Bash (Unix shell)
In computing, Bash (short for "Bourne Again SHell,")[6] is an interactive command interpreter and command programming language developed for UNIX-like operating systems.[7] Created in 1989[8] by Brian Fox for the GNU Project, it is supported by the Free Software Foundation and designed as a 100% free alternative for the Bourne shell ( Since its inception, Bash has gained widespread adoption and is commonly used as the default login shell for numerous Linux distributions.[10][11][12] 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-line interface (CLI), Bash operates within a terminal emulator, or 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, ( HistoryWhile Bash was developed for UNIX and UNIX-like operating systems, such as GNU/Linux, it is also available on Android, macOS, Windows, and numerous other current and historical operating systems.[14] "Although there have been attempts to create specialized shells, the Bourne shell derivatives continue to be the primary shells in use."[15] Timeline
FeaturesList of Short Descriptions
As a command processor, Bash can operate in two modes: an interactive and non-interactive mode. In the interactive mode a text window where users input commands to execute various tasks. The second mode, is 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:
Bash also offers...
General DiscussionThe Bash command syntax is a superset of the Bourne shell command syntax. Bash supports brace expansion,[84] command line completion (Programmable Completion),[85] basic debugging[86][87] and signal handling (using When a user presses the tab key within an interactive command-shell, Bash automatically uses command line completion, since beta version 2.04,[91] 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 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.[92] Bash supports here documents. Since version 2.05b Bash can redirect standard input (stdin) from a "here string" using the Bash 3.0 supports in-process regular expression matching using a syntax reminiscent of Perl.[93] In February 2009,[94] Bash 4.0 introduced support for associative arrays.[4] Associative array indices are strings, in a manner similar to AWK or Tcl.[95] They can be used to emulate multidimensional arrays. Bash 4 also switches its license to GPL-3.0-or-later.[96] Control StructuresBash 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 For all commands the exit status is stored in the special variable Process Management (a.k.a., "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 ";", 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.[97] A list of all processes, both in the background and stopped, can be achieved by running $ 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 The state of a process can be changed using various commands. The kill %1
Portability with POSIXInvoking Bash with the
If a piece of code uses such a feature, it is called a "bashism" – a problem for portable use. Debian's
Brace ExpansionBrace 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 $ 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
Configurable execution environment(s)Shell and Session Startup Files (a.k.a., "Dot Files")
When Bash starts, it executes the commands in a variety of dot files.[104] Unlike Bash shell scripts, dot files do typically have neither the execute permission enabled nor an interpreter directive like Legacy-compatible Bash startup exampleThe example [ -r ~/.profile ] &&. ~/.profile # set up environment, once, Bourne-sh syntax only
if [ -n "$PS1" ]; then # are we interactive?
[ -r ~/.bashrc ] &&. ~/.bashrc # tty/prompt/function setup for interactive shells
[ -r ~/.bash_login ] &&. ~/.bash_login # any at-login tasks for login shell only
fi # End of "if" block
Operating system issues in Bash startupSome versions of Unix and Linux contain Bash system startup scripts, generally under the Settings and Shell OptionsThe |
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| Feature | POSIX 2024 | Description | Bash ver. | ||
|---|---|---|---|---|---|
| Grammar type | Formal name | Syntax | |||
| Parameter Expansions | Indicate Null or Unset | "${parameter:?[word]}"
|
Yes | "Where the expansion of [word], perhaps an error message or a line number, is written to STDERR and the shell exits with a non-zero exit code." | ? |
| Special Parameters | Exit Status | "$?"
|
Yes | "Expands to the shortest representation of the decimal exit status." | ? |
| Special Parameters | PID of Invoked Shell | "$$"
|
Yes | "Expands to the shortest representation of the decimal process ID of the invoked shell." | ? |
| Special Built-In Utility | set :: xtrace | set -x
|
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." | ? |
| Special Built-In Utility | set :: verbose | set -v
|
Yes | "Writes its input to standard error as it is read." | ? |
| Special Built-In Utility | set :: pipefail | set -o pipefail
|
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." | ? |
| Special Built-In Utility | set :: nounset | set -u
|
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. | ? |
| Special Built-In Utility | set :: errexit | set -e
|
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 considered 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. | ? |
| Special Built-In Utility | trap :: EXIT | trap '[arg]' EXIT
|
Yes | "If a [sigspec] (signal specifier) is 0 or EXIT, [arg] is executed when the shell exits." If [arg] contains expansions, then [arg] should be in single quotes. | ? |
| Utility | printf | printf '<%s>\n' "${var}"
|
Yes | A means of reliably printing the contents of a variable. | ? |
| Bash Variables | BASHPID | "${BASHPID}"
|
No | "Expands to the process ID of the current bash process."[115] | ? |
| Bash Variables | BASH_ARGC | "${BASH_ARGC[@]}"
|
No | "An array variable whose values are the number of parameters in each frame of the current bash execution call stack."[116] | ? |
| Bash Variables | BASH_ARGV | "${BASH_ARGV[@]}"
|
No | "An array variable containing all of the parameters in the current bash execution call stack."[117] | ? |
| Bash Variables | BASH_LINENO | "${BASH_LINENO[@]}"
|
No | "An array variable whose members are the line numbers in source files where each corresponding member of FUNCNAME was invoked."[118] | ? |
| Bash Variables | BASH_REMATCH | "${BASH_REMATCH[@]}"
|
No | "An array variable whose members are assigned by the =~ binary operator to the [[ conditional command."[119] | ? |
| Bash Variables | BASH_SOURCE | "${BASH_SOURCE}"
|
No | "An array variable whose members are the source filenames where the corresponding shell function names in the FUNCNAME array variable are defined."[120] | ? |
| Bash Variables | BASH_XTRACEFD | "${BASH_XTRACEFD}"
|
No | "If set to an integer corresponding to a valid file descriptor, Bash will write the trace output generated when ‘set -x’ is enabled to that file descriptor."[121] | ? |
| Bash Variables | EPOCHREALTIME | "${EPOCHREALTIME}"
|
No | "Each time this parameter is referenced, it expands to the number of seconds since the Unix Epoch (see time(3)) as a floating point value with micro-second granularity."[122] | ? |
| Bash Variables | FUNCNAME | "${FUNCNAME[@]}"
|
No | "An array variable containing the names of all shell functions currently in the execution call stack."[123] | ? |
| Bash Variables | LINENO | "${LINENO}"
|
No | "Each time this parameter is referenced, the shell substitutes a decimal number representing the current sequential line number (starting with 1) within a script or function."[124] | ? |
| Bash Variables | PIPESTATUS | "${PIPESTATUS[@]}"
|
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)."[125] | ? |
| Bash Variables | PPID | "${PPID}"
|
No | "The process ID of the shell's parent."[126] | ? |
| Bash Variables | PS4 | "${PS4}"
|
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."[127] | ? |
| Shell Builtin | set :: restricted | set -r
|
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. | ? |
| Shell Builtin | shopt :: extdebug | shopt -s extdebug
|
No | "Behavior intended for use by debuggers." | ? |
| Shell Builtin | trap :: DEBUG | trap '[arg]' DEBUG
|
No | "If a sigspec is DEBUG, the command arg is executed before" certain kinds of commands. | ? |
| Shell Builtin | trap :: ERR | trap '[arg]' ERR
|
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. | ? |
| Shell Builtin | trap :: RETURN | trap '[arg]' RETURN
|
No | "If a sigspec is RETURN, the command arg is executed each time a shell function or a script executed with the. or source builtins finishes executing." | ? |
- Shell features specified by POSIX:
- Bash features not specified by POSIX:
- Third party debugging utilities:
Examples
With the :? parameter expansion, an unset or null variable can halt a script.
- ex.sh
#!/bin/bash bar="foo is not defined" echo "${foo:?$bar}" echo this message doesn't print
$ ./ex.sh ./ex.sh: line 3: foo: foo is not defined
Reliably printing the contents of an array that contains spaces and newlines first in a portable syntax, and then the same thing in Bash. Note that in Bash, the number of spaces before the newline is made clear.
$ # In POSIX shell: $ array=( "a" "b" " > c " ) $ printf ',%s,\n' "${array[@]}" ,a, , b, , c,
# In Bash: declare -p array declare -a array=([0]="a" [1]=" b" [2]=$' \n c ')
Printing an error message when there's a problem.
- error.sh
if ! lsblk | grep sdb then echo Error, line $LINENO fi
$ ./error.sh Error, line 130
Using xtrace. If errexit had been enabled, then echo quux would not have been executed.
- test.sh
#!/bin/bash set -x foo=bar; echo $foo false echo quux
$ ./test.sh + foo=bar + echo bar bar + false + echo quux quux
Deprecated syntax
- Back-tick style command substitutions:
`...`is deprecated in favor of$(...); - Use of -a or -o in
test/[/[[commands,- for example,
[ -r ./file -a ! -l ./file ]is deprecated in favor of[ -r ./file ] && ! [ -l ./file ];
- for example,
- Use of the arithmetic syntax
$[...]is deprecated in favor of$((...))or((...)), as appropriate; - Use of
^as a pipeline is deprecated in favor of|; - Any uses of
exprorlet.
Shellshock
In September 2014, a security bug was discovered[142] in the program. It was dubbed "Shellshock." Public disclosure quickly led to a range of attacks across the Internet.[143][144][145]
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.[146] The bug had been present in the source code since August 1989 (version 1.03)[147] 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 CVE-2014-6271, CVE-2014-6277 and CVE-2014-7169, among others. Under CVSS Metrics 2.x and 3.x, the bug is regarded as "high" and "critical," respectively.
Bug reporting
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).[148][149]
See also
- Comparison of command shells
- Multics § Commands, exec_com: the first command processor.[23]
Unix Shells
- Almquist shell (ash)
- Bourne shell (sh)
- BusyBox
- C shell (csh)
- Debian-Almquist Shell (dash)
- Fish shell: Friendly Interactive Shell
- Google Shell (goosh) - a UNIX-like front-end for Google Search.
- Korn shell (ksh), of which there are numerous variations.
- nsh - "A command-line shell like fish, but POSIX compatible;" available on Arch.[150]
- osh - "Oil Shell is a Bash-compatible UNIX command-line shell;" available on Arch.
- Mashey or Programmer's Workbench shell
- Qshell for IBM i
- rc from Plan 9
- RUNCOM
- rush - Restricted User Shell, available on Debian.[43]
- Stand-alone shell (sash)
- scsh - The Scheme Shell.
- TENEX C shell (tcsh)
- Thompson shell (tsh)
- Toybox
- yash - Yet Another Shell, aims "to be the most POSIX-compliant shell in the world;" available on Arch.
- Z shell (zsh)
Further reading
- Pouzin, Louis (2 April 1965). "The SHELL: A Global Tool for Calling and Chaining Procedures in the System" (PDF). mit.edu. MIT. Retrieved 5 January 2024.
- 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.
- "Learning the Bash Shell, 2e". Retrieved 14 January 2025.
This book describes the latest release of bash 2.0 (version 2.01, dated June 1997).
- "Apple Developer, Documentation Archive, Shell Style Guide". Retrieved 20 January 2025.
Copyright © 2003, 2014 Apple Inc. All Rights Reserved. ... Updated: 2014-03-10
- "Google, Shell Style Guide". Retrieved 20 January 2025.
References
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- ^ GNU Project. "README file". Archived from the original on 26 April 2019. Retrieved 16 April 2014.
Bash is free software, distributed under the terms of the [GNU] General Public License as published by the Free Software Foundation, version 3 of the License (or any later version).
- ^ "bash-1.11". oldlinux.org. Archived from the original on 15 October 2021. Retrieved 9 June 2021.
See test.c for GPL-2.0-or-later
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- ^
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- "Is there a way to download the presumably initial bash source bash-0.99?". unix.stackexchange.com.
- ^
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"Bourne Again Shell" is a play on the name Bourne Shell, which was the usual shell on Unix.
- Gattol, Markus (13 March 2011). "Bourne-again Shell". Archived from the original on 9 March 2011. Retrieved 13 March 2011.
The name is a pun on the name of the Bourne shell (sh), an early and important Unix shell written by Stephen Bourne and distributed with Version 7 Unix circa 1978, and the concept of being "born again".
- Richard Stallman (12 November 2010). "About the GNU Project". Free Software Foundation. Archived from the original on 24 April 2011. Retrieved 13 March 2011.
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The Bourne shell is an interactive command interpreter and command programming language.
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The ultimate goal is to provide free software to do all of the jobs computer users want to do—and thus make proprietary software a thing of the past.
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The Free Software Foundation (FSF) is a nonprofit with a worldwide mission to promote computer user freedom.
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GNU is an operating system which is 100% free software.
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Brian Fox has now completed GNU's version of sh, called BASH, the `Bourne Again SHell'.
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When Richard Stallman decided to create a full replacement for the then-encumbered Unix systems, he knew that he would eventually have to have replacements for all of the common utilities, especially the standard shell, and those replacements would have to have acceptable licensing.
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I've currently ported bash(1.08) and gcc(1.40), and things seem to work.
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'Guarded command,' a statement list prefixed by a Boolean expression: only when this boolean expression is initially true, is the statement list eligible for execution.
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- ^ https://v6sh.org/ V6 Sh History
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For a year and a half, the GNU shell was "just about done". The author made repeated promises to deliver what he had done, and never kept them. Finally I could no longer believe he would ever deliver anything. So Foundation staff member Brian Fox is now implementing an imitation of the Bourne shell.
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