into a single command,
separated by spaces. Bash then reads and executes this command and
returns its exit status as the exit status of ‘eval’. If there are
no arguments or only empty arguments, the return status is zero.
‘exec’
exec [-cl] [-a NAME] [COMMAND [ARGUMENTS]]
If COMMAND is supplied, it replaces the shell without creating a
new process. COMMAND cannot be a shell builtin or function. The
ARGUMENTS become the arguments to COMMAND If the ‘-l’ option is
supplied, the shell places a dash at the beginning of the zeroth
argument passed to COMMAND. This is what the ‘login’ program does.
The ‘-c’ option causes COMMAND to be executed with an empty
environment. If ‘-a’ is supplied, the shell passes NAME as the
zeroth argument to COMMAND.
If COMMAND cannot be executed for some reason, a non-interactive
shell exits, unless the ‘execfail’ shell option is enabled. In
that case, it returns a non-zero status. An interactive shell
returns a non-zero status if the file cannot be executed. A
subshell exits unconditionally if ‘exec’ fails.
If COMMAND is not specified, redirections may be used to affect the
current shell environment. If there are no redirection errors, the
return status is zero; otherwise the return status is non-zero.
‘exit’
exit [N]
Exit the shell, returning a status of N to the shell's parent. If
N is omitted, the exit status is that of the last command executed.
Any trap on ‘EXIT’ is executed before the shell terminates.
‘export’
export [-fn] [-p] [NAME[=VALUE]]
Mark each NAME to be passed to subsequently executed commands in
the environment. If the ‘-f’ option is supplied, the NAMEs refer
to shell functions; otherwise the names refer to shell variables.
The ‘-n’ option means to unexport each name: no longer mark it for
export. If no NAMEs are supplied, or if only the ‘-p’ option is
given, ‘export’ displays a list of names of all exported variables
on the standard output. Using ‘-p’ and ‘-f’ together displays
exported functions. The ‘-p’ option displays output in a form that
may be reused as input.
‘export’ allows the value of a variable to be set at the same time
it is exported or unexported by following the variable name with
=VALUE. This sets the value of the variable is to VALUE while
modifying the export attribute.
The return status is zero unless an invalid option is supplied, one
of the names is not a valid shell variable name, or ‘-f’ is
supplied with a name that is not a shell function.
‘false’
false
Does nothing; returns a non-zero status.
‘getopts’
getopts OPTSTRING NAME [ARG ...]
‘getopts’ is used by shell scripts or functions to parse positional
parameters and obtain options and their arguments. OPTSTRING
contains the option characters to be recognized; if a character is
followed by a colon, the option is expected to have an argument,
which should be separated from it by whitespace. The colon (‘:’)
and question mark (‘?’) may not be used as option characters.
Each time it is invoked, ‘getopts’ places the next option in the
shell variable NAME, initializing NAME if it does not exist, and
the index of the next argument to be processed into the variable
‘OPTIND’. ‘OPTIND’ is initialized to 1 each time the shell or a
shell script is invoked. When an option requires an argument,
‘getopts’ places that argument into the variable ‘OPTARG’.
The shell does not reset ‘OPTIND’ automatically; it must be
manually reset between multiple calls to ‘getopts’ within the same
shell invocation to use a new set of parameters.
When it reaches the end of options, ‘getopts’ exits with a return
value greater than zero. ‘OPTIND’ is set to the index of the first
non-option argument, and NAME is set to ‘?’.
‘getopts’ normally parses the positional parameters, but if more
arguments are supplied as ARG values, ‘getopts’ parses those
instead.
‘getopts’ can report errors in two ways. If the first character of
OPTSTRING is a colon, ‘getopts’ uses _silent_ error reporting. In
normal operation, ‘getopts’ prints diagnostic messages when it
encounters invalid options or missing option arguments. If the
variable ‘OPTERR’ is set to 0, ‘getopts’ does not display any error
messages, even if the first character of ‘optstring’ is not a
colon.
If ‘getopts’ detects an invalid option, it places ‘?’ into NAME
and, if not silent, prints an error message and unsets ‘OPTARG’.
If ‘getopts’ is silent, it assigns the option character found to
‘OPTARG’ and does not print a diagnostic message.
If a required argument is not found, and ‘getopts’ is not silent,
it sets the value of NAME to a question mark (‘?’), unsets
‘OPTARG’, and prints a diagnostic message. If ‘getopts’ is silent,
it sets the value of NAME to a colon (‘:’), and sets ‘OPTARG’ to
the option character found.
‘getopts’ returns true if an option, specified or unspecified, is
found. It returns false when it encounters the end of options or
if an error occurs.
‘hash’
hash [-r] [-p FILENAME] [-dt] [NAME]
Each time ‘hash’ is invoked, it remembers the full filenames of the
commands specified as NAME arguments, so they need not be searched
for on subsequent invocations. The commands are found by searching
through the directories listed in ‘$PATH’. Any
previously-remembered filename associated with NAME is discarded.
The ‘-p’ option inhibits the path search, and ‘hash’ uses FILENAME
as the location of NAME.
The ‘-r’ option causes the shell to forget all remembered
locations. Assigning to the ‘PATH’ variable also clears all hashed
filenames. The ‘-d’ option causes the shell to forget the
remembered location of each NAME.
If the ‘-t’ option is supplied, ‘hash’ prints the full pathname
corresponding to each NAME. If multiple NAME arguments are
supplied with ‘-t’, ‘hash’ prints each NAME before the
corresponding hashed full path. The ‘-l’ option displays output in
a format that may be reused as input.
If no arguments are given, or if only ‘-l’ is supplied, ‘hash’
prints information about remembered commands. The ‘-t’, ‘-d’, and
‘-p’ options (the options that act on the NAME arguments) are
mutually exclusive. Only one will be active. If more than one is
supplied, ‘-t’ has higher priority than ‘-p’, and both have higher
priority than ‘-d’.
The return status is zero unless a NAME is not found or an invalid
option is supplied.
‘pwd’
pwd [-LP]
Print the absolute pathname of the current working directory. If
the ‘-P’ option is supplied, or the ‘-o physical’ option to the
‘set’ builtin (*note The Set Builtin::) is enabled, the pathname
printed will not contain symbolic links. If the ‘-L’ option is
supplied, the pathname printed may contain symbolic links. The
return status is zero unless an error is encountered while
determining the name of the current directory or an invalid option
is supplied.
‘readonly’
readonly [-aAf] [-p] [NAME[=VALUE]] ...
Mark each NAME as readonly. The values of these names may not be
changed by subsequent assignment or unset. If the ‘-f’ option is
supplied, each NAME refers to a shell function. The ‘-a’ option
means each NAME refers to an indexed array variable; the ‘-A’
option means each NAME refers to an associative array variable. If
both options are supplied, ‘-A’ takes precedence. If no NAME
arguments are supplied, or if the ‘-p’ option is supplied, print a
list of all readonly names. The other options may be used to
restrict the output to a subset of the set of readonly names. The
‘-p’ option displays output in a format that may be reused as
input.
‘readonly’ allows the value of a variable to be set at the same
time the readonly attribute is changed by following the variable
name with =VALUE. This sets the value of the variable is to VALUE
while modifying the readonly attribute.
The return status is zero unless an invalid option is supplied, one
of the NAME arguments is not a valid shell variable or function
name, or the ‘-f’ option is supplied with a name that is not a
shell function.
‘return’
return [N]
Stop executing a shell function or sourced file and return the
value N to its caller. If N is not supplied, the return value is
the exit status of the last command executed. If ‘return’ is
executed by a trap handler, the last command used to determine the
status is the last command executed before the trap handler. If
‘return’ is executed during a ‘DEBUG’ trap, the last command used
to determine the status is the last command executed by the trap
handler before ‘return’ was invoked.
When ‘return’ is used to terminate execution of a script being
executed with the ‘.’ (‘source’) builtin, it returns either N or
the exit status of the last command executed within the script as
the exit status of the script. If N is supplied, the return value
is its least significant 8 bits.
Any command associated with the ‘RETURN’ trap is executed before
execution resumes after the function or script.
The return status is non-zero if ‘return’ is supplied a non-numeric
argument or is used outside a function and not during the execution
of a script by ‘.’ or ‘source’.
‘shift’
shift [N]
Shift the positional parameters to the left by N: the positional
parameters from N+1 ... ‘$#’ are renamed to ‘$1’ ... ‘$#’-N.
Parameters represented by the numbers ‘$#’ down to ‘$#’-N+1 are
unset. N must be a non-negative number less than or equal to ‘$#’.
If N is not supplied, it is assumed to be 1. If N is zero or
greater than ‘$#’, the positional parameters are not changed. The
return status is zero unless N is greater than ‘$#’ or less than
zero, non-zero otherwise.
‘test’
‘[’
test EXPR
Evaluate a conditional expression EXPR and return a status of 0
(true) or 1 (false). Each operator and operand must be a separate
argument. Expressions are composed of the primaries described
below in *note Bash Conditional Expressions::. ‘test’ does not
accept any options, nor does it accept and ignore an argument of
‘--’ as signifying the end of options. When using the ‘[’ form,
the last argument to the command must be a ‘]’.
Expressions may be combined using the following operators, listed
in decreasing order of precedence. The evaluation depends on the
number of arguments; see below. ‘test’ uses operator precedence
when there are five or more arguments.
‘! EXPR’
True if EXPR is false.
‘( EXPR )’
Returns the value of EXPR. This may be used to override
normal operator precedence.
‘EXPR1 -a EXPR2’
True if both EXPR1 and EXPR2 are true.
‘EXPR1 -o EXPR2’
True if either EXPR1 or EXPR2 is true.
The ‘test’ and ‘[’ builtins evaluate conditional expressions using
a set of rules based on the number of arguments.
0 arguments
The expression is false.
1 argument
The expression is true if, and only if, the argument is not
null.
2 arguments
If the first argument is ‘!’, the expression is true if and
only if the second argument is null. If the first argument is
one of the unary conditional operators (*note Bash Conditional
Expressions::), the expression is true if the unary test is
true. If the first argument is not a valid unary operator,
the expression is false.
3 arguments
The following conditions are applied in the order listed.
1. If the second argument is one of the binary conditional
operators (*note Bash Conditional Expressions::), the
result of the expression is the result of the binary test
using the first and third arguments as operands. The
‘-a’ and ‘-o’ operators are considered binary operators
when there are three arguments.
2. If the first argument is ‘!’, the value is the negation
of the two-argument test using the second and third
arguments.
3. If the first argument is exactly ‘(’ and the third
argument is exactly ‘)’, the result is the one-argument
test of the second argument.
4. Otherwise, the expression is false.
4 arguments
The following conditions are applied in the order listed.
1. If the first argument is ‘!’, the result is the negation
of the three-argument expression composed of the
remaining arguments.
2. If the first argument is exactly ‘(’ and the fourth
argument is exactly ‘)’, the result is the two-argument
test of the second and third arguments.
3. Otherwise, the expression is parsed and evaluated
according to precedence using the rules listed above.
5 or more arguments
The expression is parsed and evaluated according to precedence
using the rules listed above.
If the shell is in POSIX mode, or if the expression is part of the
‘[[’ command, the ‘<’ and ‘>’ operators sort using the current
locale. If the shell is not in POSIX mode, the ‘test’ and ‘[’
commands sort lexicographically using ASCII ordering.
The historical operator-precedence parsing with 4 or more arguments
can lead to ambiguities when it encounters strings that look like
primaries. The POSIX standard has deprecated the ‘-a’ and ‘-o’
primaries and enclosing expressions within parentheses. Scripts
should no longer use them. It's much more reliable to restrict
test invocations to a single primary, and to replace uses of ‘-a’
and ‘-o’ with the shell's ‘&&’ and ‘||’ list operators. For
example, use
test -n string1 && test -n string2
instead of
test -n string1 -a -n string2
‘times’
times
Print out the user and system times used by the shell and its
children. The return status is zero.
‘trap’
trap [-lpP] [ACTION] [SIGSPEC ...]
The ACTION is a command that is read and executed when the shell
receives any of the signals SIGSPEC. If ACTION is absent (and
there is a single SIGSPEC) or equal to ‘-’, each specified
SIGSPEC's disposition is reset to the value it had when the shell
was started. If ACTION is the null string, then the signal
specified by each SIGSPEC is ignored by the shell and commands it
invokes.
If no arguments are supplied, ‘trap’ prints the actions associated
with each trapped signal as a set of ‘trap’ commands that can be
reused as shell input to restore the current signal dispositions.
If ACTION is not present and ‘-p’ has been supplied, ‘trap’
displays the trap commands associated with each SIGSPEC, or, if no
SIGSPECs are supplied, for all trapped signals, as a set of ‘trap’
commands that can be reused as shell input to restore the current
signal dispositions. The ‘-P’ option behaves similarly, but
displays only the actions associated with each SIGSPEC argument.
‘-P’ requires at least one SIGSPEC argument. The ‘-P’ or ‘-p’
options may be used in a subshell environment (e.g., command
substitution) and, as long as they are used before ‘trap’ is used
to change a signal's handling, will display the state of its
parent's traps.
The ‘-l’ option prints a list of signal names and their
corresponding numbers. Each SIGSPEC is either a signal name or a
signal number. Signal names are case insensitive and the ‘SIG’
prefix is optional. If ‘-l’ is supplied with no SIGSPEC arguments,
it prints a list of valid signal names.
If a SIGSPEC is ‘0’ or ‘EXIT’, ACTION is executed when the shell
exits. If a SIGSPEC is ‘DEBUG’, ACTION is executed before every
simple command, ‘for’ command, ‘case’ command, ‘select’ command, ((
arithmetic command, [[ conditional command, arithmetic ‘for’
command, and before the first command executes in a shell function.
Refer to the description of the ‘extdebug’ shell option (*note The
Shopt Builtin::) for details of its effect on the ‘DEBUG’ trap. If
a SIGSPEC is ‘RETURN’, ACTION is executed each time a shell
function or a script executed with the ‘.’ or ‘source’ builtins
finishes executing.
If a SIGSPEC is ‘ERR’, ACTION is executed whenever a pipeline
(which may consist of a single simple command), a list, or a
compound command returns a non-zero exit status, subject to the
following conditions. The ‘ERR’ trap is not executed if the failed
command is part of the command list immediately following an
‘until’ or ‘while’ reserved word, part of the test following the
‘if’ or ‘elif’ reserved words, part of a command executed in a ‘&&’
or ‘||’ list except the command following the final ‘&&’ or ‘||’,
any command in a pipeline but the last, (subject to the state of
the ‘pipefail’ shell option), or if the command's return status is
being inverted using ‘!’. These are the same conditions obeyed by
the ‘errexit’ (‘-e’) option.
When the shell is not interactive, signals ignored upon entry to a
non-interactive shell cannot be trapped or reset. Interactive
shells permit trapping signals ignored on entry. Trapped signals
that are not being ignored are reset to their original values in a
subshell or subshell environment when one is created.
The return status is zero unless a SIGSPEC does not specify a valid
signal; non-zero otherwise.
‘true’
true
Does nothing, returns a 0 status.
‘umask’
umask [-p] [-S] [MODE]
Set the shell process's file creation mask to MODE. If MODE begins
with a digit, it is interpreted as an octal number; if not, it is
interpreted as a symbolic mode mask similar to that accepted by the
‘chmod’ command. If MODE is omitted, ‘umask’ prints the current
value of the mask. If the ‘-S’ option is supplied without a MODE
argument, ‘umask’ prints the mask in a symbolic format; the default
output is an octal number. If the ‘-p’ option is supplied, and
MODE is omitted, the output is in a form that may be reused as
input. The return status is zero if the mode is successfully
changed or if no MODE argument is supplied, and non-zero otherwise.
Note that when the mode is interpreted as an octal number, each
number of the umask is subtracted from ‘7’. Thus, a umask of ‘022’
results in permissions of ‘755’.
‘unset’
unset [-fnv] [NAME]
Remove each variable or function NAME. If the ‘-v’ option is
given, each NAME refers to a shell variable and that variable is
removed. If the ‘-f’ option is given, the NAMEs refer to shell
functions, and the function definition is removed. If the ‘-n’
option is supplied, and NAME is a variable with the ‘nameref’
attribute, NAME will be unset rather than the variable it
references. ‘-n’ has no effect if the ‘-f’ option is supplied. If
no options are supplied, each NAME refers to a variable; if there
is no variable by that name, a function with that name, if any, is
unset. Readonly variables and functions may not be unset. When
variables or functions are removed, they are also removed from the
environment passed to subsequent commands. Some shell variables
may not be unset. Some shell variables lose their special behavior
if they are unset; such behavior is noted in the description of the
individual variables. The return status is zero unless a NAME is
readonly or may not be unset.
File: bash.info, Node: Bash Builtins, Next: Modifying Shell Behavior, Prev: Bourne Shell Builtins, Up: Shell Builtin Commands
4.2 Bash Builtin Commands
=========================
This section describes builtin commands which are unique to or have been
extended in Bash. Some of these commands are specified in the POSIX
standard.
‘alias’
alias [-p] [NAME[=VALUE] ...]
Without arguments or with the ‘-p’ option, ‘alias’ prints the list
of aliases on the standard output in a form that allows them to be
reused as input. If arguments are supplied, define an alias for
each NAME whose VALUE is given. If no VALUE is given, print the
name and value of the alias NAME. A trailing space in VALUE causes
the next word to be checked for alias substitution when the alias
is expanded during command parsing. ‘alias’ returns true unless a
NAME is given (without a corresponding =VALUE) for which no alias
has been defined. Aliases are described in *note Aliases::.
‘bind’
bind [-m KEYMAP] [-lsvSVX]
bind [-m KEYMAP] [-q FUNCTION] [-u FUNCTION] [-r KEYSEQ]
bind [-m KEYMAP] -f FILENAME
bind [-m KEYMAP] -x KEYSEQ[: ]SHELL-COMMAND
bind [-m KEYMAP] KEYSEQ:FUNCTION-NAME
bind [-m KEYMAP] KEYSEQ:READLINE-COMMAND
bind [-m KEYMAP] -p|-P [READLINE-COMMAND]
bind READLINE-COMMAND-LINE
Display current Readline (*note Command Line Editing::) key and
function bindings, bind a key sequence to a Readline function or
macro or to a shell command, or set a Readline variable. Each
non-option argument is a key binding or command as it would appear
in a Readline initialization file (*note Readline Init File::), but
each binding or command must be passed as a separate argument;
e.g., ‘"\C-x\C-r":re-read-init-file’.
In the following descriptions, options that display output in a
form available to be re-read format their output as commands that
would appear in a Readline initialization file or that would be
supplied as individual arguments to a ‘bind’ command.
Options, if supplied, have the following meanings:
‘-m KEYMAP’
Use KEYMAP as the keymap to be affected by the subsequent
bindings. Acceptable KEYMAP names are ‘emacs’,
‘emacs-standard’, ‘emacs-meta’, ‘emacs-ctlx’, ‘vi’, ‘vi-move’,
‘vi-command’, and ‘vi-insert’. ‘vi’ is equivalent to
‘vi-command’ (‘vi-move’ is also a synonym); ‘emacs’ is
equivalent to ‘emacs-standard’.
‘-l’
List the names of all Readline functions.
‘-p’
Display Readline function names and bindings in such a way
that they can be used as an argument to a subsequent ‘bind’
command or in a Readline initialization file. If arguments
remain after option processing, ‘bind’ treats them as readline
command names and restricts output to those names.
‘-P’
List current Readline function names and bindings. If
arguments remain after option processing, ‘bind’ treats them
as readline command names and restricts output to those names.
‘-s’
Display Readline key sequences bound to macros and the strings
they output in such a way that they can be used as an argument
to a subsequent ‘bind’ command or in a Readline initialization
file.
‘-S’
Display Readline key sequences bound to macros and the strings
they output.
‘-v’
Display Readline variable names and values in such a way that
they can be used as an argument to a subsequent ‘bind’ command
or in a Readline initialization file.
‘-V’
List current Readline variable names and values.
‘-f FILENAME’
Read key bindings from FILENAME.
‘-q FUNCTION’
Display key sequences that invoke the named Readline FUNCTION.
‘-u FUNCTION’
Unbind all key sequences bound to the named Readline FUNCTION.
‘-r KEYSEQ’
Remove any current binding for KEYSEQ.
‘-x KEYSEQ:SHELL-COMMAND’
Cause SHELL-COMMAND to be executed whenever KEYSEQ is entered.
The separator between KEYSEQ and SHELL-COMMAND is either
whitespace or a colon optionally followed by whitespace. If
the separator is whitespace, SHELL-COMMAND must be enclosed in
double quotes and Readline expands any of its special
backslash-escapes in SHELL-COMMAND before saving it. If the
separator is a colon, any enclosing double quotes are
optional, and Readline does not expand the command string
before saving it. Since the entire key binding expression
must be a single argument, it should be enclosed in single
quotes. When SHELL-COMMAND is executed, the shell sets the
‘READLINE_LINE’ variable to the contents of the Readline line
buffer and the ‘READLINE_POINT’ and ‘READLINE_MARK’ variables
to the current location of the insertion point and the saved
insertion point (the MARK), respectively. The shell assigns
any numeric argument the user supplied to the
‘READLINE_ARGUMENT’ variable. If there was no argument, that
variable is not set. If the executed command changes the
value of any of ‘READLINE_LINE’, ‘READLINE_POINT’, or
‘READLINE_MARK’, those new values will be reflected in the
editing state.
‘-X’
List all key sequences bound to shell commands and the
associated commands in a format that can be reused as an
argument to a subsequent ‘bind’ command.
The return status is zero unless an invalid option is supplied or
an error occurs.
‘builtin’
builtin [SHELL-BUILTIN [ARGS]]
Execute the specified shell builtin SHELL-BUILTIN, passing it ARGS,
and return its exit status. This is useful when defining a shell
function with the same name as a shell builtin, retaining the
functionality of the builtin within the function. The return
status is non-zero if SHELL-BUILTIN is not a shell builtin command.
‘caller’
caller [EXPR]
Returns the context of any active subroutine call (a shell function
or a script executed with the ‘.’ or ‘source’ builtins).
Without EXPR, ‘caller’ displays the line number and source filename
of the current subroutine call. If a non-negative integer is
supplied as EXPR, ‘caller’ displays the line number, subroutine
name, and source file corresponding to that position in the current
execution call stack. This extra information may be used, for
example, to print a stack trace. The current frame is frame 0.
The return value is 0 unless the shell is not executing a
subroutine call or EXPR does not correspond to a valid position in
the call stack.
‘command’
command [-pVv] COMMAND [ARGUMENTS ...]
The ‘command’ builtin runs COMMAND with ARGUMENTS ignoring any
shell function named COMMAND. Only shell builtin commands or
commands found by searching the ‘PATH’ are executed. If there is a
shell function named ‘ls’, running ‘command ls’ within the function
will execute the external command ‘ls’ instead of calling the
function recursively. The ‘-p’ option means to use a default value
for ‘PATH’ that is guaranteed to find all of the standard
utilities. The return status in this case is 127 if COMMAND cannot
be found or an error occurred, and the exit status of COMMAND
otherwise.
If either the ‘-V’ or ‘-v’ option is supplied, ‘command’ prints a
description of COMMAND. The ‘-v’ option displays a single word
indicating the command or file name used to invoke COMMAND; the
‘-V’ option produces a more verbose description. In this case, the
return status is zero if COMMAND is found, and non-zero if not.
‘declare’
declare [-aAfFgiIlnrtux] [-p] [NAME[=VALUE] ...]
Declare variables and give them attributes. If no NAMEs are given,
then display the values of variables or shell functions instead.
The ‘-p’ option will display the attributes and values of each
NAME. When ‘-p’ is used with NAME arguments, additional options,
other than ‘-f’ and ‘-F’, are ignored.
When ‘-p’ is supplied without NAME arguments, ‘declare’ will
display the attributes and values of all variables having the
attributes specified by the additional options. If no other
options are supplied with ‘-p’, ‘declare’ will display the
attributes and values of all shell variables. The ‘-f’ option
restricts the display to shell functions.
The ‘-F’ option inhibits the display of function definitions; only
the function name and attributes are printed. If the ‘extdebug’
shell option is enabled using ‘shopt’ (*note The Shopt Builtin::),
the source file name and line number where each NAME is defined are
displayed as well. ‘-F’ implies ‘-f’.
The ‘-g’ option forces variables to be created or modified at the
global scope, even when ‘declare’ is executed in a shell function.
It is ignored in when ‘declare’ is not executed in a shell
function.
The ‘-I’ option causes local variables to inherit the attributes
(except the ‘nameref’ attribute) and value of any existing variable
with the same NAME at a surrounding scope. If there is no existing
variable, the local variable is initially unset.
The following options can be used to restrict output to variables
with the specified attributes or to give variables attributes:
‘-a’
Each NAME is an indexed array variable (*note Arrays::).
‘-A’
Each NAME is an associative array variable (*note Arrays::).
‘-f’
Each NAME refers to a shell function.
‘-i’
The variable is to be treated as an integer; arithmetic
evaluation (*note Shell Arithmetic::) is performed when the
variable is assigned a value.
‘-l’
When the variable is assigned a value, all upper-case
characters are converted to lower-case. The upper-case
attribute is disabled.
‘-n’
Give each NAME the ‘nameref’ attribute, making it a name
reference to another variable. That other variable is defined
by the value of NAME. All references, assignments, and
attribute modifications to NAME, except for those using or
changing the ‘-n’ attribute itself, are performed on the
variable referenced by NAME's value. The nameref attribute
cannot be applied to array variables.
‘-r’
Make NAMEs readonly. These names cannot then be assigned
values by subsequent assignment statements or unset.
‘-t’
Give each NAME the ‘trace’ attribute. Traced functions
inherit the ‘DEBUG’ and ‘RETURN’ traps from the calling shell.
The trace attribute has no special meaning for variables.
‘-u’
When the variable is assigned a value, all lower-case
characters are converted to upper-case. The lower-case
attribute is disabled.
‘-x’
Mark each NAME for export to subsequent commands via the
environment.
Using ‘+’ instead of ‘-’ turns off the specified attribute instead,
with the exceptions that ‘+a’ and ‘+A’ may not be used to destroy
array variables and ‘+r’ will not remove the readonly attribute.
When used in a function, ‘declare’ makes each NAME local, as with
the ‘local’ command, unless the ‘-g’ option is supplied. If a
variable name is followed by =VALUE, the value of the variable is
set to VALUE.
When using ‘-a’ or ‘-A’ and the compound assignment syntax to
create array variables, additional attributes do not take effect
until subsequent assignments.
The return status is zero unless an invalid option is encountered,
an attempt is made to define a function using ‘-f foo=bar’, an
attempt is made to assign a value to a readonly variable, an
attempt is made to assign a value to an array variable without
using the compound assignment syntax (*note Arrays::), one of the
NAMEs is not a valid shell variable name, an attempt is made to
turn off readonly status for a readonly variable, an attempt is
made to turn off array status for an array variable, or an attempt
is made to display a non-existent function with ‘-f’.
‘echo’
echo [-neE] [ARG ...]
Output the ARGs, separated by spaces, terminated with a newline.
The return status is 0 unless a write error occurs. If ‘-n’ is
specified, the trailing newline is not printed.
If the ‘-e’ option is given, ‘echo’ interprets the following
backslash-escaped characters. The ‘-E’ option disables
interpretation of these escape characters, even on systems where
they are interpreted by default. The ‘xpg_echo’ shell option
determines whether or not ‘echo’ interprets any options and expands
these escape characters. ‘echo’ does not interpret ‘--’ to mean
the end of options.
‘echo’ interprets the following escape sequences:
‘\a’
alert (bell)
‘\b’
backspace
‘\c’
suppress further output
‘\e’
‘\E’
escape
‘\f’
form feed
‘\n’
new line
‘\r’
carriage return
‘\t’
horizontal tab
‘\v’
vertical tab
‘\\’
backslash
‘\0NNN’
The eight-bit character whose value is the octal value NNN
(zero to three octal digits).
‘\xHH’
The eight-bit character whose value is the hexadecimal value
HH (one or two hex digits).
‘\uHHHH’
The Unicode (ISO/IEC 10646) character whose value is the
hexadecimal value HHHH (one to four hex digits).
‘\UHHHHHHHH’
The Unicode (ISO/IEC 10646) character whose value is the
hexadecimal value HHHHHHHH (one to eight hex digits).
‘echo’ writes any unrecognized backslash-escaped characters
unchanged.
‘enable’
enable [-a] [-dnps] [-f FILENAME] [NAME ...]
Enable and disable builtin shell commands. Disabling a builtin
allows an executable file which has the same name as a shell
builtin to be executed without specifying a full pathname, even
though the shell normally searches for builtins before files.
If ‘-n’ is supplied, the NAMEs are disabled. Otherwise NAMEs are
enabled. For example, to use the ‘test’ binary found using ‘$PATH’
instead of the shell builtin version, type ‘enable -n test’.
If the ‘-p’ option is supplied, or no NAME arguments are supplied,
print a list of shell builtins. With no other arguments, the list
consists of all enabled shell builtins. The ‘-n’ option means to
print only disabled builtins. The ‘-a’ option means to list each
builtin with an indication of whether or not it is enabled. The
‘-s’ option means to restrict ‘enable’ to the POSIX special
builtins.
The ‘-f’ option means to load the new builtin command NAME from
shared object FILENAME, on systems that support dynamic loading.
If FILENAME does not contain a slash. Bash will use the value of
the ‘BASH_LOADABLES_PATH’ variable as a colon-separated list of
directories in which to search for FILENAME. The default for
‘BASH_LOADABLES_PATH’ is system-dependent, and may include "." to
force a search of the current directory. The ‘-d’ option will
delete a builtin loaded with ‘-f’. If ‘-s’ is used with ‘-f’, the
new builtin becomes a POSIX special builtin (*note Special
Builtins::).
If no options are supplied and a NAME is not a shell builtin,
‘enable’ will attempt to load NAME from a shared object named NAME,
as if the command were ‘enable -f NAME NAME’.
The return status is zero unless a NAME is not a shell builtin or
there is an error loading a new builtin from a shared object.
‘help’
help [-dms] [PATTERN]
Display helpful information about builtin commands. If PATTERN is
specified, ‘help’ gives detailed help on all commands matching
PATTERN as described below; otherwise it displays a list of all
builtins and shell compound commands.
Options, if supplied, have the following meanings:
‘-d’
Display a short description of each PATTERN
‘-m’
Display the description of each PATTERN in a manpage-like
format
‘-s’
Display only a short usage synopsis for each PATTERN
If PATTERN contains pattern matching characters (*note Pattern
Matching::) it's treated as a shell pattern and ‘help’ prints the
description of each help topic matching PATTERN.
If not, and PATTERN exactly matches the name of a help topic,
‘help’ prints the description associated with that topic.
Otherwise, ‘help’ performs prefix matching and prints the
descriptions of all matching help topics.
The return status is zero unless no command matches PATTERN.
‘let’
let EXPRESSION [EXPRESSION ...]
The ‘let’ builtin allows arithmetic to be performed on shell
variables. Each EXPRESSION is evaluated as an arithmetic
expression according to the rules given below in *note Shell
Arithmetic::. If the last EXPRESSION evaluates to 0, ‘let’ returns
1; otherwise ‘let’ returns 0.
‘local’
local [OPTION] NAME[=VALUE] ...
For each argument, create a local variable named NAME, and assign
it VALUE. The OPTION can be any of the options accepted by
‘declare’. ‘local’ can only be used within a function; it makes
the variable NAME have a visible scope restricted to that function
and its children. It is an error to use ‘local’ when not within a
function.
If NAME is ‘-’, it makes the set of shell options local to the
function in which ‘local’ is invoked: any shell options changed
using the ‘set’ builtin inside the function after the call to
‘local’ are restored to their original values when the function
returns. The restore is performed as if a series of ‘set’ commands
were executed to restore the values that were in place before the
function.
With no operands, ‘local’ writes a list of local variables to the
standard output.
The return status is zero unless ‘local’ is used outside a
function, an invalid NAME is supplied, or NAME is a readonly
variable.
‘logout’
logout [N]
Exit a login shell, returning a status of N to the shell's parent.
‘mapfile’
mapfile [-d DELIM] [-n COUNT] [-O ORIGIN] [-s COUNT]
[-t] [-u FD] [-C CALLBACK] [-c QUANTUM] [ARRAY]
Read lines from the standard input, or from file descriptor FD if
the ‘-u’ option is supplied, into the indexed array variable ARRAY.
The variable ‘MAPFILE’ is the default ARRAY. Options, if supplied,
have the following meanings:
‘-d’
Use the first character of DELIM to terminate each input line,
rather than newline. If DELIM is the empty string, ‘mapfile’
will terminate a line when it reads a NUL character.
‘-n’
Copy at most COUNT lines. If COUNT is 0, copy all lines.
‘-O’
Begin assigning to ARRAY at index ORIGIN. The default index
is 0.
‘-s’
Discard the first COUNT lines read.
‘-t’
Remove a trailing DELIM (default newline) from each line read.
‘-u’
Read lines from file descriptor FD instead of the standard
input.
‘-C’
Evaluate CALLBACK each time QUANTUM lines are read. The ‘-c’
option specifies QUANTUM.
‘-c’
Specify the number of lines read between each call to
CALLBACK.
If ‘-C’ is specified without ‘-c’, the default quantum is 5000.
When CALLBACK is evaluated, it is supplied the index of the next
array element to be assigned and the line to be assigned to that
element as additional arguments. CALLBACK is evaluated after the
line is read but before the array element is assigned.
If not supplied with an explicit origin, ‘mapfile’ will clear ARRAY
before assigning to it.
‘mapfile’ returns zero unless an invalid option or option argument
is supplied, ARRAY is invalid or unassignable, or if ARRAY is not
an indexed array.
‘printf’
printf [-v VAR] FORMAT [ARGUMENTS]
Write the formatted ARGUMENTS to the standard output under the
control of the FORMAT. The ‘-v’ option assigns the output to the
variable VAR rather than printing it to the standard output.
The FORMAT is a character string which contains three types of
objects: plain characters, which are simply copied to standard
output, character escape sequences, which are converted and copied
to the standard output, and format specifications, each of which
causes printing of the next successive ARGUMENT. In addition to
the standard ‘printf(3)’ format characters ‘cCsSndiouxXeEfFgGaA’,
‘printf’ interprets the following additional format specifiers:
‘%b’
Causes ‘printf’ to expand backslash escape sequences in the
corresponding ARGUMENT in the same way as ‘echo -e’ (*note
Bash Builtins::).
‘%q’
Causes ‘printf’ to output the corresponding ARGUMENT in a
format that can be reused as shell input. ‘%q’ and ‘%Q’P use
the ANSI-C quoting style (*note ANSI-C Quoting::) if any
characters in the argument string require it, and backslash
quoting otherwise. If the format string uses the ‘printf’
_alternate form_, these two formats quote the argument string
using single quotes.
‘%Q’
like ‘%q’, but applies any supplied precision to the ARGUMENT
before quoting it.
‘%(DATEFMT)T’
Causes ‘printf’ to output the date-time string resulting from
using DATEFMT as a format string for ‘strftime’(3). The
corresponding ARGUMENT is an integer representing the number
of seconds since the epoch. This format specifier recognizes
Two special argument values: -1 represents the current time,
and -2 represents the time the shell was invoked. If no
argument is specified, conversion behaves as if -1 had been
supplied. This is an exception to the usual ‘printf’
behavior.
The %b, %q, and %T format specifiers all use the field width and
precision arguments from the format specification and write that
many bytes from (or use that wide a field for) the expanded
argument, which usually contains more characters than the original.
The %n format specifier accepts a corresponding argument that is
treated as a shell variable name.
The %s and %c format specifiers accept an l (long) modifier, which
forces them to convert the argument string to a wide-character
string and apply any supplied field width and precision in terms of
characters, not bytes. The %S and %C format specifiers are
equivalent to %ls and %lc, respectively.
Arguments to non-string format specifiers are treated as C language
constants, except that a leading plus or minus sign is allowed, and
if the leading character is a single or double quote, the value is
the numeric value of the following character, using the current
locale.
The FORMAT is reused as necessary to consume all of the ARGUMENTS.
If the FORMAT requires more ARGUMENTS than are supplied, the extra
format specifications behave as if a zero value or null string, as
appropriate, had been supplied. The return value is zero on
success, non-zero if an invalid option is supplied or a write or
assignment error occurs.
‘read’
read [-Eers] [-a ANAME] [-d DELIM] [-i TEXT] [-n NCHARS]
[-N NCHARS] [-p PROMPT] [-t TIMEOUT] [-u FD] [NAME ...]
Read one line from the standard input, or from the file descriptor
FD supplied as an argument to the ‘-u’ option, split it into words
as described above in *note Word Splitting::, and assign the first
word to the first NAME, the second word to the second NAME, and so
on. If there are more words than names, the remaining words and
their intervening delimiters are assigned to the last NAME. If
there are fewer words read from the input stream than names, the
remaining names are assigned empty values. The characters in the
value of the ‘IFS’ variable are used to split the line into words
using the same rules the shell uses for expansion (described above
in *note Word Splitting::). The backslash character ‘\’ removes
any special meaning for the next character read and is used for
line continuation.
Options, if supplied, have the following meanings:
‘-a ANAME’
The words are assigned to sequential indices of the array
variable ANAME, starting at 0. All elements are removed from
ANAME before the assignment. Other NAME arguments are
ignored.
‘-d DELIM’
The first character of DELIM terminates the input line, rather
than newline. If DELIM is the empty string, ‘read’ will
terminate a line when it reads a NUL character.
‘-e’
If the standard input is coming from a terminal, ‘read’ uses
Readline (*note Command Line Editing::) to obtain the line.
Readline uses the current (or default, if line editing was not
previously active) editing settings, but uses Readline's
default filename completion.
‘-E’
If the standard input is coming from a terminal, ‘read’ uses
Readline (*note Command Line Editing::) to obtain the line.
Readline uses the current (or default, if line editing was not
previously active) editing settings, but uses Bash's default
completion, including programmable completion.
‘-i TEXT’
If Readline is being used to read the line, ‘read’ places TEXT
into the editing buffer before editing begins.
‘-n NCHARS’
‘read’ returns after reading NCHARS characters rather than
waiting for a complete line of input, unless it encounters EOF
or ‘read’ times out, but honors a delimiter if it reads fewer
than NCHARS characters before the delimiter.
‘-N NCHARS’
‘read’ returns after reading exactly NCHARS characters rather
than waiting for a complete line of input, unless it
encounters EOF or ‘read’ times out. Delimiter characters in
the input are not treated specially and do not cause ‘read’ to
return until it has read NCHARS characters. The result is not
split on the characters in ‘IFS’; the intent is that the
variable is assigned exactly the characters read (with the
exception of backslash; see the ‘-r’ option below).
‘-p PROMPT’
Display PROMPT, without a trailing newline, before attempting
to read any input, but only if input is coming from a
terminal.
‘-r’
If this option is given, backslash does not act as an escape
character. The backslash is considered to be part of the
line. In particular, a backslash-newline pair may not then be
used as a line continuation.
‘-s’
Silent mode. If input is coming from a terminal, characters
are not echoed.
‘-t TIMEOUT’
Cause ‘read’ to time out and return failure if it does not
read a complete line of input (or a specified number of
characters) within TIMEOUT seconds. TIMEOUT may be a decimal
number with a fractional portion following the decimal point.
This option is only effective if ‘read’ is reading input from
a terminal, pipe, or other special file; it has no effect when
reading from regular files. If ‘read’ times out, it saves any
partial input read into the specified variable NAME, and
returns a status greater than 128. If TIMEOUT is 0, ‘read’
returns immediately, without trying to read any data. In this
case, the exit status is 0 if input is available on the
specified file descriptor, or the read will return EOF,
non-zero otherwise.
‘-u FD’
Read input from file descriptor FD instead of the standard
input.
Other than the case where DELIM is the empty string, ‘read’ ignores
any NUL characters in the input.
If no NAMEs are supplied, ‘read’ assigns the line read, without the
ending delimiter but otherwise unmodified, to the variable ‘REPLY’.
The exit status is zero, unless end-of-file is encountered, ‘read’
times out (in which case the status is greater than 128), a
variable assignment error (such as assigning to a readonly
variable) occurs, or an invalid file descriptor is supplied as the
argument to ‘-u’.
‘readarray’
readarray [-d DELIM] [-n COUNT] [-O ORIGIN] [-s COUNT]
[-t] [-u FD] [-C CALLBACK] [-c QUANTUM] [ARRAY]
Read lines from the standard input into the indexed array variable
ARRAY, or from file descriptor FD if the ‘-u’ option is supplied.
A synonym for ‘mapfile’.
‘source’
source [-p PATH] FILENAME [ARGUMENTS]
A synonym for ‘.’ (*note Bourne Shell Builtins::).
‘type’
type [-afptP] [NAME ...]
Indicate how each NAME would be interpreted if used as a command
name.
If the ‘-t’ option is used, ‘type’ prints a single word which is
one of ‘alias’, ‘keyword’, ‘function’, ‘builtin’, or ‘file’, if
NAME is an alias, shell reserved word, shell function, shell
builtin, or executable file, respectively. If the NAME is not
found, ‘type’ prints nothing and returns a failure status.
If the ‘-p’ option is used, ‘type’ either returns the name of the
executable file that would be found by searching ‘$PATH’ for
‘name’, or nothing if ‘-t’ would not return ‘file’.
The ‘-P’ option forces a path search for each NAME, even if ‘-t’
would not return ‘file’.
If a NAME is present in the table of hashed commands, options ‘-p’
and ‘-P’ print the hashed value, which is not necessarily the file
that appears first in ‘$PATH’.
If the ‘-a’ option is used, ‘type’ returns all of the places that
contain a command named NAME. This includes aliases, reserved
words, functions, and builtins, but the path search options (‘-p’
and ‘-P’) can be supplied to restrict the output to executable
files. If ‘-a’ is supplied with ‘-p’, ‘type’ does not look in the
table of hashed commands, and only performs a ‘PATH’ search for
NAME.
If the ‘-f’ option is used, ‘type’ does not attempt to find shell
functions, as with the ‘command’ builtin.
The return status is zero if all of the NAMEs are found, non-zero
if any are not found.
‘typeset’
typeset [-afFgrxilnrtux] [-p] [NAME[=VALUE] ...]
The ‘typeset’ command is supplied for compatibility with the Korn
shell. It is a synonym for the ‘declare’ builtin command.
‘ulimit’
ulimit [-HS] -a
ulimit [-HS] [-bcdefiklmnpqrstuvxPRT] [LIMIT]
‘ulimit’ provides control over the resources available to the shell
and to processes it starts, on systems that allow such control. If
an option is given, it is interpreted as follows:
‘-S’
Change and report the soft limit associated with a resource.
‘-H’
Change and report the hard limit associated with a resource.
‘-a’
Report all current limits; no limits are set.
‘-b’
The maximum socket buffer size.
‘-c’
The maximum size of core files created.
‘-d’
The maximum size of a process's data segment.
‘-e’
The maximum scheduling priority ("nice").
‘-f’
The maximum size of files written by the shell and its
children.
‘-i’
The maximum number of pending signals.
‘-k’
The maximum number of kqueues that may be allocated.
‘-l’
The maximum size that may be locked into memory.
‘-m’
The maximum resident set size (many systems do not honor this
limit).
‘-n’
The maximum number of open file descriptors (most systems do
not allow this value to be set).
‘-p’
The pipe buffer size.
‘-q’
The maximum number of bytes in POSIX message queues.
‘-r’
The maximum real-time scheduling priority.
‘-s’
The maximum stack size.
‘-t’
The maximum amount of cpu time in seconds.
‘-u’
The maximum number of processes available to a single user.
‘-v’
The maximum amount of virtual memory available to the shell,
and, on some systems, to its children.
‘-x’
The maximum number of file locks.
‘-P’
The maximum number of pseudoterminals.
‘-R’
The maximum time a real-time process can run before blocking,
in microseconds.
‘-T’
The maximum number of threads.
If LIMIT is supplied, and the ‘-a’ option is not used, LIMIT is the
new value of the specified resource. The special LIMIT values
‘hard’, ‘soft’, and ‘unlimited’ stand for the current hard limit,
the current soft limit, and no limit, respectively. A hard limit
cannot be increased by a non-root user once it is set; a soft limit
may be increased up to the value of the hard limit. Otherwise,
‘ulimit’ prints the current value of the soft limit for the
specified resource, unless the ‘-H’ option is supplied. When more
than one resource is specified, the limit name and unit, if
appropriate, are printed before the value. When setting new
limits, if neither ‘-H’ nor ‘-S’ is supplied, ‘ulimit’ sets both
the hard and soft limits. If no option is supplied, then ‘-f’ is
assumed.
Values are in 1024-byte increments, except for ‘-t’, which is in
seconds; ‘-R’, which is in microseconds; ‘-p’, which is in units of
512-byte blocks; ‘-P’, ‘-T’, ‘-b’, ‘-k’, ‘-n’ and ‘-u’, which are
unscaled values; and, when in POSIX mode (*note Bash POSIX Mode::),
‘-c’ and ‘-f’, which are in 512-byte increments.
The return status is zero unless an invalid option or argument is
supplied, or an error occurs while setting a new limit.
‘unalias’
unalias [-a] [NAME ... ]
Remove each NAME from the list of aliases. If ‘-a’ is supplied,
remove all aliases. The return value is true unless a supplied
NAME is not a defined alias. Aliases are described in *note
Aliases::.
File: bash.info, Node: Modifying Shell Behavior, Next: Special Builtins, Prev: Bash Builtins, Up: Shell Builtin Commands
4.3 Modifying Shell Behavior
============================
* Menu:
* The Set Builtin:: Change the values of shell attributes and
positional parameters.
* The Shopt Builtin:: Modify shell optional behavior.
File: bash.info, Node: The Set Builtin, Next: The Shopt Builtin, Up: Modifying Shell Behavior
4.3.1 The Set Builtin
---------------------
This builtin is so complicated that it deserves its own section. ‘set’
allows you to change the values of shell options and set the positional
parameters, or to display the names and values of shell variables.
‘set’
set [-abefhkmnptuvxBCEHPT] [-o OPTION-NAME] [--] [-] [ARGUMENT ...]
set [+abefhkmnptuvxBCEHPT] [+o OPTION-NAME] [--] [-] [ARGUMENT ...]
set -o
set +o
If no options or arguments are supplied, ‘set’ displays the names
and values of all shell variables and functions, sorted according
to the current locale, in a format that may be reused as input for
setting or resetting the currently-set variables. Read-only
variables cannot be reset. In POSIX mode, only shell variables are
listed.
When options are supplied, they set or unset shell attributes. Any
arguments remaining after option processing replace the positional
parameters.
Options, if specified, have the following meanings:
‘-a’
Each variable or function that is created or modified is given
the export attribute and marked for export to the environment
of subsequent commands.
‘-b’
Cause the status of terminated background jobs to be reported
immediately, rather than before printing the next primary
prompt or, under some circumstances, when a foreground command
exits. This is effective only when job control is enabled.
‘-e’
Exit immediately if a pipeline (*note Pipelines::), which may
consist of a single simple command (*note Simple Commands::),
a list (*note Lists::), or a compound command (*note Compound
Commands::) returns a non-zero status. The shell does not
exit if the command that fails is part of the command list
immediately following a ‘while’ or ‘until’ reserved word, part
of the test in an ‘if’ statement, part of any command executed
in a ‘&&’ or ‘||’ list except the command following the final
‘&&’ or ‘||’, any command in a pipeline but the last (subject
to the state of the ‘pipefail’ shell option), or if the
command's return status is being inverted with ‘!’. If a
compound command other than a subshell returns a non-zero
status because a command failed while ‘-e’ was being ignored,
the shell does not exit. A trap on ‘ERR’, if set, is executed
before the shell exits.
This option applies to the shell environment and each subshell
environment separately (*note Command Execution
Environment::), and may cause subshells to exit before
executing all the commands in the subshell.
If a compound command or shell function executes in a context
where ‘-e’ is being ignored, none of the commands executed
within the compound command or function body will be affected
by the ‘-e’ setting, even if ‘-e’ is set and a command returns
a failure status. If a compound command or shell function
sets ‘-e’ while executing in a context where ‘-e’ is ignored,
that setting will not have any effect until the compound
command or the command containing the function call completes.
‘-f’
Disable filename expansion (globbing).
‘-h’
Locate and remember (hash) commands as they are looked up for
execution. This option is enabled by default.
‘-k’
All arguments in the form of assignment statements are placed
in the environment for a command, not just those that precede
the command name.
‘-m’
Job control is enabled (*note Job Control::). All processes
run in a separate process group. When a background job
completes, the shell prints a line containing its exit status.
‘-n’
Read commands but do not execute them. This may be used to
check a script for syntax errors. This option is ignored by
interactive shells.
‘-o OPTION-NAME’
Set the option corresponding to OPTION-NAME. If ‘-o’ is
supplied with no OPTION-NAME, ‘set’ prints the current shell
options settings. If ‘+o’ is supplied with no OPTION-NAME,
‘set’ prints a series of ‘set’ commands to recreate the
current option settings on the standard output. Valid option
names are:
‘allexport’
Same as ‘-a’.
‘braceexpand’
Same as ‘-B’.
‘emacs’
Use an ‘emacs’-style line editing interface (*note
Command Line Editing::). This also affects the editing
interface used for ‘read -e’.
‘errexit’
Same as ‘-e’.
‘errtrace’
Same as ‘-E’.
‘functrace’
Same as ‘-T’.
‘hashall’
Same as ‘-h’.
‘histexpand’
Same as ‘-H’.
‘history’
Enable command history, as described in *note Bash
History Facilities::. This option is on by default in
interactive shells.
‘ignoreeof’
An interactive shell will not exit upon reading EOF.
‘keyword’
Same as ‘-k’.
‘monitor’
Same as ‘-m’.
‘noclobber’
Same as ‘-C’.
‘noexec’
Same as ‘-n’.
‘noglob’
Same as ‘-f’.
‘nolog’
Currently ignored.
‘notify’
Same as ‘-b’.
‘nounset’
Same as ‘-u’.
‘onecmd’
Same as ‘-t’.
‘physical’
Same as ‘-P’.
‘pipefail’
If set, the return value of a pipeline is the value of
the last (rightmost) command to exit with a non-zero
status, or zero if all commands in the pipeline exit
successfully. This option is disabled by default.
‘posix’
Enable POSIX mode; change the behavior of Bash where the
default operation differs from the POSIX standard to
match the standard (*note Bash POSIX Mode::). This is
intended to make Bash behave as a strict superset of that
standard.
‘privileged’
Same as ‘-p’.
‘verbose’
Same as ‘-v’.
‘vi’
Use a ‘vi’-style line editing interface. This also
affects the editing interface used for ‘read -e’.
‘xtrace’
Same as ‘-x’.
‘-p’
Turn on privileged mode. In this mode, the ‘$BASH_ENV’ and
‘$ENV’ files are not processed, shell functions are not
inherited from the environment, and the ‘SHELLOPTS’,
‘BASHOPTS’, ‘CDPATH’ and ‘GLOBIGNORE’ variables, if they
appear in the environment, are ignored. If the shell is
started with the effective user (group) id not equal to the
real user (group) id, and the ‘-p’ option is not supplied,
these actions are taken and the effective user id is set to
the real user id. If the ‘-p’ option is supplied at startup,
the effective user id is not reset. Turning this option off
causes the effective user and group ids to be set to the real
user and group ids.
‘-r’
Enable restricted shell mode (*note The Restricted Shell::).
This option cannot be unset once it has been set.
‘-t’
Exit after reading and executing one command.
‘-u’
Treat unset variables and parameters other than the special
parameters ‘@’ or ‘*’, or array variables subscripted with ‘@’
or ‘*’, as an error when performing parameter expansion. An
error message will be written to the standard error, and a
non-interactive shell will exit.
‘-v’
Print shell input lines to standard error as they are read.
‘-x’
Print a trace of simple commands, ‘for’ commands, ‘case’
commands, ‘select’ commands, and arithmetic ‘for’ commands and
their arguments or associated word lists to the standard error
after they are expanded and before they are executed. The
shell prints the expanded value of the ‘PS4’ variable before
the command and its expanded arguments.
‘-B’
The shell will perform brace expansion (*note Brace
Expansion::). This option is on by default.
‘-C’
Prevent output redirection using ‘>’, ‘>&’, and ‘<>’ from
overwriting existing files. Using the redirection operator
‘>|’ instead of ‘>’ will override this and force the creation
of an output file.
‘-E’
If set, any trap on ‘ERR’ is inherited by shell functions,
command substitutions, and commands executed in a subshell
environment. The ‘ERR’ trap is normally not inherited in such
cases.
‘-H’
Enable ‘!’ style history substitution (*note History
Interaction::). This option is on by default for interactive
shells.
‘-P’
If set, Bash does not resolve symbolic links when executing
commands such as ‘cd’ which change the current directory. It
uses the physical directory structure instead. By default,
Bash follows the logical chain of directories when performing
commands which change the current directory.
For example, if ‘/usr/sys’ is a symbolic link to
‘/usr/local/sys’ then:
$ cd /usr/sys; echo $PWD
/usr/sys
$ cd ..; pwd
/usr
If ‘set -P’ is on, then:
$ cd /usr/sys; echo $PWD
/usr/local/sys
$ cd ..; pwd
/usr/local
‘-T’
If set, any traps on ‘DEBUG’ and ‘RETURN’ are inherited by
shell functions, command substitutions, and commands executed
in a subshell environment. The ‘DEBUG’ and ‘RETURN’ traps are
normally not inherited in such cases.
‘--’
If no arguments follow this option, unset the positional
parameters. Otherwise, the positional parameters are set to
the ARGUMENTS, even if some of them begin with a ‘-’.
‘-’
Signal the end of options, and assign all remaining ARGUMENTS
to the positional parameters. The ‘-x’ and ‘-v’ options are
turned off. If there are no arguments, the positional
parameters remain unchanged.
Using ‘+’ rather than ‘-’ causes these options to be turned off.
The options can also be used upon invocation of the shell. The
current set of options may be found in ‘$-’.
The remaining N ARGUMENTS are positional parameters and are
assigned, in order, to ‘$1’, ‘$2’, ... ‘$N’. The special parameter
‘#’ is set to N.
The return status is always zero unless an invalid option is
supplied.
File: bash.info, Node: The Shopt Builtin, Prev: The Set Builtin, Up: Modifying Shell Behavior
4.3.2 The Shopt Builtin
-----------------------
This builtin allows you to change additional optional shell behavior.
‘shopt’
shopt [-pqsu] [-o] [OPTNAME ...]
Toggle the values of settings controlling optional shell behavior.
The settings can be either those listed below, or, if the ‘-o’
option is used, those available with the ‘-o’ option to the ‘set’
builtin command (*note The Set Builtin::).
With no options, or with the ‘-p’ option, display a list of all
settable options, with an indication of whether or not each is set;
if any OPTNAMEs are supplied, the output is restricted to those
options. The ‘-p’ option displays output in a form that may be
reused as input.
Other options have the following meanings:
‘-s’
Enable (set) each OPTNAME.
‘-u’
Disable (unset) each OPTNAME.
‘-q’
Suppresses normal output; the return status indicates whether
the OPTNAME is set or unset. If multiple OPTNAME arguments
are supplied with ‘-q’, the return status is zero if all
OPTNAMEs are enabled; non-zero otherwise.
‘-o’
Restricts the values of OPTNAME to be those defined for the
‘-o’ option to the ‘set’ builtin (*note The Set Builtin::).
If either ‘-s’ or ‘-u’ is used with no OPTNAME arguments, ‘shopt’
shows only those options which are set or unset, respectively.
Unless otherwise noted, the ‘shopt’ options are disabled (off) by
default.
The return status when listing options is zero if all OPTNAMEs are
enabled, non-zero otherwise. When setting or unsetting options,
the return status is zero unless an OPTNAME is not a valid shell
option.
The list of ‘shopt’ options is:
‘array_expand_once’
If set, the shell suppresses multiple evaluation of
associative and indexed array subscripts during arithmetic
expression evaluation, while executing builtins that can
perform variable assignments, and while executing builtins
that perform array dereferencing.
‘assoc_expand_once’
Deprecated; a synonym for ‘array_expand_once’.
‘autocd’
If set, a command name that is the name of a directory is
executed as if it were the argument to the ‘cd’ command. This
option is only used by interactive shells.
‘bash_source_fullpath’
If set, filenames added to the ‘BASH_SOURCE’ array variable
are converted to full pathnames (*note Bash Variables::).
‘cdable_vars’
If this is set, an argument to the ‘cd’ builtin command that
is not a directory is assumed to be the name of a variable
whose value is the directory to change to.
‘cdspell’
If set, the ‘cd’ command attempts to correct minor errors in
the spelling of a directory component. Minor errors include
transposed characters, a missing character, and one extra
character. If ‘cd’ corrects the directory name, it prints the
corrected filename, and the command proceeds. This option is
only used by interactive shells.
‘checkhash’
If this is set, Bash checks that a command found in the hash
table exists before trying to execute it. If a hashed command
no longer exists, Bash performs a normal path search.
‘checkjobs’
If set, Bash lists the status of any stopped and running jobs
before exiting an interactive shell. If any jobs are running,
Bash defers the exit until a second exit is attempted without
an intervening command (*note Job Control::). The shell
always postpones exiting if any jobs are stopped.
‘checkwinsize’
If set, Bash checks the window size after each external
(non-builtin) command and, if necessary, updates the values of
‘LINES’ and ‘COLUMNS’, using the file descriptor associated
with stderr if it is a terminal. This option is enabled by
default.
‘cmdhist’
If set, Bash attempts to save all lines of a multiple-line
command in the same history entry. This allows easy
re-editing of multi-line commands. This option is enabled by
default, but only has an effect if command history is enabled
(*note Bash History Facilities::).
‘compat31’
‘compat32’
‘compat40’
‘compat41’
‘compat42’
‘compat43’
‘compat44’
These control aspects of the shell's compatibility mode (*note
Shell Compatibility Mode::).
‘complete_fullquote’
If set, Bash quotes all shell metacharacters in filenames and
directory names when performing completion. If not set, Bash
removes metacharacters such as the dollar sign from the set of
characters that will be quoted in completed filenames when
these metacharacters appear in shell variable references in
words to be completed. This means that dollar signs in
variable names that expand to directories will not be quoted;
however, any dollar signs appearing in filenames will not be
quoted, either. This is active only when Bash is using
backslashes to quote completed filenames. This variable is
set by default, which is the default Bash behavior in versions
through 4.2.
‘direxpand’
If set, Bash replaces directory names with the results of word
expansion when performing filename completion. This changes
the contents of the Readline editing buffer. If not set, Bash
attempts to preserve what the user typed.
‘dirspell’
If set, Bash attempts spelling correction on directory names
during word completion if the directory name initially
supplied does not exist.
‘dotglob’
If set, Bash includes filenames beginning with a ‘.’ in the
results of filename expansion. The filenames ‘.’ and ‘..’
must always be matched explicitly, even if ‘dotglob’ is set.
‘execfail’
If this is set, a non-interactive shell will not exit if it
cannot execute the file specified as an argument to the ‘exec’
builtin. An interactive shell does not exit if ‘exec’ fails.
‘expand_aliases’
If set, aliases are expanded as described below under Aliases,
*note Aliases::. This option is enabled by default for
interactive shells.
‘extdebug’
If set at shell invocation, or in a shell startup file,
arrange to execute the debugger profile before the shell
starts, identical to the ‘--debugger’ option. If set after
invocation, behavior intended for use by debuggers is enabled:
1. The ‘-F’ option to the ‘declare’ builtin (*note Bash
Builtins::) displays the source file name and line number
corresponding to each function name supplied as an
argument.
2. If the command run by the ‘DEBUG’ trap returns a non-zero
value, the next command is skipped and not executed.
3. If the command run by the ‘DEBUG’ trap returns a value of
2, and the shell is executing in a subroutine (a shell
function or a shell script executed by the ‘.’ or
‘source’ builtins), the shell simulates a call to
‘return’.
4. ‘BASH_ARGC’ and ‘BASH_ARGV’ are updated as described in
their descriptions (*note Bash Variables::).
5. Function tracing is enabled: command substitution, shell
functions, and subshells invoked with ‘( COMMAND )’
inherit the ‘DEBUG’ and ‘RETURN’ traps.
6. Error tracing is enabled: command substitution, shell
functions, and subshells invoked with ‘( COMMAND )’
inherit the ‘ERR’ trap.
‘extglob’
If set, enable the extended pattern matching features
described above (*note Pattern Matching::).
‘extquote’
If set, ‘$'STRING'’ and ‘$"STRING"’ quoting is performed
within ‘${PARAMETER}’ expansions enclosed in double quotes.
This option is enabled by default.
‘failglob’
If set, patterns which fail to match filenames during filename
expansion result in an expansion error.
‘force_fignore’
If set, the suffixes specified by the ‘FIGNORE’ shell variable
cause words to be ignored when performing word completion even
if the ignored words are the only possible completions. *Note
Bash Variables::, for a description of ‘FIGNORE’. This option
is enabled by default.
‘globasciiranges’
If set, range expressions used in pattern matching bracket
expressions (*note Pattern Matching::) behave as if in the
traditional C locale when performing comparisons. That is,
pattern matching does not take the current locale's collating
sequence into account, so ‘b’ will not collate between ‘A’ and
‘B’, and upper-case and lower-case ASCII characters will
collate together.
‘globskipdots’
If set, filename expansion will never match the filenames ‘.’
and ‘..’, even if the pattern begins with a ‘.’. This option
is enabled by default.
‘globstar’
If set, the pattern ‘**’ used in a filename expansion context
will match all files and zero or more directories and
subdirectories. If the pattern is followed by a ‘/’, only
directories and subdirectories match.
‘gnu_errfmt’
If set, shell error messages are written in the standard GNU
error message format.
‘histappend’
If set, the history list is appended to the file named by the
value of the ‘HISTFILE’ variable when the shell exits, rather
than overwriting the file.
‘histreedit’
If set, and Readline is being used, the user is given the
opportunity to re-edit a failed history substitution.
‘histverify’
If set, and Readline is being used, the results of history
substitution are not immediately passed to the shell parser.
Instead, the resulting line is loaded into the Readline
editing buffer, allowing further modification.
‘hostcomplete’
If set, and Readline is being used, Bash will attempt to
perform hostname completion when a word containing a ‘@’ is
being completed (*note Commands For Completion::). This
option is enabled by default.
‘huponexit’
If set, Bash will send ‘SIGHUP’ to all jobs when an
interactive login shell exits (*note Signals::).
‘inherit_errexit’
If set, command substitution inherits the value of the
‘errexit’ option, instead of unsetting it in the subshell
environment. This option is enabled when POSIX mode is
enabled.
‘interactive_comments’
In an interactive shell, a word beginning with ‘#’ causes that
word and all remaining characters on that line to be ignored,
as in a non-interactive shell. This option is enabled by
default.
‘lastpipe’
If set, and job control is not active, the shell runs the last
command of a pipeline not executed in the background in the
current shell environment.
‘lithist’
If enabled, and the ‘cmdhist’ option is enabled, multi-line
commands are saved to the history with embedded newlines
rather than using semicolon separators where possible.
‘localvar_inherit’
If set, local variables inherit the value and attributes of a
variable of the same name that exists at a previous scope
before any new value is assigned. The ‘nameref’ attribute is
not inherited.
‘localvar_unset’
If set, calling ‘unset’ on local variables in previous
function scopes marks them so subsequent lookups find them
unset until that function returns. This is identical to the
behavior of unsetting local variables at the current function
scope.
‘login_shell’
The shell sets this option if it is started as a login shell
(*note Invoking Bash::). The value may not be changed.
‘mailwarn’
If set, and a file that Bash is checking for mail has been
accessed since the last time it was checked, Bash displays the
message ‘"The mail in MAILFILE has been read"’.
‘no_empty_cmd_completion’
If set, and Readline is being used, Bash does not search the
‘PATH’ for possible completions when completion is attempted
on an empty line.
‘nocaseglob’
If set, Bash matches filenames in a case-insensitive fashion
when performing filename expansion.
‘nocasematch’
If set, Bash matches patterns in a case-insensitive fashion
when performing matching while executing ‘case’ or ‘[[’
conditional commands (*note Conditional Constructs::, when
performing pattern substitution word expansions, or when
filtering possible completions as part of programmable
completion.
‘noexpand_translation’
If set, Bash encloses the translated results of $"..." quoting
in single quotes instead of double quotes. If the string is
not translated, this has no effect.
‘nullglob’
If set, filename expansion patterns which match no files
(*note Filename Expansion::) expand to nothing and are
removed, rather than expanding to themselves.
‘patsub_replacement’
If set, Bash expands occurrences of ‘&’ in the replacement
string of pattern substitution to the text matched by the
pattern, as described above (*note Shell Parameter
Expansion::). This option is enabled by default.
‘progcomp’
If set, enable the programmable completion facilities (*note
Programmable Completion::). This option is enabled by
default.
‘progcomp_alias’
If set, and programmable completion is enabled, Bash treats a
command name that doesn't have any completions as a possible
alias and attempts alias expansion. If it has an alias, Bash
attempts programmable completion using the command word
resulting from the expanded alias.
‘promptvars’
If set, prompt strings undergo parameter expansion, command
substitution, arithmetic expansion, and quote removal after
being expanded as described below (*note Controlling the
Prompt::). This option is enabled by default.
‘restricted_shell’
The shell sets this option if it is started in restricted mode
(*note The Restricted Shell::). The value may not be changed.
This is not reset when the startup files are executed,
allowing the startup files to discover whether or not a shell
is restricted.
‘shift_verbose’
If this is set, the ‘shift’ builtin prints an error message
when the shift count exceeds the number of positional
parameters.
‘sourcepath’
If set, the ‘.’ (‘source’) builtin uses the value of ‘PATH’ to
find the directory containing the file supplied as an argument
when the ‘-p’ option is not supplied. This option is enabled
by default.
‘varredir_close’
If set, the shell automatically closes file descriptors
assigned using the ‘{varname}’ redirection syntax (*note
Redirections::) instead of leaving them open when the command
completes.
‘xpg_echo’
If set, the ‘echo’ builtin expands backslash-escape sequences
by default. If the ‘posix’ shell option (*note The Set
Builtin::) is also enabled, ‘echo’ does not interpret any
options.
File: bash.info, Node: Special Builtins, Prev: Modifying Shell Behavior, Up: Shell Builtin Commands
4.4 Special Builtins
====================
For historical reasons, the POSIX standard has classified several
builtin commands as _special_. When Bash is executing in POSIX mode,
the special builtins differ from other builtin commands in three
respects:
1. Special builtins are found before shell functions during command
lookup.
2. If a special builtin returns an error status, a non-interactive
shell exits.
3. Assignment statements preceding the command stay in effect in the
shell environment after the command completes.
When Bash is not executing in POSIX mode, these builtins behave no
differently than the rest of the Bash builtin commands. The Bash POSIX
mode is described in *note Bash POSIX Mode::.
These are the POSIX special builtins:
break : . source continue eval exec exit export readonly return set
shift times trap unset
File: bash.info, Node: Shell Variables, Next: Bash Features, Prev: Shell Builtin Commands, Up: Top
5 Shell Variables
*****************
* Menu:
* Bourne Shell Variables:: Variables which Bash uses in the same way
as the Bourne Shell.
* Bash Variables:: List of variables that exist in Bash.
This chapter describes the shell variables that Bash uses. Bash
automatically assigns default values to a number of variables.
File: bash.info, Node: Bourne Shell Variables, Next: Bash Variables, Up: Shell Variables
5.1 Bourne Shell Variables
==========================
Bash uses certain shell variables in the same way as the Bourne shell.
In some cases, Bash assigns a default value to the variable.
‘CDPATH’
A colon-separated list of directories used as a search path for the
‘cd’ builtin command.
‘HOME’
The current user's home directory; the default for the ‘cd’ builtin
command. The value of this variable is also used by tilde
expansion (*note Tilde Expansion::).
‘IFS’
A list of characters that separate fields; used when the shell
splits words as part of expansion and by the ‘read’ builtin to
split lines into words. *Note Word Splitting::, for a description
of word splitting.
‘MAIL’
If the value is set to a filename or directory name and the
‘MAILPATH’ variable is not set, Bash informs the user of the
arrival of mail in the specified file or Maildir-format directory.
‘MAILPATH’
A colon-separated list of filenames which the shell periodically
checks for new mail. Each list entry can specify the message that
is printed when new mail arrives in the mail file by separating the
filename from the message with a ‘?’. When used in the text of the
message, ‘$_’ expands to the name of the current mail file.
‘OPTARG’
The value of the last option argument processed by the ‘getopts’
builtin.
‘OPTIND’
The index of the next argument to be processed by the ‘getopts’
builtin.
‘PATH’
A colon-separated list of directories in which the shell looks for
commands. A zero-length (null) directory name in the value of
‘PATH’ indicates the current directory. A null directory name may
appear as two adjacent colons, or as an initial or trailing colon.
The default path is system-dependent, and is set by the
administrator who installs ‘bash’. A common value is
"/usr/local/bin:/usr/local/sbin:/usr/bin:/usr/sbin:/bin:/sbin".
‘PS1’
The primary prompt string. The default value is ‘\s-\v\$ ’. *Note
Controlling the Prompt::, for the complete list of escape sequences
that are expanded before ‘PS1’ is displayed.
‘PS2’
The secondary prompt string. The default value is ‘> ’. ‘PS2’ is
expanded in the same way as ‘PS1’ before being displayed.
File: bash.info, Node: Bash Variables, Prev: Bourne Shell Variables, Up: Shell Variables
5.2 Bash Variables
==================
These variables are set or used by Bash, but other shells do not
normally treat them specially.
A few variables used by Bash are described in different chapters:
variables for controlling the job control facilities (*note Job Control
Variables::).
‘_’
($_, an underscore.) This has a number of meanings depending on
context. At shell startup, $_ set to the pathname used to invoke
the shell or shell script being executed as passed in the
environment or argument list. Subsequently, it expands to the last
argument to the previous simple command executed in the foreground,
after expansion. It is also set to the full pathname used to
invoke each command executed and placed in the environment exported
to that command. When checking mail, $_ expands to the name of the
mail file.
‘BASH’
The full pathname used to execute the current instance of Bash.
‘BASHOPTS’
A colon-separated list of enabled shell options. Each word in the
list is a valid argument for the ‘-s’ option to the ‘shopt’ builtin
command (*note The Shopt Builtin::). The options appearing in
‘BASHOPTS’ are those reported as ‘on’ by ‘shopt’. If this variable
is in the environment when Bash starts up, the shell enables each
option in the list before reading any startup files. If this
variable is exported, child shells will enable each option in the
list. This variable is readonly.
‘BASHPID’
Expands to the process ID of the current Bash process. This
differs from ‘$$’ under certain circumstances, such as subshells
that do not require Bash to be re-initialized. Assignments to
‘BASHPID’ have no effect. If ‘BASHPID’ is unset, it loses its
special properties, even if it is subsequently reset.
‘BASH_ALIASES’
An associative array variable whose members correspond to the
internal list of aliases as maintained by the ‘alias’ builtin.
(*note Bourne Shell Builtins::). Elements added to this array
appear in the alias list; however, unsetting array elements
currently does not cause aliases to be removed from the alias list.
If ‘BASH_ALIASES’ is unset, it loses its special properties, even
if it is subsequently reset.
‘BASH_ARGC’
An array variable whose values are the number of parameters in each
frame of the current Bash execution call stack. The number of
parameters to the current subroutine (shell function or script
executed with ‘.’ or ‘source’) is at the top of the stack. When a
subroutine is executed, the number of parameters passed is pushed
onto ‘BASH_ARGC’. The shell sets ‘BASH_ARGC’ only when in extended
debugging mode (see *note The Shopt Builtin:: for a description of
the ‘extdebug’ option to the ‘shopt’ builtin). Setting ‘extdebug’
after the shell has started to execute a subroutine, or referencing
this variable when ‘extdebug’ is not set, may result in
inconsistent values. Assignments to ‘BASH_ARGC’ have no effect,
and it may not be unset.
‘BASH_ARGV’
An array variable containing all of the parameters in the current
Bash execution call stack. The final parameter of the last
subroutine call is at the top of the stack; the first parameter of
the initial call is at the bottom. When a subroutine is executed,
the shell pushes the supplied parameters onto ‘BASH_ARGV’. The
shell sets ‘BASH_ARGV’ only when in extended debugging mode (see
*note The Shopt Builtin:: for a description of the ‘extdebug’
option to the ‘shopt’ builtin). Setting ‘extdebug’ after the shell
has started to execute a script, or referencing this variable when
‘extdebug’ is not set, may result in inconsistent values.
Assignments to ‘BASH_ARGV’ have no effect, and it may not be unset.
‘BASH_ARGV0’
When referenced, this variable expands to the name of the shell or
shell script (identical to ‘$0’; *Note Special Parameters::, for
the description of special parameter 0). Assigning a value to
‘BASH_ARGV0’ sets ‘$0’ to the same value. If ‘BASH_ARGV0’ is
unset, it loses its special properties, even if it is subsequently
reset.
‘BASH_CMDS’
An associative array variable whose members correspond to the
internal hash table of commands as maintained by the ‘hash’ builtin
(*note Bourne Shell Builtins::). Adding elements to this array
makes them appear in the hash table; however, unsetting array
elements currently does not remove command names from the hash
table. If ‘BASH_CMDS’ is unset, it loses its special properties,
even if it is subsequently reset.
‘BASH_COMMAND’
Expands to the command currently being executed or about to be
executed, unless the shell is executing a command as the result of
a trap, in which case it is the command executing at the time of
the trap. If ‘BASH_COMMAND’ is unset, it loses its special
properties, even if it is subsequently reset.
‘BASH_COMPAT’
The value is used to set the shell's compatibility level. *Note
Shell Compatibility Mode::, for a description of the various
compatibility levels and their effects. The value may be a decimal
number (e.g., 4.2) or an integer (e.g., 42) corresponding to the
desired compatibility level. If ‘BASH_COMPAT’ is unset or set to
the empty string, the compatibility level is set to the default for
the current version. If ‘BASH_COMPAT’ is set to a value that is
not one of the valid compatibility levels, the shell prints an
error message and sets the compatibility level to the default for
the current version. A subset of the valid values correspond to
the compatibility levels described below (*note Shell Compatibility
Mode::). For example, 4.2 and 42 are valid values that correspond
to the ‘compat42’ ‘shopt’ option and set the compatibility level to
42. The current version is also a valid value.
‘BASH_ENV’
If this variable is set when Bash is invoked to execute a shell
script, its value is expanded and used as the name of a startup
file to read before executing the script. Bash does not use ‘PATH’
to search for the resultant filename. *Note Bash Startup Files::.
‘BASH_EXECUTION_STRING’
The command argument to the ‘-c’ invocation option.
‘BASH_LINENO’
An array variable whose members are the line numbers in source
files where each corresponding member of ‘FUNCNAME’ was invoked.
‘${BASH_LINENO[$i]}’ is the line number in the source file
(‘${BASH_SOURCE[$i+1]}’) where ‘${FUNCNAME[$i]}’ was called (or
‘${BASH_LINENO[$i-1]}’ if referenced within another shell
function). Use ‘LINENO’ to obtain the current line number.
Assignments to ‘BASH_LINENO’ have no effect, and it may not be
unset.
‘BASH_LOADABLES_PATH’
A colon-separated list of directories in which the ‘enable’ command
looks for dynamically loadable builtins.
‘BASH_MONOSECONDS’
Each time this variable is referenced, it expands to the value
returned by the system's monotonic clock, if one is available. If
there is no monotonic clock, this is equivalent to ‘EPOCHSECONDS’.
If ‘BASH_MONOSECONDS’ is unset, it loses its special properties,
even if it is subsequently reset.
‘BASH_REMATCH’
An array variable whose members are assigned by the ‘=~’ binary
operator to the ‘[[’ conditional command (*note Conditional
Constructs::). The element with index 0 is the portion of the
string matching the entire regular expression. The element with
index N is the portion of the string matching the Nth parenthesized
subexpression.
‘BASH_SOURCE’
An array variable whose members are the source filenames where the
corresponding shell function names in the ‘FUNCNAME’ array variable
are defined. The shell function ‘${FUNCNAME[$i]}’ is defined in
the file ‘${BASH_SOURCE[$i]}’ and called from
‘${BASH_SOURCE[$i+1]}’ Assignments to ‘BASH_SOURCE’ have no effect,
and it may not be unset.
‘BASH_SUBSHELL’
Incremented by one within each subshell or subshell environment
when the shell begins executing in that environment. The initial
value is 0. If ‘BASH_SUBSHELL’ is unset, it loses its special
properties, even if it is subsequently reset.
‘BASH_TRAPSIG’
Set to the signal number corresponding to the trap action being
executed during its execution. See the description of ‘trap’
(*note Bourne Shell Builtins::) for information about signal
numbers and trap execution.
‘BASH_VERSINFO’
A readonly array variable (*note Arrays::) whose members hold
version information for this instance of Bash. The values assigned
to the array members are as follows:
‘BASH_VERSINFO[0]’
The major version number (the “release”).
‘BASH_VERSINFO[1]’
The minor version number (the “version”).
‘BASH_VERSINFO[2]’
The patch level.
‘BASH_VERSINFO[3]’
The build version.
‘BASH_VERSINFO[4]’
The release status (e.g., ‘beta’).
‘BASH_VERSINFO[5]’
The value of ‘MACHTYPE’.
‘BASH_VERSION’
Expands to a string describing the version of this instance of Bash
(e.g., 5.2.37(3)-release).
‘BASH_XTRACEFD’
If set to an integer corresponding to a valid file descriptor, Bash
writes the trace output generated when ‘set -x’ is enabled to that
file descriptor, instead of the standard error. This allows
tracing output to be separated from diagnostic and error messages.
The file descriptor is closed when ‘BASH_XTRACEFD’ is unset or
assigned a new value. Unsetting ‘BASH_XTRACEFD’ or assigning it
the empty string causes the trace output to be sent to the standard
error. Note that setting ‘BASH_XTRACEFD’ to 2 (the standard error
file descriptor) and then unsetting it will result in the standard
error being closed.
‘CHILD_MAX’
Set the number of exited child status values for the shell to
remember. Bash will not allow this value to be decreased below a
POSIX-mandated minimum, and there is a maximum value (currently
8192) that this may not exceed. The minimum value is
system-dependent.
‘COLUMNS’
Used by the ‘select’ command to determine the terminal width when
printing selection lists. Automatically set if the ‘checkwinsize’
option is enabled (*note The Shopt Builtin::), or in an interactive
shell upon receipt of a ‘SIGWINCH’.
‘COMP_CWORD’
An index into ‘${COMP_WORDS}’ of the word containing the current
cursor position. This variable is available only in shell
functions invoked by the programmable completion facilities (*note
Programmable Completion::).
‘COMP_KEY’
The key (or final key of a key sequence) used to invoke the current
completion function. This variable is available only in shell
functions and external commands invoked by the programmable
completion facilities (*note Programmable Completion::).
‘COMP_LINE’
The current command line. This variable is available only in shell
functions and external commands invoked by the programmable
completion facilities (*note Programmable Completion::).
‘COMP_POINT’
The index of the current cursor position relative to the beginning
of the current command. If the current cursor position is at the
end of the current command, the value of this variable is equal to
‘${#COMP_LINE}’. This variable is available only in shell
functions and external commands invoked by the programmable
completion facilities (*note Programmable Completion::).
‘COMP_TYPE’
Set to an integer value corresponding to the type of attempted
completion that caused a completion function to be called: <TAB>,
for normal completion, ‘?’, for listing completions after
successive tabs, ‘!’, for listing alternatives on partial word
completion, ‘@’, to list completions if the word is not unmodified,
or ‘%’, for menu completion. This variable is available only in
shell functions and external commands invoked by the programmable
completion facilities (*note Programmable Completion::).
‘COMP_WORDBREAKS’
The set of characters that the Readline library treats as word
separators when performing word completion. If ‘COMP_WORDBREAKS’
is unset, it loses its special properties, even if it is
subsequently reset.
‘COMP_WORDS’
An array variable consisting of the individual words in the current
command line. The line is split into words as Readline would split
it, using ‘COMP_WORDBREAKS’ as described above. This variable is
available only in shell functions invoked by the programmable
completion facilities (*note Programmable Completion::).
‘COMPREPLY’
An array variable from which Bash reads the possible completions
generated by a shell function invoked by the programmable
completion facility (*note Programmable Completion::). Each array
element contains one possible completion.
‘COPROC’
An array variable created to hold the file descriptors for output
from and input to an unnamed coprocess (*note Coprocesses::).
‘DIRSTACK’
An array variable containing the current contents of the directory
stack. Directories appear in the stack in the order they are
displayed by the ‘dirs’ builtin. Assigning to members of this
array variable may be used to modify directories already in the
stack, but the ‘pushd’ and ‘popd’ builtins must be used to add and
remove directories. Assigning to this variable does not change the
current directory. If ‘DIRSTACK’ is unset, it loses its special
properties, even if it is subsequently reset.
‘EMACS’
If Bash finds this variable in the environment when the shell
starts, and its value is ‘t’, Bash assumes that the shell is
running in an Emacs shell buffer and disables line editing.
‘ENV’
Expanded and executed similarly to ‘BASH_ENV’ (*note Bash Startup
Files::) when an interactive shell is invoked in POSIX mode (*note
Bash POSIX Mode::).
‘EPOCHREALTIME’
Each time this parameter is referenced, it expands to the number of
seconds since the Unix Epoch as a floating-point value with
micro-second granularity (see the documentation for the C library
function ‘time’ for the definition of Epoch). Assignments to
‘EPOCHREALTIME’ are ignored. If ‘EPOCHREALTIME’ is unset, it loses
its special properties, even if it is subsequently reset.
‘EPOCHSECONDS’
Each time this parameter is referenced, it expands to the number of
seconds since the Unix Epoch (see the documentation for the C
library function ‘time’ for the definition of Epoch). Assignments
to ‘EPOCHSECONDS’ are ignored. If ‘EPOCHSECONDS’ is unset, it
loses its special properties, even if it is subsequently reset.
‘EUID’
The numeric effective user id of the current user. This variable
is readonly.
‘EXECIGNORE’
A colon-separated list of shell patterns (*note Pattern Matching::)
defining the set of filenames to be ignored by command search using
‘PATH’. Files whose full pathnames match one of these patterns are
not considered executable files for the purposes of completion and
command execution via ‘PATH’ lookup. This does not affect the
behavior of the ‘[’, ‘test’, and ‘[[’ commands. Full pathnames in
the command hash table are not subject to ‘EXECIGNORE’. Use this
variable to ignore shared library files that have the executable
bit set, but are not executable files. The pattern matching honors
the setting of the ‘extglob’ shell option.
‘FCEDIT’
The editor used as a default by the ‘fc’ builtin command.
‘FIGNORE’
A colon-separated list of suffixes to ignore when performing
filename completion. A filename whose suffix matches one of the
entries in ‘FIGNORE’ is excluded from the list of matched
filenames. A sample value is ‘.o:~’
‘FUNCNAME’
An array variable containing the names of all shell functions
currently in the execution call stack. The element with index 0 is
the name of any currently-executing shell function. The
bottom-most element (the one with the highest index) is ‘"main"’.
This variable exists only when a shell function is executing.
Assignments to ‘FUNCNAME’ have no effect. If ‘FUNCNAME’ is unset,
it loses its special properties, even if it is subsequently reset.
This variable can be used with ‘BASH_LINENO’ and ‘BASH_SOURCE’.
Each element of ‘FUNCNAME’ has corresponding elements in
‘BASH_LINENO’ and ‘BASH_SOURCE’ to describe the call stack. For
instance, ‘${FUNCNAME[$i]}’ was called from the file
‘${BASH_SOURCE[$i+1]}’ at line number ‘${BASH_LINENO[$i]}’. The
‘caller’ builtin displays the current call stack using this
information.
‘FUNCNEST’
A numeric value greater than 0 defines a maximum function nesting
level. Function invocations that exceed this nesting level cause
the current command to abort.
‘GLOBIGNORE’
A colon-separated list of patterns defining the set of file names
to be ignored by filename expansion. If a file name matched by a
filename expansion pattern also matches one of the patterns in
‘GLOBIGNORE’, it is removed from the list of matches. The pattern
matching honors the setting of the ‘extglob’ shell option.
‘GLOBSORT’
Controls how the results of filename expansion are sorted. The
value of this variable specifies the sort criteria and sort order
for the results of filename expansion. If this variable is unset
or set to the null string, filename expansion uses the historical
behavior of sorting by name, in ascending lexicographic order as
determined by the ‘LC_COLLATE’ shell variable.
If set, a valid value begins with an optional ‘+’, which is
ignored, or ‘-’, which reverses the sort order from ascending to
descending, followed by a sort specifier. The valid sort
specifiers are ‘name’, ‘numeric’, ‘size’, ‘mtime’, ‘atime’,
‘ctime’, and ‘blocks’, which sort the files on name, names in
numeric rather than lexicographic order, file size, modification
time, access time, inode change time, and number of blocks,
respectively. If any of the non-name keys compare as equal (e.g.,
if two files are the same size), sorting uses the name as a
secondary sort key.
For example, a value of ‘-mtime’ sorts the results in descending
order by modification time (newest first).
The ‘numeric’ specifier treats names consisting solely of digits as
numbers and sorts them using their numeric value (so "2" sorts
before "10", for example). When using ‘numeric’, names containing
non-digits sort after all the all-digit names and are sorted by
name using the traditional behavior.
A sort specifier of ‘nosort’ disables sorting completely; Bash
returns the results in the order they are read from the file
system, ignoring any leading ‘-’.
If the sort specifier is missing, it defaults to NAME, so a value
of ‘+’ is equivalent to the null string, and a value of ‘-’ sorts
by name in descending order.
Any invalid value restores the historical sorting behavior.
‘GROUPS’
An array variable containing the list of groups of which the
current user is a member. Assignments to ‘GROUPS’ have no effect.
If ‘GROUPS’ is unset, it loses its special properties, even if it
is subsequently reset.
‘histchars’
The two or three characters which control history expansion, quick
substitution, and tokenization (*note History Interaction::). The
first character is the “history expansion” character, the character
which begins a history expansion, normally ‘!’. The second
character is the “quick substitution” character, normally ‘^’.
When it appears as the first character on the line, history
substitution repeats the previous command, replacing one string
with another. The optional third character is the “history
comment” character, normally ‘#’, which indicates that the
remainder of the line is a comment when it appears as the first
character of a word. The history comment character disables
history substitution for the remaining words on the line. It does
not necessarily cause the shell parser to treat the rest of the
line as a comment.
‘HISTCMD’
The history number, or index in the history list, of the current
command. Assignments to ‘HISTCMD’ have no effect. If ‘HISTCMD’ is
unset, it loses its special properties, even if it is subsequently
reset.
‘HISTCONTROL’
A colon-separated list of values controlling how commands are saved
on the history list. If the list of values includes ‘ignorespace’,
lines which begin with a space character are not saved in the
history list. A value of ‘ignoredups’ causes lines which match the
previous history entry not to be saved. A value of ‘ignoreboth’ is
shorthand for ‘ignorespace’ and ‘ignoredups’. A value of
‘erasedups’ causes all previous lines matching the current line to
be removed from the history list before that line is saved. Any
value not in the above list is ignored. If ‘HISTCONTROL’ is unset,
or does not include a valid value, Bash saves all lines read by the
shell parser on the history list, subject to the value of
‘HISTIGNORE’. If the first line of a multi-line compound command
was saved, the second and subsequent lines are not tested, and are
added to the history regardless of the value of ‘HISTCONTROL’. If
the first line was not saved, the second and subsequent lines of
the command are not saved either.
‘HISTFILE’
The name of the file to which the command history is saved. Bash
assigns a default value of ‘~/.bash_history’. If ‘HISTFILE’ is
unset or null, the shell does not save the command history when it
exits.
‘HISTFILESIZE’
The maximum number of lines contained in the history file. When
this variable is assigned a value, the history file is truncated,
if necessary, to contain no more than the number of history entries
that total no more than that number of lines by removing the oldest
entries. If the history list contains multi-line entries, the
history file may contain more lines than this maximum to avoid
leaving partial history entries. The history file is also
truncated to this size after writing it when a shell exits or by
the ‘history’ builtin. If the value is 0, the history file is
truncated to zero size. Non-numeric values and numeric values less
than zero inhibit truncation. The shell sets the default value to
the value of ‘HISTSIZE’ after reading any startup files.
‘HISTIGNORE’
A colon-separated list of patterns used to decide which command
lines should be saved on the history list. If a command line
matches one of the patterns in the value of ‘HISTIGNORE’, it is not
saved on the history list. Each pattern is anchored at the
beginning of the line and must match the complete line (Bash does
not implicitly append a ‘*’). Each pattern is tested against the
line after the checks specified by ‘HISTCONTROL’ are applied. In
addition to the normal shell pattern matching characters, ‘&’
matches the previous history line. A backslash escapes the ‘&’;
the backslash is removed before attempting a match. If the first
line of a multi-line compound command was saved, the second and
subsequent lines are not tested, and are added to the history
regardless of the value of ‘HISTIGNORE’. If the first line was not
saved, the second and subsequent lines of the command are not saved
either. The pattern matching honors the setting of the ‘extglob’
shell option.
‘HISTIGNORE’ subsumes some of the function of ‘HISTCONTROL’. A
pattern of ‘&’ is identical to ‘ignoredups’, and a pattern of ‘[
]*’ is identical to ‘ignorespace’. Combining these two patterns,
separating them with a colon, provides the functionality of
‘ignoreboth’.
‘HISTSIZE’
The maximum number of commands to remember on the history list. If
the value is 0, commands are not saved in the history list.
Numeric values less than zero result in every command being saved
on the history list (there is no limit). The shell sets the
default value to 500 after reading any startup files.
‘HISTTIMEFORMAT’
If this variable is set and not null, its value is used as a format
string for ‘strftime’(3) to print the time stamp associated with
each history entry displayed by the ‘history’ builtin. If this
variable is set, the shell writes time stamps to the history file
so they may be preserved across shell sessions. This uses the
history comment character to distinguish timestamps from other
history lines.
‘HOSTFILE’
Contains the name of a file in the same format as ‘/etc/hosts’ that
should be read when the shell needs to complete a hostname. The
list of possible hostname completions may be changed while the
shell is running; the next time hostname completion is attempted
after the value is changed, Bash adds the contents of the new file
to the existing list. If ‘HOSTFILE’ is set, but has no value, or
does not name a readable file, Bash attempts to read ‘/etc/hosts’
to obtain the list of possible hostname completions. When
‘HOSTFILE’ is unset, Bash clears the hostname list.
‘HOSTNAME’
The name of the current host.
‘HOSTTYPE’
A string describing the machine Bash is running on.
‘IGNOREEOF’
Controls the action of the shell on receipt of an ‘EOF’ character
as the sole input. If set, the value is the number of consecutive
‘EOF’ characters that can be read as the first character on an
input line before Bash exits. If the variable is set but does not
have a numeric value, or the value is null, then the default is 10.
If the variable is unset, then ‘EOF’ signifies the end of input to
the shell. This is only in effect for interactive shells.
‘INPUTRC’
The name of the Readline initialization file, overriding the
default of ‘~/.inputrc’.
‘INSIDE_EMACS’
If Bash finds this variable in the environment when the shell
starts, it assumes that the shell is running in an Emacs shell
buffer and may disable line editing depending on the value of
‘TERM’.
‘LANG’
Used to determine the locale category for any category not
specifically selected with a variable starting with ‘LC_’.
‘LC_ALL’
This variable overrides the value of ‘LANG’ and any other ‘LC_’
variable specifying a locale category.
‘LC_COLLATE’
This variable determines the collation order used when sorting the
results of filename expansion, and determines the behavior of range
expressions, equivalence classes, and collating sequences within
filename expansion and pattern matching (*note Filename
Expansion::).
‘LC_CTYPE’
This variable determines the interpretation of characters and the
behavior of character classes within filename expansion and pattern
matching (*note Filename Expansion::).
‘LC_MESSAGES’
This variable determines the locale used to translate double-quoted
strings preceded by a ‘$’ (*note Locale Translation::).
‘LC_NUMERIC’
This variable determines the locale category used for number
formatting.
‘LC_TIME’
This variable determines the locale category used for data and time
formatting.
‘LINENO’
The line number in the script or shell function currently
executing. Line numbers start with 1. When not in a script or
function, the value is not guaranteed to be meaningful. If
‘LINENO’ is unset, it loses its special properties, even if it is
subsequently reset.
‘LINES’
Used by the ‘select’ command to determine the column length for
printing selection lists. Automatically set if the ‘checkwinsize’
option is enabled (*note The Shopt Builtin::), or in an interactive
shell upon receipt of a ‘SIGWINCH’.
‘MACHTYPE’
A string that fully describes the system type on which Bash is
executing, in the standard GNU CPU-COMPANY-SYSTEM format.
‘MAILCHECK’
How often (in seconds) that the shell should check for mail in the
files specified in the ‘MAILPATH’ or ‘MAIL’ variables. The default
is 60 seconds. When it is time to check for mail, the shell does
so before displaying the primary prompt. If this variable is
unset, or set to a value that is not a number greater than or equal
to zero, the shell disables mail checking.
‘MAPFILE’
An array variable created to hold the text read by the ‘mapfile’
builtin when no variable name is supplied.
‘OLDPWD’
The previous working directory as set by the ‘cd’ builtin.
‘OPTERR’
If set to the value 1, Bash displays error messages generated by
the ‘getopts’ builtin command. ‘OPTERR’ is initialized to 1 each
time the shell is invoked.
‘OSTYPE’
A string describing the operating system Bash is running on.
‘PIPESTATUS’
An array variable (*note Arrays::) containing a list of exit status
values from the commands in the most-recently-executed foreground
pipeline, which may consist of only a simple command (*note Shell
Commands::). Bash sets ‘PIPESTATUS’ after executing multi-element
pipelines, timed and negated pipelines, simple commands, subshells
created with the ‘(’ operator, the ‘[[’ and ‘((’ compound commands,
and after error conditions that result in the shell aborting
command execution.
‘POSIXLY_CORRECT’
If this variable is in the environment when Bash starts, the shell
enters POSIX mode (*note Bash POSIX Mode::) before reading the
startup files, as if the ‘--posix’ invocation option had been
supplied. If it is set while the shell is running, Bash enables
POSIX mode, as if the command
set -o posix
had been executed. When the shell enters POSIX mode, it sets this
variable if it was not already set.
‘PPID’
The process ID of the shell's parent process. This variable is
readonly.
‘PROMPT_COMMAND’
If this variable is set, and is an array, the value of each set
element is interpreted as a command to execute before printing the
primary prompt (‘$PS1’). If this is set but not an array variable,
its value is used as a command to execute instead.
‘PROMPT_DIRTRIM’
If set to a number greater than zero, the value is used as the
number of trailing directory components to retain when expanding
the ‘\w’ and ‘\W’ prompt string escapes (*note Controlling the
Prompt::). Characters removed are replaced with an ellipsis.
‘PS0’
The value of this parameter is expanded like ‘PS1’ and displayed by
interactive shells after reading a command and before the command
is executed.
‘PS3’
The value of this variable is used as the prompt for the ‘select’
command. If this variable is not set, the ‘select’ command prompts
with ‘#? ’
‘PS4’
The value of this parameter is expanded like ‘PS1’ and the expanded
value is the prompt printed before the command line is echoed when
the ‘-x’ option is set (*note The Set Builtin::). The first
character of the expanded value is replicated multiple times, as
necessary, to indicate multiple levels of indirection. The default
is ‘+ ’.
‘PWD’
The current working directory as set by the ‘cd’ builtin.
‘RANDOM’
Each time this parameter is referenced, it expands to a random
integer between 0 and 32767. Assigning a value to ‘RANDOM’
initializes (seeds) the sequence of random numbers. Seeding the
random number generator with the same constant value produces the
same sequence of values. If ‘RANDOM’ is unset, it loses its
special properties, even if it is subsequently reset.
‘READLINE_ARGUMENT’
Any numeric argument given to a Readline command that was defined
using ‘bind -x’ (*note Bash Builtins:: when it was invoked.
‘READLINE_LINE’
The contents of the Readline line buffer, for use with ‘bind -x’
(*note Bash Builtins::).
‘READLINE_MARK’
The position of the “mark” (saved insertion point) in the Readline
line buffer, for use with ‘bind -x’ (*note Bash Builtins::). The
characters between the insertion point and the mark are often
called the “region”.
‘READLINE_POINT’
The position of the insertion point in the Readline line buffer,
for use with ‘bind -x’ (*note Bash Builtins::).
‘REPLY’
The default variable for the ‘read’ builtin; set to the line read
when ‘read’ is not supplied a variable name argument.
‘SECONDS’
This variable expands to the number of seconds since the shell was
started. Assignment to this variable resets the count to the value
assigned, and the expanded value becomes the value assigned plus
the number of seconds since the assignment. The number of seconds
at shell invocation and the current time are always determined by
querying the system clock at one-second resolution. If ‘SECONDS’
is unset, it loses its special properties, even if it is
subsequently reset.
‘SHELL’
This environment variable expands to the full pathname to the
shell. If it is not set when the shell starts, Bash assigns to it
the full pathname of the current user's login shell.
‘SHELLOPTS’
A colon-separated list of enabled shell options. Each word in the
list is a valid argument for the ‘-o’ option to the ‘set’ builtin
command (*note The Set Builtin::). The options appearing in
‘SHELLOPTS’ are those reported as ‘on’ by ‘set -o’. If this
variable is in the environment when Bash starts up, the shell
enables each option in the list before reading any startup files.
If this variable is exported, child shells will enable each option
in the list. This variable is readonly.
‘SHLVL’
Incremented by one each time a new instance of Bash is started.
This is intended to be a count of how deeply your Bash shells are
nested.
‘SRANDOM’
This variable expands to a 32-bit pseudo-random number each time it
is referenced. The random number generator is not linear on
systems that support ‘/dev/urandom’ or ‘arc4random’, so each
returned number has no relationship to the numbers preceding it.
The random number generator cannot be seeded, so assignments to
this variable have no effect. If ‘SRANDOM’ is unset, it loses its
special properties, even if it is subsequently reset.
‘TIMEFORMAT’
The value of this parameter is used as a format string specifying
how the timing information for pipelines prefixed with the ‘time’
reserved word should be displayed. The ‘%’ character introduces an
escape sequence that is expanded to a time value or other
information. The escape sequences and their meanings are as
follows; the brackets denote optional portions.
‘%%’
A literal ‘%’.
‘%[P][l]R’
The elapsed time in seconds.
‘%[P][l]U’
The number of CPU seconds spent in user mode.
‘%[P][l]S’
The number of CPU seconds spent in system mode.
‘%P’
The CPU percentage, computed as (%U + %S) / %R.
The optional P is a digit specifying the precision, the number of
fractional digits after a decimal point. A value of 0 causes no
decimal point or fraction to be output. ‘time’ prints at most six
digits after the decimal point; values of P greater than 6 are
changed to 6. If P is not specified, ‘time’ prints three digits
after the decimal point.
The optional ‘l’ specifies a longer format, including minutes, of
the form MMmSS.FFs. The value of P determines whether or not the
fraction is included.
If this variable is not set, Bash acts as if it had the value
$'\nreal\t%3lR\nuser\t%3lU\nsys\t%3lS'
If the value is null, Bash does not display any timing information.
A trailing newline is added when the format string is displayed.
‘TMOUT’
If set to a value greater than zero, the ‘read’ builtin uses the
value as its default timeout (*note Bash Builtins::). The ‘select’
command (*note Conditional Constructs::) terminates if input does
not arrive after ‘TMOUT’ seconds when input is coming from a
terminal.
In an interactive shell, the value is interpreted as the number of
seconds to wait for a line of input after issuing the primary
prompt. Bash terminates after waiting for that number of seconds
if a complete line of input does not arrive.
‘TMPDIR’
If set, Bash uses its value as the name of a directory in which
Bash creates temporary files for the shell's use.
‘UID’
The numeric real user id of the current user. This variable is
readonly.
File: bash.info, Node: Bash Features, Next: Job Control, Prev: Shell Variables, Up: Top
6 Bash Features
***************
This chapter describes features unique to Bash.
* Menu:
* Invoking Bash:: Command line options that you can give
to Bash.
* Bash Startup Files:: When and how Bash executes scripts.
* Interactive Shells:: What an interactive shell is.
* Bash Conditional Expressions:: Primitives used in composing expressions for
the ‘test’ builtin.
* Shell Arithmetic:: Arithmetic on shell variables.
* Aliases:: Substituting one command for another.
* Arrays:: Array Variables.
* The Directory Stack:: History of visited directories.
* Controlling the Prompt:: Customizing the various prompt strings.
* The Restricted Shell:: A more controlled mode of shell execution.
* Bash POSIX Mode:: Making Bash behave more closely to what
the POSIX standard specifies.
* Shell Compatibility Mode:: How Bash supports behavior that was present
in earlier versions and has changed.
File: bash.info, Node: Invoking Bash, Next: Bash Startup Files, Up: Bash Features
6.1 Invoking Bash
=================
bash [long-opt] [-ir] [-abefhkmnptuvxdBCDHP] [-o OPTION]
[-O SHOPT_OPTION] [ARGUMENT ...]
bash [long-opt] [-abefhkmnptuvxdBCDHP] [-o OPTION]
[-O SHOPT_OPTION] -c STRING [ARGUMENT ...]
bash [long-opt] -s [-abefhkmnptuvxdBCDHP] [-o OPTION]
[-O SHOPT_OPTION] [ARGUMENT ...]
All of the single-character options used with the ‘set’ builtin
(*note The Set Builtin::) can be used as options when the shell is
invoked. In addition, there are several multi-character options that
you can use. These options must appear on the command line before the
single-character options to be recognized.
‘--debugger’
Arrange for the debugger profile to be executed before the shell
starts. Turns on extended debugging mode (see *note The Shopt
Builtin:: for a description of the ‘extdebug’ option to the ‘shopt’
builtin).
‘--dump-po-strings’
Print a list of all double-quoted strings preceded by ‘$’ on the
standard output in the GNU ‘gettext’ PO (portable object) file
format. Equivalent to ‘-D’ except for the output format.
‘--dump-strings’
Equivalent to ‘-D’.
‘--help’
Display a usage message on standard output and exit successfully.
‘--init-file FILENAME’
‘--rcfile FILENAME’
Execute commands from FILENAME (instead of ‘~/.bashrc’) in an
interactive shell.
‘--login’
Equivalent to ‘-l’.
‘--noediting’
Do not use the GNU Readline library (*note Command Line Editing::)
to read command lines when the shell is interactive.
‘--noprofile’
Don't load the system-wide startup file ‘/etc/profile’ or any of
the personal initialization files ‘~/.bash_profile’,
‘~/.bash_login’, or ‘~/.profile’ when Bash is invoked as a login
shell.
‘--norc’
Don't read the ‘~/.bashrc’ initialization file in an interactive
shell. This is on by default if the shell is invoked as ‘sh’.
‘--posix’
Enable POSIX mode; change the behavior of Bash where the default
operation differs from the POSIX standard to match the standard.
This is intended to make Bash behave as a strict superset of that
standard. *Note Bash POSIX Mode::, for a description of the Bash
POSIX mode.
‘--restricted’
Equivalent to ‘-r’. Make the shell a restricted shell (*note The
Restricted Shell::).
‘--verbose’
Equivalent to ‘-v’. Print shell input lines as they're read.
‘--version’
Show version information for this instance of Bash on the standard
output and exit successfully.
There are several single-character options that may be supplied at
invocation which are not available with the ‘set’ builtin.
‘-c’
Read and execute commands from the first non-option argument
COMMAND_STRING, then exit. If there are arguments after the
COMMAND_STRING, the first argument is assigned to ‘$0’ and any
remaining arguments are assigned to the positional parameters. The
assignment to ‘$0’ sets the name of the shell, which is used in
warning and error messages.
‘-i’
Force the shell to run interactively. Interactive shells are
described in *note Interactive Shells::.
‘-l’
Make this shell act as if it had been directly invoked by login.
When the shell is interactive, this is equivalent to starting a
login shell with ‘exec -l bash’. When the shell is not
interactive, it will read and execute the login shell startup
files. ‘exec bash -l’ or ‘exec bash --login’ will replace the
current shell with a Bash login shell. *Note Bash Startup Files::,
for a description of the special behavior of a login shell.
‘-r’
Make the shell a restricted shell (*note The Restricted Shell::).
‘-s’
If this option is present, or if no arguments remain after option
processing, then Bash reads commands from the standard input. This
option allows the positional parameters to be set when invoking an
interactive shell or when reading input through a pipe.
‘-D’
Print a list of all double-quoted strings preceded by ‘$’ on the
standard output. These are the strings that are subject to
language translation when the current locale is not ‘C’ or ‘POSIX’
(*note Locale Translation::). This implies the ‘-n’ option; no
commands will be executed.
‘[-+]O [SHOPT_OPTION]’
SHOPT_OPTION is one of the shell options accepted by the ‘shopt’
builtin (*note The Shopt Builtin::). If SHOPT_OPTION is present,
‘-O’ sets the value of that option; ‘+O’ unsets it. If
SHOPT_OPTION is not supplied, Bash prints the names and values of
the shell options accepted by ‘shopt’ on the standard output. If
the invocation option is ‘+O’, the output is displayed in a format
that may be reused as input.
‘--’
A ‘--’ signals the end of options and disables further option
processing. Any arguments after the ‘--’ are treated as a shell
script filename (*note Shell Scripts::) and arguments passed to
that script.
‘-’
Equivalent to ‘--’.
A “login shell” is one whose first character of argument zero is ‘-’,
or one invoked with the ‘--login’ option.
An “interactive shell” is one started without non-option arguments,
unless ‘-s’ is specified, without specifying the ‘-c’ option, and whose
standard input and standard error are both connected to terminals (as
determined by isatty(3)), or one started with the ‘-i’ option. *Note
Interactive Shells::, for more information.
If arguments remain after option processing, and neither the ‘-c’ nor
the ‘-s’ option has been supplied, the first argument is treated as the
name of a file containing shell commands (*note Shell Scripts::). When
Bash is invoked in this fashion, ‘$0’ is set to the name of the file,
and the positional parameters are set to the remaining arguments. Bash
reads and executes commands from this file, then exits. Bash's exit
status is the exit status of the last command executed in the script.
If no commands are executed, the exit status is 0. Bash first attempts
to open the file in the current directory, and, if no file is found,
searches the directories in ‘PATH’ for the script.
File: bash.info, Node: Bash Startup Files, Next: Interactive Shells, Prev: Invoking Bash, Up: Bash Features
6.2 Bash Startup Files
======================
This section describes how Bash executes its startup files. If any of
the files exist but cannot be read, Bash reports an error. Tildes are
expanded in filenames as described above under Tilde Expansion (*note
Tilde Expansion::).
Interactive shells are described in *note Interactive Shells::.
Invoked as an interactive login shell, or with ‘--login’
........................................................
When Bash is invoked as an interactive login shell, or as a
non-interactive shell with the ‘--login’ option, it first reads and
executes commands from the file ‘/etc/profile’, if that file exists.
After reading that file, it looks for ‘~/.bash_profile’,
‘~/.bash_login’, and ‘~/.profile’, in that order, and reads and executes
commands from the first one that exists and is readable. The
‘--noprofile’ option inhibits this behavior.
When an interactive login shell exits, or a non-interactive login
shell executes the ‘exit’ builtin command, Bash reads and executes
commands from the file ‘~/.bash_logout’, if it exists.
Invoked as an interactive non-login shell
.........................................
When Bash runs as an interactive shell that is not a login shell, it
reads and executes commands from ‘~/.bashrc’, if that file exists. The
‘--norc’ option inhibits this behavior. The ‘--rcfile FILE’ option
causes Bash to use FILE instead of ‘~/.bashrc’.
So, typically, your ‘~/.bash_profile’ contains the line
if [ -f ~/.bashrc ]; then . ~/.bashrc; fi
after (or before) any login-specific initializations.
Invoked non-interactively
.........................
When Bash is started non-interactively, to run a shell script, for
example, it looks for the variable ‘BASH_ENV’ in the environment,
expands its value if it appears there, and uses the expanded value as
the name of a file to read and execute. Bash behaves as if the
following command were executed:
if [ -n "$BASH_ENV" ]; then . "$BASH_ENV"; fi
but does not the value of the ‘PATH’ variable to search for the
filename.
As noted above, if a non-interactive shell is invoked with the
‘--login’ option, Bash attempts to read and execute commands from the
login shell startup files.
Invoked with name ‘sh’
......................
If Bash is invoked with the name ‘sh’, it tries to mimic the startup
behavior of historical versions of ‘sh’ as closely as possible, while
conforming to the POSIX standard as well.
When invoked as an interactive login shell, or as a non-interactive
shell with the ‘--login’ option, it first attempts to read and execute
commands from ‘/etc/profile’ and ‘~/.profile’, in that order. The
‘--noprofile’ option inhibits this behavior.
When invoked as an interactive shell with the name ‘sh’, Bash looks
for the variable ‘ENV’, expands its value if it is defined, and uses the
expanded value as the name of a file to read and execute. Since a shell
invoked as ‘sh’ does not attempt to read and execute commands from any
other startup files, the ‘--rcfile’ option has no effect.
A non-interactive shell invoked with the name ‘sh’ does not attempt
to read any other startup files.
When invoked as ‘sh’, Bash enters POSIX mode after reading the
startup files.
Invoked in POSIX mode
.....................
When Bash is started in POSIX mode, as with the ‘--posix’ command line
option, it follows the POSIX standard for startup files. In this mode,
interactive shells expand the ‘ENV’ variable and read and execute
commands from the file whose name is the expanded value. No other
startup files are read.
Invoked by remote shell daemon
..............................
Bash attempts to determine when it is being run with its standard input
connected to a network connection, as when executed by the historical
and rarely-seen remote shell daemon, usually ‘rshd’, or the secure shell
daemon ‘sshd’. If Bash determines it is being run non-interactively in
this fashion, it reads and executes commands from ‘~/.bashrc’, if that
file exists and is readable. Bash does not read this file if invoked as
‘sh’. The ‘--norc’ option inhibits this behavior, and the ‘--rcfile’
option makes Bash use a different file instead of ‘~/.bashrc’, but
neither ‘rshd’ nor ‘sshd’ generally invoke the shell with those options
or allow them to be specified.
Invoked with unequal effective and real UID/GIDs
................................................
If Bash is started with the effective user (group) id not equal to the
real user (group) id, and the ‘-p’ option is not supplied, no startup
files are read, shell functions are not inherited from the environment,
the ‘SHELLOPTS’, ‘BASHOPTS’, ‘CDPATH’, and ‘GLOBIGNORE’ variables, if
they appear in the environment, are ignored, and the effective user id
is set to the real user id. If the ‘-p’ option is supplied at
invocation, the startup behavior is the same, but the effective user id
is not reset.
File: bash.info, Node: Interactive Shells, Next: Bash Conditional Expressions, Prev: Bash Startup Files, Up: Bash Features
6.3 Interactive Shells
======================
* Menu:
* What is an Interactive Shell?:: What determines whether a shell is Interactive.
* Is this Shell Interactive?:: How to tell if a shell is interactive.
* Interactive Shell Behavior:: What changes in an interactive shell?
File: bash.info, Node: What is an Interactive Shell?, Next: Is this Shell Interactive?, Up: Interactive Shells
6.3.1 What is an Interactive Shell?
-----------------------------------
An interactive shell is one started without non-option arguments (unless
‘-s’ is specified) and without specifying the ‘-c’ option, whose input
and error output are both connected to terminals (as determined by
‘isatty(3)’), or one started with the ‘-i’ option.
An interactive shell generally reads from and writes to a user's
terminal.
The ‘-s’ invocation option may be used to set the positional
parameters when an interactive shell starts.
File: bash.info, Node: Is this Shell Interactive?, Next: Interactive Shell Behavior, Prev: What is an Interactive Shell?, Up: Interactive Shells
6.3.2 Is this Shell Interactive?
--------------------------------
To determine within a startup script whether or not Bash is running
interactively, test the value of the ‘-’ special parameter. It contains
‘i’ when the shell is interactive. For example:
case "$-" in
*i*) echo This shell is interactive ;;
*) echo This shell is not interactive ;;
esac
Alternatively, startup scripts may examine the variable ‘PS1’; it is
unset in non-interactive shells, and set in interactive shells. Thus:
if [ -z "$PS1" ]; then
echo This shell is not interactive
else
echo This shell is interactive
fi
File: bash.info, Node: Interactive Shell Behavior, Prev: Is this Shell Interactive?, Up: Interactive Shells
6.3.3 Interactive Shell Behavior
--------------------------------
When the shell is running interactively, it changes its behavior in
several ways.
1. Bash reads and executes startup files as described in *note Bash
Startup Files::.
2. Job Control (*note Job Control::) is enabled by default. When job
control is in effect, Bash ignores the keyboard-generated job
control signals ‘SIGTTIN’, ‘SIGTTOU’, and ‘SIGTSTP’.
3. Bash executes the values of the set elements of the
‘PROMPT_COMMAND’ array variable as commands before printing the
primary prompt, ‘$PS1’ (*note Bash Variables::).
4. Bash expands and displays ‘PS1’ before reading the first line of a
command, and expands and displays ‘PS2’ before reading the second
and subsequent lines of a multi-line command. Bash expands and
displays ‘PS0’ after it reads a command but before executing it.
See *note Controlling the Prompt::, for a complete list of prompt
string escape sequences.
5. Bash uses Readline (*note Command Line Editing::) to read commands
from the user's terminal.
6. Bash inspects the value of the ‘ignoreeof’ option to ‘set -o’
instead of exiting immediately when it receives an ‘EOF’ on its
standard input when reading a command (*note The Set Builtin::).
7. Bash enables Command history (*note Bash History Facilities::) and
history expansion (*note History Interaction::) by default. When a
shell with history enabled exits, Bash saves the command history to
the file named by ‘$HISTFILE’.
8. Alias expansion (*note Aliases::) is performed by default.
9. In the absence of any traps, Bash ignores ‘SIGTERM’ (*note
Signals::).
10. In the absence of any traps, ‘SIGINT’ is caught and handled (*note
Signals::). ‘SIGINT’ will interrupt some shell builtins.
11. An interactive login shell sends a ‘SIGHUP’ to all jobs on exit if
the ‘huponexit’ shell option has been enabled (*note Signals::).
12. The ‘-n’ option has no effect, whether at invocation or when using
‘set -n’ (*note The Set Builtin::).
13. Bash will check for mail periodically, depending on the values of
the ‘MAIL’, ‘MAILPATH’, and ‘MAILCHECK’ shell variables (*note Bash
Variables::).
14. The shell will not exit on expansion errors due to references to
unbound shell variables after ‘set -u’ has been enabled (*note The
Set Builtin::).
15. The shell will not exit on expansion errors caused by VAR being
unset or null in ‘${VAR:?WORD}’ expansions (*note Shell Parameter
Expansion::).
16. Redirection errors encountered by shell builtins will not cause
the shell to exit.
17. When running in POSIX mode, a special builtin returning an error
status will not cause the shell to exit (*note Bash POSIX Mode::).
18. A failed ‘exec’ will not cause the shell to exit (*note Bourne
Shell Builtins::).
19. Parser syntax errors will not cause the shell to exit.
20. If the ‘cdspell’ shell option is enabled, the shell will attempt
simple spelling correction for directory arguments to the ‘cd’
builtin (see the description of the ‘cdspell’ option to the ‘shopt’
builtin in *note The Shopt Builtin::). The ‘cdspell’ option is
only effective in interactive shells.
21. The shell will check the value of the ‘TMOUT’ variable and exit if
a command is not read within the specified number of seconds after
printing ‘$PS1’ (*note Bash Variables::).
File: bash.info, Node: Bash Conditional Expressions, Next: Shell Arithmetic, Prev: Interactive Shells, Up: Bash Features
6.4 Bash Conditional Expressions
================================
Conditional expressions are used by the ‘[[’ compound command (*note
Conditional Constructs::) and the ‘test’ and ‘[’ builtin commands (*note
Bourne Shell Builtins::). The ‘test’ and ‘[’ commands determine their
behavior based on the number of arguments; see the descriptions of those
commands for any other command-specific actions.
Expressions may be unary or binary, and are formed from the primaries
listed below. Unary expressions are often used to examine the status of
a file or shell variable. Binary operators are used for string,
numeric, and file attribute comparisons.
Bash handles several filenames specially when they are used in
expressions. If the operating system on which Bash is running provides
these special files, Bash uses them; otherwise it emulates them
internally with this behavior: If the FILE argument to one of the
primaries is of the form ‘/dev/fd/N’, then Bash checks file descriptor
N. If the FILE argument to one of the primaries is one of ‘/dev/stdin’,
‘/dev/stdout’, or ‘/dev/stderr’, Bash checks file descriptor 0, 1, or 2,
respectively.
When used with ‘[[’, the ‘<’ and ‘>’ operators sort lexicographically
using the current locale. The ‘test’ command uses ASCII ordering.
Unless otherwise specified, primaries that operate on files follow
symbolic links and operate on the target of the link, rather than the
link itself.
‘-a FILE’
True if FILE exists.
‘-b FILE’
True if FILE exists and is a block special file.
‘-c FILE’
True if FILE exists and is a character special file.
‘-d FILE’
True if FILE exists and is a directory.
‘-e FILE’
True if FILE exists.
‘-f FILE’
True if FILE exists and is a regular file.
‘-g FILE’
True if FILE exists and its set-group-id bit is set.
‘-h FILE’
True if FILE exists and is a symbolic link.
‘-k FILE’
True if FILE exists and its "sticky" bit is set.
‘-p FILE’
True if FILE exists and is a named pipe (FIFO).
‘-r FILE’
True if FILE exists and is readable.
‘-s FILE’
True if FILE exists and has a size greater than zero.
‘-t FD’
True if file descriptor FD is open and refers to a terminal.
‘-u FILE’
True if FILE exists and its set-user-id bit is set.
‘-w FILE’
True if FILE exists and is writable.
‘-x FILE’
True if FILE exists and is executable.
‘-G FILE’
True if FILE exists and is owned by the effective group id.
‘-L FILE’
True if FILE exists and is a symbolic link.
‘-N FILE’
True if FILE exists and has been modified since it was last
accessed.
‘-O FILE’
True if FILE exists and is owned by the effective user id.
‘-S FILE’
True if FILE exists and is a socket.
‘FILE1 -ef FILE2’
True if FILE1 and FILE2 refer to the same device and inode numbers.
‘FILE1 -nt FILE2’
True if FILE1 is newer (according to modification date) than FILE2,
or if FILE1 exists and FILE2 does not.
‘FILE1 -ot FILE2’
True if FILE1 is older than FILE2, or if FILE2 exists and FILE1
does not.
‘-o OPTNAME’
True if the shell option OPTNAME is enabled. The list of options
appears in the description of the ‘-o’ option to the ‘set’ builtin
(*note The Set Builtin::).
‘-v VARNAME’
True if the shell variable VARNAME is set (has been assigned a
value). If VARNAME is an indexed array variable name subscripted
by ‘@’ or ‘*’, this returns true if the array has any set elements.
If VARNAME is an associative array variable name subscripted by ‘@’
or ‘*’, this returns true if an element with that key is set.
‘-R VARNAME’
True if the shell variable VARNAME is set and is a name reference.
‘-z STRING’
True if the length of STRING is zero.
‘-n STRING’
‘STRING’
True if the length of STRING is non-zero.
‘STRING1 == STRING2’
‘STRING1 = STRING2’
True if the strings are equal. When used with the ‘[[’ command,
this performs pattern matching as described above (*note
Conditional Constructs::).
‘=’ should be used with the ‘test’ command for POSIX conformance.
‘STRING1 != STRING2’
True if the strings are not equal.
‘STRING1 < STRING2’
True if STRING1 sorts before STRING2 lexicographically.
‘STRING1 > STRING2’
True if STRING1 sorts after STRING2 lexicographically.
‘ARG1 OP ARG2’
‘OP’ is one of ‘-eq’, ‘-ne’, ‘-lt’, ‘-le’, ‘-gt’, or ‘-ge’. These
arithmetic binary operators return true if ARG1 is equal to, not
equal to, less than, less than or equal to, greater than, or
greater than or equal to ARG2, respectively. ARG1 and ARG2 may be
positive or negative integers. When used with the ‘[[’ command,
ARG1 and ARG2 are evaluated as arithmetic expressions (*note Shell
Arithmetic::). Since the expansions the ‘[[’ command performs on
ARG1 and ARG2 can potentially result in empty strings, arithmetic
expression evaluation treats those as expressions that evaluate to
0.
File: bash.info, Node: Shell Arithmetic, Next: Aliases, Prev: Bash Conditional Expressions, Up: Bash Features
6.5 Shell Arithmetic
====================
The shell allows arithmetic expressions to be evaluated, as one of the
shell expansions or by using the ‘((’ compound command, the ‘let’ and
‘declare’ builtins, the arithmetic ‘for’ command, the ‘[[’ conditional
command, or the ‘-i’ option to the ‘declare’ builtin.
Evaluation is done in the largest fixed-width integers available,
with no check for overflow, though division by 0 is trapped and flagged
as an error. The operators and their precedence, associativity, and
values are the same as in the C language. The following list of
operators is grouped into levels of equal-precedence operators. The
levels are listed in order of decreasing precedence.
‘ID++ ID--’
variable post-increment and post-decrement
‘++ID --ID’
variable pre-increment and pre-decrement
‘- +’
unary minus and plus
‘! ~’
logical and bitwise negation
‘**’
exponentiation
‘* / %’
multiplication, division, remainder
‘+ -’
addition, subtraction
‘<< >>’
left and right bitwise shifts
‘<= >= < >’
comparison
‘== !=’
equality and inequality
‘&’
bitwise AND
‘^’
bitwise exclusive OR
‘|’
bitwise OR
‘&&’
logical AND
‘||’
logical OR
‘expr ? if-true-expr : if-false-expr’
conditional operator
‘= *= /= %= += -= <<= >>= &= ^= |=’
assignment
‘expr1 , expr2’
comma
Shell variables are allowed as operands; parameter expansion is
performed before the expression is evaluated. Within an expression,
shell variables may also be referenced by name without using the
parameter expansion syntax. This means you can use X, where X is a
shell variable name, in an arithmetic expression, and the shell will
evaluate its value as an expression and use the result. A shell
variable that is null or unset evaluates to 0 when referenced by name in
an expression.
The value of a variable is evaluated as an arithmetic expression when
it is referenced, or when a variable which has been given the ‘integer’
attribute using ‘declare -i’ is assigned a value. A null value
evaluates to 0. A shell variable need not have its ‘integer’ attribute
turned on to be used in an expression.
Integer constants follow the C language definition, without suffixes
or character constants. Constants with a leading 0 are interpreted as
octal numbers. A leading ‘0x’ or ‘0X’ denotes hexadecimal. Otherwise,
numbers take the form [BASE‘#’]N, where the optional BASE is a decimal
number between 2 and 64 representing the arithmetic base, and N is a
number in that base. If BASE‘#’ is omitted, then base 10 is used. When
specifying N, if a non-digit is required, the digits greater than 9 are
represented by the lowercase letters, the uppercase letters, ‘@’, and
‘_’, in that order. If BASE is less than or equal to 36, lowercase and
uppercase letters may be used interchangeably to represent numbers
between 10 and 35.
Operators are evaluated in precedence order. Sub-expressions in
parentheses are evaluated first and may override the precedence rules
above.
File: bash.info, Node: Aliases, Next: Arrays, Prev: Shell Arithmetic, Up: Bash Features
6.6 Aliases
===========
“Aliases” allow a string to be substituted for a word that is in a
position in the input where it can be the first word of a simple
command. Aliases have names and corresponding values that are set and
unset using the ‘alias’ and ‘unalias’ builtin commands (*note Shell
Builtin Commands::).
If the shell reads an unquoted word in the right position, it checks
the word to see if it matches an alias name. If it matches, the shell
replaces the word with the alias value, and reads that value as if it
had been read instead of the word. The shell doesn't look at any
characters following the word before attempting alias substitution.
The characters ‘/’, ‘$’, ‘`’, ‘=’ and any of the shell metacharacters
or quoting characters listed above may not appear in an alias name. The
replacement text may contain any valid shell input, including shell
metacharacters. The first word of the replacement text is tested for
aliases, but a word that is identical to an alias being expanded is not
expanded a second time. This means that one may alias ‘ls’ to ‘"ls
-F"’, for instance, and Bash does not try to recursively expand the
replacement text.
If the last character of the alias value is a ‘blank’, then the shell
checks the next command word following the alias for alias expansion.
Aliases are created and listed with the ‘alias’ command, and removed
with the ‘unalias’ command.
There is no mechanism for using arguments in the replacement text, as
in ‘csh’. If arguments are needed, use a shell function (*note Shell
Functions::) instead.
Aliases are not expanded when the shell is not interactive, unless
the ‘expand_aliases’ shell option is set using ‘shopt’ (*note The Shopt
Builtin::).
The rules concerning the definition and use of aliases are somewhat
confusing. Bash always reads at least one complete line of input, and
all lines that make up a compound command, before executing any of the
commands on that line or the compound command. Aliases are expanded
when a command is read, not when it is executed. Therefore, an alias
definition appearing on the same line as another command does not take
effect until the shell reads the next line of input, and an alias
definition in a compound command does not take effect until the shell
parses and executes the entire compound command. The commands following
the alias definition on that line, or in the rest of a compound command,
are not affected by the new alias. This behavior is also an issue when
functions are executed. Aliases are expanded when a function definition
is read, not when the function is executed, because a function
definition is itself a command. As a consequence, aliases defined in a
function are not available until after that function is executed. To be
safe, always put alias definitions on a separate line, and do not use
‘alias’ in compound commands.
For almost every purpose, shell functions are preferable to aliases.
File: bash.info, Node: Arrays, Next: The Directory Stack, Prev: Aliases, Up: Bash Features
6.7 Arrays
==========
Bash provides one-dimensional indexed and associative array variables.
Any variable may be used as an indexed array; the ‘declare’ builtin
explicitly declares an array. There is no maximum limit on the size of
an array, nor any requirement that members be indexed or assigned
contiguously. Indexed arrays are referenced using arithmetic
expressions that must expand to an integer (*note Shell Arithmetic::))
and are zero-based; associative arrays use arbitrary strings. Unless
otherwise noted, indexed array indices must be non-negative integers.
The shell performs parameter and variable expansion, arithmetic
expansion, command substitution, and quote removal on indexed array
subscripts. Since this can potentially result in empty strings,
subscript indexing treats those as expressions that evaluate to 0.
The shell performs tilde expansion, parameter and variable expansion,
arithmetic expansion, command substitution, and quote removal on
associative array subscripts. Empty strings cannot be used as
associative array keys.
Bash automatically creates an indexed array if any variable is
assigned to using the syntax
NAME[SUBSCRIPT]=VALUE
The SUBSCRIPT is treated as an arithmetic expression that must evaluate
to a number greater than or equal to zero. To explicitly declare an
indexed array, use
declare -a NAME
(*note Bash Builtins::). The syntax
declare -a NAME[SUBSCRIPT]
is also accepted; the SUBSCRIPT is ignored.
Associative arrays are created using
declare -A NAME
Attributes may be specified for an array variable using the ‘declare’
and ‘readonly’ builtins. Each attribute applies to all members of an
array.
Arrays are assigned using compound assignments of the form
NAME=(VALUE1 VALUE2 ... )
where each VALUE may be of the form ‘[SUBSCRIPT]=’STRING. Indexed array
assignments do not require anything but STRING.
Each VALUE in the list undergoes the shell expansions described above
(*note Shell Expansions::), but VALUEs that are valid variable
assignments including the brackets and subscript do not undergo brace
expansion and word splitting, as with individual variable assignments.
When assigning to indexed arrays, if the optional subscript is
supplied, that index is assigned to; otherwise the index of the element
assigned is the last index assigned to by the statement plus one.
Indexing starts at zero.
When assigning to an associative array, the words in a compound
assignment may be either assignment statements, for which the subscript
is required, or a list of words that is interpreted as a sequence of
alternating keys and values: NAME=(KEY1 VALUE1 KEY2 VALUE2 ... ). These
are treated identically to NAME=( [KEY1]=VALUE1 [KEY2]=VALUE2 ... ).
The first word in the list determines how the remaining words are
interpreted; all assignments in a list must be of the same type. When
using key/value pairs, the keys may not be missing or empty; a final
missing value is treated like the empty string.
This syntax is also accepted by the ‘declare’ builtin. Individual
array elements may be assigned to using the ‘NAME[SUBSCRIPT]=VALUE’
syntax introduced above.
When assigning to an indexed array, if NAME is subscripted by a
negative number, that number is interpreted as relative to one greater
than the maximum index of NAME, so negative indices count back from the
end of the array, and an index of -1 references the last element.
The ‘+=’ operator appends to an array variable when assigning using
the compound assignment syntax; see *note Shell Parameters:: above.
An array element is referenced using ‘${NAME[SUBSCRIPT]}’. The
braces are required to avoid conflicts with the shell's filename
expansion operators. If the SUBSCRIPT is ‘@’ or ‘*’, the word expands
to all members of the array NAME, unless otherwise noted in the
description of a builtin or word expansion. These subscripts differ
only when the word appears within double quotes. If the word is
double-quoted, ‘${NAME[*]}’ expands to a single word with the value of
each array member separated by the first character of the ‘IFS’
variable, and ‘${NAME[@]}’ expands each element of NAME to a separate
word. When there are no array members, ‘${NAME[@]}’ expands to nothing.
If the double-quoted expansion occurs within a word, the expansion of
the first parameter is joined with the beginning part of the expansion
of the original word, and the expansion of the last parameter is joined
with the last part of the expansion of the original word. This is
analogous to the expansion of the special parameters ‘@’ and ‘*’.
‘${#NAME[SUBSCRIPT]}’ expands to the length of ‘${NAME[SUBSCRIPT]}’.
If SUBSCRIPT is ‘@’ or ‘*’, the expansion is the number of elements in
the array.
If the SUBSCRIPT used to reference an element of an indexed array
evaluates to a number less than zero, it is interpreted as relative to
one greater than the maximum index of the array, so negative indices
count back from the end of the array, and an index of -1 refers to the
last element.
Referencing an array variable without a subscript is equivalent to
referencing with a subscript of 0. Any reference to a variable using a
valid subscript is valid; Bash creates an array if necessary.
An array variable is considered set if a subscript has been assigned
a value. The null string is a valid value.
It is possible to obtain the keys (indices) of an array as well as
the values. ${!NAME[@]} and ${!NAME[*]} expand to the indices assigned
in array variable NAME. The treatment when in double quotes is similar
to the expansion of the special parameters ‘@’ and ‘*’ within double
quotes.
The ‘unset’ builtin is used to destroy arrays. ‘unset
NAME[SUBSCRIPT]’ unsets the array element at index SUBSCRIPT. Negative
subscripts to indexed arrays are interpreted as described above.
Unsetting the last element of an array variable does not unset the
variable. ‘unset NAME’, where NAME is an array, removes the entire
array. ‘unset NAME[SUBSCRIPT]’ behaves differently depending on the
array type when SUBSCRIPT is ‘*’ or ‘@’. When NAME is an associative
array, it removes the element with key ‘*’ or ‘@’. If NAME is an
indexed array, ‘unset’ removes all of the elements, but does not remove
the array itself.
When using a variable name with a subscript as an argument to a
command, such as with ‘unset’, without using the word expansion syntax
described above (e.g., unset a[4]), the argument is subject to the
shell's filename expansion. Quote the argument if pathname expansion is
not desired (e.g., unset 'a[4]').
The ‘declare’, ‘local’, and ‘readonly’ builtins each accept a ‘-a’
option to specify an indexed array and a ‘-A’ option to specify an
associative array. If both options are supplied, ‘-A’ takes precedence.
The ‘read’ builtin accepts a ‘-a’ option to assign a list of words read
from the standard input to an array, and can read values from the
standard input into individual array elements. The ‘set’ and ‘declare’
builtins display array values in a way that allows them to be reused as
input. Other builtins accept array name arguments as well (e.g.,
‘mapfile’); see the descriptions of individual builtins for details.
The shell provides a number of builtin array variables.
File: bash.info, Node: The Directory Stack, Next: Controlling the Prompt, Prev: Arrays, Up: Bash Features
6.8 The Directory Stack
=======================
* Menu:
* Directory Stack Builtins:: Bash builtin commands to manipulate
the directory stack.
The directory stack is a list of recently-visited directories. The
‘pushd’ builtin adds directories to the stack as it changes the current
directory, and the ‘popd’ builtin removes specified directories from the
stack and changes the current directory to the directory removed. The
‘dirs’ builtin displays the contents of the directory stack. The
current directory is always the "top" of the directory stack.
The contents of the directory stack are also visible as the value of
the ‘DIRSTACK’ shell variable.
File: bash.info, Node: Directory Stack Builtins, Up: The Directory Stack
6.8.1 Directory Stack Builtins
------------------------------
‘dirs’
dirs [-clpv] [+N | -N]
Without options, display the list of currently remembered
directories. Directories are added to the list with the ‘pushd’
command; the ‘popd’ command removes directories from the list. The
current directory is always the first directory in the stack.
Options, if supplied, have the following meanings:
‘-c’
Clears the directory stack by deleting all of the elements.
‘-l’
Produces a listing using full pathnames; the default listing
format uses a tilde to denote the home directory.
‘-p’
Causes ‘dirs’ to print the directory stack with one entry per
line.
‘-v’
Causes ‘dirs’ to print the directory stack with one entry per
line, prefixing each entry with its index in the stack.
‘+N’
Displays the Nth directory (counting from the left of the list
printed by ‘dirs’ when invoked without options), starting with
zero.
‘-N’
Displays the Nth directory (counting from the right of the
list printed by ‘dirs’ when invoked without options), starting
with zero.
‘popd’
popd [-n] [+N | -N]
Remove elements from the directory stack. The elements are
numbered from 0 starting at the first directory listed by ‘dirs’;
that is, ‘popd’ is equivalent to ‘popd +0’.
When no arguments are given, ‘popd’ removes the top directory from
the stack and changes to the new top directory.
Arguments, if supplied, have the following meanings:
‘-n’
Suppress the normal change of directory when removing
directories from the stack, only manipulate the stack.
‘+N’
Remove the Nth directory (counting from the left of the list
printed by ‘dirs’), starting with zero, from the stack.
‘-N’
Remove the Nth directory (counting from the right of the list
printed by ‘dirs’), starting with zero, from the stack.
If the top element of the directory stack is modified, and the ‘-n’
option was not supplied, ‘popd’ uses the ‘cd’ builtin to change to
the directory at the top of the stack. If the ‘cd’ fails, ‘popd’
returns a non-zero value.
Otherwise, ‘popd’ returns an unsuccessful status if an invalid
option is specified, the directory stack is empty, or N specifies a
non-existent directory stack entry.
If the ‘popd’ command is successful, Bash runs ‘dirs’ to show the
final contents of the directory stack, and the return status is 0.
‘pushd’
pushd [-n] [+N | -N | DIR]
Add a directory to the top of the directory stack, or rotate the
stack, making the new top of the stack the current working
directory. With no arguments, ‘pushd’ exchanges the top two
elements of the directory stack.
Arguments, if supplied, have the following meanings:
‘-n’
Suppress the normal change of directory when rotating or
adding directories to the stack, only manipulate the stack.
‘+N’
Rotate the stack so that the Nth directory (counting from the
left of the list printed by ‘dirs’, starting with zero) is at
the top.
‘-N’
Rotate the stack so that the Nth directory (counting from the
right of the list printed by ‘dirs’, starting with zero) is at
the top.
‘DIR’
Make DIR be the top of the stack.
After the stack has been modified, if the ‘-n’ option was not
supplied, ‘pushd’ uses the ‘cd’ builtin to change to the directory
at the top of the stack. If the ‘cd’ fails, ‘pushd’ returns a
non-zero value.
Otherwise, if no arguments are supplied, ‘pushd’ returns zero
unless the directory stack is empty. When rotating the directory
stack, ‘pushd’ returns zero unless the directory stack is empty or
N specifies a non-existent directory stack element.
If the ‘pushd’ command is successful, Bash runs ‘dirs’ to show the
final contents of the directory stack.
File: bash.info, Node: Controlling the Prompt, Next: The Restricted Shell, Prev: The Directory Stack, Up: Bash Features
6.9 Controlling the Prompt
==========================
In addition, the following table describes the special characters which
can appear in the prompt variables ‘PS0’, ‘PS1’, ‘PS2’, and ‘PS4’:
‘\a’
A bell character.
‘\d’
The date, in "Weekday Month Date" format (e.g., "Tue May 26").
‘\D{FORMAT}’
The FORMAT is passed to ‘strftime’(3) and the result is inserted
into the prompt string; an empty FORMAT results in a
locale-specific time representation. The braces are required.
‘\e’
An escape character.
‘\h’
The hostname, up to the first ‘.’.
‘\H’
The hostname.
‘\j’
The number of jobs currently managed by the shell.
‘\l’
The basename of the shell's terminal device name (e.g., "ttys0").
‘\n’
A newline.
‘\r’
A carriage return.
‘\s’
The name of the shell: the basename of ‘$0’ (the portion following
the final slash).
‘\t’
The time, in 24-hour HH:MM:SS format.
‘\T’
The time, in 12-hour HH:MM:SS format.
‘\@’
The time, in 12-hour am/pm format.
‘\A’
The time, in 24-hour HH:MM format.
‘\u’
The username of the current user.
‘\v’
The Bash version (e.g., 2.00).
‘\V’
The Bash release, version + patchlevel (e.g., 2.00.0).
‘\w’
The value of the ‘PWD’ shell variable (‘$PWD’), with ‘$HOME’
abbreviated with a tilde (uses the ‘$PROMPT_DIRTRIM’ variable).
‘\W’
The basename of ‘$PWD’, with ‘$HOME’ abbreviated with a tilde.
‘\!’
The history number of this command.
‘\#’
The command number of this command.
‘\$’
If the effective uid is 0, ‘#’, otherwise ‘$’.
‘\NNN’
The character whose ASCII code is the octal value NNN.
‘\\’
A backslash.
‘\[’
Begin a sequence of non-printing characters. Thiss could be used
to embed a terminal control sequence into the prompt.
‘\]’
End a sequence of non-printing characters.
The command number and the history number are usually different: the
history number of a command is its position in the history list, which
may include commands restored from the history file (*note Bash History
Facilities::), while the command number is the position in the sequence
of commands executed during the current shell session.
After the string is decoded, it is expanded via parameter expansion,
command substitution, arithmetic expansion, and quote removal, subject
to the value of the ‘promptvars’ shell option (*note The Shopt
Builtin::). This can have unwanted side effects if escaped portions of
the string appear within command substitution or contain characters
special to word expansion.
File: bash.info, Node: The Restricted Shell, Next: Bash POSIX Mode, Prev: Controlling the Prompt, Up: Bash Features
6.10 The Restricted Shell
=========================
If Bash is started with the name ‘rbash’, or the ‘--restricted’ or ‘-r’
option is supplied at invocation, the shell becomes RESTRICTED. A
restricted shell is used to set up an environment more controlled than
the standard shell. A restricted shell behaves identically to ‘bash’
with the exception that the following are disallowed or not performed:
• Changing directories with the ‘cd’ builtin.
• Setting or unsetting the values of the ‘SHELL’, ‘PATH’, ‘HISTFILE’,
‘ENV’, or ‘BASH_ENV’ variables.
• Specifying command names containing slashes.
• Specifying a filename containing a slash as an argument to the ‘.’
builtin command.
• Using the ‘-p’ option to the ‘.’ builtin command to specify a
search path.
• Specifying a filename containing a slash as an argument to the
‘history’ builtin command.
• Specifying a filename containing a slash as an argument to the ‘-p’
option to the ‘hash’ builtin command.
• Importing function definitions from the shell environment at
startup.
• Parsing the value of ‘SHELLOPTS’ from the shell environment at
startup.
• Redirecting output using the ‘>’, ‘>|’, ‘<>’, ‘>&’, ‘&>’, and ‘>>’
redirection operators.
• Using the ‘exec’ builtin to replace the shell with another command.
• Adding or deleting builtin commands with the ‘-f’ and ‘-d’ options
to the ‘enable’ builtin.
• Using the ‘enable’ builtin command to enable disabled shell
builtins.
• Specifying the ‘-p’ option to the ‘command’ builtin.
• Turning off restricted mode with ‘set +r’ or ‘shopt -u
restricted_shell’.
These restrictions are enforced after any startup files are read.
When a command that is found to be a shell script is executed (*note
Shell Scripts::), ‘rbash’ turns off any restrictions in the shell
spawned to execute the script.
The restricted shell mode is only one component of a useful
restricted environment. It should be accompanied by setting ‘PATH’ to a
value that allows execution of only a few verified commands (commands
that allow shell escapes are particularly vulnerable), changing the
current directory to a non-writable directory other than ‘$HOME’ after
login, not allowing the restricted shell to execute shell scripts, and
cleaning the environment of variables that cause some commands to modify
their behavior (e.g., ‘VISUAL’ or ‘PAGER’).
Modern systems provide more secure ways to implement a restricted
environment, such as ‘jails’, ‘zones’, or ‘containers’.
File: bash.info, Node: Bash POSIX Mode, Next: Shell Compatibility Mode, Prev: The Restricted Shell, Up: Bash Features
6.11 Bash and POSIX
===================
6.11.1 What is POSIX?
---------------------
POSIX is the name for a family of standards based on Unix. A number of
Unix services, tools, and functions are part of the standard, ranging
from the basic system calls and C library functions to common
applications and tools to system administration and management.
The POSIX Shell and Utilities standard was originally developed by
IEEE Working Group 1003.2 (POSIX.2). The first edition of the 1003.2
standard was published in 1992. It was merged with the original IEEE
1003.1 Working Group and is currently maintained by the Austin Group (a
joint working group of the IEEE, The Open Group and ISO/IEC SC22/WG15).
Today the Shell and Utilities are a volume within the set of documents
that make up IEEE Std 1003.1-2024, and thus the former POSIX.2 (from
1992) is now part of the current unified POSIX standard.
The Shell and Utilities volume concentrates on the command
interpreter interface and utility programs commonly executed from the
command line or by other programs. The standard is freely available on
the web at
<https://pubs.opengroup.org/onlinepubs/9799919799/utilities/contents.html>.
Bash is concerned with the aspects of the shell's behavior defined by
the POSIX Shell and Utilities volume. The shell command language has of
course been standardized, including the basic flow control and program
execution constructs, I/O redirection and pipelines, argument handling,
variable expansion, and quoting.
The special builtins, which must be implemented as part of the shell
to provide the desired functionality, are specified as being part of the
shell; examples of these are ‘eval’ and ‘export’. Other utilities
appear in the sections of POSIX not devoted to the shell which are
commonly (and in some cases must be) implemented as builtin commands,
such as ‘read’ and ‘test’. POSIX also specifies aspects of the shell's
interactive behavior, including job control and command line editing.
Only vi-style line editing commands have been standardized; emacs
editing commands were left out due to objections.
6.11.2 Bash POSIX Mode
----------------------
Although Bash is an implementation of the POSIX shell specification,
there are areas where the Bash default behavior differs from the
specification. The Bash “posix mode” changes the Bash behavior in these
areas so that it conforms more strictly to the standard.
Starting Bash with the ‘--posix’ command-line option or executing
‘set -o posix’ while Bash is running will cause Bash to conform more
closely to the POSIX standard by changing the behavior to match that
specified by POSIX in areas where the Bash default differs.
When invoked as ‘sh’, Bash enters POSIX mode after reading the
startup files.
The following list is what's changed when POSIX mode is in effect:
1. Bash ensures that the ‘POSIXLY_CORRECT’ variable is set.
2. Bash reads and executes the POSIX startup files (‘$ENV’) rather
than the normal Bash files (*note Bash Startup Files::.
3. Alias expansion is always enabled, even in non-interactive shells.
4. Reserved words appearing in a context where reserved words are
recognized do not undergo alias expansion.
5. Alias expansion is performed when initially parsing a command
substitution. The default (non-posix) mode generally defers it,
when enabled, until the command substitution is executed. This
means that command substitution will not expand aliases that are
defined after the command substitution is initially parsed (e.g.,
as part of a function definition).
6. The ‘time’ reserved word may be used by itself as a simple command.
When used in this way, it displays timing statistics for the shell
and its completed children. The ‘TIMEFORMAT’ variable controls the
format of the timing information.
7. The parser does not recognize ‘time’ as a reserved word if the next
token begins with a ‘-’.
8. When parsing and expanding a ${...} expansion that appears within
double quotes, single quotes are no longer special and cannot be
used to quote a closing brace or other special character, unless
the operator is one of those defined to perform pattern removal.
In this case, they do not have to appear as matched pairs.
9. Redirection operators do not perform filename expansion on the word
in a redirection unless the shell is interactive.
10. Redirection operators do not perform word splitting on the word in
a redirection.
11. Function names may not be the same as one of the POSIX special
builtins.
12. Tilde expansion is only performed on assignments preceding a
command name, rather than on all assignment statements on the line.
13. While variable indirection is available, it may not be applied to
the ‘#’ and ‘?’ special parameters.
14. Expanding the ‘*’ special parameter in a pattern context where the
expansion is double-quoted does not treat the ‘$*’ as if it were
double-quoted.
15. A double quote character (‘"’) is treated specially when it
appears in a backquoted command substitution in the body of a
here-document that undergoes expansion. That means, for example,
that a backslash preceding a double quote character will escape it
and the backslash will be removed.
16. Command substitutions don't set the ‘?’ special parameter. The
exit status of a simple command without a command word is still the
exit status of the last command substitution that occurred while
evaluating the variable assignments and redirections in that
command, but that does not happen until after all of the
assignments and redirections.
17. Literal tildes that appear as the first character in elements of
the ‘PATH’ variable are not expanded as described above under *note
Tilde Expansion::.
18. Command lookup finds POSIX special builtins before shell
functions, including output printed by the ‘type’ and ‘command’
builtins.
19. Even if a shell function whose name contains a slash was defined
before entering POSIX mode, the shell will not execute a function
whose name contains one or more slashes.
20. When a command in the hash table no longer exists, Bash will
re-search ‘$PATH’ to find the new location. This is also available
with ‘shopt -s checkhash’.
21. Bash will not insert a command without the execute bit set into
the command hash table, even if it returns it as a (last-ditch)
result from a ‘$PATH’ search.
22. The message printed by the job control code and builtins when a
job exits with a non-zero status is 'Done(status)'.
23. The message printed by the job control code and builtins when a
job is stopped is 'Stopped(SIGNAME)', where SIGNAME is, for
example, ‘SIGTSTP’.
24. If the shell is interactive, Bash does not perform job
notifications between executing commands in lists separated by ‘;’
or newline. Non-interactive shells print status messages after a
foreground job in a list completes.
25. If the shell is interactive, Bash waits until the next prompt
before printing the status of a background job that changes status
or a foreground job that terminates due to a signal.
Non-interactive shells print status messages after a foreground job
completes.
26. Bash permanently removes jobs from the jobs table after notifying
the user of their termination via the ‘wait’ or ‘jobs’ builtins.
It removes the job from the jobs list after notifying the user of
its termination, but the status is still available via ‘wait’, as
long as ‘wait’ is supplied a PID argument.
27. The ‘vi’ editing mode will invoke the ‘vi’ editor directly when
the ‘v’ command is run, instead of checking ‘$VISUAL’ and
‘$EDITOR’.
28. Prompt expansion enables the POSIX ‘PS1’ and ‘PS2’ expansions of
‘!’ to the history number and ‘!!’ to ‘!’, and Bash performs
parameter expansion on the values of ‘PS1’ and ‘PS2’ regardless of
the setting of the ‘promptvars’ option.
29. The default history file is ‘~/.sh_history’ (this is the default
value the shell assigns to ‘$HISTFILE’).
30. The ‘!’ character does not introduce history expansion within a
double-quoted string, even if the ‘histexpand’ option is enabled.
31. When printing shell function definitions (e.g., by ‘type’), Bash
does not print the ‘function’ reserved word unless necessary.
32. Non-interactive shells exit if a syntax error in an arithmetic
expansion results in an invalid expression.
33. Non-interactive shells exit if a parameter expansion error occurs.
34. If a POSIX special builtin returns an error status, a
non-interactive shell exits. The fatal errors are those listed in
the POSIX standard, and include things like passing incorrect
options, redirection errors, variable assignment errors for
assignments preceding the command name, and so on.
35. A non-interactive shell exits with an error status if a variable
assignment error occurs when no command name follows the assignment
statements. A variable assignment error occurs, for example, when
trying to assign a value to a readonly variable.
36. A non-interactive shell exits with an error status if a variable
assignment error occurs in an assignment statement preceding a
special builtin, but not with any other simple command. For any
other simple command, the shell aborts execution of that command,
and execution continues at the top level ("the shell shall not
perform any further processing of the command in which the error
occurred").
37. A non-interactive shell exits with an error status if the
iteration variable in a ‘for’ statement or the selection variable
in a ‘select’ statement is a readonly variable or has an invalid
name.
38. Non-interactive shells exit if FILENAME in ‘.’ FILENAME is not
found.
39. Non-interactive shells exit if there is a syntax error in a script
read with the ‘.’ or ‘source’ builtins, or in a string processed by
the ‘eval’ builtin.
40. Non-interactive shells exit if the ‘export’, ‘readonly’ or ‘unset’
builtin commands get an argument that is not a valid identifier,
and they are not operating on shell functions. These errors force
an exit because these are special builtins.
41. Assignment statements preceding POSIX special builtins persist in
the shell environment after the builtin completes.
42. The ‘command’ builtin does not prevent builtins that take
assignment statements as arguments from expanding them as
assignment statements; when not in POSIX mode, declaration commands
lose their assignment statement expansion properties when preceded
by ‘command’.
43. Enabling POSIX mode has the effect of setting the
‘inherit_errexit’ option, so subshells spawned to execute command
substitutions inherit the value of the ‘-e’ option from the parent
shell. When the ‘inherit_errexit’ option is not enabled, Bash
clears the ‘-e’ option in such subshells.
44. Enabling POSIX mode has the effect of setting the ‘shift_verbose’
option, so numeric arguments to ‘shift’ that exceed the number of
positional parameters will result in an error message.
45. Enabling POSIX mode has the effect of setting the
‘interactive_comments’ option (*note Comments::).
46. The ‘.’ and ‘source’ builtins do not search the current directory
for the filename argument if it is not found by searching ‘PATH’.
47. When the ‘alias’ builtin displays alias definitions, it does not
display them with a leading ‘alias ’ unless the ‘-p’ option is
supplied.
48. The ‘bg’ builtin uses the required format to describe each job
placed in the background, which does not include an indication of
whether the job is the current or previous job.
49. When the ‘cd’ builtin is invoked in logical mode, and the pathname
constructed from ‘$PWD’ and the directory name supplied as an
argument does not refer to an existing directory, ‘cd’ will fail
instead of falling back to physical mode.
50. When the ‘cd’ builtin cannot change a directory because the length
of the pathname constructed from ‘$PWD’ and the directory name
supplied as an argument exceeds ‘PATH_MAX’ when canonicalized, ‘cd’
will attempt to use the supplied directory name.
51. When the ‘xpg_echo’ option is enabled, Bash does not attempt to
interpret any arguments to ‘echo’ as options. ‘echo’ displays each
argument after converting escape sequences.
52. The ‘export’ and ‘readonly’ builtin commands display their output
in the format required by POSIX.
53. When listing the history, the ‘fc’ builtin does not include an
indication of whether or not a history entry has been modified.
54. The default editor used by ‘fc’ is ‘ed’.
55. ‘fc’ treats extra arguments as an error instead of ignoring them.
56. If there are too many arguments supplied to ‘fc -s’, ‘fc’ prints
an error message and returns failure.
57. The output of ‘kill -l’ prints all the signal names on a single
line, separated by spaces, without the ‘SIG’ prefix.
58. The ‘kill’ builtin does not accept signal names with a ‘SIG’
prefix.
59. The ‘kill’ builtin returns a failure status if any of the pid or
job arguments are invalid or if sending the specified signal to any
of them fails. In default mode, ‘kill’ returns success if the
signal was successfully sent to any of the specified processes.
60. The ‘printf’ builtin uses ‘double’ (via ‘strtod’) to convert
arguments corresponding to floating point conversion specifiers,
instead of ‘long double’ if it's available. The ‘L’ length
modifier forces ‘printf’ to use ‘long double’ if it's available.
61. The ‘pwd’ builtin verifies that the value it prints is the same as
the current directory, even if it is not asked to check the file
system with the ‘-P’ option.
62. The ‘read’ builtin may be interrupted by a signal for which a trap
has been set. If Bash receives a trapped signal while executing
‘read’, the trap handler executes and ‘read’ returns an exit status
greater than 128.
63. When the ‘set’ builtin is invoked without options, it does not
display shell function names and definitions.
64. When the ‘set’ builtin is invoked without options, it displays
variable values without quotes, unless they contain shell
metacharacters, even if the result contains nonprinting characters.
65. The ‘test’ builtin compares strings using the current locale when
evaluating the ‘<’ and ‘>’ binary operators.
66. The ‘test’ builtin's ‘-t’ unary primary requires an argument.
Historical versions of ‘test’ made the argument optional in certain
cases, and Bash attempts to accommodate those for backwards
compatibility.
67. The ‘trap’ builtin displays signal names without the leading
‘SIG’.
68. The ‘trap’ builtin doesn't check the first argument for a possible
signal specification and revert the signal handling to the original
disposition if it is, unless that argument consists solely of
digits and is a valid signal number. If users want to reset the
handler for a given signal to the original disposition, they should
use ‘-’ as the first argument.
69. ‘trap -p’ without arguments displays signals whose dispositions
are set to SIG_DFL and those that were ignored when the shell
started, not just trapped signals.
70. The ‘type’ and ‘command’ builtins will not report a non-executable
file as having been found, though the shell will attempt to execute
such a file if it is the only so-named file found in ‘$PATH’.
71. The ‘ulimit’ builtin uses a block size of 512 bytes for the ‘-c’
and ‘-f’ options.
72. The ‘unset’ builtin with the ‘-v’ option specified returns a fatal
error if it attempts to unset a ‘readonly’ or ‘non-unsettable’
variable, which causes a non-interactive shell to exit.
73. When asked to unset a variable that appears in an assignment
statement preceding the command, the ‘unset’ builtin attempts to
unset a variable of the same name in the current or previous scope
as well. This implements the required "if an assigned variable is
further modified by the utility, the modifications made by the
utility shall persist" behavior.
74. The arrival of ‘SIGCHLD’ when a trap is set on ‘SIGCHLD’ does not
interrupt the ‘wait’ builtin and cause it to return immediately.
The trap command is run once for each child that exits.
75. Bash removes an exited background process's status from the list
of such statuses after the ‘wait’ builtin returns it.
There is additional POSIX behavior that Bash does not implement by
default even when in POSIX mode. Specifically:
1. POSIX requires that word splitting be byte-oriented. That is, each
_byte_ in the value of ‘IFS’ potentially splits a word, even if
that byte is part of a multibyte character in ‘IFS’ or part of
multibyte character in the word. Bash allows multibyte characters
in the value of ‘IFS’, treating a valid multibyte character as a
single delimiter, and will not split a valid multibyte character
even if one of the bytes composing that character appears in ‘IFS’.
This is POSIX interpretation 1560, further modified by issue 1924.
2. The ‘fc’ builtin checks ‘$EDITOR’ as a program to edit history
entries if ‘FCEDIT’ is unset, rather than defaulting directly to
‘ed’. ‘fc’ uses ‘ed’ if ‘EDITOR’ is unset.
3. As noted above, Bash requires the ‘xpg_echo’ option to be enabled
for the ‘echo’ builtin to be fully conformant.
Bash can be configured to be POSIX-conformant by default, by
specifying the ‘--enable-strict-posix-default’ to ‘configure’ when
building (*note Optional Features::).
File: bash.info, Node: Shell Compatibility Mode, Prev: Bash POSIX Mode, Up: Bash Features
6.12 Shell Compatibility Mode
=============================
Bash-4.0 introduced the concept of a “shell compatibility level”,
specified as a set of options to the shopt builtin (‘compat31’,
‘compat32’, ‘compat40’, ‘compat41’, and so on). There is only one
current compatibility level - each option is mutually exclusive. The
compatibility level is intended to allow users to select behavior from
previous versions that is incompatible with newer versions while they
migrate scripts to use current features and behavior. It's intended to
be a temporary solution.
This section does not mention behavior that is standard for a
particular version (e.g., setting ‘compat32’ means that quoting the
right hand side of the regexp matching operator quotes special regexp
characters in the word, which is default behavior in bash-3.2 and
subsequent versions).
If a user enables, say, ‘compat32’, it may affect the behavior of
other compatibility levels up to and including the current compatibility
level. The idea is that each compatibility level controls behavior that
changed in that version of Bash, but that behavior may have been present
in earlier versions. For instance, the change to use locale-based
comparisons with the ‘[[’ command came in bash-4.1, and earlier versions
used ASCII-based comparisons, so enabling ‘compat32’ will enable
ASCII-based comparisons as well. That granularity may not be sufficient
for all uses, and as a result users should employ compatibility levels
carefully. Read the documentation for a particular feature to find out
the current behavior.
Bash-4.3 introduced a new shell variable: ‘BASH_COMPAT’. The value
assigned to this variable (a decimal version number like 4.2, or an
integer corresponding to the ‘compat’NN option, like 42) determines the
compatibility level.
Starting with bash-4.4, Bash began deprecating older compatibility
levels. Eventually, the options will be removed in favor of
‘BASH_COMPAT’.
Bash-5.0 was the final version for which there was an individual
shopt option for the previous version. ‘BASH_COMPAT’ is the only
mechanism to control the compatibility level in versions newer than
bash-5.0.
The following table describes the behavior changes controlled by each
compatibility level setting. The ‘compat’NN tag is used as shorthand
for setting the compatibility level to NN using one of the following
mechanisms. For versions prior to bash-5.0, the compatibility level may
be set using the corresponding ‘compat’NN shopt option. For bash-4.3
and later versions, the ‘BASH_COMPAT’ variable is preferred, and it is
required for bash-5.1 and later versions.
‘compat31’
• Quoting the rhs of the ‘[[’ command's regexp matching operator
(=~) has no special effect
‘compat40’
• The ‘<’ and ‘>’ operators to the ‘[[’ command do not consider
the current locale when comparing strings; they use ASCII
ordering. Bash versions prior to bash-4.1 use ASCII collation
and strcmp(3); bash-4.1 and later use the current locale's
collation sequence and strcoll(3).
‘compat41’
• In POSIX mode, ‘time’ may be followed by options and still be
recognized as a reserved word (this is POSIX interpretation
267).
• In POSIX mode, the parser requires that an even number of
single quotes occur in the WORD portion of a double-quoted
${...} parameter expansion and treats them specially, so that
characters within the single quotes are considered quoted
(this is POSIX interpretation 221).
‘compat42’
• The replacement string in double-quoted pattern substitution
does not undergo quote removal, as it does in versions after
bash-4.2.
• In POSIX mode, single quotes are considered special when
expanding the WORD portion of a double-quoted ${...} parameter
expansion and can be used to quote a closing brace or other
special character (this is part of POSIX interpretation 221);
in later versions, single quotes are not special within
double-quoted word expansions.
‘compat43’
• Word expansion errors are considered non-fatal errors that
cause the current command to fail, even in POSIX mode (the
default behavior is to make them fatal errors that cause the
shell to exit).
• When executing a shell function, the loop state
(while/until/etc.) is not reset, so ‘break’ or ‘continue’ in
that function will break or continue loops in the calling
context. Bash-4.4 and later reset the loop state to prevent
this.
‘compat44’
• The shell sets up the values used by ‘BASH_ARGV’ and
‘BASH_ARGC’ so they can expand to the shell's positional
parameters even if extended debugging mode is not enabled.
• A subshell inherits loops from its parent context, so ‘break’
or ‘continue’ will cause the subshell to exit. Bash-5.0 and
later reset the loop state to prevent the exit.
• Variable assignments preceding builtins like ‘export’ and
‘readonly’ that set attributes continue to affect variables
with the same name in the calling environment even if the
shell is not in POSIX mode.
‘compat50 (set using BASH_COMPAT)’
• Bash-5.1 changed the way ‘$RANDOM’ is generated to introduce
slightly more randomness. If the shell compatibility level is
set to 50 or lower, it reverts to the method from bash-5.0 and
previous versions, so seeding the random number generator by
assigning a value to ‘RANDOM’ will produce the same sequence
as in bash-5.0.
• If the command hash table is empty, Bash versions prior to
bash-5.1 printed an informational message to that effect, even
when producing output that can be reused as input. Bash-5.1
suppresses that message when the ‘-l’ option is supplied.
‘compat51 (set using BASH_COMPAT)’
• The ‘unset’ builtin will unset the array ‘a’ given an argument
like ‘a[@]’. Bash-5.2 will unset an element with key ‘@’
(associative arrays) or remove all the elements without
unsetting the array (indexed arrays).
• Arithmetic commands ( ((...)) ) and the expressions in an
arithmetic for statement can be expanded more than once.
• Expressions used as arguments to arithmetic operators in the
‘[[’ conditional command can be expanded more than once.
• The expressions in substring parameter brace expansion can be
expanded more than once.
• The expressions in the $(( ... )) word expansion can be
expanded more than once.
• Arithmetic expressions used as indexed array subscripts can be
expanded more than once.
• ‘test -v’, when given an argument of ‘A[@]’, where A is an
existing associative array, will return true if the array has
any set elements. Bash-5.2 will look for and report on a key
named ‘@’.
• the ${PARAMETER[:]=VALUE} word expansion will return VALUE,
before any variable-specific transformations have been
performed (e.g., converting to lowercase). Bash-5.2 will
return the final value assigned to the variable.
• Parsing command substitutions will behave as if extended
globbing (*note The Shopt Builtin::) is enabled, so that
parsing a command substitution containing an extglob pattern
(say, as part of a shell function) will not fail. This
assumes the intent is to enable extglob before the command is
executed and word expansions are performed. It will fail at
word expansion time if extglob hasn't been enabled by the time
the command is executed.
‘compat52 (set using BASH_COMPAT)’
• The ‘test’ builtin uses its historical algorithm to parse
parenthesized subexpressions when given five or more
arguments.
• If the ‘-p’ or ‘-P’ option is supplied to the ‘bind’ builtin,
‘bind’ treats any arguments remaining after option processing
as bindable command names, and displays any key sequences
bound to those commands, instead of treating the arguments as
key sequences to bind.
• Interactive shells will notify the user of completed jobs
while sourcing a script. Newer versions defer notification
until script execution completes.
File: bash.info, Node: Job Control, Next: Command Line Editing, Prev: Bash Features, Up: Top
7 Job Control
*************
This chapter discusses what job control is, how it works, and how Bash
allows you to access its facilities.
* Menu:
* Job Control Basics:: How job control works.
* Job Control Builtins:: Bash builtin commands used to interact
with job control.
* Job Control Variables:: Variables Bash uses to customize job
control.
File: bash.info, Node: Job Control Basics, Next: Job Control Builtins, Up: Job Control
7.1 Job Control Basics
======================
Job control refers to the ability to selectively stop (suspend) the
execution of processes and continue (resume) their execution at a later
point. A user typically employs this facility via an interactive
interface supplied jointly by the operating system kernel's terminal
driver and Bash.
The shell associates a JOB with each pipeline. It keeps a table of
currently executing jobs, which the ‘jobs’ command will display. Each
job has a “job number”, which ‘jobs’ displays between brackets. Job
numbers start at 1. When Bash starts a job asynchronously, it prints a
line that looks like:
[1] 25647
indicating that this job is job number 1 and that the process ID of the
last process in the pipeline associated with this job is 25647. All of
the processes in a single pipeline are members of the same job. Bash
uses the JOB abstraction as the basis for job control.
To facilitate the implementation of the user interface to job
control, each process has a “process group ID”, and the operating system
maintains the notion of a current terminal process group ID. This
terminal process group ID is associated with the “controlling terminal”.
Processes that have the same process group ID are said to be part of
the same “process group”. Members of the foreground process group
(processes whose process group ID is equal to the current terminal
process group ID) receive keyboard-generated signals such as ‘SIGINT’.
Processes in the foreground process group are said to be foreground
processes. Background processes are those whose process group ID
differs from the controlling terminal's; such processes are immune to
keyboard-generated signals. Only foreground processes are allowed to
read from or, if the user so specifies with ‘stty tostop’, write to the
controlling terminal. The system sends a ‘SIGTTIN’ (‘SIGTTOU’) signal
to background processes which attempt to read from (write to when
‘tostop’ is in effect) the terminal, which, unless caught, suspends the
process.
If the operating system on which Bash is running supports job
control, Bash contains facilities to use it. Typing the “suspend”
character (typically ‘^Z’, Control-Z) while a process is running stops
that process and returns control to Bash. Typing the “delayed suspend”
character (typically ‘^Y’, Control-Y) causes the process to stop when it
attempts to read input from the terminal, and returns control to Bash.
The user then manipulates the state of this job, using the ‘bg’ command
to continue it in the background, the ‘fg’ command to continue it in the
foreground, or the ‘kill’ command to kill it. The suspend character
takes effect immediately, and has the additional side effect of
discarding any pending output and typeahead. If you want to force a
background process to stop, or stop a process that's not associated with
your terminal session, send it the ‘SIGSTOP’ signal using ‘kill’.
There are a number of ways to refer to a job in the shell. The ‘%’
character introduces a “job specification” (jobspec).
Job number ‘n’ may be referred to as ‘%n’. A job may also be
referred to using a prefix of the name used to start it, or using a
substring that appears in its command line. For example, ‘%ce’ refers
to a job whose command name begins with ‘ce’. Using ‘%?ce’, on the
other hand, refers to any job containing the string ‘ce’ in its command
line. If the prefix or substring matches more than one job, Bash
reports an error.
The symbols ‘%%’ and ‘%+’ refer to the shell's notion of the “current
job”. A single ‘%’ (with no accompanying job specification) also refers
to the current job. ‘%-’ refers to the “previous job”. When a job
starts in the background, a job stops while in the foreground, or a job
is resumed in the background, it becomes the current job. The job that
was the current job becomes the previous job. When the current job
terminates, the previous job becomes the current job. If there is only
a single job, ‘%+’ and ‘%-’ can both be used to refer to that job. In
output pertaining to jobs (e.g., the output of the ‘jobs’ command), the
current job is always marked with a ‘+’, and the previous job with a
‘-’.
Simply naming a job can be used to bring it into the foreground: ‘%1’
is a synonym for ‘fg %1’, bringing job 1 from the background into the
foreground. Similarly, ‘%1 &’ resumes job 1 in the background,
equivalent to ‘bg %1’.
The shell learns immediately whenever a job changes state. Normally,
Bash waits until it is about to print a prompt before notifying the user
about changes in a job's status so as to not interrupt any other output,
though it will notify of changes in a job's status after a foreground
command in a list completes, before executing the next command in the
list. If the ‘-b’ option to the ‘set’ builtin is enabled, Bash reports
status changes immediately (*note The Set Builtin::). Bash executes any
trap on ‘SIGCHLD’ for each child process that terminates.
When a job terminates and Bash notifies the user about it, Bash
removes the job from the jobs table. It will not appear in ‘jobs’
output, but ‘wait’ will report its exit status, as long as it's supplied
the process ID associated with the job as an argument. When the table
is empty, job numbers start over at 1.
If a user attempts to exit Bash while jobs are stopped, (or running,
if the ‘checkjobs’ option is enabled - see *note The Shopt Builtin::),
the shell prints a warning message, and if the ‘checkjobs’ option is
enabled, lists the jobs and their statuses. The ‘jobs’ command may then
be used to inspect their status. If the user immediately attempts to
exit again, without an intervening command, Bash does not print another
warning, and terminates any stopped jobs.
When the shell is waiting for a job or process using the ‘wait’
builtin, and job control is enabled, ‘wait’ will return when the job
changes state. The ‘-f’ option causes ‘wait’ to wait until the job or
process terminates before returning.
File: bash.info, Node: Job Control Builtins, Next: Job Control Variables, Prev: Job Control Basics, Up: Job Control
7.2 Job Control Builtins
========================
‘bg’
bg [JOBSPEC ...]
Resume each suspended job JOBSPEC in the background, as if it had
been started with ‘&’. If JOBSPEC is not supplied, the shell uses
its notion of the current job. ‘bg’ returns zero unless it is run
when job control is not enabled, or, when run with job control
enabled, any JOBSPEC was not found or specifies a job that was
started without job control.
‘fg’
fg [JOBSPEC]
Resume the job JOBSPEC in the foreground and make it the current
job. If JOBSPEC is not supplied, ‘fg’ resumes the current job.
The return status is that of the command placed into the
foreground, or non-zero if run when job control is disabled or,
when run with job control enabled, JOBSPEC does not specify a valid
job or JOBSPEC specifies a job that was started without job
control.
‘jobs’
jobs [-lnprs] [JOBSPEC]
jobs -x COMMAND [ARGUMENTS]
The first form lists the active jobs. The options have the
following meanings:
‘-l’
List process IDs in addition to the normal information.
‘-n’
Display information only about jobs that have changed status
since the user was last notified of their status.
‘-p’
List only the process ID of the job's process group leader.
‘-r’
Display only running jobs.
‘-s’
Display only stopped jobs.
If JOBSPEC is supplied, ‘jobs’ restricts output to information
about that job. If JOBSPEC is not supplied, ‘jobs’ lists the
status of all jobs. The return status is zero unless an invalid
option is encountered or an invalid JOBSPEC is supplied.
If the ‘-x’ option is supplied, ‘jobs’ replaces any JOBSPEC found
in COMMAND or ARGUMENTS with the corresponding process group ID,
and executes COMMAND, passing it ARGUMENTs, returning its exit
status.
‘kill’
kill [-s SIGSPEC] [-n SIGNUM] [-SIGSPEC] ID [...]
kill -l|-L [EXIT_STATUS]
Send a signal specified by SIGSPEC or SIGNUM to the processes named
by each ID. Each ID may be a job specification JOBSPEC or process
ID PID. SIGSPEC is either a case-insensitive signal name such as
‘SIGINT’ (with or without the ‘SIG’ prefix) or a signal number;
SIGNUM is a signal number. If SIGSPEC and SIGNUM are not present,
‘kill’ sends ‘SIGTERM’.
The ‘-l’ option lists the signal names. If any arguments are
supplied when ‘-l’ is supplied, ‘kill’ lists the names of the
signals corresponding to the arguments, and the return status is
zero. EXIT_STATUS is a number specifying a signal number or the
exit status of a process terminated by a signal; if it is supplied,
‘kill’ prints the name of the signal that caused the process to
terminate. ‘kill’ assumes that process exit statuses are greater
than 128; anything less than that is a signal number. The ‘-L’
option is equivalent to ‘-l’.
The return status is zero if at least one signal was successfully
sent, or non-zero if an error occurs or an invalid option is
encountered.
‘wait’
wait [-fn] [-p VARNAME] [ID ...]
Wait until the child process specified by each ID exits and return
the exit status of the last ID. Each ID may be a process ID PID or
a job specification JOBSPEC; if a jobspec is supplied, ‘wait’ waits
for all processes in the job.
If no options or IDs are supplied, ‘wait’ waits for all running
background jobs and the last-executed process substitution, if its
process id is the same as $!, and the return status is zero.
If the ‘-n’ option is supplied, ‘wait’ waits for any one of the IDs
or, if no IDs are supplied, any job or process substitution, to
complete and returns its exit status. If none of the supplied IDs
is a child of the shell, or if no arguments are supplied and the
shell has no unwaited-for children, the exit status is 127.
If the ‘-p’ option is supplied, ‘wait’ assigns the process or job
identifier of the job for which the exit status is returned to the
variable VARNAME named by the option argument. The variable, which
cannot be readonly, will be unset initially, before any assignment.
This is useful only when used with the ‘-n’ option.
Supplying the ‘-f’ option, when job control is enabled, forces
‘wait’ to wait for each ID to terminate before returning its
status, instead of returning when it changes status.
If none of the IDs specify one of the shell's an active child
processes, the return status is 127. If ‘wait’ is interrupted by a
signal, any VARNAME will remain unset, and the return status will
be greater than 128, as described above (*note Signals::).
Otherwise, the return status is the exit status of the last ID.
‘disown’
disown [-ar] [-h] [ID ...]
Without options, remove each ID from the table of active jobs.
Each ID may be a job specification JOBSPEC or a process ID PID; if
ID is a PID, ‘disown’ uses the job containing PID as JOBSPEC.
If the ‘-h’ option is supplied, ‘disown’ does not remove the jobs
corresponding to each ‘id’ from the jobs table, but rather marks
them so the shell does not send ‘SIGHUP’ to the job if the shell
receives a ‘SIGHUP’.
If no ID is supplied, the ‘-a’ option means to remove or mark all
jobs; the ‘-r’ option without an ID argument removes or marks
running jobs. If no ID is supplied, and neither the ‘-a’ nor the
‘-r’ option is supplied, ‘disown’ removes or marks the current job.
The return value is 0 unless an ID does not specify a valid job.
‘suspend’
suspend [-f]
Suspend the execution of this shell until it receives a ‘SIGCONT’
signal. A login shell, or a shell without job control enabled,
cannot be suspended; the ‘-f’ option will override this and force
the suspension. The return status is 0 unless the shell is a login
shell or job control is not enabled and ‘-f’ is not supplied.
When job control is not active, the ‘kill’ and ‘wait’ builtins do not
accept JOBSPEC arguments. They must be supplied process IDs.
File: bash.info, Node: Job Control Variables, Prev: Job Control Builtins, Up: Job Control
7.3 Job Control Variables
=========================
‘auto_resume’
This variable controls how the shell interacts with the user and
job control. If this variable exists then simple commands
consisting of only a single word, without redirections, are treated
as candidates for resumption of an existing job. There is no
ambiguity allowed; if there is more than one job beginning with or
containing the word, then this selects the most recently accessed
job. The name of a stopped job, in this context, is the command
line used to start it, as displayed by ‘jobs’. If this variable is
set to the value ‘exact’, the word must match the name of a stopped
job exactly; if set to ‘substring’, the word needs to match a
substring of the name of a stopped job. The ‘substring’ value
provides functionality analogous to the ‘%?string’ job ID (*note
Job Control Basics::). If set to any other value (e.g., ‘prefix’),
the word must be a prefix of a stopped job's name; this provides
functionality analogous to the ‘%string’ job ID.
File: bash.info, Node: Command Line Editing, Next: Using History Interactively, Prev: Job Control, Up: Top
8 Command Line Editing
**********************
This chapter describes the basic features of the GNU command line
editing interface. Command line editing is provided by the Readline
library, which is used by several different programs, including Bash.
Command line editing is enabled by default when using an interactive
shell, unless the ‘--noediting’ option is supplied at shell invocation.
Line editing is also used when using the ‘-e’ option to the ‘read’
builtin command (*note Bash Builtins::). By default, the line editing
commands are similar to those of Emacs; a vi-style line editing
interface is also available. Line editing can be enabled at any time
using the ‘-o emacs’ or ‘-o vi’ options to the ‘set’ builtin command
(*note The Set Builtin::), or disabled using the ‘+o emacs’ or ‘+o vi’
options to ‘set’.
* Menu:
* Introduction and Notation:: Notation used in this text.
* Readline Interaction:: The minimum set of commands for editing a line.
* Readline Init File:: Customizing Readline from a user's view.
* Bindable Readline Commands:: A description of most of the Readline commands
available for binding
* Readline vi Mode:: A short description of how to make Readline
behave like the vi editor.
* Programmable Completion:: How to specify the possible completions for
a specific command.
* Programmable Completion Builtins:: Builtin commands to specify how to
complete arguments for a particular command.
* A Programmable Completion Example:: An example shell function for
generating possible completions.
File: bash.info, Node: Introduction and Notation, Next: Readline Interaction, Up: Command Line Editing
8.1 Introduction to Line Editing
================================
The following paragraphs use Emacs style to describe the notation used
to represent keystrokes.
The text ‘C-k’ is read as 'Control-K' and describes the character
produced when the <k> key is pressed while the Control key is depressed.
The text ‘M-k’ is read as 'Meta-K' and describes the character
produced when the Meta key (if you have one) is depressed, and the <k>
key is pressed (a “meta character”), then both are released. The Meta
key is labeled <ALT> or <Option> on many keyboards. On keyboards with
two keys labeled <ALT> (usually to either side of the space bar), the
<ALT> on the left side is generally set to work as a Meta key. One of
the <ALT> keys may also be configured as some other modifier, such as a
Compose key for typing accented characters.
On some keyboards, the Meta key modifier produces characters with the
eighth bit (0200) set. You can use the ‘enable-meta-key’ variable to
control whether or not it does this, if the keyboard allows it. On many
others, the terminal or terminal emulator converts the metafied key to a
key sequence beginning with <ESC> as described in the next paragraph.
If you do not have a Meta or <ALT> key, or another key working as a
Meta key, you can generally achieve the latter effect by typing <ESC>
_first_, and then typing <k>. The <ESC> character is known as the “meta
prefix”).
Either process is known as “metafying” the <k> key.
If your Meta key produces a key sequence with the <ESC> meta prefix,
you can make ‘M-key’ key bindings you specify (see ‘Key Bindings’ in
*note Readline Init File Syntax::) do the same thing by setting the
‘force-meta-prefix’ variable.
The text ‘M-C-k’ is read as 'Meta-Control-k' and describes the
character produced by metafying ‘C-k’.
In addition, several keys have their own names. Specifically, <DEL>,
<ESC>, <LFD>, <SPC>, <RET>, and <TAB> all stand for themselves when seen
in this text, or in an init file (*note Readline Init File::). If your
keyboard lacks a <LFD> key, typing <C-j> will output the appropriate
character. The <RET> key may be labeled <Return> or <Enter> on some
keyboards.
File: bash.info, Node: Readline Interaction, Next: Readline Init File, Prev: Introduction and Notation, Up: Command Line Editing
8.2 Readline Interaction
========================
Often during an interactive session you type in a long line of text,
only to notice that the first word on the line is misspelled. The
Readline library gives you a set of commands for manipulating the text
as you type it in, allowing you to just fix your typo, and not forcing
you to retype the majority of the line. Using these editing commands,
you move the cursor to the place that needs correction, and delete or
insert the text of the corrections. Then, when you are satisfied with
the line, you simply press <RET>. You do not have to be at the end of
the line to press <RET>; the entire line is accepted regardless of the
location of the cursor within the line.
* Menu:
* Readline Bare Essentials:: The least you need to know about Readline.
* Readline Movement Commands:: Moving about the input line.
* Readline Killing Commands:: How to delete text, and how to get it back!
* Readline Arguments:: Giving numeric arguments to commands.
* Searching:: Searching through previous lines.
File: bash.info, Node: Readline Bare Essentials, Next: Readline Movement Commands, Up: Readline Interaction
8.2.1 Readline Bare Essentials
------------------------------
In order to enter characters into the line, simply type them. The typed
character appears where the cursor was, and then the cursor moves one
space to the right. If you mistype a character, you can use your erase
character to back up and delete the mistyped character.
Sometimes you may mistype a character, and not notice the error until
you have typed several other characters. In that case, you can type
‘C-b’ to move the cursor to the left, and then correct your mistake.
Afterwards, you can move the cursor to the right with ‘C-f’.
When you add text in the middle of a line, you will notice that
characters to the right of the cursor are 'pushed over' to make room for
the text that you have inserted. Likewise, when you delete text behind
the cursor, characters to the right of the cursor are 'pulled back' to
fill in the blank space created by the removal of the text. These are
the bare essentials for editing the text of an input line:
‘C-b’
Move back one character.
‘C-f’
Move forward one character.
<DEL> or <Backspace>
Delete the character to the left of the cursor.
‘C-d’
Delete the character underneath the cursor.
Printing characters
Insert the character into the line at the cursor.
‘C-_’ or ‘C-x C-u’
Undo the last editing command. You can undo all the way back to an
empty line.
Depending on your configuration, the <Backspace> key might be set to
delete the character to the left of the cursor and the <DEL> key set to
delete the character underneath the cursor, like ‘C-d’, rather than the
character to the left of the cursor.
File: bash.info, Node: Readline Movement Commands, Next: Readline Killing Commands, Prev: Readline Bare Essentials, Up: Readline Interaction
8.2.2 Readline Movement Commands
--------------------------------
The above table describes the most basic keystrokes that you need in
order to do editing of the input line. For your convenience, many other
commands are available in addition to ‘C-b’, ‘C-f’, ‘C-d’, and <DEL>.
Here are some commands for moving more rapidly within the line.
‘C-a’
Move to the start of the line.
‘C-e’
Move to the end of the line.
‘M-f’
Move forward a word, where a word is composed of letters and
digits.
‘M-b’
Move backward a word.
‘C-l’
Clear the screen, reprinting the current line at the top.
Notice how ‘C-f’ moves forward a character, while ‘M-f’ moves forward
a word. It is a loose convention that control keystrokes operate on
characters while meta keystrokes operate on words.
File: bash.info, Node: Readline Killing Commands, Next: Readline Arguments, Prev: Readline Movement Commands, Up: Readline Interaction
8.2.3 Readline Killing Commands
-------------------------------
“Killing” text means to delete the text from the line, but to save it
away for later use, usually by “yanking” (re-inserting) it back into the
line. ('Cut' and 'paste' are more recent jargon for 'kill' and 'yank'.)
If the description for a command says that it 'kills' text, then you
can be sure that you can get the text back in a different (or the same)
place later.
When you use a kill command, the text is saved in a “kill-ring”. Any
number of consecutive kills save all of the killed text together, so
that when you yank it back, you get it all. The kill ring is not line
specific; the text that you killed on a previously typed line is
available to be yanked back later, when you are typing another line.
Here is the list of commands for killing text.
‘C-k’
Kill the text from the current cursor position to the end of the
line.
‘M-d’
Kill from the cursor to the end of the current word, or, if between
words, to the end of the next word. Word boundaries are the same
as those used by ‘M-f’.
‘M-<DEL>’
Kill from the cursor to the start of the current word, or, if
between words, to the start of the previous word. Word boundaries
are the same as those used by ‘M-b’.
‘C-w’
Kill from the cursor to the previous whitespace. This is different
than ‘M-<DEL>’ because the word boundaries differ.
Here is how to “yank” the text back into the line. Yanking means to
copy the most-recently-killed text from the kill buffer into the line at
the current cursor position.
‘C-y’
Yank the most recently killed text back into the buffer at the
cursor.
‘M-y’
Rotate the kill-ring, and yank the new top. You can only do this
if the prior command is ‘C-y’ or ‘M-y’.
File: bash.info, Node: Readline Arguments, Next: Searching, Prev: Readline Killing Commands, Up: Readline Interaction
8.2.4 Readline Arguments
------------------------
You can pass numeric arguments to Readline commands. Sometimes the
argument acts as a repeat count, other times it is the sign of the
argument that is significant. If you pass a negative argument to a
command which normally acts in a forward direction, that command will
act in a backward direction. For example, to kill text back to the
start of the line, you might type ‘M-- C-k’.
The general way to pass numeric arguments to a command is to type
meta digits before the command. If the first 'digit' typed is a minus
sign (‘-’), then the sign of the argument will be negative. Once you
have typed one meta digit to get the argument started, you can type the
remainder of the digits, and then the command. For example, to give the
‘C-d’ command an argument of 10, you could type ‘M-1 0 C-d’, which will
delete the next ten characters on the input line.
File: bash.info, Node: Searching, Prev: Readline Arguments, Up: Readline Interaction
8.2.5 Searching for Commands in the History
-------------------------------------------
Readline provides commands for searching through the command history
(*note Bash History Facilities::) for lines containing a specified
string. There are two search modes: “incremental” and
“non-incremental”.
Incremental searches begin before the user has finished typing the
search string. As each character of the search string is typed,
Readline displays the next entry from the history matching the string
typed so far. An incremental search requires only as many characters as
needed to find the desired history entry. When using emacs editing
mode, type ‘C-r’ to search backward in the history for a particular
string. Typing ‘C-s’ searches forward through the history. The
characters present in the value of the ‘isearch-terminators’ variable
are used to terminate an incremental search. If that variable has not
been assigned a value, the <ESC> and ‘C-j’ characters terminate an
incremental search. ‘C-g’ aborts an incremental search and restores the
original line. When the search is terminated, the history entry
containing the search string becomes the current line.
To find other matching entries in the history list, type ‘C-r’ or
‘C-s’ as appropriate. This searches backward or forward in the history
for the next entry matching the search string typed so far. Any other
key sequence bound to a Readline command terminates the search and
executes that command. For instance, a <RET> terminates the search and
accepts the line, thereby executing the command from the history list.
A movement command will terminate the search, make the last line found
the current line, and begin editing.
Readline remembers the last incremental search string. If two ‘C-r’s
are typed without any intervening characters defining a new search
string, Readline uses any remembered search string.
Non-incremental searches read the entire search string before
starting to search for matching history entries. The search string may
be typed by the user or be part of the contents of the current line.
File: bash.info, Node: Readline Init File, Next: Bindable Readline Commands, Prev: Readline Interaction, Up: Command Line Editing
8.3 Readline Init File
======================
Although the Readline library comes with a set of Emacs-like keybindings
installed by default, it is possible to use a different set of
keybindings. Any user can customize programs that use Readline by
putting commands in an “inputrc” file, conventionally in their home
directory. The name of this file is taken from the value of the shell
variable ‘INPUTRC’. If that variable is unset, the default is
‘~/.inputrc’. If that file does not exist or cannot be read, Readline
looks for ‘/etc/inputrc’. The ‘bind’ builtin command can also be used
to set Readline keybindings and variables. *Note Bash Builtins::.
When a program that uses the Readline library starts up, Readline
reads the init file and sets any variables and key bindings it contains.
In addition, the ‘C-x C-r’ command re-reads this init file, thus
incorporating any changes that you might have made to it.
* Menu:
* Readline Init File Syntax:: Syntax for the commands in the inputrc file.
* Conditional Init Constructs:: Conditional key bindings in the inputrc file.
* Sample Init File:: An example inputrc file.
File: bash.info, Node: Readline Init File Syntax, Next: Conditional Init Constructs, Up: Readline Init File
8.3.1 Readline Init File Syntax
-------------------------------
There are only a few basic constructs allowed in the Readline init file.
Blank lines are ignored. Lines beginning with a ‘#’ are comments.
Lines beginning with a ‘$’ indicate conditional constructs (*note
Conditional Init Constructs::). Other lines denote variable settings
and key bindings.
Variable Settings
You can modify the run-time behavior of Readline by altering the
values of variables in Readline using the ‘set’ command within the
init file. The syntax is simple:
set VARIABLE VALUE
Here, for example, is how to change from the default Emacs-like key
binding to use ‘vi’ line editing commands:
set editing-mode vi
Variable names and values, where appropriate, are recognized
without regard to case. Unrecognized variable names are ignored.
Boolean variables (those that can be set to on or off) are set to
on if the value is null or empty, ON (case-insensitive), or 1. Any
other value results in the variable being set to off.
The ‘bind -V’ command lists the current Readline variable names and
values. *Note Bash Builtins::.
A great deal of run-time behavior is changeable with the following
variables.
‘active-region-start-color’
A string variable that controls the text color and background
when displaying the text in the active region (see the
description of ‘enable-active-region’ below). This string
must not take up any physical character positions on the
display, so it should consist only of terminal escape
sequences. It is output to the terminal before displaying the
text in the active region. This variable is reset to the
default value whenever the terminal type changes. The default
value is the string that puts the terminal in standout mode,
as obtained from the terminal's terminfo description. A
sample value might be ‘\e[01;33m’.
‘active-region-end-color’
A string variable that "undoes" the effects of
‘active-region-start-color’ and restores "normal" terminal
display appearance after displaying text in the active region.
This string must not take up any physical character positions
on the display, so it should consist only of terminal escape
sequences. It is output to the terminal after displaying the
text in the active region. This variable is reset to the
default value whenever the terminal type changes. The default
value is the string that restores the terminal from standout
mode, as obtained from the terminal's terminfo description. A
sample value might be ‘\e[0m’.
‘bell-style’
Controls what happens when Readline wants to ring the terminal
bell. If set to ‘none’, Readline never rings the bell. If
set to ‘visible’, Readline uses a visible bell if one is
available. If set to ‘audible’ (the default), Readline
attempts to ring the terminal's bell.
‘bind-tty-special-chars’
If set to ‘on’ (the default), Readline attempts to bind the
control characters that are treated specially by the kernel's
terminal driver to their Readline equivalents. These override
the default Readline bindings described here. Type ‘stty -a’
at a Bash prompt to see your current terminal settings,
including the special control characters (usually ‘cchars’).
‘blink-matching-paren’
If set to ‘on’, Readline attempts to briefly move the cursor
to an opening parenthesis when a closing parenthesis is
inserted. The default is ‘off’.
‘colored-completion-prefix’
If set to ‘on’, when listing completions, Readline displays
the common prefix of the set of possible completions using a
different color. The color definitions are taken from the
value of the ‘LS_COLORS’ environment variable. If there is a
color definition in ‘LS_COLORS’ for the custom suffix
‘readline-colored-completion-prefix’, Readline uses this color
for the common prefix instead of its default. The default is
‘off’.
‘colored-stats’
If set to ‘on’, Readline displays possible completions using
different colors to indicate their file type. The color
definitions are taken from the value of the ‘LS_COLORS’
environment variable. The default is ‘off’.
‘comment-begin’
The string to insert at the beginning of the line by the
‘insert-comment’ command. The default value is ‘"#"’.
‘completion-display-width’
The number of screen columns used to display possible matches
when performing completion. The value is ignored if it is
less than 0 or greater than the terminal screen width. A
value of 0 causes matches to be displayed one per line. The
default value is -1.
‘completion-ignore-case’
If set to ‘on’, Readline performs filename matching and
completion in a case-insensitive fashion. The default value
is ‘off’.
‘completion-map-case’
If set to ‘on’, and COMPLETION-IGNORE-CASE is enabled,
Readline treats hyphens (‘-’) and underscores (‘_’) as
equivalent when performing case-insensitive filename matching
and completion. The default value is ‘off’.
‘completion-prefix-display-length’
The maximum length in characters of the common prefix of a
list of possible completions that is displayed without
modification. When set to a value greater than zero, Readline
replaces common prefixes longer than this value with an
ellipsis when displaying possible completions. If a
completion begins with a period, and Readline is completing
filenames, it uses three underscores instead of an ellipsis.
‘completion-query-items’
The number of possible completions that determines when the
user is asked whether the list of possibilities should be
displayed. If the number of possible completions is greater
than or equal to this value, Readline asks whether or not the
user wishes to view them; otherwise, Readline simply lists the
completions. This variable must be set to an integer value
greater than or equal to zero. A zero value means Readline
should never ask; negative values are treated as zero. The
default limit is ‘100’.
‘convert-meta’
If set to ‘on’, Readline converts characters it reads that
have the eighth bit set to an ASCII key sequence by clearing
the eighth bit and prefixing an <ESC> character, converting
them to a meta-prefixed key sequence. The default value is
‘on’, but Readline sets it to ‘off’ if the locale contains
characters whose encodings may include bytes with the eighth
bit set. This variable is dependent on the ‘LC_CTYPE’ locale
category, and may change if the locale changes. This variable
also affects key bindings; see the description of
‘force-meta-prefix’ below.
‘disable-completion’
If set to ‘On’, Readline inhibits word completion. Completion
characters are inserted into the line as if they had been
mapped to ‘self-insert’. The default is ‘off’.
‘echo-control-characters’
When set to ‘on’, on operating systems that indicate they
support it, Readline echoes a character corresponding to a
signal generated from the keyboard. The default is ‘on’.
‘editing-mode’
The ‘editing-mode’ variable controls the default set of key
bindings. By default, Readline starts up in emacs editing
mode, where the keystrokes are most similar to Emacs. This
variable can be set to either ‘emacs’ or ‘vi’.
‘emacs-mode-string’
If the SHOW-MODE-IN-PROMPT variable is enabled, this string is
displayed immediately before the last line of the primary
prompt when emacs editing mode is active. The value is
expanded like a key binding, so the standard set of meta- and
control- prefixes and backslash escape sequences is available.
The ‘\1’ and ‘\2’ escapes begin and end sequences of
non-printing characters, which can be used to embed a terminal
control sequence into the mode string. The default is ‘@’.
‘enable-active-region’
“point” is the current cursor position, and “mark” refers to a
saved cursor position (*note Commands For Moving::). The text
between the point and mark is referred to as the “region”.
When this variable is set to ‘On’, Readline allows certain
commands to designate the region as “active”. When the region
is active, Readline highlights the text in the region using
the value of the ‘active-region-start-color’, which defaults
to the string that enables the terminal's standout mode. The
active region shows the text inserted by bracketed-paste and
any matching text found by incremental and non-incremental
history searches. The default is ‘On’.
‘enable-bracketed-paste’
When set to ‘On’, Readline configures the terminal to insert
each paste into the editing buffer as a single string of
characters, instead of treating each character as if it had
been read from the keyboard. This is called putting the
terminal into “bracketed paste mode”; it prevents Readline
from executing any editing commands bound to key sequences
appearing in the pasted text. The default is ‘On’.
‘enable-keypad’
When set to ‘on’, Readline tries to enable the application
keypad when it is called. Some systems need this to enable
the arrow keys. The default is ‘off’.
‘enable-meta-key’
When set to ‘on’, Readline tries to enable any meta modifier
key the terminal claims to support when it is called. On many
terminals, the Meta key is used to send eight-bit characters;
this variable checks for the terminal capability that
indicates the terminal can enable and disable a mode that sets
the eighth bit of a character (0200) if the Meta key is held
down when the character is typed (a meta character). The
default is ‘on’.
‘expand-tilde’
If set to ‘on’, Readline attempts tilde expansion when it
attempts word completion. The default is ‘off’.
‘force-meta-prefix’
If set to ‘on’, Readline modifies its behavior when binding
key sequences containing ‘\M-’ or ‘Meta-’ (see ‘Key Bindings’
in *note Readline Init File Syntax::) by converting a key
sequence of the form ‘\M-’C or ‘Meta-’C to the two-character
sequence ‘ESC’ C (adding the meta prefix). If
‘force-meta-prefix’ is set to ‘off’ (the default), Readline
uses the value of the ‘convert-meta’ variable to determine
whether to perform this conversion: if ‘convert-meta’ is ‘on’,
Readline performs the conversion described above; if it is
‘off’, Readline converts C to a meta character by setting the
eighth bit (0200). The default is ‘off’.
‘history-preserve-point’
If set to ‘on’, the history code attempts to place the point
(the current cursor position) at the same location on each
history line retrieved with ‘previous-history’ or
‘next-history’. The default is ‘off’.
‘history-size’
Set the maximum number of history entries saved in the history
list. If set to zero, any existing history entries are
deleted and no new entries are saved. If set to a value less
than zero, the number of history entries is not limited. By
default, Bash sets the maximum number of history entries to
the value of the ‘HISTSIZE’ shell variable. If you try to set
HISTORY-SIZE to a non-numeric value, the maximum number of
history entries will be set to 500.
‘horizontal-scroll-mode’
Setting this variable to ‘on’ means that the text of the lines
being edited will scroll horizontally on a single screen line
when the lines are longer than the width of the screen,
instead of wrapping onto a new screen line. This variable is
automatically set to ‘on’ for terminals of height 1. By
default, this variable is set to ‘off’.
‘input-meta’
If set to ‘on’, Readline enables eight-bit input (that is, it
does not clear the eighth bit in the characters it reads),
regardless of what the terminal claims it can support. The
default value is ‘off’, but Readline sets it to ‘on’ if the
locale contains characters whose encodings may include bytes
with the eighth bit set. This variable is dependent on the
‘LC_CTYPE’ locale category, and its value may change if the
locale changes. The name ‘meta-flag’ is a synonym for
‘input-meta’.
‘isearch-terminators’
The string of characters that should terminate an incremental
search without subsequently executing the character as a
command (*note Searching::). If this variable has not been
given a value, the characters <ESC> and ‘C-j’ terminate an
incremental search.
‘keymap’
Sets Readline's idea of the current keymap for key binding
commands. Built-in ‘keymap’ names are ‘emacs’,
‘emacs-standard’, ‘emacs-meta’, ‘emacs-ctlx’, ‘vi’, ‘vi-move’,
‘vi-command’, and ‘vi-insert’. ‘vi’ is equivalent to
‘vi-command’ (‘vi-move’ is also a synonym); ‘emacs’ is
equivalent to ‘emacs-standard’. Applications may add
additional names. The default value is ‘emacs’; the value of
the ‘editing-mode’ variable also affects the default keymap.
‘keyseq-timeout’
Specifies the duration Readline will wait for a character when
reading an ambiguous key sequence (one that can form a
complete key sequence using the input read so far, or can take
additional input to complete a longer key sequence). If
Readline doesn't receive any input within the timeout, it uses
the shorter but complete key sequence. Readline uses this
value to determine whether or not input is available on the
current input source (‘rl_instream’ by default). The value is
specified in milliseconds, so a value of 1000 means that
Readline will wait one second for additional input. If this
variable is set to a value less than or equal to zero, or to a
non-numeric value, Readline waits until another key is pressed
to decide which key sequence to complete. The default value
is ‘500’.
‘mark-directories’
If set to ‘on’, completed directory names have a slash
appended. The default is ‘on’.
‘mark-modified-lines’
When this variable is set to ‘on’, Readline displays an
asterisk (‘*’) at the start of history lines which have been
modified. This variable is ‘off’ by default.
‘mark-symlinked-directories’
If set to ‘on’, completed names which are symbolic links to
directories have a slash appended, subject to the value of
‘mark-directories’. The default is ‘off’.
‘match-hidden-files’
This variable, when set to ‘on’, forces Readline to match
files whose names begin with a ‘.’ (hidden files) when
performing filename completion. If set to ‘off’, the user
must include the leading ‘.’ in the filename to be completed.
This variable is ‘on’ by default.
‘menu-complete-display-prefix’
If set to ‘on’, menu completion displays the common prefix of
the list of possible completions (which may be empty) before
cycling through the list. The default is ‘off’.
‘output-meta’
If set to ‘on’, Readline displays characters with the eighth
bit set directly rather than as a meta-prefixed escape
sequence. The default is ‘off’, but Readline sets it to ‘on’
if the locale contains characters whose encodings may include
bytes with the eighth bit set. This variable is dependent on
the ‘LC_CTYPE’ locale category, and its value may change if
the locale changes.
‘page-completions’
If set to ‘on’, Readline uses an internal pager resembling
more(1) to display a screenful of possible completions at a
time. This variable is ‘on’ by default.
‘prefer-visible-bell’
See ‘bell-style’.
‘print-completions-horizontally’
If set to ‘on’, Readline displays completions with matches
sorted horizontally in alphabetical order, rather than down
the screen. The default is ‘off’.
‘revert-all-at-newline’
If set to ‘on’, Readline will undo all changes to history
lines before returning when executing ‘accept-line’. By
default, history lines may be modified and retain individual
undo lists across calls to ‘readline()’. The default is
‘off’.
‘search-ignore-case’
If set to ‘on’, Readline performs incremental and
non-incremental history list searches in a case-insensitive
fashion. The default value is ‘off’.
‘show-all-if-ambiguous’
This alters the default behavior of the completion functions.
If set to ‘on’, words which have more than one possible
completion cause the matches to be listed immediately instead
of ringing the bell. The default value is ‘off’.
‘show-all-if-unmodified’
This alters the default behavior of the completion functions
in a fashion similar to SHOW-ALL-IF-AMBIGUOUS. If set to
‘on’, words which have more than one possible completion
without any possible partial completion (the possible
completions don't share a common prefix) cause the matches to
be listed immediately instead of ringing the bell. The
default value is ‘off’.
‘show-mode-in-prompt’
If set to ‘on’, add a string to the beginning of the prompt
indicating the editing mode: emacs, vi command, or vi
insertion. The mode strings are user-settable (e.g.,
EMACS-MODE-STRING). The default value is ‘off’.
‘skip-completed-text’
If set to ‘on’, this alters the default completion behavior
when inserting a single match into the line. It's only active
when performing completion in the middle of a word. If
enabled, Readline does not insert characters from the
completion that match characters after point in the word being
completed, so portions of the word following the cursor are
not duplicated. For instance, if this is enabled, attempting
completion when the cursor is after the first ‘e’ in
‘Makefile’ will result in ‘Makefile’ rather than
‘Makefilefile’, assuming there is a single possible
completion. The default value is ‘off’.
‘vi-cmd-mode-string’
If the SHOW-MODE-IN-PROMPT variable is enabled, this string is
displayed immediately before the last line of the primary
prompt when vi editing mode is active and in command mode.
The value is expanded like a key binding, so the standard set
of meta- and control- prefixes and backslash escape sequences
is available. The ‘\1’ and ‘\2’ escapes begin and end
sequences of non-printing characters, which can be used to
embed a terminal control sequence into the mode string. The
default is ‘(cmd)’.
‘vi-ins-mode-string’
If the SHOW-MODE-IN-PROMPT variable is enabled, this string is
displayed immediately before the last line of the primary
prompt when vi editing mode is active and in insertion mode.
The value is expanded like a key binding, so the standard set
of meta- and control- prefixes and backslash escape sequences
is available. The ‘\1’ and ‘\2’ escapes begin and end
sequences of non-printing characters, which can be used to
embed a terminal control sequence into the mode string. The
default is ‘(ins)’.
‘visible-stats’
If set to ‘on’, a character denoting a file's type is appended
to the filename when listing possible completions. The
default is ‘off’.
Key Bindings
The syntax for controlling key bindings in the init file is simple.
First you need to find the name of the command that you want to
change. The following sections contain tables of the command name,
the default keybinding, if any, and a short description of what the
command does.
Once you know the name of the command, simply place on a line in
the init file the name of the key you wish to bind the command to,
a colon, and then the name of the command. There can be no space
between the key name and the colon - that will be interpreted as
part of the key name. The name of the key can be expressed in
different ways, depending on what you find most comfortable.
In addition to command names, Readline allows keys to be bound to a
string that is inserted when the key is pressed (a MACRO). The
difference between a macro and a command is that a macro is
enclosed in single or double quotes.
The ‘bind -p’ command displays Readline function names and bindings
in a format that can be put directly into an initialization file.
*Note Bash Builtins::.
KEYNAME: FUNCTION-NAME or MACRO
KEYNAME is the name of a key spelled out in English. For
example:
Control-u: universal-argument
Meta-Rubout: backward-kill-word
Control-o: "> output"
In the example above, ‘C-u’ is bound to the function
‘universal-argument’, ‘M-DEL’ is bound to the function
‘backward-kill-word’, and ‘C-o’ is bound to run the macro
expressed on the right hand side (that is, to insert the text
‘> output’ into the line).
This key binding syntax recognizes a number of symbolic
character names: DEL, ESC, ESCAPE, LFD, NEWLINE, RET, RETURN,
RUBOUT (a destructive backspace), SPACE, SPC, and TAB.
"KEYSEQ": FUNCTION-NAME or MACRO
KEYSEQ differs from KEYNAME above in that strings denoting an
entire key sequence can be specified, by placing the key
sequence in double quotes. Some GNU Emacs style key escapes
can be used, as in the following example, but none of the
special character names are recognized.
"\C-u": universal-argument
"\C-x\C-r": re-read-init-file
"\e[11~": "Function Key 1"
In the above example, ‘C-u’ is again bound to the function
‘universal-argument’ (just as it was in the first example),
‘‘C-x’ ‘C-r’’ is bound to the function ‘re-read-init-file’,
and ‘<ESC> <[> <1> <1> <~>’ is bound to insert the text
‘Function Key 1’.
The following GNU Emacs style escape sequences are available when
specifying key sequences:
‘\C-’
A control prefix.
‘\M-’
Adding the meta prefix or converting the following character
to a meta character, as described above under
‘force-meta-prefix’ (see ‘Variable Settings’ in *note Readline
Init File Syntax::).
‘\e’
An escape character.
‘\\’
Backslash.
‘\"’
<">, a double quotation mark.
‘\'’
<'>, a single quote or apostrophe.
In addition to the GNU Emacs style escape sequences, a second set
of backslash escapes is available:
‘\a’
alert (bell)
‘\b’
backspace
‘\d’
delete
‘\f’
form feed
‘\n’
newline
‘\r’
carriage return
‘\t’
horizontal tab
‘\v’
vertical tab
‘\NNN’
The eight-bit character whose value is the octal value NNN
(one to three digits).
‘\xHH’
The eight-bit character whose value is the hexadecimal value
HH (one or two hex digits).
When entering the text of a macro, single or double quotes must be
used to indicate a macro definition. Unquoted text is assumed to
be a function name. The backslash escapes described above are
expanded in the macro body. Backslash will quote any other
character in the macro text, including ‘"’ and ‘'’. For example,
the following binding will make ‘‘C-x’ \’ insert a single ‘\’ into
the line:
"\C-x\\": "\\"
File: bash.info, Node: Conditional Init Constructs, Next: Sample Init File, Prev: Readline Init File Syntax, Up: Readline Init File
8.3.2 Conditional Init Constructs
---------------------------------
Readline implements a facility similar in spirit to the conditional
compilation features of the C preprocessor which allows key bindings and
variable settings to be performed as the result of tests. There are
four parser directives available.
‘$if’
The ‘$if’ construct allows bindings to be made based on the editing
mode, the terminal being used, or the application using Readline.
The text of the test, after any comparison operator, extends to the
end of the line; unless otherwise noted, no characters are required
to isolate it.
‘mode’
The ‘mode=’ form of the ‘$if’ directive is used to test
whether Readline is in ‘emacs’ or ‘vi’ mode. This may be used
in conjunction with the ‘set keymap’ command, for instance, to
set bindings in the ‘emacs-standard’ and ‘emacs-ctlx’ keymaps
only if Readline is starting out in ‘emacs’ mode.
‘term’
The ‘term=’ form may be used to include terminal-specific key
bindings, perhaps to bind the key sequences output by the
terminal's function keys. The word on the right side of the
‘=’ is tested against both the full name of the terminal and
the portion of the terminal name before the first ‘-’. This
allows ‘xterm’ to match both ‘xterm’ and ‘xterm-256color’, for
instance.
‘version’
The ‘version’ test may be used to perform comparisons against
specific Readline versions. The ‘version’ expands to the
current Readline version. The set of comparison operators
includes ‘=’ (and ‘==’), ‘!=’, ‘<=’, ‘>=’, ‘<’, and ‘>’. The
version number supplied on the right side of the operator
consists of a major version number, an optional decimal point,
and an optional minor version (e.g., ‘7.1’). If the minor
version is omitted, it defaults to ‘0’. The operator may be
separated from the string ‘version’ and from the version
number argument by whitespace. The following example sets a
variable if the Readline version being used is 7.0 or newer:
$if version >= 7.0
set show-mode-in-prompt on
$endif
‘application’
The APPLICATION construct is used to include
application-specific settings. Each program using the
Readline library sets the APPLICATION NAME, and you can test
for a particular value. This could be used to bind key
sequences to functions useful for a specific program. For
instance, the following command adds a key sequence that
quotes the current or previous word in Bash:
$if Bash
# Quote the current or previous word
"\C-xq": "\eb\"\ef\""
$endif
‘variable’
The VARIABLE construct provides simple equality tests for
Readline variables and values. The permitted comparison
operators are ‘=’, ‘==’, and ‘!=’. The variable name must be
separated from the comparison operator by whitespace; the
operator may be separated from the value on the right hand
side by whitespace. String and boolean variables may be
tested. Boolean variables must be tested against the values
ON and OFF. The following example is equivalent to the
‘mode=emacs’ test described above:
$if editing-mode == emacs
set show-mode-in-prompt on
$endif
‘$else’
Commands in this branch of the ‘$if’ directive are executed if the
test fails.
‘$endif’
This command, as seen in the previous example, terminates an ‘$if’
command.
‘$include’
This directive takes a single filename as an argument and reads
commands and key bindings from that file. For example, the
following directive reads from ‘/etc/inputrc’:
$include /etc/inputrc
File: bash.info, Node: Sample Init File, Prev: Conditional Init Constructs, Up: Readline Init File
8.3.3 Sample Init File
----------------------
Here is an example of an INPUTRC file. This illustrates key binding,
variable assignment, and conditional syntax.
# This file controls the behavior of line input editing for
# programs that use the GNU Readline library. Existing
# programs include FTP, Bash, and GDB.
#
# You can re-read the inputrc file with C-x C-r.
# Lines beginning with '#' are comments.
#
# First, include any system-wide bindings and variable
# assignments from /etc/Inputrc
$include /etc/Inputrc
#
# Set various bindings for emacs mode.
set editing-mode emacs
$if mode=emacs
Meta-Control-h: backward-kill-word Text after the function name is ignored
#
# Arrow keys in keypad mode
#
#"\M-OD": backward-char
#"\M-OC": forward-char
#"\M-OA": previous-history
#"\M-OB": next-history
#
# Arrow keys in ANSI mode
#
"\M-[D": backward-char
"\M-[C": forward-char
"\M-[A": previous-history
"\M-[B": next-history
#
# Arrow keys in 8 bit keypad mode
#
#"\M-\C-OD": backward-char
#"\M-\C-OC": forward-char
#"\M-\C-OA": previous-history
#"\M-\C-OB": next-history
#
# Arrow keys in 8 bit ANSI mode
#
#"\M-\C-[D": backward-char
#"\M-\C-[C": forward-char
#"\M-\C-[A": previous-history
#"\M-\C-[B": next-history
C-q: quoted-insert
$endif
# An old-style binding. This happens to be the default.
TAB: complete
# Macros that are convenient for shell interaction
$if Bash
# edit the path
"\C-xp": "PATH=${PATH}\e\C-e\C-a\ef\C-f"
# prepare to type a quoted word --
# insert open and close double quotes
# and move to just after the open quote
"\C-x\"": "\"\"\C-b"
# insert a backslash (testing backslash escapes
# in sequences and macros)
"\C-x\\": "\\"
# Quote the current or previous word
"\C-xq": "\eb\"\ef\""
# Add a binding to refresh the line, which is unbound
"\C-xr": redraw-current-line
# Edit variable on current line.
"\M-\C-v": "\C-a\C-k$\C-y\M-\C-e\C-a\C-y="
$endif
# use a visible bell if one is available
set bell-style visible
# don't strip characters to 7 bits when reading
set input-meta on
# allow iso-latin1 characters to be inserted rather
# than converted to prefix-meta sequences
set convert-meta off
# display characters with the eighth bit set directly
# rather than as meta-prefixed characters
set output-meta on
# if there are 150 or more possible completions for a word,
# ask whether or not the user wants to see all of them
set completion-query-items 150
# For FTP
$if Ftp
"\C-xg": "get \M-?"
"\C-xt": "put \M-?"
"\M-.": yank-last-arg
$endif
File: bash.info, Node: Bindable Readline Commands, Next: Readline vi Mode, Prev: Readline Init File, Up: Command Line Editing
8.4 Bindable Readline Commands
==============================
* Menu:
* Commands For Moving:: Moving about the line.
* Commands For History:: Getting at previous lines.
* Commands For Text:: Commands for changing text.
* Commands For Killing:: Commands for killing and yanking.
* Numeric Arguments:: Specifying numeric arguments, repeat counts.
* Commands For Completion:: Getting Readline to do the typing for you.
* Keyboard Macros:: Saving and re-executing typed characters
* Miscellaneous Commands:: Other miscellaneous commands.
This section describes Readline commands that may be bound to key
sequences. You can list your key bindings by executing ‘bind -P’ or,
for a more terse format, suitable for an INPUTRC file, ‘bind -p’.
(*Note Bash Builtins::.) Command names without an accompanying key
sequence are unbound by default.
In the following descriptions, “point” refers to the current cursor
position, and “mark” refers to a cursor position saved by the ‘set-mark’
command. The text between the point and mark is referred to as the
“region”. Readline has the concept of an _active region_: when the
region is active, Readline redisplay highlights the region using the
value of the ‘active-region-start-color’ variable. The
‘enable-active-region’ variable turns this on and off. Several commands
set the region to active; those are noted below.
File: bash.info, Node: Commands For Moving, Next: Commands For History, Up: Bindable Readline Commands
8.4.1 Commands For Moving
-------------------------
‘beginning-of-line (C-a)’
Move to the start of the current line. This may also be bound to
the Home key on some keyboards.
‘end-of-line (C-e)’
Move to the end of the line. This may also be bound to the End key
on some keyboards.
‘forward-char (C-f)’
Move forward a character. This may also be bound to the right
arrow key on some keyboards.
‘backward-char (C-b)’
Move back a character. This may also be bound to the left arrow
key on some keyboards.
‘forward-word (M-f)’
Move forward to the end of the next word. Words are composed of
letters and digits.
‘backward-word (M-b)’
Move back to the start of the current or previous word. Words are
composed of letters and digits.
‘shell-forward-word (M-C-f)’
Move forward to the end of the next word. Words are delimited by
non-quoted shell metacharacters.
‘shell-backward-word (M-C-b)’
Move back to the start of the current or previous word. Words are
delimited by non-quoted shell metacharacters.
‘previous-screen-line ()’
Attempt to move point to the same physical screen column on the
previous physical screen line. This will not have the desired
effect if the current Readline line does not take up more than one
physical line or if point is not greater than the length of the
prompt plus the screen width.
‘next-screen-line ()’
Attempt to move point to the same physical screen column on the
next physical screen line. This will not have the desired effect
if the current Readline line does not take up more than one
physical line or if the length of the current Readline line is not
greater than the length of the prompt plus the screen width.
‘clear-display (M-C-l)’
Clear the screen and, if possible, the terminal's scrollback
buffer, then redraw the current line, leaving the current line at
the top of the screen.
‘clear-screen (C-l)’
Clear the screen, then redraw the current line, leaving the current
line at the top of the screen. If given a numeric argument, this
refreshes the current line without clearing the screen.
‘redraw-current-line ()’
Refresh the current line. By default, this is unbound.
File: bash.info, Node: Commands For History, Next: Commands For Text, Prev: Commands For Moving, Up: Bindable Readline Commands
8.4.2 Commands For Manipulating The History
-------------------------------------------
‘accept-line (Newline or Return)’
Accept the line regardless of where the cursor is. If this line is
non-empty, add it to the history list according to the setting of
the ‘HISTCONTROL’ and ‘HISTIGNORE’ variables. If this line is a
modified history line, then restore the history line to its
original state.
‘previous-history (C-p)’
Move 'back' through the history list, fetching the previous
command. This may also be bound to the up arrow key on some
keyboards.
‘next-history (C-n)’
Move 'forward' through the history list, fetching the next command.
This may also be bound to the down arrow key on some keyboards.
‘beginning-of-history (M-<)’
Move to the first line in the history.
‘end-of-history (M->)’
Move to the end of the input history, i.e., the line currently
being entered.
‘reverse-search-history (C-r)’
Search backward starting at the current line and moving 'up'
through the history as necessary. This is an incremental search.
This command sets the region to the matched text and activates the
region.
‘forward-search-history (C-s)’
Search forward starting at the current line and moving 'down'
through the history as necessary. This is an incremental search.
This command sets the region to the matched text and activates the
region.
‘non-incremental-reverse-search-history (M-p)’
Search backward starting at the current line and moving 'up'
through the history as necessary using a non-incremental search for
a string supplied by the user. The search string may match
anywhere in a history line.
‘non-incremental-forward-search-history (M-n)’
Search forward starting at the current line and moving 'down'
through the history as necessary using a non-incremental search for
a string supplied by the user. The search string may match
anywhere in a history line.
‘history-search-backward ()’
Search backward through the history for the string of characters
between the start of the current line and the point. The search
string must match at the beginning of a history line. This is a
non-incremental search. By default, this command is unbound, but
may be bound to the Page Down key on some keyboards.
‘history-search-forward ()’
Search forward through the history for the string of characters
between the start of the current line and the point. The search
string must match at the beginning of a history line. This is a
non-incremental search. By default, this command is unbound, but
may be bound to the Page Up key on some keyboards.
‘history-substring-search-backward ()’
Search backward through the history for the string of characters
between the start of the current line and the point. The search
string may match anywhere in a history line. This is a
non-incremental search. By default, this command is unbound.
‘history-substring-search-forward ()’
Search forward through the history for the string of characters
between the start of the current line and the point. The search
string may match anywhere in a history line. This is a
non-incremental search. By default, this command is unbound.
‘yank-nth-arg (M-C-y)’
Insert the first argument to the previous command (usually the
second word on the previous line) at point. With an argument N,
insert the Nth word from the previous command (the words in the
previous command begin with word 0). A negative argument inserts
the Nth word from the end of the previous command. Once the
argument N is computed, this uses the history expansion facilities
to extract the Nth word, as if the ‘!N’ history expansion had been
specified.
‘yank-last-arg (M-. or M-_)’
Insert last argument to the previous command (the last word of the
previous history entry). With a numeric argument, behave exactly
like ‘yank-nth-arg’. Successive calls to ‘yank-last-arg’ move back
through the history list, inserting the last word (or the word
specified by the argument to the first call) of each line in turn.
Any numeric argument supplied to these successive calls determines
the direction to move through the history. A negative argument
switches the direction through the history (back or forward). This
uses the history expansion facilities to extract the last word, as
if the ‘!$’ history expansion had been specified.
‘operate-and-get-next (C-o)’
Accept the current line for return to the calling application as if
a newline had been entered, and fetch the next line relative to the
current line from the history for editing. A numeric argument, if
supplied, specifies the history entry to use instead of the current
line.
‘fetch-history ()’
With a numeric argument, fetch that entry from the history list and
make it the current line. Without an argument, move back to the
first entry in the history list.
File: bash.info, Node: Commands For Text, Next: Commands For Killing, Prev: Commands For History, Up: Bindable Readline Commands
8.4.3 Commands For Changing Text
--------------------------------
‘end-of-file (usually C-d)’
The character indicating end-of-file as set, for example, by
‘stty’. If this character is read when there are no characters on
the line, and point is at the beginning of the line, Readline
interprets it as the end of input and returns EOF.
‘delete-char (C-d)’
Delete the character at point. If this function is bound to the
same character as the tty EOF character, as ‘C-d’ commonly is, see
above for the effects. This may also be bound to the Delete key on
some keyboards.
‘backward-delete-char (Rubout)’
Delete the character behind the cursor. A numeric argument means
to kill the characters, saving them on the kill ring, instead of
deleting them.
‘forward-backward-delete-char ()’
Delete the character under the cursor, unless the cursor is at the
end of the line, in which case the character behind the cursor is
deleted. By default, this is not bound to a key.
‘quoted-insert (C-q or C-v)’
Add the next character typed to the line verbatim. This is how to
insert key sequences like ‘C-q’, for example.
‘self-insert (a, b, A, 1, !, ...)’
Insert the character typed.
‘bracketed-paste-begin ()’
This function is intended to be bound to the "bracketed paste"
escape sequence sent by some terminals, and such a binding is
assigned by default. It allows Readline to insert the pasted text
as a single unit without treating each character as if it had been
read from the keyboard. The characters are inserted as if each one
was bound to ‘self-insert’ instead of executing any editing
commands.
Bracketed paste sets the region (the characters between point and
the mark) to the inserted text. It sets the _active region_.
‘transpose-chars (C-t)’
Drag the character before the cursor forward over the character at
the cursor, moving the cursor forward as well. If the insertion
point is at the end of the line, then this transposes the last two
characters of the line. Negative arguments have no effect.
‘transpose-words (M-t)’
Drag the word before point past the word after point, moving point
past that word as well. If the insertion point is at the end of
the line, this transposes the last two words on the line.
‘shell-transpose-words (M-C-t)’
Drag the word before point past the word after point, moving point
past that word as well. If the insertion point is at the end of
the line, this transposes the last two words on the line. Word
boundaries are the same as ‘shell-forward-word’ and
‘shell-backward-word’.
‘upcase-word (M-u)’
Uppercase the current (or following) word. With a negative
argument, uppercase the previous word, but do not move the cursor.
‘downcase-word (M-l)’
Lowercase the current (or following) word. With a negative
argument, lowercase the previous word, but do not move the cursor.
‘capitalize-word (M-c)’
Capitalize the current (or following) word. With a negative
argument, capitalize the previous word, but do not move the cursor.
‘overwrite-mode ()’
Toggle overwrite mode. With an explicit positive numeric argument,
switches to overwrite mode. With an explicit non-positive numeric
argument, switches to insert mode. This command affects only
‘emacs’ mode; ‘vi’ mode does overwrite differently. Each call to
‘readline()’ starts in insert mode.
In overwrite mode, characters bound to ‘self-insert’ replace the
text at point rather than pushing the text to the right.
Characters bound to ‘backward-delete-char’ replace the character
before point with a space.
By default, this command is unbound, but may be bound to the Insert
key on some keyboards.
File: bash.info, Node: Commands For Killing, Next: Numeric Arguments, Prev: Commands For Text, Up: Bindable Readline Commands
8.4.4 Killing And Yanking
-------------------------
‘kill-line (C-k)’
Kill the text from point to the end of the current line. With a
negative numeric argument, kill backward from the cursor to the
beginning of the line.
‘backward-kill-line (C-x Rubout)’
Kill backward from the cursor to the beginning of the current line.
With a negative numeric argument, kill forward from the cursor to
the end of the line.
‘unix-line-discard (C-u)’
Kill backward from the cursor to the beginning of the current line.
‘kill-whole-line ()’
Kill all characters on the current line, no matter where point is.
By default, this is unbound.
‘kill-word (M-d)’
Kill from point to the end of the current word, or if between
words, to the end of the next word. Word boundaries are the same
as ‘forward-word’.
‘backward-kill-word (M-<DEL>)’
Kill the word behind point. Word boundaries are the same as
‘backward-word’.
‘shell-kill-word (M-C-d)’
Kill from point to the end of the current word, or if between
words, to the end of the next word. Word boundaries are the same
as ‘shell-forward-word’.
‘shell-backward-kill-word ()’
Kill the word behind point. Word boundaries are the same as
‘shell-backward-word’.
‘unix-word-rubout (C-w)’
Kill the word behind point, using white space as a word boundary,
saving the killed text on the kill-ring.
‘unix-filename-rubout ()’
Kill the word behind point, using white space and the slash
character as the word boundaries, saving the killed text on the
kill-ring.
‘delete-horizontal-space ()’
Delete all spaces and tabs around point. By default, this is
unbound.
‘kill-region ()’
Kill the text in the current region. By default, this command is
unbound.
‘copy-region-as-kill ()’
Copy the text in the region to the kill buffer, so it can be yanked
right away. By default, this command is unbound.
‘copy-backward-word ()’
Copy the word before point to the kill buffer. The word boundaries
are the same as ‘backward-word’. By default, this command is
unbound.
‘copy-forward-word ()’
Copy the word following point to the kill buffer. The word
boundaries are the same as ‘forward-word’. By default, this
command is unbound.
‘yank (C-y)’
Yank the top of the kill ring into the buffer at point.
‘yank-pop (M-y)’
Rotate the kill-ring, and yank the new top. You can only do this
if the prior command is ‘yank’ or ‘yank-pop’.
File: bash.info, Node: Numeric Arguments, Next: Commands For Completion, Prev: Commands For Killing, Up: Bindable Readline Commands
8.4.5 Specifying Numeric Arguments
----------------------------------
‘digit-argument (M-0, M-1, ... M--)’
Add this digit to the argument already accumulating, or start a new
argument. ‘M--’ starts a negative argument.
‘universal-argument ()’
This is another way to specify an argument. If this command is
followed by one or more digits, optionally with a leading minus
sign, those digits define the argument. If the command is followed
by digits, executing ‘universal-argument’ again ends the numeric
argument, but is otherwise ignored. As a special case, if this
command is immediately followed by a character that is neither a
digit nor minus sign, the argument count for the next command is
multiplied by four. The argument count is initially one, so
executing this function the first time makes the argument count
four, a second time makes the argument count sixteen, and so on.
By default, this is not bound to a key.
File: bash.info, Node: Commands For Completion, Next: Keyboard Macros, Prev: Numeric Arguments, Up: Bindable Readline Commands
8.4.6 Letting Readline Type For You
-----------------------------------
‘complete (<TAB>)’
Attempt to perform completion on the text before point. The actual
completion performed is application-specific. Bash attempts
completion by first checking for any programmable completions for
the command word (*note Programmable Completion::), otherwise
treating the text as a variable (if the text begins with ‘$’),
username (if the text begins with ‘~’), hostname (if the text
begins with ‘@’), or command (including aliases, functions, and
builtins) in turn. If none of these produces a match, it falls
back to filename completion.
‘possible-completions (M-?)’
List the possible completions of the text before point. When
displaying completions, Readline sets the number of columns used
for display to the value of ‘completion-display-width’, the value
of the environment variable ‘COLUMNS’, or the screen width, in that
order.
‘insert-completions (M-*)’
Insert all completions of the text before point that would have
been generated by ‘possible-completions’, separated by a space.
‘menu-complete ()’
Similar to ‘complete’, but replaces the word to be completed with a
single match from the list of possible completions. Repeatedly
executing ‘menu-complete’ steps through the list of possible
completions, inserting each match in turn. At the end of the list
of completions, ‘menu-complete’ rings the bell (subject to the
setting of ‘bell-style’) and restores the original text. An
argument of N moves N positions forward in the list of matches; a
negative argument moves backward through the list. This command is
intended to be bound to <TAB>, but is unbound by default.
‘menu-complete-backward ()’
Identical to ‘menu-complete’, but moves backward through the list
of possible completions, as if ‘menu-complete’ had been given a
negative argument. This command is unbound by default.
‘export-completions ()’
Perform completion on the word before point as described above and
write the list of possible completions to Readline's output stream
using the following format, writing information on separate lines:
• the number of matches N;
• the word being completed;
• S:E, where S and E are the start and end offsets of the word
in the Readline line buffer; then
• each match, one per line
If there are no matches, the first line will be "0", and this
command does not print any output after the S:E. If there is only
a single match, this prints a single line containing it. If there
is more than one match, this prints the common prefix of the
matches, which may be empty, on the first line after the S:E, then
the matches on subsequent lines. In this case, N will include the
first line with the common prefix.
The user or application should be able to accommodate the
possibility of a blank line. The intent is that the user or
application reads N lines after the line containing S:E to obtain
the match list. This command is unbound by default.
‘delete-char-or-list ()’
Deletes the character under the cursor if not at the beginning or
end of the line (like ‘delete-char’). At the end of the line, it
behaves identically to ‘possible-completions’. This command is
unbound by default.
‘complete-filename (M-/)’
Attempt filename completion on the text before point.
‘possible-filename-completions (C-x /)’
List the possible completions of the text before point, treating it
as a filename.
‘complete-username (M-~)’
Attempt completion on the text before point, treating it as a
username.
‘possible-username-completions (C-x ~)’
List the possible completions of the text before point, treating it
as a username.
‘complete-variable (M-$)’
Attempt completion on the text before point, treating it as a shell
variable.
‘possible-variable-completions (C-x $)’
List the possible completions of the text before point, treating it
as a shell variable.
‘complete-hostname (M-@)’
Attempt completion on the text before point, treating it as a
hostname.
‘possible-hostname-completions (C-x @)’
List the possible completions of the text before point, treating it
as a hostname.
‘complete-command (M-!)’
Attempt completion on the text before point, treating it as a
command name. Command completion attempts to match the text
against aliases, reserved words, shell functions, shell builtins,
and finally executable filenames, in that order.
‘possible-command-completions (C-x !)’
List the possible completions of the text before point, treating it
as a command name.
‘dynamic-complete-history (M-<TAB>)’
Attempt completion on the text before point, comparing the text
against history list entries for possible completion matches.
‘dabbrev-expand ()’
Attempt menu completion on the text before point, comparing the
text against lines from the history list for possible completion
matches.
‘complete-into-braces (M-{)’
Perform filename completion and insert the list of possible
completions enclosed within braces so the list is available to the
shell (*note Brace Expansion::).
File: bash.info, Node: Keyboard Macros, Next: Miscellaneous Commands, Prev: Commands For Completion, Up: Bindable Readline Commands
8.4.7 Keyboard Macros
---------------------
‘start-kbd-macro (C-x ()’
Begin saving the characters typed into the current keyboard macro.
‘end-kbd-macro (C-x ))’
Stop saving the characters typed into the current keyboard macro
and save the definition.
‘call-last-kbd-macro (C-x e)’
Re-execute the last keyboard macro defined, by making the
characters in the macro appear as if typed at the keyboard.
‘print-last-kbd-macro ()’
Print the last keyboard macro defined in a format suitable for the
INPUTRC file.
File: bash.info, Node: Miscellaneous Commands, Prev: Keyboard Macros, Up: Bindable Readline Commands
8.4.8 Some Miscellaneous Commands
---------------------------------
‘re-read-init-file (C-x C-r)’
Read in the contents of the INPUTRC file, and incorporate any
bindings or variable assignments found there.
‘abort (C-g)’
Abort the current editing command and ring the terminal's bell
(subject to the setting of ‘bell-style’).
‘do-lowercase-version (M-A, M-B, M-X, ...)’
If the metafied character X is upper case, run the command that is
bound to the corresponding metafied lower case character. The
behavior is undefined if X is already lower case.
‘prefix-meta (<ESC>)’
Metafy the next character typed. Typing ‘<ESC> f’ is equivalent to
typing ‘M-f’.
‘undo (C-_ or C-x C-u)’
Incremental undo, separately remembered for each line.
‘revert-line (M-r)’
Undo all changes made to this line. This is like executing the
‘undo’ command enough times to get back to the initial state.
‘tilde-expand (M-&)’
Perform tilde expansion on the current word.
‘set-mark (C-@)’
Set the mark to the point. If a numeric argument is supplied, set
the mark to that position.
‘exchange-point-and-mark (C-x C-x)’
Swap the point with the mark. Set the current cursor position to
the saved position, then set the mark to the old cursor position.
‘character-search (C-])’
Read a character and move point to the next occurrence of that
character. A negative argument searches for previous occurrences.
‘character-search-backward (M-C-])’
Read a character and move point to the previous occurrence of that
character. A negative argument searches for subsequent
occurrences.
‘skip-csi-sequence ()’
Read enough characters to consume a multi-key sequence such as
those defined for keys like Home and End. CSI sequences begin with
a Control Sequence Indicator (CSI), usually ‘ESC [’. If this
sequence is bound to "\e[", keys producing CSI sequences have no
effect unless explicitly bound to a Readline command, instead of
inserting stray characters into the editing buffer. This is
unbound by default, but usually bound to ‘ESC [’.
‘insert-comment (M-#)’
Without a numeric argument, insert the value of the ‘comment-begin’
variable at the beginning of the current line. If a numeric
argument is supplied, this command acts as a toggle: if the
characters at the beginning of the line do not match the value of
‘comment-begin’, insert the value; otherwise delete the characters
in ‘comment-begin’ from the beginning of the line. In either case,
the line is accepted as if a newline had been typed. The default
value of ‘comment-begin’ causes this command to make the current
line a shell comment. If a numeric argument causes the comment
character to be removed, the line will be executed by the shell.
‘dump-functions ()’
Print all of the functions and their key bindings to the Readline
output stream. If a numeric argument is supplied, the output is
formatted in such a way that it can be made part of an INPUTRC
file. This command is unbound by default.
‘dump-variables ()’
Print all of the settable variables and their values to the
Readline output stream. If a numeric argument is supplied, the
output is formatted in such a way that it can be made part of an
INPUTRC file. This command is unbound by default.
‘dump-macros ()’
Print all of the Readline key sequences bound to macros and the
strings they output to the Readline output stream. If a numeric
argument is supplied, the output is formatted in such a way that it
can be made part of an INPUTRC file. This command is unbound by
default.
‘execute-named-command (M-x)’
Read a bindable Readline command name from the input and execute
the function to which it's bound, as if the key sequence to which
it was bound appeared in the input. If this function is supplied
with a numeric argument, it passes that argument to the function it
executes.
‘spell-correct-word (C-x s)’
Perform spelling correction on the current word, treating it as a
directory or filename, in the same way as the ‘cdspell’ shell
option. Word boundaries are the same as those used by
‘shell-forward-word’.
‘glob-complete-word (M-g)’
Treat the word before point as a pattern for pathname expansion,
with an asterisk implicitly appended, then use the pattern to
generate a list of matching file names for possible completions.
‘glob-expand-word (C-x *)’
Treat the word before point as a pattern for pathname expansion,
and insert the list of matching file names, replacing the word. If
a numeric argument is supplied, append a ‘*’ before pathname
expansion.
‘glob-list-expansions (C-x g)’
Display the list of expansions that would have been generated by
‘glob-expand-word’, and redisplay the line. If a numeric argument
is supplied, append a ‘*’ before pathname expansion.
‘shell-expand-line (M-C-e)’
Expand the line by performing shell word expansions. This performs
alias and history expansion, $'STRING' and $"STRING" quoting, tilde
expansion, parameter and variable expansion, arithmetic expansion,
command and process substitution, word splitting, and quote
removal. An explicit argument suppresses command and process
substitution.
‘history-expand-line (M-^)’
Perform history expansion on the current line.
‘magic-space ()’
Perform history expansion on the current line and insert a space
(*note History Interaction::).
‘alias-expand-line ()’
Perform alias expansion on the current line (*note Aliases::).
‘history-and-alias-expand-line ()’
Perform history and alias expansion on the current line.
‘insert-last-argument (M-. or M-_)’
A synonym for ‘yank-last-arg’.
‘edit-and-execute-command (C-x C-e)’
Invoke an editor on the current command line, and execute the
result as shell commands. Bash attempts to invoke ‘$VISUAL’,
‘$EDITOR’, and ‘emacs’ as the editor, in that order.
‘display-shell-version (C-x C-v)’
Display version information about the current instance of Bash.
File: bash.info, Node: Readline vi Mode, Next: Programmable Completion, Prev: Bindable Readline Commands, Up: Command Line Editing
8.5 Readline vi Mode
====================
While the Readline library does not have a full set of ‘vi’ editing
functions, it does contain enough to allow simple editing of the line.
The Readline ‘vi’ mode behaves as specified in the ‘sh’ description in
the POSIX standard.
You can use the ‘set -o emacs’ and ‘set -o vi’ commands (*note The
Set Builtin::) to switch interactively between ‘emacs’ and ‘vi’ editing
modes, The Readline default is ‘emacs’ mode.
When you enter a line in ‘vi’ mode, you are already placed in
'insertion' mode, as if you had typed an ‘i’. Pressing <ESC> switches
you into 'command' mode, where you can edit the text of the line with
the standard ‘vi’ movement keys, move to previous history lines with ‘k’
and subsequent lines with ‘j’, and so forth.
File: bash.info, Node: Programmable Completion, Next: Programmable Completion Builtins, Prev: Readline vi Mode, Up: Command Line Editing
8.6 Programmable Completion
===========================
When the user attempts word completion for a command or an argument to a
command for which a completion specification (a “compspec”) has been
defined using the ‘complete’ builtin (*note Programmable Completion
Builtins::), Readline invokes the programmable completion facilities.
First, Bash identifies the command name. If a compspec has been
defined for that command, the compspec is used to generate the list of
possible completions for the word. If the command word is the empty
string (completion attempted at the beginning of an empty line), Bash
uses any compspec defined with the ‘-E’ option to ‘complete’. The ‘-I’
option to ‘complete’ indicates that the command word is the first
non-assignment word on the line, or after a command delimiter such as
‘;’ or ‘|’. This usually indicates command name completion.
If the command word is a full pathname, Bash searches for a compspec
for the full pathname first. If there is no compspec for the full
pathname, Bash attempts to find a compspec for the portion following the
final slash. If those searches do not result in a compspec, or if there
is no compspec for the command word, Bash uses any compspec defined with
the ‘-D’ option to ‘complete’ as the default. If there is no default
compspec, Bash performs alias expansion on the command word as a final
resort, and attempts to find a compspec for the command word resulting
from any successful expansion.
If a compspec is not found, Bash performs its default completion
described above (*note Commands For Completion::). Otherwise, once a
compspec has been found, Bash uses it to generate the list of matching
words.
First, Bash performs the ACTIONS specified by the compspec. This
only returns matches which are prefixes of the word being completed.
When the ‘-f’ or ‘-d’ option is used for filename or directory name
completion, Bash uses shell the variable ‘FIGNORE’ to filter the
matches. *Note Bash Variables::, for a description of ‘FIGNORE’.
Next, programmable completion generates matches specified by a
pathname expansion pattern supplied as an argument to the ‘-G’ option.
The words generated by the pattern need not match the word being
completed. Bash uses the ‘FIGNORE’ variable to filter the matches, but
does not use the ‘GLOBIGNORE’ shell variable.
Next, completion considers the string specified as the argument to
the ‘-W’ option. The string is first split using the characters in the
‘IFS’ special variable as delimiters. This honors shell quoting within
the string, in order to provide a mechanism for the words to contain
shell metacharacters or characters in the value of ‘IFS’. Each word is
then expanded using brace expansion, tilde expansion, parameter and
variable expansion, command substitution, and arithmetic expansion, as
described above (*note Shell Expansions::). The results are split using
the rules described above (*note Word Splitting::). The results of the
expansion are prefix-matched against the word being completed, and the
matching words become possible completions.
After these matches have been generated, Bash executes any shell
function or command specified with the ‘-F’ and ‘-C’ options. When the
command or function is invoked, Bash assigns values to the ‘COMP_LINE’,
‘COMP_POINT’, ‘COMP_KEY’, and ‘COMP_TYPE’ variables as described above
(*note Bash Variables::). If a shell function is being invoked, Bash
also sets the ‘COMP_WORDS’ and ‘COMP_CWORD’ variables. When the
function or command is invoked, the first argument ($1) is the name of
the command whose arguments are being completed, the second argument
($2) is the word being completed, and the third argument ($3) is the
word preceding the word being completed on the current command line.
There is no filtering of the generated completions against the word
being completed; the function or command has complete freedom in
generating the matches and they do not need to match a prefix of the
word.
Any function specified with ‘-F’ is invoked first. The function may
use any of the shell facilities, including the ‘compgen’ and ‘compopt’
builtins described below (*note Programmable Completion Builtins::), to
generate the matches. It must put the possible completions in the
‘COMPREPLY’ array variable, one per array element.
Next, any command specified with the ‘-C’ option is invoked in an
environment equivalent to command substitution. It should print a list
of completions, one per line, to the standard output. Backslash will
escape a newline, if necessary. These are added to the set of possible
completions.
After generating all of the possible completions, Bash applies any
filter specified with the ‘-X’ option to the completions in the list.
The filter is a pattern as used for pathname expansion; a ‘&’ in the
pattern is replaced with the text of the word being completed. A
literal ‘&’ may be escaped with a backslash; the backslash is removed
before attempting a match. Any completion that matches the pattern is
removed from the list. A leading ‘!’ negates the pattern; in this case
Bash removes any completion that does not match the pattern. If the
‘nocasematch’ shell option is enabled (see the description of ‘shopt’ in
*note The Shopt Builtin::), Bash performs the match without regard to
the case of alphabetic characters.
Finally, programmable completion adds any prefix and suffix specified
with the ‘-P’ and ‘-S’ options, respectively, to each completion, and
returns the result to Readline as the list of possible completions.
If the previously-applied actions do not generate any matches, and
the ‘-o dirnames’ option was supplied to ‘complete’ when the compspec
was defined, Bash attempts directory name completion.
If the ‘-o plusdirs’ option was supplied to ‘complete’ when the
compspec was defined, Bash attempts directory name completion and adds
any matches to the set of possible completions.
By default, if a compspec is found, whatever it generates is returned
to the completion code as the full set of possible completions. The
default Bash completions and the Readline default of filename completion
are disabled. If the ‘-o bashdefault’ option was supplied to ‘complete’
when the compspec was defined, and the compspec generates no matches,
Bash attempts its default completions. If the compspec and, if
attempted, the default Bash completions generate no matches, and the ‘-o
default’ option was supplied to ‘complete’ when the compspec was
defined, programmable completion performs Readline's default completion.
The options supplied to ‘complete’ and ‘compopt’ can control how
Readline treats the completions. For instance, the ‘-o fullquote’
option tells Readline to quote the matches as if they were filenames.
See the description of ‘complete’ (*note Programmable Completion
Builtins::) for details.
When a compspec indicates that it wants directory name completion,
the programmable completion functions force Readline to append a slash
to completed names which are symbolic links to directories, subject to
the value of the MARK-DIRECTORIES Readline variable, regardless of the
setting of the MARK-SYMLINKED-DIRECTORIES Readline variable.
There is some support for dynamically modifying completions. This is
most useful when used in combination with a default completion specified
with ‘-D’. It's possible for shell functions executed as completion
functions to indicate that completion should be retried by returning an
exit status of 124. If a shell function returns 124, and changes the
compspec associated with the command on which completion is being
attempted (supplied as the first argument when the function is
executed), programmable completion restarts from the beginning, with an
attempt to find a new compspec for that command. This can be used to
build a set of completions dynamically as completion is attempted,
rather than loading them all at once.
For instance, assuming that there is a library of compspecs, each
kept in a file corresponding to the name of the command, the following
default completion function would load completions dynamically:
_completion_loader()
{
. "/etc/bash_completion.d/$1.sh" >/dev/null 2>&1 && return 124
}
complete -D -F _completion_loader -o bashdefault -o default
File: bash.info, Node: Programmable Completion Builtins, Next: A Programmable Completion Example, Prev: Programmable Completion, Up: Command Line Editing
8.7 Programmable Completion Builtins
====================================
Three builtin commands are available to manipulate the programmable
completion facilities: one to specify how the arguments to a particular
command are to be completed, and two to modify the completion as it is
happening.
‘compgen’
compgen [-V VARNAME] [OPTION] [WORD]
Generate possible completion matches for WORD according to the
OPTIONs, which may be any option accepted by the ‘complete’ builtin
with the exceptions of ‘-p’, ‘-r’, ‘-D’, ‘-E’, and ‘-I’, and write
the matches to the standard output.
If the ‘-V’ option is supplied, ‘compgen’ stores the generated
completions into the indexed array variable VARNAME instead of
writing them to the standard output.
When using the ‘-F’ or ‘-C’ options, the various shell variables
set by the programmable completion facilities, while available,
will not have useful values.
The matches will be generated in the same way as if the
programmable completion code had generated them directly from a
completion specification with the same flags. If WORD is
specified, only those completions matching WORD will be displayed
or stored.
The return value is true unless an invalid option is supplied, or
no matches were generated.
‘complete’
complete [-abcdefgjksuv] [-o COMP-OPTION] [-DEI] [-A ACTION]
[-G GLOBPAT] [-W WORDLIST] [-F FUNCTION] [-C COMMAND]
[-X FILTERPAT] [-P PREFIX] [-S SUFFIX] NAME [NAME ...]
complete -pr [-DEI] [NAME ...]
Specify how arguments to each NAME should be completed.
If the ‘-p’ option is supplied, or if no options or NAMEs are
supplied, print existing completion specifications in a way that
allows them to be reused as input. The ‘-r’ option removes a
completion specification for each NAME, or, if no NAMEs are
supplied, all completion specifications.
The ‘-D’ option indicates that other supplied options and actions
should apply to the "default" command completion; that is,
completion attempted on a command for which no completion has
previously been defined. The ‘-E’ option indicates that other
supplied options and actions should apply to "empty" command
completion; that is, completion attempted on a blank line. The
‘-I’ option indicates that other supplied options and actions
should apply to completion on the initial non-assignment word on
the line, or after a command delimiter such as ‘;’ or ‘|’, which is
usually command name completion. If multiple options are supplied,
the ‘-D’ option takes precedence over ‘-E’, and both take
precedence over ‘-I’. If any of ‘-D’, ‘-E’, or ‘-I’ are supplied,
any other NAME arguments are ignored; these completions only apply
to the case specified by the option.
The process of applying these completion specifications when word
completion is attempted is described above (*note Programmable
Completion::).
Other options, if specified, have the following meanings. The
arguments to the ‘-G’, ‘-W’, and ‘-X’ options (and, if necessary,
the ‘-P’ and ‘-S’ options) should be quoted to protect them from
expansion before the ‘complete’ builtin is invoked.
‘-o COMP-OPTION’
The COMP-OPTION controls several aspects of the compspec's
behavior beyond the simple generation of completions.
COMP-OPTION may be one of:
‘bashdefault’
Perform the rest of the default Bash completions if the
compspec generates no matches.
‘default’
Use Readline's default filename completion if the
compspec generates no matches.
‘dirnames’
Perform directory name completion if the compspec
generates no matches.
‘filenames’
Tell Readline that the compspec generates filenames, so
it can perform any filename-specific processing (such as
adding a slash to directory names, quoting special
characters, or suppressing trailing spaces). This option
is intended to be used with shell functions specified
with ‘-F’.
‘fullquote’
Tell Readline to quote all the completed words even if
they are not filenames.
‘noquote’
Tell Readline not to quote the completed words if they
are filenames (quoting filenames is the default).
‘nosort’
Tell Readline not to sort the list of possible
completions alphabetically.
‘nospace’
Tell Readline not to append a space (the default) to
words completed at the end of the line.
‘plusdirs’
After generating any matches defined by the compspec,
attempt directory name completion and add any matches to
the results of the other actions.
‘-A ACTION’
The ACTION may be one of the following to generate a list of
possible completions:
‘alias’
Alias names. May also be specified as ‘-a’.
‘arrayvar’
Array variable names.
‘binding’
Readline key binding names (*note Bindable Readline
Commands::).
‘builtin’
Names of shell builtin commands. May also be specified
as ‘-b’.
‘command’
Command names. May also be specified as ‘-c’.
‘directory’
Directory names. May also be specified as ‘-d’.
‘disabled’
Names of disabled shell builtins.
‘enabled’
Names of enabled shell builtins.
‘export’
Names of exported shell variables. May also be specified
as ‘-e’.
‘file’
File and directory names, similar to Readline's filename
completion. May also be specified as ‘-f’.
‘function’
Names of shell functions.
‘group’
Group names. May also be specified as ‘-g’.
‘helptopic’
Help topics as accepted by the ‘help’ builtin (*note Bash
Builtins::).
‘hostname’
Hostnames, as taken from the file specified by the
‘HOSTFILE’ shell variable (*note Bash Variables::).
‘job’
Job names, if job control is active. May also be
specified as ‘-j’.
‘keyword’
Shell reserved words. May also be specified as ‘-k’.
‘running’
Names of running jobs, if job control is active.
‘service’
Service names. May also be specified as ‘-s’.
‘setopt’
Valid arguments for the ‘-o’ option to the ‘set’ builtin
(*note The Set Builtin::).
‘shopt’
Shell option names as accepted by the ‘shopt’ builtin
(*note Bash Builtins::).
‘signal’
Signal names.
‘stopped’
Names of stopped jobs, if job control is active.
‘user’
User names. May also be specified as ‘-u’.
‘variable’
Names of all shell variables. May also be specified as
‘-v’.
‘-C COMMAND’
COMMAND is executed in a subshell environment, and its output
is used as the possible completions. Arguments are passed as
with the ‘-F’ option.
‘-F FUNCTION’
The shell function FUNCTION is executed in the current shell
environment. When it is executed, the first argument ($1) is
the name of the command whose arguments are being completed,
the second argument ($2) is the word being completed, and the
third argument ($3) is the word preceding the word being
completed, as described above (*note Programmable
Completion::). When ‘function’ finishes, programmable
completion retrieves the possible completions from the value
of the ‘COMPREPLY’ array variable.
‘-G GLOBPAT’
Expand the filename expansion pattern GLOBPAT to generate the
possible completions.
‘-P PREFIX’
Add PREFIX to the beginning of each possible completion after
all other options have been applied.
‘-S SUFFIX’
Append SUFFIX to each possible completion after all other
options have been applied.
‘-W WORDLIST’
Split the WORDLIST using the characters in the ‘IFS’ special
variable as delimiters, and expand each resulting word. Shell
quoting is honored within WORDLIST in order to provide a
mechanism for the words to contain shell metacharacters or
characters in the value of ‘IFS’. The possible completions
are the members of the resultant list which match a prefix of
the word being completed.
‘-X FILTERPAT’
FILTERPAT is a pattern as used for filename expansion. It is
applied to the list of possible completions generated by the
preceding options and arguments, and each completion matching
FILTERPAT is removed from the list. A leading ‘!’ in
FILTERPAT negates the pattern; in this case, any completion
not matching FILTERPAT is removed.
The return value is true unless an invalid option is supplied, an
option other than ‘-p’, ‘-r’, ‘-D’, ‘-E’, or ‘-I’ is supplied
without a NAME argument, an attempt is made to remove a completion
specification for a NAME for which no specification exists, or an
error occurs adding a completion specification.
‘compopt’
compopt [-o OPTION] [-DEI] [+o OPTION] [NAME]
Modify completion options for each NAME according to the OPTIONs,
or for the currently-executing completion if no NAMEs are supplied.
If no OPTIONs are given, display the completion options for each
NAME or the current completion. The possible values of OPTION are
those valid for the ‘complete’ builtin described above.
The ‘-D’ option indicates that other supplied options should apply
to the "default" command completion; the ‘-E’ option indicates that
other supplied options should apply to "empty" command completion;
and the ‘-I’ option indicates that other supplied options should
apply to completion on the initial word on the line. These are
determined in the same way as the ‘complete’ builtin.
If multiple options are supplied, the ‘-D’ option takes precedence
over ‘-E’, and both take precedence over ‘-I’
The return value is true unless an invalid option is supplied, an
attempt is made to modify the options for a NAME for which no
completion specification exists, or an output error occurs.
File: bash.info, Node: A Programmable Completion Example, Prev: Programmable Completion Builtins, Up: Command Line Editing
8.8 A Programmable Completion Example
=====================================
The most common way to obtain additional completion functionality beyond
the default actions ‘complete’ and ‘compgen’ provide is to use a shell
function and bind it to a particular command using ‘complete -F’.
The following function provides completions for the ‘cd’ builtin. It
is a reasonably good example of what shell functions must do when used
for completion. This function uses the word passed as ‘$2’ to determine
the directory name to complete. You can also use the ‘COMP_WORDS’ array
variable; the current word is indexed by the ‘COMP_CWORD’ variable.
The function relies on the ‘complete’ and ‘compgen’ builtins to do
much of the work, adding only the things that the Bash ‘cd’ does beyond
accepting basic directory names: tilde expansion (*note Tilde
Expansion::), searching directories in $CDPATH, which is described above
(*note Bourne Shell Builtins::), and basic support for the ‘cdable_vars’
shell option (*note The Shopt Builtin::). ‘_comp_cd’ modifies the value
of IFS so that it contains only a newline to accommodate file names
containing spaces and tabs - ‘compgen’ prints the possible completions
it generates one per line.
Possible completions go into the COMPREPLY array variable, one
completion per array element. The programmable completion system
retrieves the completions from there when the function returns.
# A completion function for the cd builtin
# based on the cd completion function from the bash_completion package
_comp_cd()
{
local IFS=$' \t\n' # normalize IFS
local cur _skipdot _cdpath
local i j k
# Tilde expansion, which also expands tilde to full pathname
case "$2" in
\~*) eval cur="$2" ;;
*) cur=$2 ;;
esac
# no cdpath or absolute pathname -- straight directory completion
if [[ -z "${CDPATH:-}" ]] || [[ "$cur" == @(./*|../*|/*) ]]; then
# compgen prints paths one per line; could also use while loop
IFS=$'\n'
COMPREPLY=( $(compgen -d -- "$cur") )
IFS=$' \t\n'
# CDPATH+directories in the current directory if not in CDPATH
else
IFS=$'\n'
_skipdot=false
# preprocess CDPATH to convert null directory names to .
_cdpath=${CDPATH/#:/.:}
_cdpath=${_cdpath//::/:.:}
_cdpath=${_cdpath/%:/:.}
for i in ${_cdpath//:/$'\n'}; do
if [[ $i -ef . ]]; then _skipdot=true; fi
k="${#COMPREPLY[@]}"
for j in $( compgen -d -- "$i/$cur" ); do
COMPREPLY[k++]=${j#$i/} # cut off directory
done
done
$_skipdot || COMPREPLY+=( $(compgen -d -- "$cur") )
IFS=$' \t\n'
fi
# variable names if appropriate shell option set and no completions
if shopt -q cdable_vars && [[ ${#COMPREPLY[@]} -eq 0 ]]; then
COMPREPLY=( $(compgen -v -- "$cur") )
fi
return 0
}
We install the completion function using the ‘-F’ option to
‘complete’:
# Tell readline to quote appropriate and append slashes to directories;
# use the bash default completion for other arguments
complete -o filenames -o nospace -o bashdefault -F _comp_cd cd
Since we'd like Bash and Readline to take care of some of the other
details for us, we use several other options to tell Bash and Readline
what to do. The ‘-o filenames’ option tells Readline that the possible
completions should be treated as filenames, and quoted appropriately.
That option will also cause Readline to append a slash to filenames it
can determine are directories (which is why we might want to extend
‘_comp_cd’ to append a slash if we're using directories found via
CDPATH: Readline can't tell those completions are directories). The ‘-o
nospace’ option tells Readline to not append a space character to the
directory name, in case we want to append to it. The ‘-o bashdefault’
option brings in the rest of the "Bash default" completions - possible
completions that Bash adds to the default Readline set. These include
things like command name completion, variable completion for words
beginning with ‘$’ or ‘${’, completions containing pathname expansion
patterns (*note Filename Expansion::), and so on.
Once installed using ‘complete’, ‘_comp_cd’ will be called every time
we attempt word completion for a ‘cd’ command.
Many more examples - an extensive collection of completions for most
of the common GNU, Unix, and Linux commands - are available as part of
the bash_completion project. This is installed by default on many
GNU/Linux distributions. Originally written by Ian Macdonald, the
project now lives at <https://github.com/scop/bash-completion/>. There
are ports for other systems such as Solaris and Mac OS X.
An older version of the bash_completion package is distributed with
bash in the ‘examples/complete’ subdirectory.
File: bash.info, Node: Using History Interactively, Next: Installing Bash, Prev: Command Line Editing, Up: Top
9 Using History Interactively
*****************************
This chapter describes how to use the GNU History Library interactively,
from a user's standpoint. It should be considered a user's guide. For
information on using the GNU History Library in other programs, see the
GNU Readline Library Manual.
* Menu:
* Bash History Facilities:: How Bash lets you manipulate your command
history.
* Bash History Builtins:: The Bash builtin commands that manipulate
the command history.
* History Interaction:: What it feels like using History as a user.
File: bash.info, Node: Bash History Facilities, Next: Bash History Builtins, Up: Using History Interactively
9.1 Bash History Facilities
===========================
When the ‘-o history’ option to the ‘set’ builtin is enabled (*note The
Set Builtin::), the shell provides access to the “command history”, the
list of commands previously typed. The value of the ‘HISTSIZE’ shell
variable is used as the number of commands to save in a history list:
the shell saves the text of the last ‘$HISTSIZE’ commands (default 500).
The shell stores each command in the history list prior to parameter and
variable expansion but after history expansion is performed, subject to
the values of the shell variables ‘HISTIGNORE’ and ‘HISTCONTROL’.
When the shell starts up, Bash initializes the history list by
reading history entries from the file named by the ‘HISTFILE’ variable
(default ‘~/.bash_history’). This is referred to as the “history file”.
The history file is truncated, if necessary, to contain no more than the
number of history entries specified by the value of the ‘HISTFILESIZE’
variable. If ‘HISTFILESIZE’ is unset, or set to null, a non-numeric
value, or a numeric value less than zero, the history file is not
truncated.
When the history file is read, lines beginning with the history
comment character followed immediately by a digit are interpreted as
timestamps for the following history entry. These timestamps are
optionally displayed depending on the value of the ‘HISTTIMEFORMAT’
variable (*note Bash Variables::). When present, history timestamps
delimit history entries, making multi-line entries possible.
When a shell with history enabled exits, Bash copies the last
‘$HISTSIZE’ entries from the history list to the file named by
‘$HISTFILE’. If the ‘histappend’ shell option is set (*note Bash
Builtins::), Bash appends the entries to the history file, otherwise it
overwrites the history file. If ‘HISTFILE’ is unset or null, or if the
history file is unwritable, the history is not saved. After saving the
history, Bash truncates the history file to contain no more than
‘$HISTFILESIZE’ lines as described above.
If the ‘HISTTIMEFORMAT’ variable is set, the shell writes the
timestamp information associated with each history entry to the history
file, marked with the history comment character, so timestamps are
preserved across shell sessions. When the history file is read, lines
beginning with the history comment character followed immediately by a
digit are interpreted as timestamps for the following history entry. As
above, when using ‘HISTTIMEFORMAT’, the timestamps delimit multi-line
history entries.
The ‘fc’ builtin command will list or edit and re-execute a portion
of the history list. The ‘history’ builtin can display or modify the
history list and manipulate the history file. When using command-line
editing, search commands are available in each editing mode that provide
access to the history list (*note Commands For History::).
The shell allows control over which commands are saved on the history
list. The ‘HISTCONTROL’ and ‘HISTIGNORE’ variables are used to save
only a subset of the commands entered. If the ‘cmdhist’ shell option is
enabled, the shell attempts to save each line of a multi-line command in
the same history entry, adding semicolons where necessary to preserve
syntactic correctness. The ‘lithist’ shell option modifies ‘cmdhist’ by
saving the command with embedded newlines instead of semicolons. The
‘shopt’ builtin is used to set these options. *Note The Shopt
Builtin::, for a description of ‘shopt’.
File: bash.info, Node: Bash History Builtins, Next: History Interaction, Prev: Bash History Facilities, Up: Using History Interactively
9.2 Bash History Builtins
=========================
Bash provides two builtin commands which manipulate the history list and
history file.
‘fc’
fc [-e ENAME] [-lnr] [FIRST] [LAST]
fc -s [PAT=REP] [COMMAND]
The first form selects a range of commands from FIRST to LAST from
the history list and displays or edits and re-executes them. Both
FIRST and LAST may be specified as a string (to locate the most
recent command beginning with that string) or as a number (an index
into the history list, where a negative number is used as an offset
from the current command number).
When listing, a FIRST or LAST of 0 is equivalent to -1 and -0 is
equivalent to the current command (usually the ‘fc’ command);
otherwise 0 is equivalent to -1 and -0 is invalid.
If LAST is not specified, it is set to the current command for
listing and to FIRST otherwise. If FIRST is not specified, it is
set to the previous command for editing and −16 for listing.
If the ‘-l’ flag is supplied, the commands are listed on standard
output. The ‘-n’ flag suppresses the command numbers when listing.
The ‘-r’ flag reverses the order of the listing.
Otherwise, ‘fc’ invokes the editor named by ENAME on a file
containing those commands. If ENAME is not supplied, ‘fc’ uses the
value of the following variable expansion:
‘${FCEDIT:-${EDITOR:-vi}}’. This says to use the value of the
‘FCEDIT’ variable if set, or the value of the ‘EDITOR’ variable if
that is set, or ‘vi’ if neither is set. When editing is complete,
‘fc’ reads the file of edited commands and echoes and executes
them.
In the second form, ‘fc’ re-executes COMMAND after replacing each
instance of PAT in the selected command with REP. COMMAND is
interpreted the same as FIRST above.
A useful alias to use with the ‘fc’ command is ‘r='fc -s'’, so that
typing ‘r cc’ runs the last command beginning with ‘cc’ and typing
‘r’ re-executes the last command (*note Aliases::).
If the first form is used, the return value is zero unless an
invalid option is encountered or FIRST or LAST specify history
lines out of range. When editing and re-executing a file of
commands, the return value is the value of the last command
executed or failure if an error occurs with the temporary file. If
the second form is used, the return status is that of the
re-executed command, unless COMMAND does not specify a valid
history entry, in which case ‘fc’ returns a non-zero status.
‘history’
history [N]
history -c
history -d OFFSET
history -d START-END
history [-anrw] [FILENAME]
history -ps ARG
With no options, display the history list with numbers. Entries
prefixed with a ‘*’ have been modified. An argument of N lists
only the last N entries. If the shell variable ‘HISTTIMEFORMAT’ is
set and not null, it is used as a format string for ‘strftime’(3)
to display the time stamp associated with each displayed history
entry. If ‘history’ uses ‘HISTTIMEFORMAT’, it does not print an
intervening space between the formatted time stamp and the history
entry.
Options, if supplied, have the following meanings:
‘-c’
Clear the history list. This may be combined with the other
options to replace the history list.
‘-d OFFSET’
Delete the history entry at position OFFSET. If OFFSET is
positive, it should be specified as it appears when the
history is displayed. If OFFSET is negative, it is
interpreted as relative to one greater than the last history
position, so negative indices count back from the end of the
history, and an index of ‘-1’ refers to the current ‘history
-d’ command.
‘-d START-END’
Delete the range of history entries between positions START
and END, inclusive. Positive and negative values for START
and END are interpreted as described above.
‘-a’
Append the "new" history lines to the history file. These are
history lines entered since the beginning of the current Bash
session, but not already appended to the history file.
‘-n’
Read the history lines not already read from the history file
and add them to the current history list. These are lines
appended to the history file since the beginning of the
current Bash session.
‘-r’
Read the history file and append its contents to the history
list.
‘-w’
Write the current history list to the history file,
overwriting the history file.
‘-p’
Perform history substitution on the ARGs and display the
result on the standard output, without storing the results in
the history list.
‘-s’
Add the ARGs to the end of the history list as a single entry.
The last command in the history list is removed before adding
the ARGs.
If a FILENAME argument is supplied with any of the ‘-w’, ‘-r’,
‘-a’, or ‘-n’ options, Bash uses FILENAME as the history file. If
not, it uses the value of the ‘HISTFILE’ variable. If ‘HISTFILE’
is unset or null, these options have no effect.
If the ‘HISTTIMEFORMAT’ variable is set, ‘history’ writes the time
stamp information associated with each history entry to the history
file, marked with the history comment character as described above.
When the history file is read, lines beginning with the history
comment character followed immediately by a digit are interpreted
as timestamps for the following history entry.
The return value is 0 unless an invalid option is encountered, an
error occurs while reading or writing the history file, an invalid
OFFSET or range is supplied as an argument to ‘-d’, or the history
expansion supplied as an argument to ‘-p’ fails.
File: bash.info, Node: History Interaction, Prev: Bash History Builtins, Up: Using History Interactively
9.3 History Expansion
=====================
The shell provides a history expansion feature that is similar to the
history expansion provided by ‘csh’ (also referred to as history
substitution where appropriate). This section describes the syntax used
to manipulate the history information.
History expansion is enabled by default for interactive shells, and
can be disabled using the ‘+H’ option to the ‘set’ builtin command
(*note The Set Builtin::). Non-interactive shells do not perform
history expansion by default, but it can be enabled with ‘set -H’.
History expansions introduce words from the history list into the
input stream, making it easy to repeat commands, insert the arguments to
a previous command into the current input line, or fix errors in
previous commands quickly.
History expansion is performed immediately after a complete line is
read, before the shell breaks it into words, and is performed on each
line individually. Bash attempts to inform the history expansion
functions about quoting still in effect from previous lines.
History expansion takes place in two parts. The first is to
determine which entry from the history list should be used during
substitution. The second is to select portions of that entry to include
into the current one.
The entry selected from the history is called the “event”, and the
portions of that entry that are acted upon are “words”. Various
“modifiers” are available to manipulate the selected words. The entry
is split into words in the same fashion that Bash does when reading
input, so that several words surrounded by quotes are considered one
word. The “event designator” selects the event, the optional “word
designator” selects words from the event, and various optional
“modifiers” are available to manipulate the selected words.
History expansions are introduced by the appearance of the history
expansion character, which is ‘!’ by default. History expansions may
appear anywhere in the input, but do not nest.
History expansion implements shell-like quoting conventions: a
backslash can be used to remove the special handling for the next
character; single quotes enclose verbatim sequences of characters, and
can be used to inhibit history expansion; and characters enclosed within
double quotes may be subject to history expansion, since backslash can
escape the history expansion character, but single quotes may not, since
they are not treated specially within double quotes.
When using the shell, only ‘\’ and ‘'’ may be used to escape the
history expansion character, but the history expansion character is also
treated as quoted if it immediately precedes the closing double quote in
a double-quoted string.
Several characters inhibit history expansion if found immediately
following the history expansion character, even if it is unquoted:
space, tab, newline, carriage return, ‘=’, and the other shell
metacharacters.
There is a special abbreviation for substitution, active when the
QUICK SUBSTITUTION character (described above under ‘histchars’) is the
first character on the line. It selects the previous history list
entry, using an event designator equivalent to ‘!!’, and substitutes one
string for another in that entry. It is described below (*note Event
Designators::). This is the only history expansion that does not begin
with the history expansion character.
Several shell options settable with the ‘shopt’ builtin (*note The
Shopt Builtin::) modify history expansion behavior If the ‘histverify’
shell option is enabled, and Readline is being used, history
substitutions are not immediately passed to the shell parser. Instead,
the expanded line is reloaded into the Readline editing buffer for
further modification. If Readline is being used, and the ‘histreedit’
shell option is enabled, a failed history expansion is reloaded into the
Readline editing buffer for correction.
The ‘-p’ option to the ‘history’ builtin command shows what a history
expansion will do before using it. The ‘-s’ option to the ‘history’
builtin may be used to add commands to the end of the history list
without actually executing them, so that they are available for
subsequent recall. This is most useful in conjunction with Readline.
The shell allows control of the various characters used by the
history expansion mechanism with the ‘histchars’ variable, as explained
above (*note Bash Variables::). The shell uses the history comment
character to mark history timestamps when writing the history file.
* Menu:
* Event Designators:: How to specify which history line to use.
* Word Designators:: Specifying which words are of interest.
* Modifiers:: Modifying the results of substitution.
File: bash.info, Node: Event Designators, Next: Word Designators, Up: History Interaction
9.3.1 Event Designators
-----------------------
An event designator is a reference to an entry in the history list. The
event designator consists of the portion of the word beginning with the
history expansion character, and ending with the word designator if one
is present, or the end of the word. Unless the reference is absolute,
events are relative to the current position in the history list.
‘!’
Start a history substitution, except when followed by a space, tab,
the end of the line, ‘=’, or the rest of the shell metacharacters
defined above (*note Definitions::).
‘!N’
Refer to history list entry N.
‘!-N’
Refer to the history entry minus N.
‘!!’
Refer to the previous entry. This is a synonym for ‘!-1’.
‘!STRING’
Refer to the most recent command preceding the current position in
the history list starting with STRING.
‘!?STRING[?]’
Refer to the most recent command preceding the current position in
the history list containing STRING. The trailing ‘?’ may be
omitted if the STRING is followed immediately by a newline. If
STRING is missing, this uses the string from the most recent
search; it is an error if there is no previous search string.
‘^STRING1^STRING2^’
Quick Substitution. Repeat the last command, replacing STRING1
with STRING2. Equivalent to ‘!!:s^STRING1^STRING2^’.
‘!#’
The entire command line typed so far.
File: bash.info, Node: Word Designators, Next: Modifiers, Prev: Event Designators, Up: History Interaction
9.3.2 Word Designators
----------------------
Word designators are used to select desired words from the event. They
are optional; if the word designator isn't supplied, the history
expansion uses the entire event. A ‘:’ separates the event
specification from the word designator. It may be omitted if the word
designator begins with a ‘^’, ‘$’, ‘*’, ‘-’, or ‘%’. Words are numbered
from the beginning of the line, with the first word being denoted by 0
(zero). That first word is usually the command word, and the arguments
begin with the second word. Words are inserted into the current line
separated by single spaces.
For example,
‘!!’
designates the preceding command. When you type this, the
preceding command is repeated in toto.
‘!!:$’
designates the last word of the preceding command. This may be
shortened to ‘!$’.
‘!fi:2’
designates the second argument of the most recent command starting
with the letters ‘fi’.
Here are the word designators:
‘0 (zero)’
The ‘0’th word. For the shell, and many other, applications, this
is the command word.
‘N’
The Nth word.
‘^’
The first argument: word 1.
‘$’
The last word. This is usually the last argument, but expands to
the zeroth word if there is only one word in the line.
‘%’
The first word matched by the most recent ‘?STRING?’ search, if the
search string begins with a character that is part of a word. By
default, searches begin at the end of each line and proceed to the
beginning, so the first word matched is the one closest to the end
of the line.
‘X-Y’
A range of words; ‘-Y’ abbreviates ‘0-Y’.
‘*’
All of the words, except the ‘0’th. This is a synonym for ‘1-$’.
It is not an error to use ‘*’ if there is just one word in the
event; it expands to the empty string in that case.
‘X*’
Abbreviates ‘X-$’.
‘X-’
Abbreviates ‘X-$’ like ‘X*’, but omits the last word. If ‘x’ is
missing, it defaults to 0.
If a word designator is supplied without an event specification, the
previous command is used as the event, equivalent to ‘!!’.
File: bash.info, Node: Modifiers, Prev: Word Designators, Up: History Interaction
9.3.3 Modifiers
---------------
After the optional word designator, you can add a sequence of one or
more of the following modifiers, each preceded by a ‘:’. These modify,
or edit, the word or words selected from the history event.
‘h’
Remove a trailing filename component, leaving only the head.
‘t’
Remove all leading filename components, leaving the tail.
‘r’
Remove a trailing suffix of the form ‘.SUFFIX’, leaving the
basename.
‘e’
Remove all but the trailing suffix.
‘p’
Print the new command but do not execute it.
‘q’
Quote the substituted words, escaping further substitutions.
‘x’
Quote the substituted words as with ‘q’, but break into words at
spaces, tabs, and newlines. The ‘q’ and ‘x’ modifiers are mutually
exclusive; expansion uses the last one supplied.
‘s/OLD/NEW/’
Substitute NEW for the first occurrence of OLD in the event line.
Any character may be used as the delimiter in place of ‘/’. The
delimiter may be quoted in OLD and NEW with a single backslash. If
‘&’ appears in NEW, it is replaced with OLD. A single backslash
quotes the ‘&’ in OLD and NEW. If OLD is null, it is set to the
last OLD substituted, or, if no previous history substitutions took
place, the last STRING in a !?STRING‘[?]’ search. If NEW is null,
each matching OLD is deleted. The final delimiter is optional if
it is the last character on the input line.
‘&’
Repeat the previous substitution.
‘g’
‘a’
Cause changes to be applied over the entire event line. This is
used in conjunction with ‘s’, as in ‘gs/OLD/NEW/’, or with ‘&’.
‘G’
Apply the following ‘s’ or ‘&’ modifier once to each word in the
event.
File: bash.info, Node: Installing Bash, Next: Reporting Bugs, Prev: Using History Interactively, Up: Top
10 Installing Bash
******************
This chapter provides basic instructions for installing Bash on the
various supported platforms. The distribution supports the GNU
operating systems, nearly every version of Unix, and several non-Unix
systems such as BeOS and Interix. Other independent ports exist for
Windows platforms.
* Menu:
* Basic Installation:: Installation instructions.
* Compilers and Options:: How to set special options for various
systems.
* Compiling For Multiple Architectures:: How to compile Bash for more
than one kind of system from
the same source tree.
* Installation Names:: How to set the various paths used by the installation.
* Specifying the System Type:: How to configure Bash for a particular system.
* Sharing Defaults:: How to share default configuration values among GNU
programs.
* Operation Controls:: Options recognized by the configuration program.
* Optional Features:: How to enable and disable optional features when
building Bash.
File: bash.info, Node: Basic Installation, Next: Compilers and Options, Up: Installing Bash
10.1 Basic Installation
=======================
These are installation instructions for Bash.
The simplest way to compile Bash is:
1. ‘cd’ to the directory containing the source code and type
‘./configure’ to configure Bash for your system. If you're using
‘csh’ on an old version of System V, you might need to type ‘sh
./configure’ instead to prevent ‘csh’ from trying to execute
‘configure’ itself.
Running ‘configure’ takes some time. While running, it prints
messages telling which features it is checking for.
2. Type ‘make’ to compile Bash and build the ‘bashbug’ bug reporting
script.
3. Optionally, type ‘make tests’ to run the Bash test suite.
4. Type ‘make install’ to install ‘bash’ and ‘bashbug’. This will
also install the manual pages and Info file, message translation
files, some supplemental documentation, a number of example
loadable builtin commands, and a set of header files for developing
loadable builtins. You may need additional privileges to install
‘bash’ to your desired destination, which may require ‘sudo make
install’. More information about controlling the locations where
‘bash’ and other files are installed is below (*note Installation
Names::).
The ‘configure’ shell script attempts to guess correct values for
various system-dependent variables used during compilation. It uses
those values to create a ‘Makefile’ in each directory of the package
(the top directory, the ‘builtins’, ‘doc’, ‘po’, and ‘support’
directories, each directory under ‘lib’, and several others). It also
creates a ‘config.h’ file containing system-dependent definitions.
Finally, it creates a shell script named ‘config.status’ that you can
run in the future to recreate the current configuration, a file
‘config.cache’ that saves the results of its tests to speed up
reconfiguring, and a file ‘config.log’ containing compiler output
(useful mainly for debugging ‘configure’). If at some point
‘config.cache’ contains results you don't want to keep, you may remove
or edit it.
To find out more about the options and arguments that the ‘configure’
script understands, type
bash-4.2$ ./configure --help
at the Bash prompt in your Bash source directory.
If you want to build Bash in a directory separate from the source
directory - to build for multiple architectures, for example - just use
the full path to the configure script. The following commands will
build Bash in a directory under ‘/usr/local/build’ from the source code
in ‘/usr/local/src/bash-4.4’:
mkdir /usr/local/build/bash-4.4
cd /usr/local/build/bash-4.4
bash /usr/local/src/bash-4.4/configure
make
See *note Compiling For Multiple Architectures:: for more information
about building in a directory separate from the source.
If you need to do unusual things to compile Bash, please try to
figure out how ‘configure’ could check whether or not to do them, and
mail diffs or instructions to <bash-maintainers@gnu.org> so they can be
considered for the next release.
The file ‘configure.ac’ is used to create ‘configure’ by a program
called Autoconf. You only need ‘configure.ac’ if you want to change it
or regenerate ‘configure’ using a newer version of Autoconf. If you do
this, make sure you are using Autoconf version 2.69 or newer.
You can remove the program binaries and object files from the source
code directory by typing ‘make clean’. To also remove the files that
‘configure’ created (so you can compile Bash for a different kind of
computer), type ‘make distclean’.
File: bash.info, Node: Compilers and Options, Next: Compiling For Multiple Architectures, Prev: Basic Installation, Up: Installing Bash
10.2 Compilers and Options
==========================
Some systems require unusual options for compilation or linking that the
‘configure’ script does not know about. You can give ‘configure’
initial values for variables by setting them in the environment. Using
a Bourne-compatible shell, you can do that on the command line like
this:
CC=c89 CFLAGS=-O2 LIBS=-lposix ./configure
On systems that have the ‘env’ program, you can do it like this:
env CPPFLAGS=-I/usr/local/include LDFLAGS=-s ./configure
The configuration process uses GCC to build Bash if it is available.
File: bash.info, Node: Compiling For Multiple Architectures, Next: Installation Names, Prev: Compilers and Options, Up: Installing Bash
10.3 Compiling For Multiple Architectures
=========================================
You can compile Bash for more than one kind of computer at the same
time, by placing the object files for each architecture in their own
directory. To do this, you must use a version of ‘make’ that supports
the ‘VPATH’ variable, such as GNU ‘make’. ‘cd’ to the directory where
you want the object files and executables to go and run the ‘configure’
script from the source directory (*note Basic Installation::). You may
need to supply the ‘--srcdir=PATH’ argument to tell ‘configure’ where
the source files are. ‘configure’ automatically checks for the source
code in the directory that ‘configure’ is in and in ‘..’.
If you have to use a ‘make’ that does not support the ‘VPATH’
variable, you can compile Bash for one architecture at a time in the
source code directory. After you have installed Bash for one
architecture, use ‘make distclean’ before reconfiguring for another
architecture.
Alternatively, if your system supports symbolic links, you can use
the ‘support/mkclone’ script to create a build tree which has symbolic
links back to each file in the source directory. Here's an example that
creates a build directory in the current directory from a source
directory ‘/usr/gnu/src/bash-2.0’:
bash /usr/gnu/src/bash-2.0/support/mkclone -s /usr/gnu/src/bash-2.0 .
The ‘mkclone’ script requires Bash, so you must have already built Bash
for at least one architecture before you can create build directories
for other architectures.
File: bash.info, Node: Installation Names, Next: Specifying the System Type, Prev: Compiling For Multiple Architectures, Up: Installing Bash
10.4 Installation Names
=======================
By default, ‘make install’ will install into ‘/usr/local/bin’,
‘/usr/local/man’, etc.; that is, the “installation prefix” defaults to
‘/usr/local’. You can specify an installation prefix other than
‘/usr/local’ by giving ‘configure’ the option ‘--prefix=PATH’, or by
specifying a value for the ‘prefix’ ‘make’ variable when running ‘make
install’ (e.g., ‘make install prefix=PATH’). The ‘prefix’ variable
provides a default for ‘exec_prefix’ and other variables used when
installing Bash.
You can specify separate installation prefixes for
architecture-specific files and architecture-independent files. If you
give ‘configure’ the option ‘--exec-prefix=PATH’, ‘make install’ will
use PATH as the prefix for installing programs and libraries.
Documentation and other data files will still use the regular prefix.
If you would like to change the installation locations for a single
run, you can specify these variables as arguments to ‘make’: ‘make
install exec_prefix=/’ will install ‘bash’ and ‘bashbug’ into ‘/bin’
instead of the default ‘/usr/local/bin’.
If you want to see the files Bash will install and where it will
install them without changing anything on your system, specify the
variable ‘DESTDIR’ as an argument to ‘make’. Its value should be the
absolute directory path you'd like to use as the root of your sample
installation tree. For example,
mkdir /fs1/bash-install
make install DESTDIR=/fs1/bash-install
will install ‘bash’ into ‘/fs1/bash-install/usr/local/bin/bash’, the
documentation into directories within
‘/fs1/bash-install/usr/local/share’, the example loadable builtins into
‘/fs1/bash-install/usr/local/lib/bash’, and so on. You can use the
usual ‘exec_prefix’ and ‘prefix’ variables to alter the directory paths
beneath the value of ‘DESTDIR’.
The GNU Makefile standards provide a more complete description of
these variables and their effects.
File: bash.info, Node: Specifying the System Type, Next: Sharing Defaults, Prev: Installation Names, Up: Installing Bash
10.5 Specifying the System Type
===============================
There may be some features ‘configure’ can not figure out automatically,
but needs to determine by the type of host Bash will run on. Usually
‘configure’ can figure that out, but if it prints a message saying it
can not guess the host type, give it the ‘--host=TYPE’ option. ‘TYPE’
can either be a short name for the system type, such as ‘sun4’, or a
canonical name with three fields: ‘CPU-COMPANY-SYSTEM’ (e.g.,
‘i386-unknown-freebsd4.2’).
See the file ‘support/config.sub’ for the possible values of each
field.
File: bash.info, Node: Sharing Defaults, Next: Operation Controls, Prev: Specifying the System Type, Up: Installing Bash
10.6 Sharing Defaults
=====================
If you want to set default values for ‘configure’ scripts to share, you
can create a site shell script called ‘config.site’ that gives default
values for variables like ‘CC’, ‘cache_file’, and ‘prefix’. ‘configure’
looks for ‘PREFIX/share/config.site’ if it exists, then
‘PREFIX/etc/config.site’ if it exists. Or, you can set the
‘CONFIG_SITE’ environment variable to the location of the site script.
A warning: the Bash ‘configure’ looks for a site script, but not all
‘configure’ scripts do.
File: bash.info, Node: Operation Controls, Next: Optional Features, Prev: Sharing Defaults, Up: Installing Bash
10.7 Operation Controls
=======================
‘configure’ recognizes the following options to control how it operates.
‘--cache-file=FILE’
Use and save the results of the tests in FILE instead of
‘./config.cache’. Set FILE to ‘/dev/null’ to disable caching, for
debugging ‘configure’.
‘--help’
Print a summary of the options to ‘configure’, and exit.
‘--quiet’
‘--silent’
‘-q’
Do not print messages saying which checks are being made.
‘--srcdir=DIR’
Look for the Bash source code in directory DIR. Usually
‘configure’ can determine that directory automatically.
‘--version’
Print the version of Autoconf used to generate the ‘configure’
script, and exit.
‘configure’ also accepts some other, not widely used, boilerplate
options. ‘configure --help’ prints the complete list.
File: bash.info, Node: Optional Features, Prev: Operation Controls, Up: Installing Bash
10.8 Optional Features
======================
The Bash ‘configure’ has a number of ‘--enable-FEATURE’ options, where
FEATURE indicates an optional part of Bash. There are also several
‘--with-PACKAGE’ options, where PACKAGE is something like ‘bash-malloc’
or ‘afs’. To turn off the default use of a package, use
‘--without-PACKAGE’. To configure Bash without a feature that is
enabled by default, use ‘--disable-FEATURE’.
Here is a complete list of the ‘--enable-’ and ‘--with-’ options that
the Bash ‘configure’ recognizes.
‘--with-afs’
Define if you are using the Andrew File System from Transarc.
‘--with-bash-malloc’
Use the Bash version of ‘malloc’ in the directory ‘lib/malloc’.
This is not the same ‘malloc’ that appears in GNU libc, but a
custom version originally derived from the 4.2 BSD ‘malloc’. This
‘malloc’ is very fast, but wastes some space on each allocation,
though it uses several techniques to minimize the waste. This
option is enabled by default. The ‘NOTES’ file contains a list of
systems for which this should be turned off, and ‘configure’
disables this option automatically for a number of systems.
‘--with-curses’
Use the curses library instead of the termcap library. ‘configure’
usually chooses this automatically, since most systems include the
termcap functions in the curses library.
‘--with-gnu-malloc’
A synonym for ‘--with-bash-malloc’.
‘--with-installed-readline[=PREFIX]’
Define this to make Bash link with a locally-installed version of
Readline rather than the version in ‘lib/readline’. This works
only with Readline 5.0 and later versions. If PREFIX is ‘yes’ or
not supplied, ‘configure’ uses the values of the make variables
‘includedir’ and ‘libdir’, which are subdirectories of ‘prefix’ by
default, to find the installed version of Readline if it is not in
the standard system include and library directories. If PREFIX is
‘no’, Bash links with the version in ‘lib/readline’. If PREFIX is
set to any other value, ‘configure’ treats it as a directory
pathname and looks for the installed version of Readline in
subdirectories of that directory (include files in PREFIX/‘include’
and the library in PREFIX/‘lib’). The Bash default is to link with
a static library built in the ‘lib/readline’ subdirectory of the
build directory.
‘--with-libintl-prefix[=PREFIX]’
Define this to make Bash link with a locally-installed version of
the libintl library instead of the version in ‘lib/intl’.
‘--with-libiconv-prefix[=PREFIX]’
Define this to make Bash look for libiconv in PREFIX instead of the
standard system locations. The Bash distribution does not include
this library.
‘--enable-minimal-config’
This produces a shell with minimal features, closer to the
historical Bourne shell.
There are several ‘--enable-’ options that alter how Bash is
compiled, linked, and installed, rather than changing run-time features.
‘--enable-largefile’
Enable support for large files
(http://www.unix.org/version2/whatsnew/lfs20mar.html) if the
operating system requires special compiler options to build
programs which can access large files. This is enabled by default,
if the operating system provides large file support.
‘--enable-profiling’
This builds a Bash binary that produces profiling information to be
processed by ‘gprof’ each time it is executed.
‘--enable-separate-helpfiles’
Use external files for the documentation displayed by the ‘help’
builtin instead of storing the text internally.
‘--enable-static-link’
This causes Bash to be linked statically, if ‘gcc’ is being used.
This could be used to build a version to use as root's shell.
The ‘minimal-config’ option can be used to disable all of the
following options, but it is processed first, so individual options may
be enabled using ‘enable-FEATURE’.
All of the following options except for ‘alt-array-implementation’,
‘disabled-builtins’, ‘direxpand-default’, ‘strict-posix-default’, and
‘xpg-echo-default’ are enabled by default, unless the operating system
does not provide the necessary support.
‘--enable-alias’
Allow alias expansion and include the ‘alias’ and ‘unalias’
builtins (*note Aliases::).
‘--enable-alt-array-implementation’
This builds Bash using an alternate implementation of arrays (*note
Arrays::) that provides faster access at the expense of using more
memory (sometimes many times more, depending on how sparse an array
is).
‘--enable-arith-for-command’
Include support for the alternate form of the ‘for’ command that
behaves like the C language ‘for’ statement (*note Looping
Constructs::).
‘--enable-array-variables’
Include support for one-dimensional array shell variables (*note
Arrays::).
‘--enable-bang-history’
Include support for ‘csh’-like history substitution (*note History
Interaction::).
‘--enable-bash-source-fullpath-default’
Set the default value of the ‘bash_source_fullpath’ shell option
described above under *note The Shopt Builtin:: to be enabled.
This controls how filenames are assigned to the ‘BASH_SOURCE’ array
variable.
‘--enable-brace-expansion’
Include ‘csh’-like brace expansion ( ‘b{a,b}c’ ↦ ‘bac bbc’ ). See
*note Brace Expansion::, for a complete description.
‘--enable-casemod-attributes’
Include support for case-modifying attributes in the ‘declare’
builtin and assignment statements. Variables with the ‘uppercase’
attribute, for example, will have their values converted to
uppercase upon assignment.
‘--enable-casemod-expansion’
Include support for case-modifying word expansions.
‘--enable-command-timing’
Include support for recognizing ‘time’ as a reserved word and for
displaying timing statistics for the pipeline following ‘time’
(*note Pipelines::). This allows timing pipelines, shell compound
commands, shell builtins, and shell functions, which an external
command cannot do easily.
‘--enable-cond-command’
Include support for the ‘[[’ conditional command. (*note
Conditional Constructs::).
‘--enable-cond-regexp’
Include support for matching POSIX regular expressions using the
‘=~’ binary operator in the ‘[[’ conditional command. (*note
Conditional Constructs::).
‘--enable-coprocesses’
Include support for coprocesses and the ‘coproc’ reserved word
(*note Pipelines::).
‘--enable-debugger’
Include support for the Bash debugger (distributed separately).
‘--enable-dev-fd-stat-broken’
If calling ‘stat’ on /dev/fd/N returns different results than
calling ‘fstat’ on file descriptor N, supply this option to enable
a workaround. This has implications for conditional commands that
test file attributes.
‘--enable-direxpand-default’
Cause the ‘direxpand’ shell option (*note The Shopt Builtin::) to
be enabled by default when the shell starts. It is normally
disabled by default.
‘--enable-directory-stack’
Include support for a ‘csh’-like directory stack and the ‘pushd’,
‘popd’, and ‘dirs’ builtins (*note The Directory Stack::).
‘--enable-disabled-builtins’
Allow builtin commands to be invoked via ‘builtin xxx’ even after
‘xxx’ has been disabled using ‘enable -n xxx’. See *note Bash
Builtins::, for details of the ‘builtin’ and ‘enable’ builtin
commands.
‘--enable-dparen-arithmetic’
Include support for the ‘((...))’ command (*note Conditional
Constructs::).
‘--enable-extended-glob’
Include support for the extended pattern matching features
described above under *note Pattern Matching::.
‘--enable-extended-glob-default’
Set the default value of the ‘extglob’ shell option described above
under *note The Shopt Builtin:: to be enabled.
‘--enable-function-import’
Include support for importing function definitions exported by
another instance of the shell from the environment. This option is
enabled by default.
‘--enable-glob-asciiranges-default’
Set the default value of the ‘globasciiranges’ shell option
described above under *note The Shopt Builtin:: to be enabled.
This controls the behavior of character ranges when used in pattern
matching bracket expressions.
‘--enable-help-builtin’
Include the ‘help’ builtin, which displays help on shell builtins
and variables (*note Bash Builtins::).
‘--enable-history’
Include command history and the ‘fc’ and ‘history’ builtin commands
(*note Bash History Facilities::).
‘--enable-job-control’
This enables the job control features (*note Job Control::), if the
operating system supports them.
‘--enable-multibyte’
This enables support for multibyte characters if the operating
system provides the necessary support.
‘--enable-net-redirections’
This enables the special handling of filenames of the form
‘/dev/tcp/HOST/PORT’ and ‘/dev/udp/HOST/PORT’ when used in
redirections (*note Redirections::).
‘--enable-process-substitution’
This enables process substitution (*note Process Substitution::) if
the operating system provides the necessary support.
‘--enable-progcomp’
Enable the programmable completion facilities (*note Programmable
Completion::). If Readline is not enabled, this option has no
effect.
‘--enable-prompt-string-decoding’
Turn on the interpretation of a number of backslash-escaped
characters in the ‘$PS0’, ‘$PS1’, ‘$PS2’, and ‘$PS4’ prompt
strings. See *note Controlling the Prompt::, for a complete list
of prompt string escape sequences.
‘--enable-readline’
Include support for command-line editing and history with the Bash
version of the Readline library (*note Command Line Editing::).
‘--enable-restricted’
Include support for a “restricted shell”. If this is enabled, Bash
enters a restricted mode when called as ‘rbash’. See *note The
Restricted Shell::, for a description of restricted mode.
‘--enable-select’
Include the ‘select’ compound command, which allows generation of
simple menus (*note Conditional Constructs::).
‘--enable-single-help-strings’
Store the text displayed by the ‘help’ builtin as a single string
for each help topic. This aids in translating the text to
different languages. You may need to disable this if your compiler
cannot handle very long string literals.
‘--enable-strict-posix-default’
Make Bash POSIX-conformant by default (*note Bash POSIX Mode::).
‘--enable-translatable-strings’
Enable support for ‘$"STRING"’ translatable strings (*note Locale
Translation::).
‘--enable-usg-echo-default’
A synonym for ‘--enable-xpg-echo-default’.
‘--enable-xpg-echo-default’
Make the ‘echo’ builtin expand backslash-escaped characters by
default, without requiring the ‘-e’ option. This sets the default
value of the ‘xpg_echo’ shell option to ‘on’, which makes the Bash
‘echo’ behave more like the version specified in the Single Unix
Specification, version 3. *Note Bash Builtins::, for a description
of the escape sequences that ‘echo’ recognizes.
The file ‘config-top.h’ contains C Preprocessor ‘#define’ statements
for options which are not settable from ‘configure’. Some of these are
not meant to be changed; beware of the consequences if you do. Read the
comments associated with each definition for more information about its
effect.
File: bash.info, Node: Reporting Bugs, Next: Major Differences From The Bourne Shell, Prev: Installing Bash, Up: Top
Appendix A Reporting Bugs
*************************
Please report all bugs you find in Bash. But first, you should make
sure that it really is a bug, and that it appears in the latest version
of Bash. The latest released version of Bash is always available for
FTP from <ftp://ftp.gnu.org/pub/gnu/bash/> and from
<http://git.savannah.gnu.org/cgit/bash.git/snapshot/bash-master.tar.gz>.
Once you have determined that a bug actually exists, use the
‘bashbug’ command to submit a bug report or use the form at the Bash
project page (https://savannah.gnu.org/projects/bash/). If you have a
fix, you are encouraged to submit that as well! Suggestions and
'philosophical' bug reports may be mailed to <bug-bash@gnu.org> or
<help-bash@gnu.org>.
All bug reports should include:
• The version number of Bash.
• The hardware and operating system.
• The compiler used to compile Bash.
• A description of the bug behavior.
• A short script or 'recipe' which exercises the bug and may be used
to reproduce it.
‘bashbug’ inserts the first three items automatically into the template
it provides for filing a bug report.
Please send all reports concerning this manual to <bug-bash@gnu.org>.
File: bash.info, Node: Major Differences From The Bourne Shell, Next: GNU Free Documentation License, Prev: Reporting Bugs, Up: Top
Appendix B Major Differences From The Bourne Shell
**************************************************
Bash implements essentially the same grammar, parameter and variable
expansion, redirection, and quoting as the Bourne Shell. Bash uses the
POSIX standard as the specification of how these features are to be
implemented and how they should behave. There are some differences
between the traditional Bourne shell and Bash; this section quickly
details the differences of significance. A number of these differences
are explained in greater depth in previous sections. This section uses
the version of ‘sh’ included in SVR4.2 (the last version of the
historical Bourne shell) as the baseline reference.
• Bash is POSIX-conformant, even where the POSIX specification
differs from traditional ‘sh’ behavior (*note Bash POSIX Mode::).
• Bash has multi-character invocation options (*note Invoking
Bash::).
• The Bash restricted mode is more useful (*note The Restricted
Shell::); the SVR4.2 shell restricted mode is too limited.
• Bash has command-line editing (*note Command Line Editing::) and
the ‘bind’ builtin.
• Bash provides a programmable word completion mechanism (*note
Programmable Completion::), and builtin commands ‘complete’,
‘compgen’, and ‘compopt’, to manipulate it.
• Bash decodes a number of backslash-escape sequences in the prompt
string variables (‘PS0’, ‘PS1’, ‘PS2’, and ‘PS4’) (*note
Controlling the Prompt::).
• Bash expands and displays the ‘PS0’ prompt string variable.
• Bash runs commands from the ‘PROMPT_COMMAND’ array variable before
issuing each primary prompt.
• Bash has command history (*note Bash History Facilities::) and the
‘history’ and ‘fc’ builtins to manipulate it. The Bash history
list maintains timestamp information and uses the value of the
‘HISTTIMEFORMAT’ variable to display it.
• Bash implements ‘csh’-like history expansion (*note History
Interaction::).
• Bash supports the ‘$'...'’ quoting syntax, which expands ANSI-C
backslash-escaped characters in the text between the single quotes
(*note ANSI-C Quoting::).
• Bash supports the ‘$"..."’ quoting syntax and performs
locale-specific translation of the characters between the double
quotes. The ‘-D’, ‘--dump-strings’, and ‘--dump-po-strings’
invocation options list the translatable strings found in a script
(*note Locale Translation::).
• Bash includes brace expansion (*note Brace Expansion::) and tilde
expansion (*note Tilde Expansion::).
• Bash implements command aliases and the ‘alias’ and ‘unalias’
builtins (*note Aliases::).
• Bash implements the ‘!’ reserved word to negate the return value of
a pipeline (*note Pipelines::). This is very useful when an ‘if’
statement needs to act only if a test fails. The Bash ‘-o
pipefail’ option to ‘set’ will cause a pipeline to return a failure
status if any command fails (*note The Set Builtin::).
• Bash has the ‘time’ reserved word and command timing (*note
Pipelines::). The display of the timing statistics may be
controlled with the ‘TIMEFORMAT’ variable.
• Bash provides coprocesses and the ‘coproc’ reserved word (*note
Coprocesses::).
• Bash implements the ‘for (( EXPR1 ; EXPR2 ; EXPR3 ))’ arithmetic
for command, similar to the C language (*note Looping
Constructs::).
• Bash includes the ‘select’ compound command, which allows the
generation of simple menus (*note Conditional Constructs::).
• Bash includes the ‘[[’ compound command, which makes conditional
testing part of the shell grammar (*note Conditional Constructs::),
including optional regular expression matching.
• Bash provides optional case-insensitive matching for the ‘case’ and
‘[[’ constructs (*note Conditional Constructs::).
• Bash provides additional ‘case’ statement action list terminators:
‘;&’ and ‘;;&’ (*note Conditional Constructs::).
• Bash provides shell arithmetic, the ‘((’ compound command (*note
Conditional Constructs::), the ‘let’ builtin, and arithmetic
expansion (*note Shell Arithmetic::).
• Bash has one-dimensional array variables (*note Arrays::), and the
appropriate variable expansions and assignment syntax to use them.
Several of the Bash builtins take options to act on arrays. Bash
provides a number of built-in array variables.
• Variables present in the shell's initial environment are
automatically exported to child processes (*note Command Execution
Environment::). The Bourne shell does not normally do this unless
the variables are explicitly marked using the ‘export’ command.
• Bash can expand positional parameters beyond ‘$9’ using ‘${NUM}’
(*note Shell Parameter Expansion::).
• Bash supports the ‘+=’ assignment operator, which appends to the
value of the variable named on the left hand side (*note Shell
Parameters::).
• Bash includes the POSIX pattern removal ‘%’, ‘#’, ‘%%’ and ‘##’
expansions to remove leading or trailing substrings from variable
values (*note Shell Parameter Expansion::).
• The expansion ‘${#xx}’, which returns the length of ‘${xx}’, is
supported (*note Shell Parameter Expansion::).
• The expansion ‘${var:’OFFSET‘[:’LENGTH‘]}’, which expands to the
substring of ‘var’'s value of length LENGTH, beginning at OFFSET,
is present (*note Shell Parameter Expansion::).
• The expansion ‘${VAR/[/]’PATTERN‘[/’REPLACEMENT‘]}’, which matches
PATTERN and replaces it with REPLACEMENT in the value of VAR, is
available (*note Shell Parameter Expansion::), with a mechanism to
use the matched text in REPLACEMENT.
• The expansion ‘${!PREFIX*}’ expansion, which expands to the names
of all shell variables whose names begin with PREFIX, is available
(*note Shell Parameter Expansion::).
• Bash has indirect variable expansion using ‘${!word}’ (*note Shell
Parameter Expansion::) and implements the ‘nameref’ variable
attribute for automatic indirect variable expansion.
• Bash includes a set of parameter transformation word expansions of
the form ‘${var@X}’, where ‘X’ specifies the transformation (*note
Shell Parameter Expansion::).
• The POSIX ‘$()’ form of command substitution is implemented (*note
Command Substitution::), and preferred to the Bourne shell's ‘``’
(which is also implemented for backwards compatibility).
• Bash implements a variant of command substitution that runs the
enclosed command in the current shell execution environment: ‘${
COMMAND;}’ or ‘${|COMMAND;}’ (*note Command Substitution::).
• Bash has process substitution (*note Process Substitution::).
• Bash automatically assigns variables that provide information about
the current user (‘UID’, ‘EUID’, and ‘GROUPS’), the current host
(‘HOSTTYPE’, ‘OSTYPE’, ‘MACHTYPE’, and ‘HOSTNAME’), and the
instance of Bash that is running (‘BASH’, ‘BASH_VERSION’, and
‘BASH_VERSINFO’). *Note Bash Variables::, for details.
• Bash uses many variables to provide functionality and customize
shell behavior that the Bourne shell does not. Examples include
‘RANDOM’, ‘SRANDOM’, ‘EPOCHSECONDS’, ‘EPOCHREALTIME’, ‘TIMEFORMAT’,
‘BASHPID’, ‘BASH_XTRACEFD’, ‘GLOBIGNORE’, ‘HISTIGNORE’, and
‘BASH_VERSION’. *Note Bash Variables::, for a complete list.
• Bash uses the ‘GLOBSORT’ shell variable to control how to sort the
results of filename expansion (*note Filename Expansion::).
• Bash uses the ‘IFS’ variable to split only the results of
expansion, not all words (*note Word Splitting::). This closes a
longstanding shell security hole.
• The filename expansion bracket expression code uses ‘!’ and ‘^’ to
negate the set of characters between the brackets (*note Filename
Expansion::). The Bourne shell uses only ‘!’.
• Bash implements the full set of POSIX filename expansion operators,
including character classes, equivalence classes, and collating
symbols (*note Filename Expansion::).
• Bash implements extended pattern matching features when the
‘extglob’ shell option is enabled (*note Pattern Matching::).
• The ‘globstar’ shell option extends filename expansion to
recursively scan directories and subdirectories for matching
filenames (*note Pattern Matching::).
• It is possible to have a variable and a function with the same
name; ‘sh’ does not separate the two name spaces.
• Bash functions are permitted to have local variables using the
‘local’ builtin, and thus users can write useful recursive
functions (*note Bash Builtins::).
• Bash performs filename expansion on filenames specified as operands
to input and output redirection operators (*note Redirections::).
• Bash contains the ‘<>’ redirection operator, allowing a file to be
opened for both reading and writing, and the ‘&>’ redirection
operator, for directing standard output and standard error to the
same file (*note Redirections::).
• Bash includes the ‘<<<’ redirection operator, allowing a string to
be used as the standard input to a command (*note Redirections::).
• Bash implements the ‘[n]<&WORD’ and ‘[n]>&WORD’ redirection
operators, which move one file descriptor to another.
• Bash treats a number of filenames specially when they are used in
redirection operators (*note Redirections::).
• Bash provides the {VAR}<WORD capability to have the shell allocate
file descriptors for redirections and assign them to VAR (*note
Redirections::). This works with multiple redirection operators.
• Bash can open network connections to arbitrary machines and
services with the redirection operators (*note Redirections::).
• The ‘noclobber’ option is available to avoid overwriting existing
files with output redirection (*note The Set Builtin::). The ‘>|’
redirection operator may be used to override ‘noclobber’.
• Variable assignments preceding commands affect only that command,
even builtins and functions (*note Environment::). In ‘sh’, all
variable assignments preceding commands are global unless the
command is executed from the file system.
• Bash includes a number of features to support a separate debugger
for shell scripts: variables (‘BASH_ARGC’, ‘BASH_ARGV’,
‘BASH_LINENO’, ‘BASH_SOURCE’), the ‘DEBUG’, ‘RETURN’, and ‘ERR’
traps, ‘declare -F’, and the ‘caller’ builtin.
• Bash implements a ‘csh’-like directory stack, and provides the
‘pushd’, ‘popd’, and ‘dirs’ builtins to manipulate it (*note The
Directory Stack::). Bash also makes the directory stack visible as
the value of the ‘DIRSTACK’ shell variable.
• Bash allows a function to override a builtin with the same name,
and provides access to that builtin's functionality within the
function via the ‘builtin’ and ‘command’ builtins (*note Bash
Builtins::).
• Bash includes the ‘caller’ builtin (*note Bash Builtins::), which
displays the context of any active subroutine call (a shell
function or a script executed with the ‘.’ or ‘source’ builtins).
This supports the Bash debugger.
• The Bash ‘cd’ and ‘pwd’ builtins (*note Bourne Shell Builtins::)
each take ‘-L’ and ‘-P’ options to switch between logical and
physical modes.
• The ‘command’ builtin allows selectively skipping shell functions
when performing command lookup (*note Bash Builtins::).
• Bash uses the ‘declare’ builtin to modify the full set of variable
and function attributes, and to assign values to variables.
• The ‘disown’ builtin can remove a job from the internal shell job
table (*note Job Control Builtins::) or suppress sending ‘SIGHUP’
to a job when the shell exits as the result of a ‘SIGHUP’.
• The ‘enable’ builtin (*note Bash Builtins::) can enable or disable
individual builtins and implements support for dynamically loading
builtin commands from shared objects.
• The Bash ‘exec’ builtin takes additional options that allow users
to control the contents of the environment passed to the executed
command, and what the zeroth argument to the command is to be
(*note Bourne Shell Builtins::).
• Shell functions may be exported to children via the environment
using ‘export -f’ (*note Shell Functions::).
• The Bash ‘export’ and ‘readonly’ builtins (*note Bourne Shell
Builtins:: can take a ‘-f’ option to act on shell functions, a ‘-p’
option to display variables with various attributes set in a format
that can be used as shell input, a ‘-n’ option to remove various
variable attributes, and ‘name=value’ arguments to set variable
attributes and values simultaneously.
• The Bash ‘hash’ builtin allows a name to be associated with an
arbitrary filename, even when that filename cannot be found by
searching the ‘$PATH’, using ‘hash -p’ (*note Bourne Shell
Builtins::).
• Bash includes a ‘help’ builtin for quick reference to shell
facilities (*note Bash Builtins::).
• Bash includes the ‘mapfile’ builtin to quickly read the contents of
a file into an indexed array variable (*note Bash Builtins::).
• The ‘printf’ builtin is available to display formatted output
(*note Bash Builtins::), and has additional custom format
specifiers and an option to assign the formatted output directly to
a shell variable.
• The Bash ‘read’ builtin (*note Bash Builtins::) will read a line
ending in ‘\’ with the ‘-r’ option, and will use the ‘REPLY’
variable as a default if no non-option arguments are supplied.
• The ‘read’ builtin (*note Bash Builtins::) accepts a prompt string
with the ‘-p’ option and will use Readline to obtain the line when
given the ‘-e’ or ‘-E’ options, with the ability to insert text
into the line using the ‘-i’ option. The ‘read’ builtin also has
additional options to control input: the ‘-s’ option will turn off
echoing of input characters as they are read, the ‘-t’ option will
allow ‘read’ to time out if input does not arrive within a
specified number of seconds, the ‘-n’ option will allow reading
only a specified number of characters rather than a full line, and
the ‘-d’ option will read until a particular character rather than
newline.
• The ‘return’ builtin may be used to abort execution of scripts
executed with the ‘.’ or ‘source’ builtins (*note Bourne Shell
Builtins::).
• Bash has much more optional behavior controllable with the ‘set’
builtin (*note The Set Builtin::).
• The ‘-x’ (‘xtrace’) option displays commands other than simple
commands when performing an execution trace (*note The Set
Builtin::).
• Bash includes the ‘shopt’ builtin, for finer control of shell
optional capabilities (*note The Shopt Builtin::), and allows these
options to be set and unset at shell invocation (*note Invoking
Bash::).
• The ‘test’ builtin (*note Bourne Shell Builtins::) is slightly
different, as it implements the POSIX algorithm, which specifies
the behavior based on the number of arguments.
• The ‘trap’ builtin (*note Bourne Shell Builtins::) allows a ‘DEBUG’
pseudo-signal specification, similar to ‘EXIT’. Commands specified
with a ‘DEBUG’ trap are executed before every simple command, ‘for’
command, ‘case’ command, ‘select’ command, every arithmetic ‘for’
command, and before the first command executes in a shell function.
The ‘DEBUG’ trap is not inherited by shell functions unless the
function has been given the ‘trace’ attribute or the ‘functrace’
option has been enabled using the ‘shopt’ builtin. The ‘extdebug’
shell option has additional effects on the ‘DEBUG’ trap.
The ‘trap’ builtin (*note Bourne Shell Builtins::) allows an ‘ERR’
pseudo-signal specification, similar to ‘EXIT’ and ‘DEBUG’.
Commands specified with an ‘ERR’ trap are executed after a simple
command fails, with a few exceptions. The ‘ERR’ trap is not
inherited by shell functions unless the ‘-o errtrace’ option to the
‘set’ builtin is enabled.
The ‘trap’ builtin (*note Bourne Shell Builtins::) allows a
‘RETURN’ pseudo-signal specification, similar to ‘EXIT’ and
‘DEBUG’. Commands specified with a ‘RETURN’ trap are executed
before execution resumes after a shell function or a shell script
executed with ‘.’ or ‘source’ returns. The ‘RETURN’ trap is not
inherited by shell functions unless the function has been given the
‘trace’ attribute or the ‘functrace’ option has been enabled using
the ‘shopt’ builtin.
• The Bash ‘type’ builtin is more extensive and gives more
information about the names it finds (*note Bash Builtins::).
• The ‘ulimit’ builtin provides control over many more per-process
resources (*note Bash Builtins::).
• The Bash ‘umask’ builtin uses the ‘-p’ option to display the output
in the form of a ‘umask’ command that may be reused as input (*note
Bourne Shell Builtins::).
• The Bash ‘wait’ builtin has a ‘-n’ option to wait for the next
child to exit, possibly selecting from a list of supplied jobs, and
the ‘-p’ option to store information about a terminated child
process in a shell variable.
• The SVR4.2 shell behaves differently when invoked as ‘jsh’ (it
turns on job control).
• The SVR4.2 shell has two privilege-related builtins (‘mldmode’ and
‘priv’) not present in Bash.
• Bash does not have the ‘stop’ or ‘newgrp’ builtins.
• Bash does not use the ‘SHACCT’ variable or perform shell
accounting.
• The SVR4.2 ‘sh’ uses a ‘TIMEOUT’ variable like Bash uses ‘TMOUT’.
More features unique to Bash may be found in *note Bash Features::.
B.1 Implementation Differences From The SVR4.2 Shell
====================================================
Since Bash is a completely new implementation, it does not suffer from
many of the limitations of the SVR4.2 shell. For instance:
• Bash does not fork a subshell when redirecting into or out of a
shell control structure such as an ‘if’ or ‘while’ statement.
• Bash does not allow unbalanced quotes. The SVR4.2 shell will
silently insert a needed closing quote at ‘EOF’ under certain
circumstances. This can be the cause of some hard-to-find errors.
• The SVR4.2 shell uses a baroque memory management scheme based on
trapping ‘SIGSEGV’. If the shell is started from a process with
‘SIGSEGV’ blocked (e.g., by using the ‘system()’ C library function
call), it misbehaves badly.
• In a questionable attempt at security, the SVR4.2 shell, when
invoked without the ‘-p’ option, will alter its real and effective
UID and GID if they are less than some magic threshold value,
commonly 100. This can lead to unexpected results.
• The SVR4.2 shell does not allow users to trap ‘SIGSEGV’, ‘SIGALRM’,
or ‘SIGCHLD’.
• The SVR4.2 shell does not allow the ‘IFS’, ‘MAILCHECK’, ‘PATH’,
‘PS1’, or ‘PS2’ variables to be unset.
• The SVR4.2 shell treats ‘^’ as the undocumented equivalent of ‘|’.
• Bash allows multiple option arguments when it is invoked (‘-x -v’);
the SVR4.2 shell allows only one option argument (‘-xv’). In fact,
some versions of the shell dump core if the second argument begins
with a ‘-’.
• The SVR4.2 shell exits a script if any builtin fails; Bash exits a
script only if one of the POSIX special builtins fails, and only
for certain failures, as enumerated in the POSIX standard.
• If the ‘lastpipe’ option is enabled, and job control is not active,
Bash runs the last element of a pipeline in the current shell
execution environment.
File: bash.info, Node: GNU Free Documentation License, Next: Indexes, Prev: Major Differences From The Bourne Shell, Up: Top
Appendix C GNU Free Documentation License
*****************************************
Version 1.3, 3 November 2008
Copyright © 2000, 2001, 2002, 2007, 2008 Free Software Foundation, Inc.
<http://fsf.org/>
Everyone is permitted to copy and distribute verbatim copies
of this license document, but changing it is not allowed.
0. PREAMBLE
The purpose of this License is to make a manual, textbook, or other
functional and useful document “free” in the sense of freedom: to
assure everyone the effective freedom to copy and redistribute it,
with or without modifying it, either commercially or
noncommercially. Secondarily, this License preserves for the
author and publisher a way to get credit for their work, while not
being considered responsible for modifications made by others.
This License is a kind of "copyleft", which means that derivative
works of the document must themselves be free in the same sense.
It complements the GNU General Public License, which is a copyleft
license designed for free software.
We have designed this License in order to use it for manuals for
free software, because free software needs free documentation: a
free program should come with manuals providing the same freedoms
that the software does. But this License is not limited to
software manuals; it can be used for any textual work, regardless
of subject matter or whether it is published as a printed book. We
recommend this License principally for works whose purpose is
instruction or reference.
1. APPLICABILITY AND DEFINITIONS
This License applies to any manual or other work, in any medium,
that contains a notice placed by the copyright holder saying it can
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"Document", below, refers to any such manual or work. Any member
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A "Modified Version" of the Document means any work containing the
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A "Secondary Section" is a named appendix or a front-matter section
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To "Preserve the Title" of such a section when you modify the
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The Document may include Warranty Disclaimers next to the notice
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2. VERBATIM COPYING
You may copy and distribute the Document in any medium, either
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may not use technical measures to obstruct or control the reading
or further copying of the copies you make or distribute. However,
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You may also lend copies, under the same conditions stated above,
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4. MODIFICATIONS
You may copy and distribute a Modified Version of the Document
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E. Add an appropriate copyright notice for your modifications
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notice giving the public permission to use the Modified
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Sections and required Cover Texts given in the Document's
license notice.
H. Include an unaltered copy of this License.
I. Preserve the section Entitled "History", Preserve its Title,
and add to it an item stating at least the title, year, new
authors, and publisher of the Modified Version as given on the
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an item describing the Modified Version as stated in the
previous sentence.
J. Preserve the network location, if any, given in the Document
for public access to a Transparent copy of the Document, and
likewise the network locations given in the Document for
previous versions it was based on. These may be placed in the
"History" section. You may omit a network location for a work
that was published at least four years before the Document
itself, or if the original publisher of the version it refers
to gives permission.
K. For any section Entitled "Acknowledgements" or "Dedications",
Preserve the Title of the section, and preserve in the section
all the substance and tone of each of the contributor
acknowledgements and/or dedications given therein.
L. Preserve all the Invariant Sections of the Document, unaltered
in their text and in their titles. Section numbers or the
equivalent are not considered part of the section titles.
M. Delete any section Entitled "Endorsements". Such a section
may not be included in the Modified Version.
N. Do not retitle any existing section to be Entitled
"Endorsements" or to conflict in title with any Invariant
Section.
O. Preserve any Warranty Disclaimers.
If the Modified Version includes new front-matter sections or
appendices that qualify as Secondary Sections and contain no
material copied from the Document, you may at your option designate
some or all of these sections as invariant. To do this, add their
titles to the list of Invariant Sections in the Modified Version's
license notice. These titles must be distinct from any other
section titles.
You may add a section Entitled "Endorsements", provided it contains
nothing but endorsements of your Modified Version by various
parties--for example, statements of peer review or that the text
has been approved by an organization as the authoritative
definition of a standard.
You may add a passage of up to five words as a Front-Cover Text,
and a passage of up to 25 words as a Back-Cover Text, to the end of
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File: bash.info, Node: Indexes, Prev: GNU Free Documentation License, Up: Top
Appendix D Indexes
******************
* Menu:
* Builtin Index:: Index of Bash builtin commands.
* Reserved Word Index:: Index of Bash reserved words.
* Variable Index:: Quick reference helps you find the
variable you want.
* Function Index:: Index of bindable Readline functions.
* Concept Index:: General index for concepts described in
this manual.
File: bash.info, Node: Builtin Index, Next: Reserved Word Index, Up: Indexes
D.1 Index of Shell Builtin Commands
===================================
[index ]
* Menu:
* :: Bourne Shell Builtins.
(line 11)
* .: Bourne Shell Builtins.
(line 17)
* [: Bourne Shell Builtins.
(line 339)
* alias: Bash Builtins. (line 11)
* bg: Job Control Builtins.
(line 7)
* bind: Bash Builtins. (line 24)
* break: Bourne Shell Builtins.
(line 47)
* builtin: Bash Builtins. (line 133)
* caller: Bash Builtins. (line 142)
* cd: Bourne Shell Builtins.
(line 55)
* command: Bash Builtins. (line 159)
* compgen: Programmable Completion Builtins.
(line 12)
* complete: Programmable Completion Builtins.
(line 37)
* compopt: Programmable Completion Builtins.
(line 258)
* continue: Bourne Shell Builtins.
(line 106)
* declare: Bash Builtins. (line 179)
* dirs: Directory Stack Builtins.
(line 7)
* disown: Job Control Builtins.
(line 120)
* echo: Bash Builtins. (line 284)
* enable: Bash Builtins. (line 337)
* eval: Bourne Shell Builtins.
(line 115)
* exec: Bourne Shell Builtins.
(line 123)
* exit: Bourne Shell Builtins.
(line 145)
* export: Bourne Shell Builtins.
(line 152)
* false: Bourne Shell Builtins.
(line 175)
* fc: Bash History Builtins.
(line 10)
* fg: Job Control Builtins.
(line 17)
* getopts: Bourne Shell Builtins.
(line 180)
* hash: Bourne Shell Builtins.
(line 232)
* help: Bash Builtins. (line 375)
* history: Bash History Builtins.
(line 59)
* jobs: Job Control Builtins.
(line 28)
* kill: Job Control Builtins.
(line 61)
* let: Bash Builtins. (line 404)
* local: Bash Builtins. (line 413)
* logout: Bash Builtins. (line 438)
* mapfile: Bash Builtins. (line 443)
* popd: Directory Stack Builtins.
(line 37)
* printf: Bash Builtins. (line 488)
* pushd: Directory Stack Builtins.
(line 71)
* pwd: Bourne Shell Builtins.
(line 264)
* read: Bash Builtins. (line 558)
* readarray: Bash Builtins. (line 669)
* readonly: Bourne Shell Builtins.
(line 276)
* return: Bourne Shell Builtins.
(line 301)
* set: The Set Builtin. (line 11)
* shift: Bourne Shell Builtins.
(line 326)
* shopt: The Shopt Builtin. (line 9)
* source: Bash Builtins. (line 678)
* suspend: Job Control Builtins.
(line 139)
* test: Bourne Shell Builtins.
(line 339)
* times: Bourne Shell Builtins.
(line 439)
* trap: Bourne Shell Builtins.
(line 445)
* true: Bourne Shell Builtins.
(line 511)
* type: Bash Builtins. (line 683)
* typeset: Bash Builtins. (line 720)
* ulimit: Bash Builtins. (line 726)
* umask: Bourne Shell Builtins.
(line 516)
* unalias: Bash Builtins. (line 834)
* unset: Bourne Shell Builtins.
(line 534)
* wait: Job Control Builtins.
(line 86)
File: bash.info, Node: Reserved Word Index, Next: Variable Index, Prev: Builtin Index, Up: Indexes
D.2 Index of Shell Reserved Words
=================================
[index ]
* Menu:
* !: Pipelines. (line 9)
* [[: Conditional Constructs.
(line 128)
* ]]: Conditional Constructs.
(line 128)
* {: Command Grouping. (line 21)
* }: Command Grouping. (line 21)
* case: Conditional Constructs.
(line 28)
* do: Looping Constructs. (line 12)
* done: Looping Constructs. (line 12)
* elif: Conditional Constructs.
(line 7)
* else: Conditional Constructs.
(line 7)
* esac: Conditional Constructs.
(line 28)
* fi: Conditional Constructs.
(line 7)
* for: Looping Constructs. (line 32)
* function: Shell Functions. (line 13)
* if: Conditional Constructs.
(line 7)
* in: Conditional Constructs.
(line 28)
* select: Conditional Constructs.
(line 84)
* then: Conditional Constructs.
(line 7)
* time: Pipelines. (line 9)
* until: Looping Constructs. (line 12)
* while: Looping Constructs. (line 22)
File: bash.info, Node: Variable Index, Next: Function Index, Prev: Reserved Word Index, Up: Indexes
D.3 Parameter and Variable Index
================================
[index ]
* Menu:
* _: Bash Variables. (line 13)
* -: Special Parameters. (line 48)
* !: Special Parameters. (line 57)
* ?: Special Parameters. (line 44)
* @: Special Parameters. (line 23)
* *: Special Parameters. (line 10)
* #: Special Parameters. (line 41)
* $: Special Parameters. (line 53)
* $_: Bash Variables. (line 14)
* $-: Special Parameters. (line 49)
* $!: Special Parameters. (line 58)
* $?: Special Parameters. (line 45)
* $@: Special Parameters. (line 24)
* $*: Special Parameters. (line 11)
* $#: Special Parameters. (line 42)
* $$: Special Parameters. (line 54)
* $0: Special Parameters. (line 63)
* 0: Special Parameters. (line 62)
* active-region-end-color: Readline Init File Syntax.
(line 51)
* active-region-start-color: Readline Init File Syntax.
(line 38)
* auto_resume: Job Control Variables.
(line 6)
* BASH: Bash Variables. (line 24)
* BASH_ALIASES: Bash Variables. (line 44)
* BASH_ARGC: Bash Variables. (line 53)
* BASH_ARGV: Bash Variables. (line 67)
* BASH_ARGV0: Bash Variables. (line 80)
* BASH_CMDS: Bash Variables. (line 88)
* BASH_COMMAND: Bash Variables. (line 97)
* BASH_COMPAT: Bash Variables. (line 104)
* BASH_ENV: Bash Variables. (line 120)
* BASH_EXECUTION_STRING: Bash Variables. (line 126)
* BASH_LINENO: Bash Variables. (line 129)
* BASH_LOADABLES_PATH: Bash Variables. (line 139)
* BASH_MONOSECONDS: Bash Variables. (line 143)
* BASH_REMATCH: Bash Variables. (line 150)
* BASH_SOURCE: Bash Variables. (line 158)
* BASH_SUBSHELL: Bash Variables. (line 166)
* BASH_TRAPSIG: Bash Variables. (line 172)
* BASH_VERSINFO: Bash Variables. (line 178)
* BASH_VERSION: Bash Variables. (line 201)
* BASH_XTRACEFD: Bash Variables. (line 205)
* BASHOPTS: Bash Variables. (line 27)
* BASHPID: Bash Variables. (line 37)
* bell-style: Readline Init File Syntax.
(line 64)
* bind-tty-special-chars: Readline Init File Syntax.
(line 71)
* blink-matching-paren: Readline Init File Syntax.
(line 79)
* CDPATH: Bourne Shell Variables.
(line 9)
* CHILD_MAX: Bash Variables. (line 217)
* colored-completion-prefix: Readline Init File Syntax.
(line 84)
* colored-stats: Readline Init File Syntax.
(line 94)
* COLUMNS: Bash Variables. (line 224)
* comment-begin: Readline Init File Syntax.
(line 100)
* COMP_CWORD: Bash Variables. (line 230)
* COMP_KEY: Bash Variables. (line 236)
* COMP_LINE: Bash Variables. (line 242)
* COMP_POINT: Bash Variables. (line 247)
* COMP_TYPE: Bash Variables. (line 255)
* COMP_WORDBREAKS: Bash Variables. (line 265)
* COMP_WORDS: Bash Variables. (line 271)
* completion-display-width: Readline Init File Syntax.
(line 104)
* completion-ignore-case: Readline Init File Syntax.
(line 111)
* completion-map-case: Readline Init File Syntax.
(line 116)
* completion-prefix-display-length: Readline Init File Syntax.
(line 122)
* completion-query-items: Readline Init File Syntax.
(line 131)
* COMPREPLY: Bash Variables. (line 278)
* convert-meta: Readline Init File Syntax.
(line 142)
* COPROC: Bash Variables. (line 284)
* DIRSTACK: Bash Variables. (line 288)
* disable-completion: Readline Init File Syntax.
(line 154)
* echo-control-characters: Readline Init File Syntax.
(line 159)
* editing-mode: Readline Init File Syntax.
(line 164)
* EMACS: Bash Variables. (line 298)
* emacs-mode-string: Readline Init File Syntax.
(line 170)
* enable-active-region The: Readline Init File Syntax.
(line 180)
* enable-bracketed-paste: Readline Init File Syntax.
(line 193)
* enable-keypad: Readline Init File Syntax.
(line 202)
* enable-meta-key: Readline Init File Syntax.
(line 207)
* ENV: Bash Variables. (line 303)
* EPOCHREALTIME: Bash Variables. (line 308)
* EPOCHSECONDS: Bash Variables. (line 316)
* EUID: Bash Variables. (line 323)
* EXECIGNORE: Bash Variables. (line 327)
* expand-tilde: Readline Init File Syntax.
(line 217)
* FCEDIT: Bash Variables. (line 339)
* FIGNORE: Bash Variables. (line 342)
* force-meta-prefix: Readline Init File Syntax.
(line 221)
* FUNCNAME: Bash Variables. (line 348)
* FUNCNEST: Bash Variables. (line 365)
* GLOBIGNORE: Bash Variables. (line 370)
* GLOBSORT: Bash Variables. (line 377)
* GROUPS: Bash Variables. (line 415)
* histchars: Bash Variables. (line 421)
* HISTCMD: Bash Variables. (line 437)
* HISTCONTROL: Bash Variables. (line 443)
* HISTFILE: Bash Variables. (line 461)
* HISTFILESIZE: Bash Variables. (line 467)
* HISTIGNORE: Bash Variables. (line 481)
* history-preserve-point: Readline Init File Syntax.
(line 234)
* history-size: Readline Init File Syntax.
(line 240)
* HISTSIZE: Bash Variables. (line 505)
* HISTTIMEFORMAT: Bash Variables. (line 512)
* HOME: Bourne Shell Variables.
(line 13)
* horizontal-scroll-mode: Readline Init File Syntax.
(line 250)
* HOSTFILE: Bash Variables. (line 521)
* HOSTNAME: Bash Variables. (line 532)
* HOSTTYPE: Bash Variables. (line 535)
* IFS: Bourne Shell Variables.
(line 18)
* IGNOREEOF: Bash Variables. (line 538)
* input-meta: Readline Init File Syntax.
(line 258)
* INPUTRC: Bash Variables. (line 547)
* INSIDE_EMACS: Bash Variables. (line 551)
* isearch-terminators: Readline Init File Syntax.
(line 269)
* keymap: Readline Init File Syntax.
(line 276)
* LANG: Creating Internationalized Scripts.
(line 51)
* LANG <1>: Bash Variables. (line 557)
* LC_ALL: Bash Variables. (line 561)
* LC_COLLATE: Bash Variables. (line 565)
* LC_CTYPE: Bash Variables. (line 572)
* LC_MESSAGES: Creating Internationalized Scripts.
(line 51)
* LC_MESSAGES <1>: Bash Variables. (line 577)
* LC_NUMERIC: Bash Variables. (line 581)
* LC_TIME: Bash Variables. (line 585)
* LINENO: Bash Variables. (line 589)
* LINES: Bash Variables. (line 596)
* MACHTYPE: Bash Variables. (line 602)
* MAIL: Bourne Shell Variables.
(line 24)
* MAILCHECK: Bash Variables. (line 606)
* MAILPATH: Bourne Shell Variables.
(line 29)
* MAPFILE: Bash Variables. (line 614)
* mark-modified-lines: Readline Init File Syntax.
(line 306)
* mark-symlinked-directories: Readline Init File Syntax.
(line 311)
* match-hidden-files: Readline Init File Syntax.
(line 316)
* menu-complete-display-prefix: Readline Init File Syntax.
(line 323)
* meta-flag: Readline Init File Syntax.
(line 258)
* OLDPWD: Bash Variables. (line 618)
* OPTARG: Bourne Shell Variables.
(line 36)
* OPTERR: Bash Variables. (line 621)
* OPTIND: Bourne Shell Variables.
(line 40)
* OSTYPE: Bash Variables. (line 626)
* output-meta: Readline Init File Syntax.
(line 328)
* page-completions: Readline Init File Syntax.
(line 337)
* PATH: Bourne Shell Variables.
(line 44)
* PIPESTATUS: Bash Variables. (line 629)
* POSIXLY_CORRECT: Bash Variables. (line 639)
* PPID: Bash Variables. (line 649)
* PROMPT_COMMAND: Bash Variables. (line 653)
* PROMPT_DIRTRIM: Bash Variables. (line 659)
* PS0: Bash Variables. (line 665)
* PS1: Bourne Shell Variables.
(line 53)
* PS2: Bourne Shell Variables.
(line 58)
* PS3: Bash Variables. (line 670)
* PS4: Bash Variables. (line 675)
* PWD: Bash Variables. (line 683)
* RANDOM: Bash Variables. (line 686)
* READLINE_ARGUMENT: Bash Variables. (line 694)
* READLINE_LINE: Bash Variables. (line 698)
* READLINE_MARK: Bash Variables. (line 702)
* READLINE_POINT: Bash Variables. (line 708)
* REPLY: Bash Variables. (line 712)
* revert-all-at-newline: Readline Init File Syntax.
(line 350)
* search-ignore-case: Readline Init File Syntax.
(line 357)
* SECONDS: Bash Variables. (line 716)
* SHELL: Bash Variables. (line 726)
* SHELLOPTS: Bash Variables. (line 731)
* SHLVL: Bash Variables. (line 741)
* show-all-if-ambiguous: Readline Init File Syntax.
(line 362)
* show-all-if-unmodified: Readline Init File Syntax.
(line 368)
* show-mode-in-prompt: Readline Init File Syntax.
(line 377)
* skip-completed-text: Readline Init File Syntax.
(line 383)
* SRANDOM: Bash Variables. (line 746)
* TEXTDOMAIN: Creating Internationalized Scripts.
(line 51)
* TEXTDOMAINDIR: Creating Internationalized Scripts.
(line 51)
* TIMEFORMAT: Bash Variables. (line 755)
* TMOUT: Bash Variables. (line 794)
* TMPDIR: Bash Variables. (line 806)
* UID: Bash Variables. (line 810)
* vi-cmd-mode-string: Readline Init File Syntax.
(line 396)
* vi-ins-mode-string: Readline Init File Syntax.
(line 407)
* visible-stats: Readline Init File Syntax.
(line 418)
File: bash.info, Node: Function Index, Next: Concept Index, Prev: Variable Index, Up: Indexes
D.4 Function Index
==================
[index ]
* Menu:
* abort (C-g): Miscellaneous Commands.
(line 10)
* accept-line (Newline or Return): Commands For History.
(line 6)
* alias-expand-line (): Miscellaneous Commands.
(line 134)
* backward-char (C-b): Commands For Moving. (line 18)
* backward-delete-char (Rubout): Commands For Text. (line 18)
* backward-kill-line (C-x Rubout): Commands For Killing.
(line 11)
* backward-kill-word (M-<DEL>): Commands For Killing.
(line 28)
* backward-word (M-b): Commands For Moving. (line 26)
* beginning-of-history (M-<): Commands For History.
(line 22)
* beginning-of-line (C-a): Commands For Moving. (line 6)
* bracketed-paste-begin (): Commands For Text. (line 35)
* call-last-kbd-macro (C-x e): Keyboard Macros. (line 13)
* capitalize-word (M-c): Commands For Text. (line 73)
* character-search (C-]): Miscellaneous Commands.
(line 41)
* character-search-backward (M-C-]): Miscellaneous Commands.
(line 45)
* clear-display (M-C-l): Commands For Moving. (line 52)
* clear-screen (C-l): Commands For Moving. (line 57)
* complete (<TAB>): Commands For Completion.
(line 6)
* complete-command (M-!): Commands For Completion.
(line 105)
* complete-filename (M-/): Commands For Completion.
(line 74)
* complete-hostname (M-@): Commands For Completion.
(line 97)
* complete-into-braces (M-{): Commands For Completion.
(line 124)
* complete-username (M-~): Commands For Completion.
(line 81)
* complete-variable (M-$): Commands For Completion.
(line 89)
* copy-backward-word (): Commands For Killing.
(line 62)
* copy-forward-word (): Commands For Killing.
(line 67)
* copy-region-as-kill (): Commands For Killing.
(line 58)
* dabbrev-expand (): Commands For Completion.
(line 119)
* delete-char (C-d): Commands For Text. (line 12)
* delete-char-or-list (): Commands For Completion.
(line 68)
* delete-horizontal-space (): Commands For Killing.
(line 50)
* digit-argument (M-0, M-1, ... M--): Numeric Arguments. (line 6)
* display-shell-version (C-x C-v): Miscellaneous Commands.
(line 148)
* do-lowercase-version (M-A, M-B, M-X, ...): Miscellaneous Commands.
(line 14)
* downcase-word (M-l): Commands For Text. (line 69)
* dump-functions (): Miscellaneous Commands.
(line 71)
* dump-macros (): Miscellaneous Commands.
(line 83)
* dump-variables (): Miscellaneous Commands.
(line 77)
* dynamic-complete-history (M-<TAB>): Commands For Completion.
(line 115)
* edit-and-execute-command (C-x C-e): Miscellaneous Commands.
(line 143)
* end-kbd-macro (C-x )): Keyboard Macros. (line 9)
* end-of-file (usually C-d): Commands For Text. (line 6)
* end-of-history (M->): Commands For History.
(line 25)
* end-of-line (C-e): Commands For Moving. (line 10)
* exchange-point-and-mark (C-x C-x): Miscellaneous Commands.
(line 37)
* execute-named-command (M-x): Miscellaneous Commands.
(line 90)
* export-completions (): Commands For Completion.
(line 44)
* fetch-history (): Commands For History.
(line 108)
* forward-backward-delete-char (): Commands For Text. (line 23)
* forward-char (C-f): Commands For Moving. (line 14)
* forward-search-history (C-s): Commands For History.
(line 35)
* forward-word (M-f): Commands For Moving. (line 22)
* glob-complete-word (M-g): Miscellaneous Commands.
(line 103)
* glob-expand-word (C-x *): Miscellaneous Commands.
(line 108)
* glob-list-expansions (C-x g): Miscellaneous Commands.
(line 114)
* history-and-alias-expand-line (): Miscellaneous Commands.
(line 137)
* history-expand-line (M-^): Miscellaneous Commands.
(line 127)
* history-search-backward (): Commands For History.
(line 53)
* history-search-forward (): Commands For History.
(line 60)
* history-substring-search-backward (): Commands For History.
(line 67)
* history-substring-search-forward (): Commands For History.
(line 73)
* insert-comment (M-#): Miscellaneous Commands.
(line 59)
* insert-completions (M-*): Commands For Completion.
(line 24)
* insert-last-argument (M-. or M-_): Miscellaneous Commands.
(line 140)
* kill-line (C-k): Commands For Killing.
(line 6)
* kill-region (): Commands For Killing.
(line 54)
* kill-whole-line (): Commands For Killing.
(line 19)
* kill-word (M-d): Commands For Killing.
(line 23)
* magic-space (): Miscellaneous Commands.
(line 130)
* menu-complete (): Commands For Completion.
(line 28)
* menu-complete-backward (): Commands For Completion.
(line 39)
* next-history (C-n): Commands For History.
(line 18)
* next-screen-line (): Commands For Moving. (line 45)
* non-incremental-forward-search-history (M-n): Commands For History.
(line 47)
* non-incremental-reverse-search-history (M-p): Commands For History.
(line 41)
* operate-and-get-next (C-o): Commands For History.
(line 101)
* overwrite-mode (): Commands For Text. (line 77)
* possible-command-completions (C-x !): Commands For Completion.
(line 111)
* possible-completions (M-?): Commands For Completion.
(line 17)
* possible-filename-completions (C-x /): Commands For Completion.
(line 77)
* possible-hostname-completions (C-x @): Commands For Completion.
(line 101)
* possible-username-completions (C-x ~): Commands For Completion.
(line 85)
* possible-variable-completions (C-x $): Commands For Completion.
(line 93)
* prefix-meta (<ESC>): Miscellaneous Commands.
(line 19)
* previous-history (C-p): Commands For History.
(line 13)
* previous-screen-line (): Commands For Moving. (line 38)
* print-last-kbd-macro (): Keyboard Macros. (line 17)
* quoted-insert (C-q or C-v): Commands For Text. (line 28)
* re-read-init-file (C-x C-r): Miscellaneous Commands.
(line 6)
* redraw-current-line (): Commands For Moving. (line 62)
* reverse-search-history (C-r): Commands For History.
(line 29)
* revert-line (M-r): Miscellaneous Commands.
(line 26)
* self-insert (a, b, A, 1, !, ...): Commands For Text. (line 32)
* set-mark (C-@): Miscellaneous Commands.
(line 33)
* shell-backward-kill-word (): Commands For Killing.
(line 37)
* shell-backward-word (M-C-b): Commands For Moving. (line 34)
* shell-expand-line (M-C-e): Miscellaneous Commands.
(line 119)
* shell-forward-word (M-C-f): Commands For Moving. (line 30)
* shell-kill-word (M-C-d): Commands For Killing.
(line 32)
* shell-transpose-words (M-C-t): Commands For Text. (line 58)
* skip-csi-sequence (): Miscellaneous Commands.
(line 50)
* spell-correct-word (C-x s): Miscellaneous Commands.
(line 97)
* start-kbd-macro (C-x (): Keyboard Macros. (line 6)
* tilde-expand (M-&): Miscellaneous Commands.
(line 30)
* transpose-chars (C-t): Commands For Text. (line 47)
* transpose-words (M-t): Commands For Text. (line 53)
* undo (C-_ or C-x C-u): Miscellaneous Commands.
(line 23)
* universal-argument (): Numeric Arguments. (line 10)
* unix-filename-rubout (): Commands For Killing.
(line 45)
* unix-line-discard (C-u): Commands For Killing.
(line 16)
* unix-word-rubout (C-w): Commands For Killing.
(line 41)
* upcase-word (M-u): Commands For Text. (line 65)
* yank (C-y): Commands For Killing.
(line 72)
* yank-last-arg (M-. or M-_): Commands For History.
(line 89)
* yank-nth-arg (M-C-y): Commands For History.
(line 79)
* yank-pop (M-y): Commands For Killing.
(line 75)
File: bash.info, Node: Concept Index, Prev: Function Index, Up: Indexes
D.5 Concept Index
=================
[index ]
* Menu:
* alias expansion: Aliases. (line 6)
* arithmetic evaluation: Shell Arithmetic. (line 6)
* arithmetic expansion: Arithmetic Expansion.
(line 6)
* arithmetic operators: Shell Arithmetic. (line 18)
* arithmetic, shell: Shell Arithmetic. (line 6)
* arrays: Arrays. (line 6)
* background: Job Control Basics. (line 6)
* Bash configuration: Basic Installation. (line 6)
* Bash installation: Basic Installation. (line 6)
* binary arithmetic operators: Shell Arithmetic. (line 18)
* bitwise arithmetic operators: Shell Arithmetic. (line 18)
* Bourne shell: Basic Shell Features.
(line 6)
* brace expansion: Brace Expansion. (line 6)
* builtin: Definitions. (line 21)
* command editing: Readline Bare Essentials.
(line 6)
* command execution: Command Search and Execution.
(line 6)
* command expansion: Simple Command Expansion.
(line 6)
* command history: Bash History Facilities.
(line 6)
* command search: Command Search and Execution.
(line 6)
* command substitution: Command Substitution.
(line 6)
* command timing: Pipelines. (line 9)
* commands, compound: Compound Commands. (line 6)
* commands, conditional: Conditional Constructs.
(line 6)
* commands, grouping: Command Grouping. (line 6)
* commands, lists: Lists. (line 6)
* commands, looping: Looping Constructs. (line 6)
* commands, pipelines: Pipelines. (line 6)
* commands, shell: Shell Commands. (line 6)
* commands, simple: Simple Commands. (line 6)
* comments, shell: Comments. (line 6)
* Compatibility Level: Shell Compatibility Mode.
(line 6)
* Compatibility Mode: Shell Compatibility Mode.
(line 6)
* completion builtins: Programmable Completion Builtins.
(line 6)
* conditional arithmetic operator: Shell Arithmetic. (line 18)
* configuration: Basic Installation. (line 6)
* control operator: Definitions. (line 25)
* coprocess: Coprocesses. (line 6)
* directory stack: The Directory Stack. (line 6)
* dollar-single quote quoting: ANSI-C Quoting. (line 6)
* editing command lines: Readline Bare Essentials.
(line 6)
* environment: Environment. (line 6)
* evaluation, arithmetic: Shell Arithmetic. (line 6)
* event designators: Event Designators. (line 6)
* execution environment: Command Execution Environment.
(line 6)
* exit status: Definitions. (line 30)
* exit status <1>: Exit Status. (line 6)
* expansion: Shell Expansions. (line 6)
* expansion, arithmetic: Arithmetic Expansion.
(line 6)
* expansion, brace: Brace Expansion. (line 6)
* expansion, filename: Filename Expansion. (line 9)
* expansion, parameter: Shell Parameter Expansion.
(line 6)
* expansion, pathname: Filename Expansion. (line 9)
* expansion, tilde: Tilde Expansion. (line 6)
* expressions, arithmetic: Shell Arithmetic. (line 6)
* expressions, conditional: Bash Conditional Expressions.
(line 6)
* field: Definitions. (line 34)
* filename: Definitions. (line 39)
* filename expansion: Filename Expansion. (line 9)
* foreground: Job Control Basics. (line 6)
* functions, shell: Shell Functions. (line 6)
* history builtins: Bash History Builtins.
(line 6)
* history events: Event Designators. (line 10)
* history expansion: History Interaction. (line 6)
* history list: Bash History Facilities.
(line 6)
* History, how to use: A Programmable Completion Example.
(line 113)
* identifier: Definitions. (line 55)
* initialization file, readline: Readline Init File. (line 6)
* installation: Basic Installation. (line 6)
* interaction, readline: Readline Interaction.
(line 6)
* interactive shell: Invoking Bash. (line 137)
* interactive shell <1>: Interactive Shells. (line 6)
* internationalization: Locale Translation. (line 6)
* internationalized scripts: Creating Internationalized Scripts.
(line 3)
* job: Definitions. (line 42)
* job control: Definitions. (line 46)
* job control <1>: Job Control Basics. (line 6)
* kill ring: Readline Killing Commands.
(line 18)
* killing text: Readline Killing Commands.
(line 6)
* localization: Locale Translation. (line 6)
* login shell: Invoking Bash. (line 134)
* matching, pattern: Pattern Matching. (line 6)
* metacharacter: Definitions. (line 50)
* name: Definitions. (line 55)
* native languages: Locale Translation. (line 6)
* notation, readline: Readline Bare Essentials.
(line 6)
* operator, shell: Definitions. (line 61)
* parameter expansion: Shell Parameter Expansion.
(line 6)
* parameters: Shell Parameters. (line 6)
* parameters, positional: Positional Parameters.
(line 6)
* parameters, special: Special Parameters. (line 6)
* pathname expansion: Filename Expansion. (line 9)
* pattern matching: Pattern Matching. (line 6)
* pipeline: Pipelines. (line 6)
* POSIX: Definitions. (line 9)
* POSIX description: Bash POSIX Mode. (line 9)
* POSIX Mode: Bash POSIX Mode. (line 48)
* process group: Definitions. (line 66)
* process group ID: Definitions. (line 70)
* process substitution: Process Substitution.
(line 6)
* programmable completion: Programmable Completion.
(line 6)
* prompting: Controlling the Prompt.
(line 6)
* quoting: Quoting. (line 6)
* quoting, ANSI: ANSI-C Quoting. (line 6)
* Readline, how to use: Job Control Variables.
(line 23)
* redirection: Redirections. (line 6)
* reserved word: Definitions. (line 74)
* reserved words: Reserved Words. (line 6)
* restricted shell: The Restricted Shell.
(line 6)
* return status: Definitions. (line 79)
* shell arithmetic: Shell Arithmetic. (line 6)
* shell function: Shell Functions. (line 6)
* shell script: Shell Scripts. (line 6)
* shell variable: Shell Parameters. (line 6)
* shell, interactive: Interactive Shells. (line 6)
* signal: Definitions. (line 82)
* signal handling: Signals. (line 6)
* special builtin: Definitions. (line 86)
* special builtin <1>: Special Builtins. (line 6)
* startup files: Bash Startup Files. (line 6)
* string translations: Creating Internationalized Scripts.
(line 3)
* suspending jobs: Job Control Basics. (line 6)
* tilde expansion: Tilde Expansion. (line 6)
* token: Definitions. (line 90)
* translation, native languages: Locale Translation. (line 6)
* unary arithmetic operators: Shell Arithmetic. (line 18)
* variable, shell: Shell Parameters. (line 6)
* variables, readline: Readline Init File Syntax.
(line 37)
* word: Definitions. (line 94)
* word splitting: Word Splitting. (line 6)
* yanking text: Readline Killing Commands.
(line 6)
Tag Table:
Node: Top891
Node: Introduction2822
Node: What is Bash?3035
Node: What is a shell?4168
Node: Definitions6778
Node: Basic Shell Features10105
Node: Shell Syntax11329
Node: Shell Operation12356
Node: Quoting13647
Node: Escape Character14985
Node: Single Quotes15520
Node: Double Quotes15869
Node: ANSI-C Quoting17214
Node: Locale Translation18608
Node: Creating Internationalized Scripts20011
Node: Comments24209
Node: Shell Commands24976
Node: Reserved Words25915
Node: Simple Commands26780
Node: Pipelines27442
Node: Lists30698
Node: Compound Commands32570
Node: Looping Constructs33579
Node: Conditional Constructs36128
Node: Command Grouping51198
Node: Coprocesses52690
Node: GNU Parallel55376
Node: Shell Functions56294
Node: Shell Parameters64742
Node: Positional Parameters69643
Node: Special Parameters70733
Node: Shell Expansions74194
Node: Brace Expansion76383
Node: Tilde Expansion79719
Node: Shell Parameter Expansion82674
Node: Command Substitution103317
Node: Arithmetic Expansion106846
Node: Process Substitution108022
Node: Word Splitting109130
Node: Filename Expansion111574
Node: Pattern Matching114798
Node: Quote Removal120521
Node: Redirections120825
Node: Executing Commands131088
Node: Simple Command Expansion131755
Node: Command Search and Execution133863
Node: Command Execution Environment136307
Node: Environment139755
Node: Exit Status141658
Node: Signals143716
Node: Shell Scripts148645
Node: Shell Builtin Commands151943
Node: Bourne Shell Builtins154054
Node: Bash Builtins180701
Node: Modifying Shell Behavior217625
Node: The Set Builtin217967
Node: The Shopt Builtin229961
Node: Special Builtins247013
Node: Shell Variables248002
Node: Bourne Shell Variables248436
Node: Bash Variables250944
Node: Bash Features290068
Node: Invoking Bash291082
Node: Bash Startup Files297666
Node: Interactive Shells302908
Node: What is an Interactive Shell?303316
Node: Is this Shell Interactive?303978
Node: Interactive Shell Behavior304802
Node: Bash Conditional Expressions308563
Node: Shell Arithmetic313980
Node: Aliases317309
Node: Arrays320443
Node: The Directory Stack328031
Node: Directory Stack Builtins328828
Node: Controlling the Prompt333273
Node: The Restricted Shell336158
Node: Bash POSIX Mode339040
Node: Shell Compatibility Mode357986
Node: Job Control366993
Node: Job Control Basics367450
Node: Job Control Builtins373818
Node: Job Control Variables380500
Node: Command Line Editing381731
Node: Introduction and Notation383434
Node: Readline Interaction385786
Node: Readline Bare Essentials386974
Node: Readline Movement Commands388782
Node: Readline Killing Commands389778
Node: Readline Arguments391801
Node: Searching392858
Node: Readline Init File395101
Node: Readline Init File Syntax396404
Node: Conditional Init Constructs423229
Node: Sample Init File427614
Node: Bindable Readline Commands430734
Node: Commands For Moving432272
Node: Commands For History434736
Node: Commands For Text440126
Node: Commands For Killing444251
Node: Numeric Arguments447039
Node: Commands For Completion448191
Node: Keyboard Macros453887
Node: Miscellaneous Commands454588
Node: Readline vi Mode461155
Node: Programmable Completion462132
Node: Programmable Completion Builtins470869
Node: A Programmable Completion Example482606
Node: Using History Interactively487951
Node: Bash History Facilities488632
Node: Bash History Builtins492367
Node: History Interaction498838
Node: Event Designators503788
Node: Word Designators505366
Node: Modifiers507758
Node: Installing Bash509695
Node: Basic Installation510811
Node: Compilers and Options514687
Node: Compiling For Multiple Architectures515437
Node: Installation Names517190
Node: Specifying the System Type519424
Node: Sharing Defaults520170
Node: Operation Controls520884
Node: Optional Features521903
Node: Reporting Bugs534283
Node: Major Differences From The Bourne Shell535640
Node: GNU Free Documentation License557066
Node: Indexes582243
Node: Builtin Index582694
Node: Reserved Word Index589792
Node: Variable Index592237
Node: Function Index609650
Node: Concept Index623645
End Tag Table
Local Variables:
coding: utf-8
End: