Bash (Unix shell)

<syntaxhighlight lang="console">$ printf '<newline>: <%b>\n' $'\n' <newline>: < > $ printf '<tab>: <%b>\n' $'\t' <tab>: < > $ printf '<space>: <%s>\n' " " <space>: < > $ printf '<NUL>: <%b>\n' $'\0' <NUL>: <> </syntaxhighlight>

Any series of characters is called a "string", or sometimes a "string literal". In Unix-like operating systems, all characters, printable and non-printing, except for a few such as the null character and forward slash Template:Char, can be used in filenames. In addition, all strings are case-sensitive.<ref> Template:Cite book </ref>

Bash, like many other programming languages, uses zero-based numbering.

Control+key combinations

The Control+key functionality is provided by GNU Readline and is available in interactive mode only. Certain keypress combinations allow a user to operate Bash to use tab completion and to search the command history.

Template:Keypress – Activate tab completion
Template:Keypress – Scroll up (i.e. backward) in the command history
Template:Keypress – Scroll down (i.e. forward) in the command history
Template:Keypress – Search the command history

Some keypress combinations also allow a user to operate the terminal emulator in order to move the cursor within the terminal window and to control the emulator program. By default, these keypress combinations in Bash mirror those of Emacs.<ref> Template:Cite web </ref>

Default keybindings for control codes include:

Template:Keypress – Move the cursor one character to the right
Template:Keypress – Move the cursor one character to the left
Template:Keypress – Move the cursor one word to the right
Template:Keypress – Move the cursor one word to the left
Template:Keypress – Move the cursor to the beginning of the current commandline
Template:Keypress – Cancels the current command and presents a new prompt
Template:Keypress – Closes the current Bash instance, possibly also closing the terminal-emulator
Template:Keypress – Move the cursor to the end of the current commandline
Template:Keypress – Wake the terminal; buffered keypresses are then processed
Template:Keypress – Put the terminal to sleep
Template:Keypress – Remove one word to the left of the cursor
Template:Keypress – Stop a foregrounded process

Vi keybindings are also available and can be enabled by running <syntaxhighlight lang="bash" class="" style="" inline="1">set -o vi</syntaxhighlight>.<ref> Template:Cite web</ref><ref> Template:Cite web</ref>

Syntax

When Bash reads a full command line, the complete string is broken down into tokens. "Tokens" are identified using, and separated from each other using metacharacters.

As of Bash 5.3, the 10 metacharacters are the space, tab, and newline, as well as the following characters: <syntaxhighlight lang="text" class="" style="" inline="1">|&;()<></syntaxhighlight>

"Blanks" are composed entirely of unquoted metacharacters, "operators" each contain at least one unquoted metacharacter and "words" may not include any unquoted metacharacters.

In practice, Bash breaks down full command strings into tokens or groups of tokens that do contain metacharacters and tokens or groups of tokens that do not contain any metacharacters—called "words". From there it further breaks words down into more specific, meaningful pieces like command names, variable assignment statements, etc.

The two blanks are space and tab.

Operators

Control operators perform a control function. They can be either a newline or one of the following: ||, &&, &, ;, ;;, ;&, ;;&, |, |&, (, or ).

Redirection operators redirect the input or output streams. They include <, >, &>, <<, and <<<.

Words

A word is a sequence of (non-meta-) characters treated as a single unit by the shell. A reserved word is a kind of a word that has a special meaning to the shell.<ref> Template:Cite web </ref> A name is a kind of a word separate from reserved words. Names consist solely of letters, underscores and numbers; which begins with either a letter or an underscore; which, however, may not begin with a number. Names also called identifiers, may be used for naming variables and functions.

Sixteen of the twenty-two "reserved words", which may be characters or words are as follows:

Names may only contain the characters ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789_.

In the following example of a full command string, metacharacters have a comma placed above them, <syntaxhighlight lang="text" class="" style="" inline="1"> ,</syntaxhighlight>, reserved words have a caret placed beneath them, <syntaxhighlight lang="text" class="" style="" inline="1"> ^</syntaxhighlight>, and other tokens have a backtick placed also beneath them, <syntaxhighlight lang="text" class="" style="" inline="1"> `</syntaxhighlight>.

Subshells

A "subshell" is an additional instance of the shell which has been initialized by a current instance of the shell. When a "parent" shell creates a subshell, or a "child" shell, an exact copy of the parent's environment information is re-created and becomes the environment of the subshell.

In Bash, in non-arithmetic contexts, one can force the use of a subshell by enclosing a full command string in single parentheses.

For this simple case, the preceding two commands are equivalent, however, use of subshells can have certain unexpected side effects. There are numerous different forms of syntax which can cause the initialization of a subshell.Template:Clarify

Expansion

Data structures

Bash offers variables and arrays as data structures, and though there are numerous kinds of each of these available, the data structures are relatively simple compared to other languages like C or Java.<ref name="gnu manual" /> All data is stored in memory as a string.

Beginning a word with a dollar character signifies that the word is the name of a variable or array. Surrounding the dollar / variable name syntax in double quotes is always advised. This practice shields the value(s) held by the parameter(s) from unwanted side effects.Template:Clarify

Wrapping the variable name in curly brackets (braces) <syntaxhighlight lang="text" class="" style="" inline="1">{} </syntaxhighlight> is recommended for readability and consistency between variables and arrays. When writing variables, curly brackets are optional and square brackets would be a syntax error. The parameter names are always on the left side of the equals sign and values are always on the right.

Variables

A variable is assigned to using the syntax name=value.

To use a variable, the syntax $name is used, or ${name}, which expands to the value assigned to the variable.

The latter syntax must be used for certain names to prevent unwanted side effects. For example, $10 will be parsed as ${1}0, so using ${10} means it will be parsed as intended.

Positional parameters, usually passed to a bash script, are denoted by the variables numbered starting from $0. Special parameters are signified by punctuation characters.<ref name="gnu manual" /> For example, <syntaxhighlight lang="text" class="" style="" inline="1">$@</syntaxhighlight> expands to a list of the first through last positional parameters, "individually requoted, separated by spaces".Template:Attribution needed

Environment variables are signified by all capital letters. Environment variables include UNIX variables like <syntaxhighlight lang="text" class="" style="" inline="1">LESS_SIGUSR1</syntaxhighlight>, and Bourne shell variables such as <syntaxhighlight lang="text" class="" style="" inline="1">HOME</syntaxhighlight>.<ref name="gnu manual" /> Scripting variables are signified by all lower case letters or CamelCase. This is only convention; any variable can be passed to the <syntaxhighlight lang="text" class="" style="" inline="1">export</syntaxhighlight> command to be made into an environment variable.

Arrays

Arrays are data structures which hold multiple values.<ref>https://www.gnu.org/software/bash/manual/html_node/Arrays.html</ref> Arrays have a set of square brackets placed at the end of the variable name and inside the curly braces. When writing arrays, curly braces and square brackets are required.

An array is assigned using the syntax name=( one or more elements ). It is expanded using <syntaxhighlight lang="bash" class="" style="" inline="1"> ${quux[@]} </syntaxhighlight> or <syntaxhighlight lang="bash" class="" style="" inline="1"> ${quux[*]} </syntaxhighlight> or <syntaxhighlight lang="bash" class="" style="" inline="1"> ${quux[1]} </syntaxhighlight>, depending on the use case.

Each kind of parameter is distinguished by a specific naming convention.<ref name="gnu manual">https://www.gnu.org/software/bash/manual/bash.html</ref>

Since Bash 4.0,<ref>https://www.linux-magazine.com/Online/News/Bash-4.0-Introduces-Associative-Arrays</ref> Bash also supports associative arrays.

In this article, examples of variables from this section include Template:Mono, Template:Mono and Template:Mono.

Execution

"Execution" of a given program occurs when a user (or some other program) asks the operating system to act upon the instructions contained in the given program.

By default, Bash reads user code one line at a time, interprets any newline or semi-colon character <syntaxhighlight lang="text" class="" style="" inline="1">;</syntaxhighlight> as the end of the current command, and executes commands in sequence. If an interactive command extends beyond the width of the terminal emulator, it is usually possible to keep typing and the command will wrap around. To extend a command beyond a newline onto an additional line, it is necessary that the final character of the first line be an unescaped backslash, <syntaxhighlight lang="text" class="" style="" inline="1">\</syntaxhighlight>, which signals "line continuation". Bash always finishes parsing and executing one full commandline before moving on to and beginning with the parsing of the next commandline.

<syntaxhighlight lang="console"> $ foo=aa bar=bb quux=cc zork=dd; set -o xtrace $ : "${foo}"; : "${bar}" + : aa + : bb $ : "${quux}" \ > : "${zork}" + : cc : dd $ </syntaxhighlight>

The first word of a command line is known as the "command position". Under UNIX coventionality, the first word of the command line is always some kind of command, and the rest of the words in the command line string are either options for the command, arguments for the options, or some kind of input upon which the command will operate. "Options" are also called "flags", "switches", or, more formally, "operators". When Bash attempts to locate a command for execution, the directories it searches are those listed in the <syntaxhighlight lang="text" class="" style="" inline="1">$PATH</syntaxhighlight> variable and the current working directory.<ref name="tldp_3.2.1"> Template:Cite book</ref>

<syntaxhighlight lang="console"> $ # [COMMAND POSITION] [OPTION] [ARGUMENTS] $ # ,--^ ,------------^ ,----^ $ declare -p USER BASH_VERSION declare -x USER="liveuser" declare -- BASH_VERSION="5.2.37(1)-release" $ </syntaxhighlight>

Users and PS1

A user account can be created for either a human or a programmatic user. In Unix-like operating systems, there are two kinds of users: "privileged" and "regular". A privileged user, such as root or the operating system kernel, is allowed to do anything whatsoever on the machine. Unprivileged users are limited in various ways.

When an interactive shell session waits for user input, by default it prints a particular string of characters to the screen. In Bash, the value of this waiting-string is held in the shell variable <syntaxhighlight lang="text" class="" style="" inline="1"> $PS1</syntaxhighlight>. For regular users, a common default value for <syntaxhighlight lang="text" class="" style="" inline="1"> $PS1</syntaxhighlight> is the dollar character, Template:Char.Template:Efn For the superuser, a common default value is hashtag (Template:Char)

<syntaxhighlight lang="console"> $ sudo --login --user root [sudo] password for liveuser:

vim /home/liveuser/names.txt
exit

$ grep -e bob ./names.txt grep: ./names.txt: Permission denied </syntaxhighlight>

Modes

Programming paradigm

Bash is written in C. A modular style can be approximated through good style and careful design.<ref> Template:Cite web</ref> It is often used in an imperative or procedural style.

Interactive and non-interactive modes

As a command processor, Bash can operate in two modes: interactive or non-interactive. In interactive mode, commands are usually read from a terminal emulator. In non-interactive mode, which facilitates automation, commands are usually read from named files known today as shell scripts. When executed as a standalone command at the command-line interface (CLI), by default Bash opens a new shell in interactive mode.

Scripts

Shell scripts are text files that contain code, often commands, intended to be read and acted upon by some particular interpreter in a batch process in a non-interactive mode and without any further user interaction. Interpreted scripts are programs that do not require their source code to be compiled: all of the relevant source code is contained within the script. There are many programs which can serve as a script interpreter: Perl, AWK, etc. Interpreted scripts are most often written for Unix shells.

The first two characters of the first line of any (executable) shell script begins with a something called a shebang: literally the characters hashtag (Template:Char) and bang (Template:Char) side by side.

<syntaxhighlight lang=console> $ cat ./example.sh

! /bin/env bash

echo foo exit

$ </syntaxhighlight>

If a script is intended to be run by a user as a stand-alone program on the commandline, then it is referred to as an "executable". By convention, the filenames of executable unix shell scripts are identified the suffix <syntaxhighlight lang="text" class="" style="" inline="1">.sh</syntaxhighlight>. The "execute" bit can be enabled on a shell script with the utility <syntaxhighlight lang="text" class="" style="" inline="1">chmod</syntaxhighlight>:

<syntaxhighlight lang=console> $ ls -l ./example.sh -rw-r--r--.1 liveuser liveuser 32 Aug 3 22:33 example.sh $ ./example.sh bash: ./example.sh: Permission denied $ chmod 0744 ./example.sh $ ls -l ./example.sh -rwxr--r--.1 liveuser liveuser 32 Aug 3 22:33 example.sh $ ./example.sh foo $ </syntaxhighlight>

The <syntaxhighlight lang="text" class="" style="" inline="1">source</syntaxhighlight> builtin

With the <syntaxhighlight lang="text" class="" style="" inline="1"> source</syntaxhighlight>, or synonymous <syntaxhighlight lang="text" class="" style="" inline="1"> .</syntaxhighlight> command, Bash reads and executes shell commands from any text file by name.<ref> Template:Cite web </ref>

Login and non-login shells

Bash can be executed as a login shell, or "session leader," in both interactive and non-interactive modes via the <syntaxhighlight lang="text" class="" style="" inline="1"> --login</syntaxhighlight> option. "Logging in" requires user authentication. For this reason, only one login shell exists per user session. In GNU/Linux, a user's login shell is identified in the /etc/passwd file.

<syntaxhighlight lang="console"> $ awk -F ':' '$1 ~ /root/' /etc/passwd root:x:0:0:Super User:/root:/bin/bash </syntaxhighlight>

When a human user initiates a login session, this procedure often occurs in a graphical user interface (GUI). When a user opens a terminal emulator, the emulator executes a non-login instance of the user's login shell.

Logging out of a shell session from within a terminal emulator can be accomplished with the Template:Mono command or, by default in Bash, pressing Template:Keypress.

Startup <syntaxhighlight lang="text" class="" style="" inline="1">source</syntaxhighlight> files

When Bash starts, it uses <syntaxhighlight lang="text" class="" style="" inline="1">source</syntaxhighlight> to execute commands in a variety of dotfiles (see lists below).<ref name="drdobbs"> Template:Cite web</ref> These dotfiles, unlike shell scripts, typically have neither the execute permission enabled nor a hash-bang. By default Bash will source a somewhat different set of files, and in a different sequence, depending on:<ref> Template:Cite web</ref>

How Bash is called: interactively, non-interactively, invoked with name <syntaxhighlight lang="text" class="" style="" inline="1"> sh</syntaxhighlight>
Which options are used: <syntaxhighlight lang="text" class="" style="" inline="1"> --login</syntaxhighlight>, <syntaxhighlight lang="text" class="" style="" inline="1"> --rcfile</syntaxhighlight>, <syntaxhighlight lang="text" class="" style="" inline="1"> --norc</syntaxhighlight>, <syntaxhighlight lang="text" class="" style="" inline="1"> --posix</syntaxhighlight>
Which environment variables are defined: <syntaxhighlight lang="text" class="" style="" inline="1">BASH_ENV</syntaxhighlight>, <syntaxhighlight lang="text" class="" style="" inline="1">ENV</syntaxhighlight>, and
Which files exist:
- <syntaxhighlight lang="text" class="" style="" inline="1"> /etc/profile</syntaxhighlight>
- <syntaxhighlight lang="text" class="" style="" inline="1"> ~/.bash_profile</syntaxhighlight>
- <syntaxhighlight lang="text" class="" style="" inline="1"> ~/.bash_login</syntaxhighlight>
- <syntaxhighlight lang="text" class="" style="" inline="1"> ~/.profile</syntaxhighlight>
- <syntaxhighlight lang="text" class="" style="" inline="1"> ~/.bash_logout</syntaxhighlight>, and
- <syntaxhighlight lang="text" class="" style="" inline="1"> ~/.bashrc</syntaxhighlight> among others.

Of course, any startup file can also execute commands from any other file. Startup files can affect shell behavior, terminal emulators, the X window system and the window manager.

POSIX mode

The POSIX IEEE 1003.1 standard specifies a common set of definitions that any shell system application (bash, dash, zsh, etc.) may conform to. Any shell user script (<syntaxhighlight lang="text" class="" style="" inline="1"> ./myscript.sh</syntaxhighlight>) written in conformance with POSIX guidelines should be executable by any shell system application that has implemented the POSIX specification. As a result, there can be a reasonable expectation that POSIX-compliant scripts can be executed with success on any Unix or Unix-like operating systems which implements the POSIX standard (Linux, OpenBSD, Oracle Linux, HP-UX, etc.). These scripts are considered "portable" as they are and without any further modifications. The portion of POSIX that applies to shells and command line utilities is a subset of a larger group of POSIX standards that further specify how terminals and terminal emulators aught to function in order to also be considered portable.

When Bash is operating in POSIX mode, fewer features are available but the resulting code can be executed on a greater variety of operating systems.

To enable POSIX mode at the initialization of an interactive shell, Bash can be executed as either <syntaxhighlight lang="text" class="" style="" inline="1">sh</syntaxhighlight>, <syntaxhighlight lang="text" class="" style="" inline="1">bash --posix</syntaxhighlight> or <syntaxhighlight lang="text" class="" style="" inline="1">bash -o posix</syntaxhighlight>.<ref name="tldp_psx"> Template:Cite book</ref> To cause a script to be initialized in POSIX mode, one would use the either the hashbang <syntaxhighlight lang="text" class="" style="" inline="1">#! /bin/env sh</syntaxhighlight> or the less portable <syntaxhighlight lang="text" class="" style="" inline="1">#!/bin/sh</syntaxhighlight>. When an instance of Bash is operating in POSIX mode, the environment variable <syntaxhighlight lang="text" class="" style="" inline="1">$POSIXLY_CORRECT</syntaxhighlight> is defined, and the value of the environment variable <syntaxhighlight lang="text" class="" style="" inline="1">SHELLOPTS</syntaxhighlight> includes the string Template:Mono.

<syntaxhighlight lang="console"> $ declare -p POSIXLY_CORRECT bash: declare: POSIXLY_CORRECT: not found $ sh $ declare -p POSIXLY_CORRECT declare -- POSIXLY_CORRECT="y" $ </syntaxhighlight>

The full list of features available in Bash which are not specified by POSIX is considerable.<ref> Template:Cite web</ref> Here is a partial list:

Any arrays other than the array of positional parameters, <syntaxhighlight lang="text" class="" style="" inline="1">$@</syntaxhighlight>, are not POSIX
The double bracket extended test construct, <syntaxhighlight lang="text" class="" style="" inline="1">...</syntaxhighlight>, is not POSIX
- <syntaxhighlight lang="text" class="" style="" inline="1">[...]</syntaxhighlight> and <syntaxhighlight lang="text" class="" style="" inline="1">test</syntaxhighlight> are POSIX
One of the double-parentheses arithmetic-evaluation syntaxes, <syntaxhighlight lang="text" class="" style="" inline="1">((...))</syntaxhighlight>, is not POSIX
- <syntaxhighlight lang="text" class="" style="" inline="1">$((...))</syntaxhighlight> is POSIX
Brace expansion, kernel{,-headers}, is not POSIX
Dynamic scoping of parameters and the <syntaxhighlight lang="text" class="" style="" inline="1">local</syntaxhighlight> builtin are not POSIX
Process substitution, <syntaxhighlight lang="text" class="" style="" inline="1"><(...)</syntaxhighlight>, is not POSIX
Certain string-manipulation operations in parameter expansions are not POSIX
Most Bash builtin commands are not POSIX
- The command <syntaxhighlight lang="bash" class="" style="" inline="1">enable -s</syntaxhighlight> prints the list of Bourne special builtins, which are POSIX<syntaxhighlight lang="console">

$ enable -s | wc --lines 16 $ enable | wc --lines 61 </syntaxhighlight>

- The <syntaxhighlight lang="text" class="" style="" inline="1">enable</syntaxhighlight> builtin itself is not POSIX
- In Bash, in non-POSIX mode, the <syntaxhighlight lang="text" class="" style="" inline="1">.</syntaxhighlight> and <syntaxhighlight lang="text" class="" style="" inline="1">source</syntaxhighlight> builtins are synonymous
  - The <syntaxhighlight lang="text" class="" style="" inline="1">.</syntaxhighlight> (i.e., 'dot') builtin is POSIX, however
  - The <syntaxhighlight lang="text" class="" style="" inline="1">source</syntaxhighlight> builtin is not POSIX
The <syntaxhighlight lang="text" class="" style="" inline="1">$EPOCHSECONDS</syntaxhighlight> and <syntaxhighlight lang="text" class="" style="" inline="1">$EPOCHREALTIME</syntaxhighlight> shell variables are not POSIX

System commands which are available in modern Unix-like operating systems, and which are also specified by POSIX, may have fewer option flags or fewer relevant environment variables available under POSIX. Most (such as <syntaxhighlight lang="text" class="" style="" inline="1">ls</syntaxhighlight>) are standalone programs in the <syntaxhighlight lang="text" class="" style="" inline="1">/bin</syntaxhighlight>, <syntaxhighlight lang="text" class="" style="" inline="1">/usr/bin</syntaxhighlight>, <syntaxhighlight lang="text" class="" style="" inline="1">/sbin</syntaxhighlight> or <syntaxhighlight lang="text" class="" style="" inline="1">/usr/sbin</syntaxhighlight> directories (in Linux, typically provided by GNU coreutils or BusyBox) rather than Bash builtins.

Because of these and other differences, modern (version 5) Bash shell scripts are rarely runnable "as-is" under the Bourne or legacy Korn shell interpreters. Scripting with portability in mind is becoming less common as GNU/Linux becomes more widespread.<ref name =tldp_psx/><ref name=deb_pol> Template:Cite book</ref>

Code that is valid syntax in Bash but not specified by POSIX is called a "bashism". The program <syntaxhighlight lang="text" class="" style="" inline="1">checkbashisms</syntaxhighlight> can be used to make sure that a script can be executed in Debian Linux without any portability errors.<ref name="checkbashisms-1"> Template:Man</ref> Vidar Holen's <syntaxhighlight lang="text" class="" style="" inline="1">shellcheck</syntaxhighlight> is another static linter written in Haskell which can parse script syntax for compatibility with any or all of bash, dash, ksh, and Bourne sh.<ref name="shellcheck-1">Template:Man</ref>

The syntax requirements for each shell are each a little different. For example, Debian's policy allows some extensions in their scripts (as they are in the dash shell),<ref name =deb_pol/> while a script intending to support pre-POSIX Bourne shells, like autoconf's Template:Mono, are even more limited in the features they can use.<ref> Template:Cite book</ref>

Other modes

Restricted mode

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 numerous actions are disallowed or not performed, including:

Changing directories with the <syntaxhighlight lang="text" class="" style="" inline="1">cd</syntaxhighlight> builtin.
Setting or unsetting the values of the <syntaxhighlight lang="bash" class="" style="" inline="1">SHELL</syntaxhighlight>, <syntaxhighlight lang="bash" class="" style="" inline="1">PATH</syntaxhighlight>, <syntaxhighlight lang="bash" class="" style="" inline="1">HISTFILE</syntaxhighlight>, <syntaxhighlight lang="bash" class="" style="" inline="1">ENV</syntaxhighlight>, or <syntaxhighlight lang="bash" class="" style="" inline="1">BASH_ENV</syntaxhighlight> variables.
Specifying command names containing slashes on the CLI.
Using absolute pathnames as arguments to the <syntaxhighlight lang="text" class="" style="" inline="1">.</syntaxhighlight>, <syntaxhighlight lang="text" class="" style="" inline="1">history</syntaxhighlight>, or <syntaxhighlight lang="bash" class="" style="" inline="1">hash -p</syntaxhighlight> commands.
Specifying a path search with <syntaxhighlight lang="text" class="" style="" inline="1">. -p</syntaxhighlight> or <syntaxhighlight lang="bash" class="" style="" inline="1">command -p</syntaxhighlight>.
Importing function definitions and parsing the value of <syntaxhighlight lang="bash" class="" style="" inline="1">SHELLOPTS</syntaxhighlight> from the shell environment at startup.
Redirecting output using the <syntaxhighlight lang="text" class="" style="" inline="1">></syntaxhighlight>, <syntaxhighlight lang="text" class="" style="" inline="1">></syntaxhighlight>, <syntaxhighlight lang="text" class="" style="" inline="1"><></syntaxhighlight>, <syntaxhighlight lang="text" class="" style="" inline="1">>&</syntaxhighlight>, <syntaxhighlight lang="text" class="" style="" inline="1">&></syntaxhighlight>, and <syntaxhighlight lang="text" class="" style="" inline="1">>></syntaxhighlight> redirection operators.
Using the <syntaxhighlight lang="text" class="" style="" inline="1">exec</syntaxhighlight> builtin to replace the shell with another command.
Altering shell builtins.

Once restricted mode is enabled, it cannot be disabled. These restrictions are enforced after any startup files are read, and it does not apply to shell scripts. Restricted mode is rarely used.

Privileged mode

In Bash, "privileged mode" is a rarely used option inheritedTemplate:Citation needed from the SVR4.2 UNIX System V shell (circa 1992).<ref>Template:Cite book</ref> It can be enabled with <syntaxhighlight lang="bash" class="" style="" inline="1"> set -p</syntaxhighlight> and disabled with <syntaxhighlight lang="bash" class="" style="" inline="1"> set +p</syntaxhighlight>.<ref> Template:Cite book</ref> When privileged mode is enabled, the <syntaxhighlight lang="bash" class="" style="" inline="1">SHELLOPTS</syntaxhighlight> shell variables includes the string "privileged".

Extended debugging mode

Enabled via <syntaxhighlight lang="text" class="" style="" inline="1">bash --debugger</syntaxhighlight> at invocation or via <syntaxhighlight lang="bash" class="" style="" inline="1">shopt -s extdebug</syntaxhighlight> during either interactive or non-interactive modes. It uses a separate program called Template:Mono.<ref name="ref_bashdb"> Template:Cite web</ref> Template:Mono is not available in POSIX mode. See documentation for more information. See also Template:Section link.

Compatibility modes

Observability

The Template:Mono option

When Template:Mono is enabled, simple debugging content is printed to the terminal. It can be enabled with <syntaxhighlight lang="text" class="" style="" inline="1"> set -o xtrace</syntaxhighlight> or <syntaxhighlight lang="text" class="" style="" inline="1"> set -x</syntaxhighlight>, and disabled with <syntaxhighlight lang="text" class="" style="" inline="1"> set +o xtrace</syntaxhighlight>, <syntaxhighlight lang="text" class="" style="" inline="1"> set +x</syntaxhighlight> or <syntaxhighlight lang="text" class="" style="" inline="1"> set -</syntaxhighlight>. These options are also accepted at the commandline and at hash-bangs: <syntaxhighlight lang="text" class="" style="" inline="1"> #!/bin/bash -x</syntaxhighlight>, etc.

<syntaxhighlight lang="console"> $ bash -x $ echo $(( 2 + 2 )) + echo 4 4 $ set -- 1 2 3 $ printf '<%s>\n' "$@" + printf '<%s>\n' 1 2 3 <1> <2> <3> $ </syntaxhighlight>

The Template:Mono shell setting is specified by POSIX. See also Template:Section link.

The Template:Mono option

The verbose option prints strings to the terminal as they are read, and before any expansions are performed. Rarely used.<ref>See <syntaxhighlight lang="bash" class="" style="" inline="1"> set -v</syntaxhighlight> in the documentation.</ref>

Comments

Comments can be a valuable way of clarifying information or explaining a script or source file to someone else who might not be familiar with the scripter's intentions or context.

Standard comments in Bash are denoted with a hash character: Template:Char. Any text to the right of the hash to the end of the line will be ignored. Inline comments are allowed, but hash comments will not print during debugging. See also: Template:Section link.

Comments denoted with a colon character, Template:Char, originated with the Thompson shell. Any arguments to the right of colon Template:Char builtin are ignored. Inline comments are not possible, but colon comments will print during debugging and any parameters will have been expanded. <ref> Template:Cite web</ref>

$ # Define foo $ foo=bar # An inline hash comment occurs on the same line as a command $ set -x $ # A regular comment (no output) $ : "${foo}" + : bar $ </syntaxhighlight>

Exit codes

When bash executes commands, exit status codes, also called "return codes," are produced which can offer some insight into the manner in which a program ceased running. The value of the most recently captured exit code is held within the shell parameter, 'question mark:' <syntaxhighlight lang="text" class="" style="" inline="1"> $? </syntaxhighlight>. In non-arithmetic contexts, (i.e., most of the time) the numerical or "Boolean" value of "true" is zero (0), and the value of "false" is one (1).

When a system command has executed, the intended meaning of its exit status can most often be found in its man page; usually a zero indicates success and a nonzero exit status indicates some kind of failure condition or partial success. <syntaxhighlight lang="text" class="" style="" inline="1"> ping</syntaxhighlight> is a well known command with three meaningful exit codes: 0, 1, and 2.

In Bash, within arithmetic contexts, the numerical truth values are reversed: "true" is one and "false" is zero. An arithmetic context can usually be identified by the syntax <syntaxhighlight lang="text" class="" style="" inline="1"> ((...)) </syntaxhighlight> or <syntaxhighlight lang="text" class="" style="" inline="1"> $((...)) </syntaxhighlight>. If an arithmetic statement evaluates to the integer zero, then the statement is considered "true," and the exit code is one. If the statement evaluates to any number other than zero the arithmetic statement is "false" and the exit code is zero.

Not all Linux/UNIX commands provide meaningful exit codes beyond zero and one, and there is no standard system for definitions of exit codes in Linux.

$ true; echo "$?" # Exit code means "true" 0 $ false; echo "$?"; echo # Exit code means "false" 1 $ $ bash -c 'exit 99'; printf 'exit-code: %d\n\n' "$?" exit-code: 99 $ $ (( 1 - 1 )); printf '%d\n' "$?" # This exit code means "true" 1 $ (( 1 + 1 )); printf '%d\n' "$?" # ...and this exit code means "false" 0 </syntaxhighlight>

Job control

The Bash shell has two modes of execution for commands: batch (asynchronous), and concurrent (synchronous). To execute commands in batch mode (i.e., in sequence) they must be separated by the character Template:Char, or on separate lines:

$ command1; command2 $ command3 $ </syntaxhighlight>

In this example, when Template:Mono is finished, Template:Mono is executed, and when Template:Mono has completed, Template:Mono will execute. A background execution of Template:Mono can occur using symbol Template:Char at the end of an execution command, and process will be executed in background while immediately returning control to the shell and allowing continued execution of commands.

$ command1 & $ </syntaxhighlight>

Or to have a concurrent execution of Template:Mono and Template:Mono, they must be executed in the Bash shell in the following way:

$ command1 & command2 $ </syntaxhighlight>

In this case Template:Mono is executed in the background, Template:Char symbol, returning immediate control to the shell that executes Template:Mono in the foreground. A process can be stopped and control returned to bash by typing Template:Key press while the process is running in the foreground.<ref> Template:Cite web</ref> A list of all processes, both in the background and stopped, can be achieved by running Template:Mono:

$ jobs [1]- Running command1 & $ </syntaxhighlight>

In the output, the number in brackets refers to the job id. The plus sign signifies the default process for Template:Mono and Template:Mono. The text "Running" and "Stopped" refer to the process state. The last string is the command that started the process.

The state of a process can be changed using various commands. The Template:Mono command brings a process to the foreground, while Template:Mono sets a stopped process running in the background. Template:Mono and Template:Mono can take a job id as their first argument, to specify the process to act on. Without one, they use the default process, identified by a plus sign in the output of Template:Mono. The Template:Mono command can be used to end a process prematurely, by sending it a signal. The job id must be specified after a percent sign:

$ sleep 100 & [1] 4904 $ kill %1 $ jobs [1]+ Terminated sleep 100 $ </syntaxhighlight>

Job control, also known as "Monitor mode," is enabled by default in interactive shells, and can be disabled with <syntaxhighlight lang="bash" class="" style="" inline="1">set +m</syntaxhighlight>.

Signals

Signaling is a means of inter-process communication (IPC). Sometimes a commandline process may seem to freeze in the middle of execution. In these instances it may become necessary to identify which process may be blocked and to manually end the offending process.

At an interactive terminal, it is usually sufficient to press Template:Key to end the current foreground process and return control back to the user prompt, or to press Template:Key to suspend it. Occasionally attempting to suspend a process will succeed when attempts to cancel a process appear unresponsive. In other cases it may be necessary to use the Template:Mono program to send an IPC signal. In this example, we use the Template:Mono command from a second terminal screen to terminate the process with PID 4331.

$ tty # Terminal one /dev/pts/0 $ whoami liveuser $ sleep 1000 # Command hangs </syntaxhighlight>

$ tty # Terminal two /dev/pts/1 $ whoami liveuser $ ps aux | grep -e sleep -e PID USER PID %CPU %MEM VSZ RSS TTY STAT START TIME COMMAND liveuser 4331 0.0 0.0 230336 2312 pts/1 S+ 11:19 0:00 sleep 1000 liveuser 4333 0.0 0.0 231248 2516 pts/0 S+ 11:19 0:00 grep --color=auto -e sleep -e PID $ kill 4331 $ ps aux | grep -e sleep -e PID # The sleep process has ended USER PID %CPU %MEM VSZ RSS TTY STAT START TIME COMMAND liveuser 4333 0.0 0.0 231248 2516 pts/0 S+ 11:19 0:00 grep --color=auto -e sleep -e PID $ </syntaxhighlight>

$ tty # Terminal one again /dev/pts/0 $ whoami liveuser $ sleep 1000 Terminated $ </syntaxhighlight>

In Unix-like operating systems, a user is allowed to instruct the kernel to send a signal to a process that is owned by the user. A regular user may not send a signal to a privileged process. Signals can be sent to a process using the Template:Mono builtin or using the system binary of the same name.

$ whoami liveuser $ ps aux | awk '$2 ~ /\<1\>/' # Let\s view some info on the kernel process, process 1. root 1 0.0 0.2 37140 20440 ? Ss 04:44 0:18 /usr/lib/systemd/systemd --switched-root --system --deserialize=53 rhgb $ kill -s SIGKILL 1 bash: kill: (1) - Operation not permitted $ type -a kill kill is a shell builtin kill is /usr/bin/kill $ /usr/bin/kill -s SIGKILL 1 kill: sending signal to 1 failed: Operation not permitted $ </syntaxhighlight>

The most commonly used signals can be viewed with kill -L | head -n 4. Each IPC signal is associated with a signal number, but exit codes and signal codes are two different things. While sending a process an IPC signal of 9 (a "KILL" signal) will almost certainly terminate the process immediately, it will most likely not result in the process returning an exit code of 9.

By default in Bash, builtin kill sends a TERM ("terminate") signal. It's common for commandline utilities to respond to a SIGTERM by shutting down and exiting cleanly. (TERM and SIGTERM are the same, the SIG- prefix to all signal names can be omitted.) The Ctrl-c keypress sequence in Bash sends a SIGINT, interrupt signal, to the foreground process. The Ctrl-z keypress sequence sends the SIGSTOP, stop signal.<ref> Template:Cite book</ref> When a process receives a SIGKILL, the process terminates immediately and messily. It is recommended to use SIGKILL only as a last resort.<ref> Template:Cite book</ref> The SIGKILL signal cannot be blocked or handled.

Processes can "catch" and "handle" IPC signals they receive. A user can use the kill builtin to "send" an IPC signal to another process. That target process can set up a mechanism, some plan beforehand, for how to repsond whenever any particular signal might be received, or "caught." The way a target program responds is referred to as how the program "handles" receiving the signal. In the man pages one can see how some system commands will print out certain information to the terminal when they receive a SIGHUP: for example, the <syntaxhighlight lang="bash" class="" style="" inline="1"> dd</syntaxhighlight> command.<ref>Template:Cite web</ref>

By default Bash shell scripts receive and respond to any and all IPC signals sent to them, however, Bash scripts can utilize the Template:Mono builtin to catch and handle signals.<ref> Template:Cite web</ref>

$ cat ./trap-example.sh

! /usr/bin/env bash

trap umask EXIT echo bar exit 0 $ chmod 0700 trap-example.sh $ ./trap-example.sh bar 0077 $ </syntaxhighlight>

There are a few signals which are only available from within Bash as GNU extensions: Template:Mono, Template:Mono, Template:Mono and Template:Mono. These signals can be useful in debugging, and can only be sent and handled by shell builtins. See also Template:Section link.

Values of parameters

There are many different implementations of Template:Mono. Some have the <syntaxhighlight lang="text" class="" style="" inline="1">-e</syntaxhighlight> option, and some don't.<ref> Template:Cite book</ref> The list of options is not uniform across implementations, though Template:Mono and Template:Mono are both specified by POSIX. If a scripter wishes to know the precise value of a string contained by a variable, then the most consistent way of doing so is to use Template:Mono.

For any string containing any character (besides null?) including digits, the format specifier is Template:Mono.Template:Citation needed

$ foo=abc bar=123 $ printf '<%s>\n' "${foo}" "${bar}" <abc> <123> $ </syntaxhighlight>

For digits only, the format specifier is Template:Mono.

$ printf '<%d>\n' "${foo}" "${bar}" bash: printf: abc: invalid number <0> <123> $ </syntaxhighlight>

With Template:Mono, a newline is never included in the output unless the scripter includes a newline in the format string. In the example below, where a newline has been omitted from the format string, the value of PS1 is printed on the same line as the output of the previous command.

$ printf '<%s>' "${foo}" "${bar}" <abc><123>$ </syntaxhighlight>

Another very consistent method is to use <syntaxhighlight lang="bash" class="" style="" inline="1">declare -p</syntaxhighlight>. The output of <syntaxhighlight lang="bash" class="" style="" inline="1">declare -p</syntaxhighlight> can be reused as input. However, not all variables and parameters can be printed using <syntaxhighlight lang="bash" class="" style="" inline="1">declare -p</syntaxhighlight>, for example, the values of the Special Parameters. The Special Parameter hashtag, <syntaxhighlight lang="bash" class="" style="" inline="1">"$#"</syntaxhighlight>, reports how many Positional Parameters are currently defined.

$ declare -p foo bar declare -- foo="abc" declare -- bar="123" $ declare -p "$#" bash: declare: 0: not found $ </syntaxhighlight>

For a full string of input at an interactive shell...

$ declare -p # </syntaxhighlight>

...the hashtag would be interpreted by Bash as an inline comment. With the comment and all text to the right of it removed, the command that Bash would execute would be <syntaxhighlight lang="bash" class="" style="" inline="1">declare -p</syntaxhighlight>. This command would, according to <syntaxhighlight lang="text" class="" style="" inline="1">help declare</syntaxhighlight>, "display the values and attributes of each NAME," i.e., each variable, and, "if no NAMEs are given, display the values and attributes and values of all variables," which can be over 100 lines of output.

On the other hand, Template:Mono cannot display variables' attributes. See also Template:Section link.

$ readonly foo $ declare -p foo declare -r foo="abc" $ printf '<%s>' "${foo}" <abc> $ </syntaxhighlight>

Environment

Template:How-to Configurable execution environment(s):<ref> Template:Cite web</ref>

Shell and session startup files such as <syntaxhighlight lang="text" class="" style="" inline="1"> ~/.bashrc</syntaxhighlight> and <syntaxhighlight lang="text" class="" style="" inline="1"> ~/.profile</syntaxhighlight> (i.e., dotfiles);
Settings (Template:Mono built-in) and shell options (Template:Mono built-in) which alter shell behavior;

Shell and session startup Files (a.k.a., "dot files")

When Bash starts, it executes the commands in a variety of dot files.<ref name="drdobbs" /> Unlike Bash shell scripts, dot files typically have neither the execute permission enabled nor an interpreter directive like <syntaxhighlight lang="text" class="" style="" inline="1"> #!/bin/bash</syntaxhighlight>.

Legacy-compatible Bash startup example

The example <syntaxhighlight lang="text" class="" style="" inline="1"> ~/.bash_profile</syntaxhighlight> below is compatible with the Bourne shell and gives semantics similar to csh for the <syntaxhighlight lang="text" class="" style="" inline="1"> ~/.bashrc</syntaxhighlight> and <syntaxhighlight lang="text" class="" style="" inline="1"> ~/.bash_login</syntaxhighlight>. The [ -r filename ] && cmd is a short-circuit evaluation that tests if filename exists and is readable, skipping the part after the <syntaxhighlight lang="text" class="" style="" inline="1"> &&</syntaxhighlight> if it is not.

[ -r ~/.profile ] && ~/.profile # set up environment, once, Bourne-sh syntax only if [ -n "$PS1" ]; then # are we interactive?

  [ -r ~/.bashrc     ] && ~/.bashrc        # tty/prompt/function setup for interactive shells
  [ -r ~/.bash_login ] && ~/.bash_login    # any at-login tasks for login shell only

fi # End of "if" block </syntaxhighlight>

Operating system issues in Bash startup

Some versions of Unix and Linux contain Bash system startup scripts, generally under the <syntaxhighlight lang="text" class="" style="" inline="1"> /etc</syntaxhighlight> directory. Bash executes these files as part of its standard initialization, but other startup files can read them in a different order than the documented Bash startup sequence. The default content of the root user's files may also have issues, as well as the skeleton files the system provides to new user accounts upon setup. The startup scripts that launch the X window system may also do surprising things with the user's Bash startup scripts in an attempt to set up user-environment variables before launching the window manager. These issues can often be addressed using a <syntaxhighlight lang="text" class="" style="" inline="1"> ~/.xsession</syntaxhighlight> or <syntaxhighlight lang="text" class="" style="" inline="1"> ~/.xprofile</syntaxhighlight> file to read the <syntaxhighlight lang="text" class="" style="" inline="1"> ~/.profile</syntaxhighlight> — which provides the environment variables that Bash shell windows spawned from the window manager need, such as xterm or Gnome Terminal.

Standard streams

Standard streams - STDIN, STDOUT and STDERR

Commands

System commands

Aliases

Keywords and reversed words

<syntaxhighlight lang="text" class="" style="" inline="1"> function</syntaxhighlight>
- Bash function declarations which include this particular keyword are not compatible with Bourne/Korn/POSIX scripts, however, Bash does accepts the function declaration syntax used by Bourne, Korn and POSIX-compliant shells.

Functions

Builtin commands

Various Built-In Commands:
- POSIX Special builtins:<ref>

- - Template:Mono, Template:Mono, etc.
- Template:Mono<ref>

- - Xtrace: [ <syntaxhighlight lang="bash" class="" style="" inline="1"> set -x</syntaxhighlight> | <syntaxhighlight lang="bash" class="" style="" inline="1"> set -o xtrace</syntaxhighlight> ]. The shell's primary means of debugging. Both xtrace and verbose can be turned off at the same time with the command <syntaxhighlight lang="bash" class="" style="" inline="1"> set -</syntaxhighlight>.
  - Verbose: [ <syntaxhighlight lang="bash" class="" style="" inline="1"> set -v</syntaxhighlight> | <syntaxhighlight lang="bash" class="" style="" inline="1"> set -o verbose</syntaxhighlight> ]. Prints a command to the terminal as Bash reads it. Bash reads constructs all at once, such as compound commands which include if-fi and case-esac blocks. If a <syntaxhighlight lang="bash" class="" style="" inline="1"> set -v</syntaxhighlight> is included within a compound command, then "verbose" will be enabled the next time Bash reads code as input, i.e., after the end of the currently executing construct.<ref>

Template:Cite mailing list</ref>

- - Both xtrace and verbose can be turned off at the same time with the command <syntaxhighlight lang="bash" class="" style="" inline="1"> set -</syntaxhighlight>.
- Template:Mono<ref name="bash-hackers.org_changes">

- - expand-aliases: On by default in interactive shells. Some developers discourage its use in scripts.

PATH and system commands

When the shell looks for external commands, it relies on the Bourne shell variable <syntaxhighlight lang="bash" class="" style="" inline="1"> $PATH</syntaxhighlight>. <syntaxhighlight lang="bash" class="" style="" inline="1"> $PATH</syntaxhighlight> contains a list of directories separated by colons, <syntaxhighlight lang="bash" class="" style="" inline="1"> :</syntaxhighlight>. Beginning with the leftmost directory and selecting directories in a left to right pattern, each directory is searched until a match is found. In Linux, so that a user can locate additional commands, it's common practice for distribution administrators and package developers to alter the value of an end user's <syntaxhighlight lang="bash" class="" style="" inline="1"> $PATH</syntaxhighlight> by including source files in <syntaxhighlight lang="bash" class="" style="" inline="1"> /etc/profile.d</syntaxhighlight> and other locations.

When looking for the command, <syntaxhighlight lang="bash" class="" style="" inline="1"> chmod</syntaxhighlight>, for instance, after considering internal commands and finding nothing, Bash will search the directories in <syntaxhighlight lang="bash" class="" style="" inline="1"> $PATH</syntaxhighlight> and will select the absolute path of the first executable found that has a basename which matches the search string.<ref name="tldp_3.2.1"/>

If there is more than one command <syntaxhighlight lang="bash" class="" style="" inline="1"> echo</syntaxhighlight> available in the directories listed in <syntaxhighlight lang="bash" class="" style="" inline="1"> $PATH</syntaxhighlight>, during the process of parsing and executing a commandline, by default only the first command found will be selected. <syntaxhighlight lang="bash" class="" style="" inline="1"> $PATH</syntaxhighlight> lookups are slow. The shell speeds up the commandline execution process by remembering command locations in a hash table. To perform a full <syntaxhighlight lang="bash" class="" style="" inline="1"> $PATH</syntaxhighlight> search without any interference from the hash table, remove the current table with <syntaxhighlight lang="bash" class="" style="" inline="1"> hash -r</syntaxhighlight> and search for all kinds of commands with <syntaxhighlight lang="bash" class="" style="" inline="1"> type -a</syntaxhighlight>.

$ # Force a full path search $ PATH=${PATH}:${HOME} $ printf 'echo script_file: "$@"\n' > ./echo $ chmod 0700 ./echo $ hash -r; type -a echo echo is a shell builtin echo is /usr/bin/echo echo is /home/liveuser/echo $ </syntaxhighlight>

In order to execute a commandline with a command found later in the <syntaxhighlight lang="bash" class="" style="" inline="1"> $PATH</syntaxhighlight> string, you can specify an absolute path or you can anchor path resolution relative to the current working directory.

$ /home/liveuser/echo foo script_file: foo $ ./echo bar script_file: bar $ </syntaxhighlight>

For security reasons it is advisable to make sure the directories in PATH are not world-writeable, or are writeable only by root and trusted users.

Command lookup

Command position: after expansions, the first word of the full text of the command line.
Command name lookup is performed, in the following order:
- Commands internal to the shell:
  - Shell aliases,
  - Shell reserved words,
  - Shell functions, and
  - Shell built-in commands;
- Commands external to the shell, using the PATH shell variable:
  - Separate UNIX-style programs such as [[ls|Template:Mono]] or [[Ln (Unix)|Template:Mono]], and
  - Shell scripts, which are files containing executable commands.Template:Efn
The resulting string is executed as a command.

Control structures

Template:See also-text

Subshells

Subshells: <syntaxhighlight lang="text" class="" style="" inline="1"> (...)</syntaxhighlight>;

Pipelines

However, by using a pipeline, they can engage in multiple cycles of computation at the same time, substantially increasing their speed. In a pipelined control unit, different instructions simultaneously go through the process but at different points. While one instruction is being fetched, a second is being decoded, and so forth. Unix-style pipelines: |.

Logical operators

AND (<syntaxhighlight lang="text" class="" style="" inline="1">&&</syntaxhighlight>)
OR (<syntaxhighlight lang="text" class="" style="" inline="1">||</syntaxhighlight>)
NOT (<syntaxhighlight lang="text" class="" style="" inline="1">!</syntaxhighlight>)

Bash supplies "conditional execution" command separators that make execution of a command contingent on the exit code set by a precedent command. For example:

<syntaxhighlight lang="console"> $ cd "$SOMEWHERE" && ./do_something || echo "An error occurred" >&2 </syntaxhighlight>

Where <syntaxhighlight lang="text" class="" style="" inline="1">./do_something</syntaxhighlight> is only executed if the Template:Mono (change directory) command was "successful" (returned an exit status of zero) and the Template:Mono command would only be executed if either the Template:Mono or the <syntaxhighlight lang="text" class="" style="" inline="1">./do_something</syntaxhighlight> command return an "error" (non-zero exit status).

Iteration

Template:Mono, Template:Mono, and Template:Mono loop compound commands;
Arithmetic C-style and list-enumerating Template:Mono loop compound commands; and
Template:Mono, Template:Mono, Template:Mono, and Template:Mono flow control commands;

Compound commands

Template:Blockquote Bash also supports <syntaxhighlight lang="bash" class="" style="" inline="1"> if ... fi</syntaxhighlight> and <syntaxhighlight lang="bash" class="" style="" inline="1"> case ... esac</syntaxhighlight> forms of conditional command evaluation.Template:Efn

Testing

Built in commands for testing file attributes, comparing string and integer values, etc.:

Traditional Template:Mono command,
Traditional single bracket test: <syntaxhighlight lang="text" class="" style="" inline="1"> [</syntaxhighlight>,
Modern double bracket test: [[...]], which includes advanced features:
- Extended regular expression and extglob matching
- Lexicographic comparisons with <syntaxhighlight lang="text" class="" style="" inline="1"> <</syntaxhighlight> and <syntaxhighlight lang="text" class="" style="" inline="1"> ></syntaxhighlight>;
<syntaxhighlight lang="text" class="" style="" inline="1"> ((...))</syntaxhighlight> numeric evaluation and testing; this includes almost all "C" language operators for arithmetic and numeric comparison;

For all commands the exit status is stored in the special variable <syntaxhighlight lang="bash" class="" style="" inline="1"> $?</syntaxhighlight>.

Regular Expressions

Bash 3.0 supports in-process regular expression matching using a syntax reminiscent of Perl.<ref> Template:Cite web</ref> Regexp matching is limited to strings on the right side of the =~ operator in the <syntaxhighlight lang="text" class="" style="" inline="1"> [[..]]</syntaxhighlight> extended test construct.<ref> Template:Cite web</ref>

<syntaxhighlight lang="bash" inline>[[ $line =~ space:*(a)?b ]]</syntaxhighlight> means values for line like 'aab', ' aaaaaab', 'xaby', and ' ab' will all match, as will a line containing a 'b' anywhere in its value.

Coprocesses

Data manipulation

Word Splitting

Split into words (i.e., word splitting)

Quoting

Bash has certain quoting rules: uses of

single quotes <syntaxhighlight lang="bash" class="" style="" inline="1">'...'</syntaxhighlight>
double quotes <syntaxhighlight lang="bash" class="" style="" inline="1">"..."</syntaxhighlight>
backslashes <syntaxhighlight lang="bash" class="" style="" inline="1">\</syntaxhighlight>, and
ANSI-C quoting <syntaxhighlight lang="text" class="" style="" inline="1"> $'...'</syntaxhighlight>.

See also Template:Section link, <syntaxhighlight lang="text" class="" style="" inline="1"> $"..."</syntaxhighlight>

See also backticks <syntaxhighlight lang="text" class="" style="" inline="1"> `...`</syntaxhighlight>: Template:Section link.

Unicode

Support for Unicode in <syntaxhighlight lang="bash" class="" style="" inline="1"> echo -e</syntaxhighlight> and ANSI-C quoting.

Brace Expansion

$ echo kernel{,-headers} kernel kernel-headers </syntaxhighlight>

Brace expansion, also called alternation, is a feature copied from the C shell. It generates a set of alternative combinations.<ref> Template:Cite web</ref> Generated results need not exist as files. The results of each expanded string are not sorted and left to right order is preserved:

$ echo a{p,c,d,b}e ape ace ade abe $ echo {a,b,c}{d,e,f} ad ae af bd be bf cd ce cf </syntaxhighlight>

Users should not use brace expansions in portable shell scripts, because the Bourne shell does not produce the same output.

$ # bash shell $/bin/bash -c 'echo a{p,c,d,b}e' ape ace ade abe $ # A traditional shell does not produce the same output $ /bin/sh -c 'echo a{p,c,d,b}e' a{p,c,d,b}e </syntaxhighlight>

When brace expansion is combined with wildcards, the braces are expanded first, and then the resulting wildcards are substituted normally. Hence, a listing of JPEG and PNG images in the current directory could be obtained using:

ls *.{jpg,jpeg,png} # expands to *.jpg *.jpeg *.png – after which,

                      # the wildcards are processed

echo *.{png,jp{e,}g} # echo just shows the expansions –

                      # and braces in braces are possible.

</syntaxhighlight>

In addition to alternation, brace expansion can be used for sequential ranges between two integers or characters separated by double dots. Newer versions of Bash allow a third integer to specify the increment.

$ echo {1..10} 1 2 3 4 5 6 7 8 9 10 $ echo {01..10} 01 02 03 04 05 06 07 08 09 10 $ echo file{1..4}.txt file1.txt file2.txt file3.txt file4.txt $ echo {a..e} a b c d e $ echo {1..10..3} 1 4 7 10 $ echo {a..j..3} a d g j </syntaxhighlight>

When brace expansion is combined with variable expansion (a.k.a., parameter expansion and parameter substitution) the variable expansion is performed after the brace expansion, which in some cases may necessitate the use of the Template:Mono built-in, thus:

$ start=1; end=10 $ echo {$start..$end} # fails to expand due to the evaluation order {1..10} $ eval echo {$start..$end} # variable expansion occurs then resulting string is evaluated 1 2 3 4 5 6 7 8 9 10 </syntaxhighlight>

Tilde Expansion

Template:Empty section

Parameter and variable expansion

Type
Shell parameters
Environment variables
User variables
Scope
Arrays
- Indexed arrays: size is unlimited.
- Associative arrays: via <syntaxhighlight lang="bash" class="" style="" inline="1"> declare -A</syntaxhighlight>Template:Efn
Parameter Expansion
Expansion syntaxes which can perform some tasks more quickly than external utilities, including, among others:
- Pattern Substitution
  - <syntaxhighlight lang="bash" class="" style="" inline="1"> ${foo//x/y}</syntaxhighlight> for sed 'Template:Codett',
- Remove Matching Prefix or Suffix Pattern
  - <syntaxhighlight lang="bash" class="" style="" inline="1"> ${bar##[a-zA-Z0-9]*}</syntaxhighlight> for <syntaxhighlight lang="text" class="" style="" inline="1"> cut -c8-</syntaxhighlight>,
- Enumerate Array Keys
  - <syntaxhighlight lang="bash" class="" style="" inline="1"> ${!array[@]}</syntaxhighlight>, and
- Display Error if Null or Unset
  - <syntaxhighlight lang="bash" class="" style="" inline="1"> ${var:?error message}</syntaxhighlight>,

Pathname expansion

Pathname expansion, i.e., shell-style globbing and pattern matching using <syntaxhighlight lang="text" class="" style="" inline="1"> *</syntaxhighlight>, <syntaxhighlight lang="text" class="" style="" inline="1"> ?</syntaxhighlight>, <syntaxhighlight lang="text" class="" style="" inline="1"> [...]</syntaxhighlight>.Template:Efn

Locales

Locale-specific translation via <syntaxhighlight lang="bash" class="" style="" inline="1"> $"..."</syntaxhighlight> quoting syntax.<ref> Template:Cite web</ref>

Process redirections and parsing

Command substitution

Command substitution: <syntaxhighlight lang="bash" class="" style="" inline="1"> $(...)</syntaxhighlight>,

Process substitution

Process substitution, <syntaxhighlight lang="bash" class="" style="" inline="1"> <()</syntaxhighlight> or <syntaxhighlight lang="bash" class="" style="" inline="1"> >()</syntaxhighlight>, when a system supports it:

Bash supports process substitution using the <syntaxhighlight lang="bash" class="" style="" inline="1"> <(command)</syntaxhighlight> and <syntaxhighlight lang="bash" class="" style="" inline="1"> >(command)</syntaxhighlight> syntax, which substitutes the output of (or input to) a command where a filename is normally used. (This is implemented through /proc/fd/ unnamed pipes on systems that support that, or via temporary named pipes where necessary).

Arithmetic expansion

Arithmetic expansion, <syntaxhighlight lang="text" class="" style="" inline="1"> ((...))</syntaxhighlight> or <syntaxhighlight lang="bash" class="" style="" inline="1"> $((...))</syntaxhighlight>, including

Integer arithmetic in any base from two to sixty-four, although
Floating-point arithmetic is not available from within the shell itself (for this functionality, see current versions of [[Bc (programming language)|Template:Mono]] and [[AWK|Template:Mono]], among others),

Bash can perform integer calculations ("arithmetic evaluation") without spawning external processes. It uses the <syntaxhighlight lang="text" class="" style="" inline="1"> ((...))</syntaxhighlight> command and the <syntaxhighlight lang="bash" class="" style="" inline="1"> $((...))</syntaxhighlight> variable syntax for this purpose.

Redirection

Redirections of Standard Input, Standard Output and Standard Error data streams are performed, including

File writing, <syntaxhighlight lang="text" class="" style="" inline="1"> ></syntaxhighlight>, and appending, <syntaxhighlight lang="text" class="" style="" inline="1"> ></syntaxhighlight>,
Here documents, <syntaxhighlight lang="text" class="" style="" inline="1"> <<</syntaxhighlight>,
Here strings, <syntaxhighlight lang="text" class="" style="" inline="1"> <<<</syntaxhighlight>, which allow parameters to be used as input, and
A redirection operator, <syntaxhighlight lang="text" class="" style="" inline="1"> ></syntaxhighlight>, which can force overwriting of a file when a shell's Template:Mono setting is enabled;

Its syntax simplifies I/O redirection. For example, it can redirect standard output (stdout) and standard error (stderr) at the same time using the <syntaxhighlight lang="bash" class="" style="" inline="1"> &></syntaxhighlight> operator. This is simpler to type than the Bourne shell equivalent '<syntaxhighlight lang="bash" class="" style="" inline="1"> command > file 2>&1</syntaxhighlight>'. Bash supports here documents. Since version 2.05b Bash can redirect standard input (stdin) from a "here string" using the <syntaxhighlight lang="text" class="" style="" inline="1"> <<<</syntaxhighlight> operator.

Command parsing

(A) Comments are ignored, from an unquoted <syntaxhighlight lang="text" class="" style="" inline="1">#</syntaxhighlight> (hash) to the end of the same line;<ref>

Template:Cite web</ref><ref> Template:Cite web</ref>

(B) Commands are parsed one line at a time:
- Control structures are honored, and
- Backslash <syntaxhighlight lang="text" class="" style="" inline="1">\</syntaxhighlight> escapes are also honored at the ends of lines;
(C) Split into words (i.e., word splitting) according to quoting rules,
- Including ANSI-C quoting <syntaxhighlight lang="text" class="" style="" inline="1">$'...'</syntaxhighlight>;
(D) Seven types of expansions are performed in the following order on the resulting string:
1. (Type 1) Brace expansion kernel{-headers},
2. (Type 2) Tilde expansion <syntaxhighlight lang="text" class="" style="" inline="1">~</syntaxhighlight>,
3. In a left-to-right fashion:
  - (Type 3) Parameter and variable expansion <syntaxhighlight lang="text" class="" style="" inline="1">$foo</syntaxhighlight> or <syntaxhighlight lang="bash" class="" style="" inline="1">${bar}</syntaxhighlight>, including
  - (Type 4) Command substitution: <syntaxhighlight lang="bash" class="" style="" inline="1">$(...)</syntaxhighlight>,
  - (Type 5) Process substitution, <syntaxhighlight lang="bash" class="" style="" inline="1"><()</syntaxhighlight> or <syntaxhighlight lang="bash" class="" style="" inline="1">>()</syntaxhighlight>, when a system supports it:
  - (Type 6) Arithmetic expansion, <syntaxhighlight lang="text" class="" style="" inline="1">((...))</syntaxhighlight> or <syntaxhighlight lang="bash" class="" style="" inline="1">$((...))</syntaxhighlight>, including
    - Integer arithmetic in any base from two to sixty-four, although
    - Floating-point arithmetic is not available from within the shell itself.Template:Efn
4. Word splitting (again),
5. (Type 7) Pathname expansion, i.e., shell-style globbing and pattern matching using <syntaxhighlight lang="text" class="" style="" inline="1">*</syntaxhighlight>, <syntaxhighlight lang="text" class="" style="" inline="1">?</syntaxhighlight>, <syntaxhighlight lang="bash" class="" style="" inline="1">[...]</syntaxhighlight>,Template:Efn
6. Quote removal;
(E) Redirections of Standard Input, Standard Output and Standard Error data streams are performed, including
- File writing, <syntaxhighlight lang="text" class="" style="" inline="1">></syntaxhighlight>, and appending, <syntaxhighlight lang="text" class="" style="" inline="1">>></syntaxhighlight>,
- Here documents, <syntaxhighlight lang="text" class="" style="" inline="1"><<</syntaxhighlight>,
- Here strings, <syntaxhighlight lang="text" class="" style="" inline="1"><<<</syntaxhighlight>, which allow parameters to be used as input, and
- A redirection operator, <syntaxhighlight lang="text" class="" style="" inline="1">></syntaxhighlight>, which can force overwriting of a file when a shell's <syntaxhighlight lang="text" class="" style="" inline="1">noclobber</syntaxhighlight> setting is enabled;
(F) Command name lookup is performed, in the following order:
1. Commands internal to the shell:
  - Shell aliases,
  - Shell reserved words,
  - Shell functions, and
  - Shell built-in commands;
2. Commands external to the shell:
  - Separate UNIX-style programs such as <syntaxhighlight lang="text" class="" style="" inline="1">ls</syntaxhighlight> or <syntaxhighlight lang="text" class="" style="" inline="1">ln</syntaxhighlight>, and
  - Shell scripts, which are files containing executable commands.Template:Efn
(G) The resulting string is executed as a command.

Interactive-only features

Command History

Unlimited size command history.<ref> Template:Cite web</ref> This feature is available in interactive mode only.

Directory stack

A directory stack (<syntaxhighlight lang="text" class="" style="" inline="1">pushd</syntaxhighlight> and <syntaxhighlight lang="text" class="" style="" inline="1">popd</syntaxhighlight> built-ins) feature is available in interactive mode only.

Programmable completion

Also known as "tab completion" or "command-line completion", when a user presses the Template:Keypress, within an interactive command-shell Bash automatically uses any available completion scripts to suggest partly typed program names, filenames and variable names.<ref> Template:Cite web</ref> <ref name =bashfaq061/> The Bash command-line completion system is very flexible and customizable, and is often packaged with functions that complete arguments and filenames for specific programs and tasks.

Bash supports programmable completion via built-in <syntaxhighlight lang="text" class="" style="" inline="1"> complete</syntaxhighlight>, <syntaxhighlight lang="text" class="" style="" inline="1"> compopt</syntaxhighlight>, and <syntaxhighlight lang="text" class="" style="" inline="1"> compgen</syntaxhighlight> commands.<ref> Template:Cite web</ref> The feature has been available since the beta version of 2.04 released in 2000.<ref> Template:Cite web</ref> These commands enable complex and intelligent completion specification for commands (i.e., installed programs), functions, variables, and filenames.<ref name="tldp_pc"> Template:Cite web</ref>

The <syntaxhighlight lang="text" class="" style="" inline="1"> complete</syntaxhighlight> and <syntaxhighlight lang="text" class="" style="" inline="1"> compopt</syntaxhighlight> two commands specify how arguments of some available commands or options are going to be listed in the readline input.As of version 5.1 completion of the command or the option is usually activated by the Template:Keypress keystroke after typing its name.<ref name =tldp_pc/> This feature is available in interactive mode only.

Prompts

Configurable prompts. This feature is available in interactive mode only.

Documentation

User Manual

A user manual for Bash is provided by the GNU Project. It is sometimes considered to be a more user-friendly document than the man page. "You may also find information about Bash ...by looking at <syntaxhighlight lang="text" class="" style="" inline="1"> /usr/share/doc/bash</syntaxhighlight>, <syntaxhighlight lang="text" class="" style="" inline="1"> /usr/local/share/doc/bash</syntaxhighlight>, or similar directories on your system."<ref name =gnu.org_bash-hm> Template:Cite web </ref> On GNU/Linux systems, if the Template:Mono program is available then the GNU Manual version relevant for your installation should also be available at <syntaxhighlight lang="text" class="" style="" inline="1"> info bash</syntaxhighlight>.<ref name="gnu.org_info"> Template:Cite web </ref><ref> Template:Cite web</ref>

Man page

The most recent technical manual, or 'man page', is intended to be the authoritative explanatory technical document for the understanding of how bash operates. On GNU/Linux systems, the version relevant for your installation is usually available through the Template:Mono program at <syntaxhighlight lang="bash" class="" style="" inline="1"> man bash</syntaxhighlight>.<ref name =gnu.org_info/><ref name =case.edu_bash(1)/><ref> Template:Cite web </ref>

help builtin

With recent versions of Bash, information on shell built-in commands can be found by executing <syntaxhighlight lang="bash" class="" style="" inline="1"> help</syntaxhighlight>, <syntaxhighlight lang="bash" class="" style="" inline="1"> help [name of builtin]</syntaxhighlight> or <syntaxhighlight lang="text" class="" style="" inline="1"> man builtins</syntaxhighlight> at a terminal prompt where bash is installed.

The Template:Mono command can be invoked via Template:Mono to ensure that you run the program found via your shell's search path, and not a shell alias or built-in function: <syntaxhighlight lang="bash" class="" style="" inline="1"> env printf --help</syntaxhighlight>.<ref> Template:Cite web</ref>

POSIX Specification

For the purpose of allowing inter-operability among different shell programs running on different operating systems, the POSIX Specification influences how modern UNIX-like shells are written. Bash "is intended to be a conformant implementation of the IEEE POSIX "Shell and Utilities" portion of the IEEE POSIX specification (IEEE Standard 1003.1)."<ref> Template:Cite web </ref> The most recent publication of the standard (2024) is available online.<ref> Template:Cite web </ref>

As the standard upon which bash is based, the POSIX Standard, or IEEE Std 1003.1,<ref> Template:Cite web</ref> et seq, is especially informative.

Further resources

"The project maintainer also has a Bash page which includes Frequently Asked Questions",<ref name="gnu.org_bash-hm"/><ref name="bash-top"> Template:Cite web</ref><ref> Template:Cite web</ref> this FAQ is current as of bash version 5.1 and is no longer updated.

Informal avenues of support are available via IRC at libera.chat, in the #bash channel, and mailing lists are available at Bash - GNU Project - Free Software Foundation.

Security and vulnerabilities

Root scripts

Running any shell scripts as the root user has, for years, been widely criticized as poor security practice. One commonly given reason is that, when a script is executed as root, the negative effects of any bugs in a script would be magnified by root's elevated privileges.

One common example: a script contains the command, <syntaxhighlight lang="bash" class="" style="" inline="1"> rm -rf ${dir}/</syntaxhighlight>, but the variable <syntaxhighlight lang="text" class="" style="" inline="1"> $dir</syntaxhighlight> is left undefined. In Linux, if the script was executed by a regular user, the shell would attempt to execute the command <syntaxhighlight lang="text" class="" style="" inline="1"> rm -rf /</syntaxhighlight> as a regular user, and the command would fail. However, if the script was executed by the root user, then the command would likely succeed and the filesystem would be erased.

It is recommended to use Template:Mono on a per-command basis instead.

CGI scripts

CGI scripts are a significant source of vulnerability.<ref> Template:Cite web</ref><ref> Template:Cite web</ref><ref> Template:Cite web</ref> Template:Clarify

builtin Template:Mono

"The eval command is extremely powerful and extremely easy to abuse."<ref> Template:Cite web</ref>

Input validation

"Input validation is the process of ensuring data has undergone data cleansing to confirm it has data quality, that is, that it is both correct and useful."

Template:Unreferenced section

Shellshock

In September 2014, a security bug was discovered<ref> Template:Cite news</ref> in the program. It was dubbed "Shellshock." Public disclosure quickly led to a range of attacks across the Internet.<ref> Template:Cite web </ref><ref> Template:Cite news</ref><ref> Template:Cite web</ref>

Exploitation of the vulnerability could enable arbitrary code execution in CGI scripts executable by certain versions of Bash. The bug involved how Bash passed function definitions to subshells through environment variables.<ref> Template:Cite web</ref> The bug had been present in the source code since August 1989 (version 1.03)<ref> Template:Cite web</ref> and was patched in September 2014 (version 4.3).

Patches to fix the bugs were made available soon after the bugs were identified. Upgrading to a current version is strongly advised.

It was assigned the Common Vulnerability identifiers Template:CVE, among others. Under CVSS Metrics 2.x and 3.x, the bug is regarded as "high" and "critical", respectively.

Deprecated syntax

Backtick style command substitutions: <syntaxhighlight lang="bash" class="" style="" inline="1"> `...`</syntaxhighlight> is deprecated in favor of
- <syntaxhighlight lang="bash" class="" style="" inline="1"> $(...)</syntaxhighlight>;
Use of <syntaxhighlight lang="text" class="" style="" inline="1"> -a</syntaxhighlight> or <syntaxhighlight lang="text" class="" style="" inline="1"> -o</syntaxhighlight> in <syntaxhighlight lang="text" class="" style="" inline="1">test</syntaxhighlight>/<syntaxhighlight lang="text" class="" style="" inline="1"> [</syntaxhighlight>/[[ commands,
- for example, <syntaxhighlight lang="bash" class="" style="" inline="1"> [ -r ./file -a ! -l ./file ]</syntaxhighlight> is deprecated in favor of
  - <syntaxhighlight lang="bash" class="" style="" inline="1"> [ -r ./file ] && ! [ -l ./file ]</syntaxhighlight>;
Use of the arithmetic syntax <syntaxhighlight lang="bash" class="" style="" inline="1"> $[...]</syntaxhighlight> is deprecated in favor of
- <syntaxhighlight lang="bash" class="" style="" inline="1"> $((...))</syntaxhighlight> or
- <syntaxhighlight lang="text" class="" style="" inline="1"> ((...))</syntaxhighlight>, as appropriate;
Use of <syntaxhighlight lang="text" class="" style="" inline="1"> ^</syntaxhighlight> as a pipeline is deprecated in favor of |;
Any uses of Template:Mono or Template:Mono.

Debugging

Table of Features

Template:Sort-under

Bash features which can be useful during debugging.<ref name =bashfaq061/><ref name =bash-hackers.org_changes/><ref> Template:Cite web</ref>
Feature			POSIX 2024	Description	Bash ver.
Grammar type	Formal name	Syntax	POSIX 2024	Description	Bash ver.
Special Built-In Utility	set / xtrace	<syntaxhighlight lang="bash" class="" style="" inline="1"> set -x</syntaxhighlight>	Template:Yes	The shell's primary means of debugging. It "writes to standard error a trace for each command after it expands the command and before it executes it."	Template:Dunno
Special Parameters	Exit Status	<syntaxhighlight lang="bash" class="" style="" inline="1"> "$?"</syntaxhighlight>	Template:Yes	"Expands to the shortest representation of the decimal exit status."	Template:Dunno
Parameter Expansions	Indicate Null or Unset	<syntaxhighlight lang="bash" class="" style="" inline="1"> "${parameter:?[word]}"</syntaxhighlight>	Template:Yes	"Where the expansion of <syntaxhighlight lang="text" class="" style="" inline="1"> [word]</syntaxhighlight>, perhaps an error message or a line number, is written to standard error and the shell exits with a non-zero exit code."	Template:Dunno
Special Parameters	PID of Invoked Shell	<syntaxhighlight lang="text" class="" style="" inline="1"> "$$"</syntaxhighlight>	Template:Yes	"Expands to the shortest representation of the decimal process ID of the invoked shell."	Template:Dunno
Special Built-In Utility	set / verbose	<syntaxhighlight lang="bash" class="" style="" inline="1"> set -v</syntaxhighlight>	Template:Yes	"Writes its input to standard error as it is read."	Template:Dunno
Special Built-In Utility	set / pipefail	<syntaxhighlight lang="bash" class="" style="" inline="1"> set -o pipefail</syntaxhighlight>	Template:Yes	"Derive the exit status of a pipeline from the exit statuses of all of the commands in the pipeline, not just the last (rightmost) command."	Template:Dunno
Special Built-In Utility	set / nounset	<syntaxhighlight lang="bash" class="" style="" inline="1"> set -u</syntaxhighlight>	Template:Yes	When enabled, will cause the shell to exit with an error message when it encounters an unset variable expansion. Its use has a number of counter-intuitive pitfalls.	Template:Dunno
Special Built-In Utility	set / errexit	<syntaxhighlight lang="bash" class="" style="" inline="1"> set -e</syntaxhighlight>	Template:Yes	Errexit is a setting that, when enabled, will, under certain very specific conditions, cause the shell to exit without an error message whenever the shell receives a non-zero exit code. Its use is somewhat controversial, to the extent that any somewhat obscure computer program can be controversial. Adherents claim that Errexit provides an assurance of verifiability in situations where shell scripts "must not fail." However, opponents claim that its use is unreliable, deceptively simple, highly counter-intuitive, rife with gotchas and pitfalls, and in essence "security theater." Numerous developers of Bash have strongly discouraged the use of this particular setting.	Template:Dunno
Special Built-In Utility	trap / EXIT	<syntaxhighlight lang="bash" class="" style="" inline="1"> trap '[arg]' EXIT</syntaxhighlight>	Template:Yes	"If a signal specifier is <syntaxhighlight lang="text" class="" style="" inline="1"> 0</syntaxhighlight> or <syntaxhighlight lang="text" class="" style="" inline="1"> EXIT</syntaxhighlight>, <syntaxhighlight lang="text" class="" style="" inline="1"> [arg]</syntaxhighlight> is executed when the shell exits." If <syntaxhighlight lang="text" class="" style="" inline="1"> [arg]</syntaxhighlight> contains expansions, then <syntaxhighlight lang="text" class="" style="" inline="1"> [arg]</syntaxhighlight> should be in single quotes.	Template:Dunno
Utility	printf	<syntaxhighlight lang="bash" class="" style="" inline="1"> printf '<%s>\n' "${var}"</syntaxhighlight>	Template:Yes	A means of reliably printing the contents of a variable.	Template:Dunno
Bash Variables	BASHPID	<syntaxhighlight lang="bash" class="" style="" inline="1"> "${BASHPID}"</syntaxhighlight>	Template:No	"Expands to the process ID of the current bash process."<ref> Template:Cite web</ref>	Template:Dunno
Bash Variables	BASH_ARGC	<syntaxhighlight lang="bash" class="" style="" inline="1"> "${BASH_ARGC[@]}"</syntaxhighlight>	Template:No	"An array variable whose values are the number of parameters in each frame of the current bash execution call stack."<ref> Template:Cite web</ref>	Template:Dunno
Bash Variables	BASH_ARGV	<syntaxhighlight lang="bash" class="" style="" inline="1"> "${BASH_ARGV[@]}"</syntaxhighlight>	Template:No	"An array variable containing all of the parameters in the current bash execution call stack."<ref> Template:Cite web</ref>	Template:Dunno
Bash Variables	BASH_LINENO	<syntaxhighlight lang="bash" class="" style="" inline="1"> "${BASH_LINENO[@]}"</syntaxhighlight>	Template:No	"An array variable whose members are the line numbers in source files where each corresponding member of <syntaxhighlight lang="bash" class="" style="" inline="1"> "${FUNCNAME[@]}"</syntaxhighlight> was invoked."<ref> Template:Cite web</ref>	Template:Dunno
Bash Variables	BASH_REMATCH	<syntaxhighlight lang="bash" class="" style="" inline="1"> "${BASH_REMATCH[@]}"</syntaxhighlight>	Template:No	"An array variable whose members are assigned by the `=~` binary operator to the `[[` conditional command."<ref> Template:Cite web</ref>	Template:Dunno
Bash Variables	BASH_SOURCE	<syntaxhighlight lang="bash" class="" style="" inline="1"> "${BASH_SOURCE[@]}"</syntaxhighlight>	Template:No	"An array variable whose members are the source filenames where the corresponding shell function names in the <syntaxhighlight lang="bash" class="" style="" inline="1"> "${FUNCNAME[@]}"</syntaxhighlight> array variable are defined."<ref> Template:Cite web</ref>	Template:Dunno
Bash Variables	BASH_XTRACEFD	<syntaxhighlight lang="bash" class="" style="" inline="1"> "${BASH_XTRACEFD}"</syntaxhighlight>	Template:No	"If set to an integer corresponding to a valid file descriptor, Bash will write the trace output generated when <syntaxhighlight lang="bash" class="" style="" inline="1"> set -x</syntaxhighlight> is enabled to that file descriptor."<ref> Template:Cite web</ref>	Template:Dunno
Bash Variables	EPOCHREALTIME	<syntaxhighlight lang="bash" class="" style="" inline="1"> "${EPOCHREALTIME}"</syntaxhighlight>	Template:No	"Each time this parameter is referenced, it expands to the number of seconds since the Unix Epoch (see <syntaxhighlight lang="bash" class="" style="" inline="1"> time(3)</syntaxhighlight>) as a floating point value with micro-second granularity."<ref> Template:Cite web</ref>	Template:Dunno
Bash Variables	FUNCNAME	<syntaxhighlight lang="bash" class="" style="" inline="1"> "${FUNCNAME[@]}"</syntaxhighlight>	Template:No	"An array variable containing the names of all shell functions currently in the execution call stack."<ref> Template:Cite web</ref>	Template:Dunno
Bash Variables	LINENO	<syntaxhighlight lang="bash" class="" style="" inline="1"> "${LINENO}"</syntaxhighlight>	Template:No	"Each time this parameter is referenced, the shell substitutes a decimal number representing the current sequential line number (starting with 1) within a script or function."<ref> Template:Cite web</ref>	Template:Dunno
Bash Variables	PIPESTATUS	<syntaxhighlight lang="bash" class="" style="" inline="1"> "${PIPESTATUS[@]}"</syntaxhighlight>	Template:No	"An array variable containing a list of exit status values from the processes in the most-recently-executed foreground pipeline (which may contain only a single command)."<ref> Template:Cite web</ref>	Template:Dunno
Bash Variables	PPID	<syntaxhighlight lang="bash" class="" style="" inline="1"> "${PPID}"</syntaxhighlight>	Template:No	"The process ID of the shell's parent."<ref> Template:Cite web</ref>	Template:Dunno
Bash Variables	PS4	<syntaxhighlight lang="bash" class="" style="" inline="1"> "${PS4}"</syntaxhighlight>	Template:No	"The value of this parameter is expanded as with PS1 and the value is printed before each command bash displays during an execution trace."<ref> Template:Cite web</ref>	Template:Dunno
Shell Builtin	set / restricted	<syntaxhighlight lang="bash" class="" style="" inline="1"> set -r</syntaxhighlight>	Template:No	Restricted mode is intended to improve the security of an individual shell instance from a malicious human with physical access to a machine. As threat models have changed, it has become less commonly used now than it once was.	Template:Dunno
Shell Builtin	shopt / extdebug	<syntaxhighlight lang="bash" class="" style="" inline="1"> shopt -s extdebug</syntaxhighlight>	Template:No	"Behavior intended for use by debuggers."	Template:Dunno
Shell Builtin	trap / DEBUG	<syntaxhighlight lang="bash" class="" style="" inline="1"> trap '[arg]' DEBUG</syntaxhighlight>	Template:No	"If a sigspec is DEBUG, the command arg is executed before" certain kinds of commands.	Template:Dunno
Shell Builtin	trap / ERR	<syntaxhighlight lang="bash" class="" style="" inline="1"> trap '[arg]' ERR</syntaxhighlight>	Template:No	"If a sigspec is ERR, the command arg is executed whenever..." certain kinds of commands "return a non-zero exit status", subject to similar restrictions as with ErrExit.	Template:Dunno
Shell Builtin	trap / RETURN	<syntaxhighlight lang="bash" class="" style="" inline="1">trap '[arg]' RETURN</syntaxhighlight>	Template:No	"If a sigspec is RETURN, the command arg is executed each time a shell function or a script executed with the <syntaxhighlight lang="bash" class="" style="" inline="1"> .</syntaxhighlight> or Template:Mono builtins finishes executing."	Template:Dunno

Shell features specified by POSIX:
- Parameter Expansions:<ref>

Template:Multiref2</ref>

- Special Parameters:<ref>

Template:Cite web</ref><ref> Template:Cite web</ref>

- Special Built-In Utility Template:Mono:<ref name="infoandposixset">

Template:Multiref2</ref><ref name="case.edu_bash1"> Template:Cite web</ref>

- Special Built-In Utility Template:Mono:<ref name =case.edu_bash1/><ref name="gnu.org_trap">

- - POSIX does specify certain uses of the Template:Mono builtin: ...
- Utility Template:Mono: a means of reliably printing the contents of a variable:
Bash features not specified by POSIX:
- Bash Variables:<ref>

Template:Cite web</ref><ref> Template:Cite web</ref>

- Shell Builtin Template:Mono:<ref name =infoandposixset/><ref name =case.edu_bash1/>
- Shell Builtin Template:Mono:<ref>

Template:Cite web</ref><ref name =case.edu_bash1/>

- Shell Builtin Template:Mono:<ref name =gnu.org_trap/><ref name =case.edu_bash1/>
  - While POSIX does specify certain uses of the Template:Mono builtin, the following signal specs are Bash extensions: ...
Third party debugging utilities:
- ShellCheck: Shell script analysis tool;<ref>

Template:Multiref2</ref><ref name =shellcheck-1/>

- devscripts-checkbashisms: Check whether a /bin/sh script contains any common bash-specific constructs;<ref>

Template:Cite web</ref><ref name =checkbashisms-1/>

- kcov: Code coverage tool without special compilation options;<ref>