Bash (Unix shell)

Bash

Screenshot of a Bash session
Original author(s)	Brian Fox
Developer(s)	Chet Ramey
Initial release	8 June 1989; 35 years ago (8 June 1989)
Stable release	5.2.37[1]  / 23 September 2024
Repository	git.savannah.gnu.org/gitweb/?p=bash.git
Written in	C
Operating system	BeOS Haiku macOS ( GPL-2.0-or-later ; GPL-3.0-or-later through 3rd parties) Unix-like Windows ( GPL-3.0-or-later )
Platform	GNU
Available in	Multilingual (gettext)
Type	Shell (computing), Unix shell, command language
License	Since 4.0: GPL-3.0-or-later, 1.11? to 3.2: GPL-2.0-or-later, 0.99? to 1.05?: GPL-1.0-or-later
Website	www.gnu.org/software/bash/

Bash, short for Bourne-Again SHell, is a shell program and command language supported by the Free Software Foundation^[2] and first developed for the GNU Project^[3] by Brian Fox.^[4] Designed as a 100%^[5] free software alternative for the Bourne shell,^[6][7][8] it was initially released in 1989.^[9] Its moniker is a play on words, referencing both its predecessor, the Bourne shell,^[10] and the concept of rebirth.^[11][12]

Since its inception, Bash has gained widespread adoption and is commonly used as the default login shell for numerous Linux distributions. It holds historical significance as one of the earliest programs ported to Linux by Linus Torvalds, alongside the GNU Compiler (GCC).^[13] It is available on nearly all modern operating systems, making it a versatile tool in various computing environments.

As a command processor, Bash operates within a text window where users input commands to execute various tasks. It also supports the execution of commands from files, known as shell scripts, facilitating automation. In keeping with Unix shell conventions, Bash incorporates a rich set of features. The keywords, syntax, dynamically scoped variables and other basic features of the language are all copied from the Bourne shell, sh. Other features, e.g., history, are copied from the C shell, csh, and the Korn Shell, ksh. Bash is a POSIX-compliant shell with a number of extensions.

While Bash was developed for UNIX and UNIX-like operating systems such as GNU/Linux,^[14] it is also available on Windows, BeOS,^[15][16] and Haiku.^[17]

Brian Fox began coding Bash on January 10, 1988,^[18] after Richard Stallman became dissatisfied with the lack of progress being made by a prior developer.^[7] Stallman and the FSF considered a free shell that could run existing shell scripts so strategic to a completely free system built from BSD and GNU code that this was one of the few projects they funded themselves, with Fox undertaking the work as an employee of FSF.^[7][19] Fox released Bash as a beta, version .99, on June 8, 1989,^[9] and remained the primary maintainer until sometime between mid-1992^[20] and mid-1994,^[21] when he was laid off from FSF^[22] and his responsibility was transitioned to another early contributor, Chet Ramey.^{[23][24][25][26][27]}

Since then, Bash has become by far the most popular shell among users of Linux, becoming the default interactive shell on that operating system's various distributions^[28][29] and on Apple's macOS releases before Catalina in October 2019.^[30][31][32] Bash has also been ported to Microsoft Windows^[33][34] and distributed with Cygwin and MinGW, to DOS by the DJGPP project, to Novell NetWare, to OpenVMS by the GNU project,^[35] to ArcaOS,^[36] and to Android via various terminal emulation applications.

In September 2014, Stéphane Chazelas, a Unix/Linux specialist,^[37] discovered a security bug in the program. The bug, first disclosed on September 24, was named Shellshock and assigned the numbers CVE-2014-6271, CVE-2014-6277 and CVE-2014-7169. The bug was regarded as severe, since CGI scripts using Bash could be vulnerable, enabling arbitrary code execution. The bug was related to how Bash passes function definitions to subshells through environment variables.^[38]

As a command processor, Bash operates within a text window where users input commands to execute various tasks. It also supports the execution of commands from files, known as shell scripts, facilitating automation. In keeping with Unix shell conventions, Bash incorporates a rich set of features, including:

• Invocation as a...
  • Interactive shell,
  • Non-interactive shell, or
  • Login shell;
• A command-line interface;
• Exit status codes;
• Control structures for
  • Condition testing,
    • if, case, select,
    • logical AND (&&) and OR (||), and
  • Iteration
    • for, while, until loops, and
    • Arithmetic C-style loop: for ((;
• Syntaxes for file type, string value and integer value testing
  • Traditional single bracket test: [,
  • Modern double bracket test: [[ ... ]], including
    • Extended regular expression and extglob matching
    • Lexicographic sorting with < and >;
• UNIX-style pipelines: |;
• Subshells: ( ... );
• Signaling as a means of inter-process communication using the trap builtin;
• Asynchronous execution: job_spec &;
• A shell portability mode where commands can be interpreted in conformance with the POSIX standard;
• Command parsing:
  • Comments are ignored:
    • Bourne-style # hashtag comments, and
    • Thompson-style : colon comments;
  • Commands are parsed one line at a time,
    • Control structures are honored, and
    • Backslash \ escapes are also honored at the ends of lines,
  • Split into words (i.e., word splitting) according to quoting rules,
    • Including ANSI-C quoting $'...', and
  • Seven kinds of expansions are performed on the resulting string in the following order:
    • (Step 1) Brace expansion kernel{-headers},
    • (Step 2) Tilde expansion ~,
    • (Step 3) In a left-to-right fashion:
      • Parameter and variable expansion $foo or ${bar}, including
        • Dynamically scoped variables,
        • Indexed arrays of unlimited size,
        • Associative arrays via declare -A, and
        • Expansion syntaxes which can perform some tasks more quickly than external utilities, such as
          • Pattern Substitution
            • ${foo//x/y} for sed 's/x/y/g',
          • Remove Matching Prefix or Suffix Pattern
            • ${bar##[a-zA-Z0-9]*} for cut -c8-,
          • Print Array Keys
            • ${!array[@]}, and
          • Display Error if Null or Unset
            • ${var:?error message}, among others,
      • Command substitution: $( ... ),
      • Process substitution, <() or >(), when a system supports it:
      • Arithmetic expansion, (( ... )) or $(( ... )), 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 and awk, among others),
    • (Step 4) Word splitting (again),
    • (Step 5) Pathname expansion,
      • Meaning shell-style globbing and pattern matching using *, ?, [...], and
    • Quote removal;
  • Redirections of Standard Input, Standard Output and Standard Error data streams are performed, including
    • File writing, >, and appending, >>,
    • Here documents, <<,
    • Here strings, <<<, which allow parameters to be used as input, and
    • A redirection operator, >|, which can force overwriting of a file when a shell's "noclobber" setting is enabled;
  • 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:
      • Separate UNIX-style programs such as ls or ln, and
      • Shell scripts, which are files containing executable commands. (Shell scripts do not require compilation before execution and, when certain requirements are met, can be invoked as commands by using their filename.)
  • The resulting string is executed as a command.

Bash also offers...

• Configurable execution environment(s):^[39]
  • Shell and session startup files such as ~/.bashrc and ~/.profile (i.e., dotfiles);
• Settings (set built-in) and shell options (shopt built-in) which alter shell behavior;
• With interactive invocation only,
  • Unlimited size command history,
  • Jobs and job control,
  • A directory stack (see pushd and popd built-ins),
  • Tab completion,
  • Configurable prompts, and
  • Command line editing with GNU readline;
• Lightweight logging for debugging purposes (xtrace), and other lightweight debugging options (errexit, noexec, nounset, pipefail, etc.);
• Shell compatibility modes: bash 5.1 can operate as if it were bash 4.2, etc.;
• Documentation:
  • A built-in help command,
  • A man page, and
  • An info page which is the same as the GNU manual;
• Informal avenues of support via:
  • IRC at libera.chat #bash
  • Mailing lists at https://www.gnu.org/software/bash/

The keywords, syntax, dynamically scoped variables and other basic features of the language are all copied from sh. Other features, e.g., history, are copied from csh and ksh.

The Bash command syntax is a superset of the Bourne shell command syntax. Bash supports brace expansion,^[40] command line completion (Programmable Completion),^[41] basic debugging^[42][43] and signal handling (using trap) since bash 2.05a^[44][45] among other features. Bash can execute the vast majority of Bourne shell scripts without modification, with the exception of Bourne shell scripts stumbling into fringe syntax behavior interpreted differently in Bash or attempting to run a system command matching a newer Bash builtin, etc. Bash command syntax includes ideas drawn from the Korn Shell (ksh) and the C shell (csh) such as command line editing, command history (history command),^[46] the directory stack, the $RANDOM and $PPID variables, and POSIX command substitution syntax $(...).

When a user presses the tab key within an interactive command-shell, Bash automatically uses command line completion, since beta version 2.04,^[47] to match partly typed program names, filenames and variable names. 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's syntax has many extensions lacking in the Bourne shell. Bash can perform integer calculations ("arithmetic evaluation") without spawning external processes. It uses the ((...)) command and the $((...)) variable syntax for this purpose. Its syntax simplifies I/O redirection. For example, it can redirect standard output (stdout) and standard error (stderr) at the same time using the &> operator. This is simpler to type than the Bourne shell equivalent 'command > file 2>&1'. Bash supports process substitution using the <(command) and >(command)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).

When using the 'function' keyword, Bash function declarations are not compatible with Bourne/Korn/POSIX scripts (the KornShell has the same problem when using 'function'), but Bash accepts the same function declaration syntax as the Bourne and Korn shells, and is POSIX-conformant. Because of these and other differences, Bash shell scripts are rarely runnable under the Bourne or Korn shell interpreters unless deliberately written with that compatibility in mind, which is becoming less common as Linux becomes more widespread. But in POSIX mode, Bash conforms with POSIX more closely.^[48]

Bash supports here documents. Since version 2.05b Bash can redirect standard input (stdin) from a "here string" using the <<< operator.

Bash 3.0 supports in-process regular expression matching using a syntax reminiscent of Perl.^[49]

In February 2009,^[50] Bash 4.0 introduced support for associative arrays.^[51] Associative array indices are strings, in a manner similar to AWK or Tcl.^[52] They can be used to emulate multidimensional arrays. Bash 4 also switches its license to GPL-3.0-or-later; some users suspect this licensing change is why macOS continues to use older versions.^[53] Zsh became the default shell in macOS with the release of macOS Catalina in 2019.^[54]

Brace expansion, also called alternation, is a feature copied from the C shell. It generates a set of alternative combinations. 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

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

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.

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

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 eval 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

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When Bash starts, it executes the commands in a variety of dot files. Unlike Bash shell scripts, dot files do typically have neither the execute permission enabled nor an interpreter directive like #!/bin/bash.

The example ~/.bash_profile below is compatible with the Bourne shell and gives semantics similar to csh for the ~/.bashrc and ~/.bash_login. The [ -r filename ] && cmd is a short-circuit evaluation that tests if filename exists and is readable, skipping the part after the && 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

Some versions of Unix and Linux contain Bash system startup scripts, generally under the /etc 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 ~/.xsession or ~/.xprofile file to read the ~/.profile — which provides the environment variables that Bash shell windows spawned from the window manager need, such as xterm or Gnome Terminal.

Invoking Bash with the --posix option or stating set -o posix in a script causes Bash to conform very closely to the POSIX 1003.2 standard.^[55] Bash shell scripts intended for portability should take into account at least the POSIX shell standard. Some bash features not found in POSIX are:^[55][56]

If a piece of code uses such a feature, it is called a "bashism" – a problem for portable use. Debian's checkbashisms and Vidar Holen's shellcheck can be used to make sure that a script does not contain these parts.^[58][59] The list varies depending on the actual target shell: Debian's policy allows some extensions in their scripts (as they are in the dash shell),^[56] while a script intending to support pre-POSIX Bourne shells, like autoconf's configure, are even more limited in the features they can use.^[60]

Bash uses GNU Readline to provide keyboard shortcuts for command line editing using the default ( Emacs ) key bindings. Vi-bindings can be enabled by running set -o vi.^[61]

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 ";", or on separate lines:

command1; command2
command3

In this example, when command1 is finished, command2 is executed, and when command2 has completed, command3 will execute.

A background execution of command1 can occur using (symbol &) 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 &

Or to have a concurrent execution of command1 and command2, they must be executed in the Bash shell in the following way:

command1 & command2

In this case command1 is executed in the background & symbol, returning immediately control to the shell that executes command2 in the foreground.

A process can be stopped and control returned to bash by typing Ctrl+z while the process is running in the foreground.^[62]

A list of all processes, both in the background and stopped, can be achieved by running jobs:

$ jobs
[1]-  Running                  command1 &
[2]+  Stopped                  command2

In the output, the number in brackets refers to the job id. The plus sign signifies the default process for bg and fg. 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 fg command brings a process to the foreground, while bg sets a stopped process running in the background. bg and fg 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 jobs. The kill command can be used to end a process prematurely, by sending it a signal. The job id must be specified after a percent sign:

kill %1

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

cd "$SOMEWHERE" && ./do_something || echo "An error occurred" >&2

Where ./do_something is only executed if the cd (change directory) command was "successful" (returned an exit status of zero) and the echo command would only be executed if either the cd or the ./do_something command return an "error" (non-zero exit status).

For all commands the exit status is stored in the special variable $?. Bash also supports if ...;then ...;else ...;fi and case $VARIABLE in $pattern)...;;$other_pattern)...;; esac forms of conditional command evaluation.

An external command called bashbug reports Bash shell bugs. When the command is invoked, it brings up the user's default editor with a form to fill in. The form is mailed to the Bash maintainers (or optionally to other email addresses).^[63][64]

Bash supports programmable completion via built-in complete, compopt, and compgen commands.^[65] The feature has been available since the beta version of 2.04 released in 2000.^[66][67] These commands enable complex and intelligent completion specification for commands (i.e. installed programs), functions, variables, and filenames.^[68]

The complete and compopt 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 Tab ↹ keystroke after typing its name.^[68]

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The program's name is a figure of speech or witticism which begins with an homage to Stephen Bourne, the creator of one of the shell programs which have sometimes been considered superseded by the bash shell. His name is used as a pun on the image of childbirth. With that pun, it would seem, is added an allusion: possibly to the Hindu or Buddhist idea of reincarnation; possibly to the Christian idiom known as "being born again;" or quite possibly just to the more abstract idea of renewal. While numerous English translations of the Christian New Testament, Book of John, chapter 3 do contain the words "born again," Merriam-Webster's dictionary has "born-again" defined as a "...person who has made a renewed or confirmed commitment....^[69]" Whatever the original touchstone may have been, in the end the program received the name, "the Bourne Again SHell."^[70][71]

The acronym of that name then is "bash," a word meaning "to strike violently.^[72]" In the context of computer programming, to "violently hit something," such as a computer keyboard, could be considered a hyperbolic image of some frustration. Such imagery of negative emotionality could be seen as standing in direct juxtaposition to the idea of becoming "born again."

The naming could be considered an instance of verbal irony^[73] or accidental innuendo.^[74] Bash grammar was initially based on the grammars of the most popular Unix shell programs then currently in use, some of which were considered particularly difficult to use or frustrating at that time. As the years progressed, bash development has made its grammar more user-friendly,^{[75][76][77][78][79][80]} so much so that it seems likely that the bash project has been committed to improving its usability. Since then, bash has become the de facto default shell program in most Linux and Unix operating systems.

As the standard upon which bash is based, the POSIX, or IEEE Std 1003.1,^[81] et seq, is informative.

The Linux man page^[82][83] is intended to be the authoritative explanatory document for the understanding of how bash operates, while the GNU manual is sometimes considered more user-friendly for reading. "You may also find information about Bash by running info bash ... or by looking at /usr/share/doc/bash/, /usr/local/share/doc/bash/, or similar directories on your system. A brief summary is available by running bash --help.^[70]"

On modern Linuxes, information on shell built-in commands can be found by executing help, help [built-in name]or man builtins at a terminal prompt where bash is installed. Some commands, such as echo, false, kill, printf, test or true, depending on your system and on your locally installed version of bash, can refer to either a shell built-in or a system binary executable file. When one of these command name collisions occurs, bash will by default execute a given command line using the shell built-in. Specifying a binary executable's absolute path (i.e., /bin/printf) is one way of ensuring that the shell uses a system binary. This name collision issue also effects any "help summaries" viewed with kill --help and /bin/kill --help. Shell built-ins and system binary executable files of the same name often have differing options.

"The project maintainer also has a Bash page which includes Frequently Asked Questions",^[84][85][70] this FAQ is current as of bash version 5.1 and is no longer updated.

A security hole in Bash dating from version 1.03 (August 1989),^[86] dubbed Shellshock, was discovered in early September 2014 and quickly led to a range of attacks across the Internet.^[87][88][89] Patches to fix the bugs were made available soon after the bugs were identified.

Since 4.0: GPL-3.0-or-later ^[90]

1.11? to 3.2: GPL-2.0-or-later ^[91]

0.99? to 1.05?: GPL-1.0-or-later ^{[92] [51] [93]}

A version is also available for Windows 10 and Windows 11 via the Windows Subsystem for Linux.^[94][95] It is also the default user shell in Solaris 11.^[96] Bash was also the default shell in BeOS,^[15] and in versions of Apple macOS from 10.3 (originally, the default shell was tcsh) to 10.15 (macOS Catalina), which changed the default shell to zsh,^[32] although Bash remains available as an alternative shell.^[54]

• Comparison of command shells

• Stephenson, Neal (2003). In the Beginning... Was the Command Line. HarperCollins. ISBN 978-0380815937.
• "Evolution of shells in Linux". ibm.com. Retrieved 19 May 2024.
• "Scripting Reference :: Scripting with the Bourne-Again Shell (Bash)". berkeley.edu. Retrieved 19 May 2024.
• "IRIS :: Instructional & Research Information Systems :: FAQ: Unix :: About UNIX Shells". berkeley.edu. Retrieved 19 May 2024.