View source for Bash (Unix shell)

{{Short description|GNU replacement for the Bourne shell}}
{{Use dmy dates|date=March 2014}}
{{Infobox software
| author                 = [[Brian Fox (programmer)|Brian Fox]]
| caption                = Screenshot of a Bash session
| developer              = Chet Ramey
| genre                  = [[Shell (computing)]],  [[Unix shell]],  [[command language]]
| language               = Multilingual ([[gettext]])
| latest preview date    = {{wikidata| qualifier | preferred | single | Q189248 | P348 | P548 = Q51930650 | P577 }}
| latest preview version = {{wikidata | property | preferred | references | edit | Q189248 | P348 | P548 = Q51930650 }}
| latest release date    = {{wikidata| qualifier | preferred | single | Q189248 | P348 | P548 = Q2804309 | P577 }}
| latest release version = {{wikidata| property | preferred | references | edit | Q189248 | P348 | P548 = Q2804309 }}
| license                = Since 4.0:  [[GNU General Public License#Version 3|GPL-3.0-or-later]], <br />1.11? to 3.2:  [[GNU General Public License#Version 2|GPL-2.0-or-later]], <br />0.99? to 1.05?:  [[GNU General Public License#Version 1|GPL-1.0-or-later]]
| logo                   = Gnu-bash-logo.svg
| name                   = Bash
| operating system       = {{Plainlist |
* [[BeOS]]
* [[Haiku (operating system)|Haiku]]
* [[macOS]] ( [[GNU General Public License#Version 2|GPL-2.0-or-later]] ; GPL-3.0-or-later through 3rd parties)
* [[Unix-like]]
* [[Windows]] ( [[GNU General Public License#Version 3|GPL-3.0-or-later]] )
}}
| platform               = [[GNU]]
| programming language   = [[C (programming language)|C]]
| released               = {{start date and age| 8 June 1989 }}
| screenshot             = Bash screenshot.png
| website                = {{URL|www.gnu.org/software/bash/}}
}}
'''Bash''', short for ''Bourne-Again SHell'', is a [[Shell (computing)|shell]] [[Unix shell|program]] and [[command language]]  supported by the [[Free Software Foundation]]<ref>
{{ cite web
| access-date = 19 May 2024
| quote = The Free Software Foundation (FSF) is a nonprofit with a worldwide mission to promote computer user freedom.
| title = Free Software Foundation — working together for free software — Front Page
| url = https://www.fsf.org/
| website = fsf.org
}}
</ref> and first developed for the [[GNU Project]]<ref>
{{ cite web
| access-date = 19 May 2024
| quote = 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.
| title = GNU in a Nutshell
| url = https://www.gnu.org/gnu/about-gnu.html
| website = gnu.org
}}
</ref> by [[Brian Fox (programmer)|Brian Fox]].<ref>
{{ cite web
| access-date = 19 May 2024
| quote = Brian Fox has now completed GNU's version of sh, called BASH, the `Bourne Again SHell'.
| title = GNU's Bulletin, vol. 1 no. 7, June, 1989 :: GNU's Who
| url = https://www.gnu.org/bulletins/bull7.html#SEC3
| website = gnu.org
}}
</ref> Designed as a 100%<ref>
{{ cite web
| access-date = 19 May 2024
| quote = GNU is an operating system which is 100% free software.
| title = GNU Software
| url = https://www.gnu.org/software/software.en.html
| website = gnu.org
}}
</ref> [[free software]] alternative for the [[Bourne shell]],<ref>
{{ cite web
| access-date = 19 May 2024
| quote = The Bourne shell is an interactive command interpreter and command programming language.
| title = Bourne shell
| url = https://www.ibm.com/docs/en/aix/latest?topic=shells-bourne-shell
| website = ibm.com
}}
</ref><ref name = "GNUBSD">
{{ cite newsgroup
| access-date = December 28, 2021
| archive-date = December 28, 2021
| archive-url = https://web.archive.org/web/20211228023030/https://groups.google.com/g/comp.unix.questions/c/iNjWwkyroR8/m/yedr9yDWSuQJ
| author = [[Richard Stallman]] (forwarded with comments by Chet Ramey)
| date = February 10, 1988
| message-id = 2362@mandrill.CWRU.Edu
| newsgroup = comp.unix.questions
| title = GNU + BSD = ?
| url = https://groups.google.com/g/comp.unix.questions/c/iNjWwkyroR8/m/yedr9yDWSuQJ
| quote = 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.
| url-status = live
}}
</ref><ref>
{{ citation
| access-date = March 21, 2011
| archive-date = July 6, 2011
| archive-url = https://web.archive.org/web/20110706103704/http://www.computerworld.com.au/article/222764/a-z_programming_languages_bash_bourne-again_shell/?pp=2&fp=16&fpid=1
| date = May 30, 2008
| first = Naomi
| journal = Computerworld
| last = Hamilton
| page = 2
| quote = 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.
| title = The A-Z of Programming Languages: BASH/Bourne-Again Shell
| url = http://www.computerworld.com.au/article/222764/a-z_programming_languages_bash_bourne-again_shell/?pp=2&fp=16&fpid=1
| url-status = dead
}}
NOTE: Original computerworld.com.au link is dead: see also copies of original material at
*     [https://a-z.readthedocs.io/en/latest/bash.html readthedocs.io],
*     [https://www2.computerworld.com.au/article/222764/a-z_programming_languages_bash_bourne-again_shell/ computerworld.com.au] and
*     [https://cse.sc.edu/~mgv/csce330f16/pres/az.pdf the University of South Carolina].
</ref> it was initially released in 1989.<ref name = "BashBeta">
{{ cite newsgroup
| access-date = October 28, 2010
| archive-date = May 4, 2013
| archive-url = https://web.archive.org/web/20130504075535/http://groups.google.com/group/gnu.announce/msg/a509f48ffb298c35?hl=en
| author = Brian Fox (forwarded by Leonard H. Tower Jr.)
| date = June 8, 1989
| newsgroup = gnu.announce
| title = Bash is in beta release!
| url = https://groups.google.com/group/gnu.announce/msg/a509f48ffb298c35?hl=en
| url-status = live
}}
</ref> Its moniker is a play on words, referencing both its predecessor, the Bourne shell,<ref>
{{ cite web
| archive-url = https://web.archive.org/web/20210302212410/https://www.drdobbs.com/i-almost-get-a-linux-editor-and-compiler/184404693
| access-date = 2020-09-12
| archive-date = March 2, 2021
| title = I Almost Get a Linux Editor and Compiler
| url = http://www.drdobbs.com/i-almost-get-a-linux-editor-and-compiler/184404693
| url-status = live
| website = Dr. Dobb's
}}
</ref> and the concept of [[Born again|rebirth]].<ref>
{{ cite web
| access-date = March 13, 2011
| archive-date = April 24, 2011
| archive-url = https://web.archive.org/web/20110424064815/https://www.gnu.org/gnu/thegnuproject.html
| author = [[Richard Stallman]]
| date = November 12, 2010
| publisher = [[Free Software Foundation]]
| quote = "Bourne Again Shell" is a play on the name ''Bourne Shell'', which was the usual shell on Unix.
| title = About the GNU Project
| url = https://www.gnu.org/gnu/thegnuproject.html
| url-status = live
}}
</ref><ref>
{{ citation
| access-date = March 13, 2011
| archive-date = March 9, 2011
| archive-url = https://web.archive.org/web/20110309092607/http://www.markus-gattol.name/ws/bash.html
| date = March 13, 2011
| df = dmy-all
| first = Markus
| last = Gattol
| quote = 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".
| title = Bourne-again Shell
| url = http://www.markus-gattol.name/ws/bash.html
| url-status = dead
}}
</ref>
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 ([[GNU Compiler Collection|GCC]]).<ref name="groups.google.com">
{{ cite web
| access-date = 2009-09-06
| date = August 1991
| first = Linus Benedict
| last = Torvalds
| quote = I've currently ported bash(1.08) and gcc(1.40), and things seem to work.
| title = comp.os.minix
| url = https://groups.google.com/group/comp.os.minix/msg/b813d52cbc5a044b
}}
</ref> It is available on nearly all modern operating systems, making it a versatile tool in various computing environments.
As a [[Command-line interface|command processor]], Bash operates within a [[Terminal emulator|text window]] where users input commands to execute various tasks. It also supports the execution of commands from files, known as [[shell script]]s, facilitating [[automation]]. In keeping with [[Unix shell]] conventions, Bash incorporates a rich set of features. The [[Keyword (computer programming)|keywords]], [[Syntax (programming languages)|syntax]], [[Scope (computer science)#Dynamic scoping|dynamically scoped]] variables and other basic features of the [[formal language|language]] are all copied from the [[Bourne shell]], <code>sh</code>. Other features, e.g., [[C shell#History|history]], are copied from the [[C shell]], <code>csh</code>, and the [[Korn Shell]], <code>ksh</code>. It is a [[POSIX]]-compliant shell with extensions.
== History ==
While Bash was developed for UNIX and UNIX-like operating systems such as GNU/Linux,<ref>
{{ cite web
| access-date = April 9, 2016
| archive-date = September 1, 2018
| archive-url = https://web.archive.org/web/20180901171316/ftp://ftp.cwru.edu/pub/bash/FAQ
| df = mdy-all
| title = Bash FAQ, version 4.14
| url = ftp://ftp.cwru.edu/pub/bash/FAQ
| url-status = dead
}}
</ref> it is also available on Windows, BeOS,<ref name = "forb">
{{ cite web
| title = A desktop alternative
| url = https://www.forbes.com/1999/01/25/feat.html
| website = Forbes
}}
</ref><ref>
{{ cite web
| title = Appendix A: Using the BeOS Command Line Shell
| url = http://testou.free.fr/www.beatjapan.org/mirror/www.be.com/documentation/user_docs/app_a_commandline.html
| website = testou.free.fr
}}
</ref> and Haiku.<ref>
{{ cite web
| title = Terminal
| url = https://www.haiku-os.org/docs/userguide/en/applications/terminal.html
}}
</ref>
[[Brian Fox (computer programmer)|Brian Fox]] began [[Computer programming|coding]] Bash on January 10, 1988,<ref>
{{ citation
| access-date = November 1, 2010
| archive-date = September 28, 2018
| archive-url = https://web.archive.org/web/20180928193215/http://ftp.gnu.org/gnu/bash/bash-1.14.7.tar.gz
| author = Brian Fox
| date = August 29, 1996
| publisher = [[Free Software Foundation]]
| quote = Birthdate: Sunday, January 10th, 1988. Initial author: Brian Fox
| title = shell.c
| url = http://ftp.gnu.org/gnu/bash/bash-1.14.7.tar.gz
| url-status = live
}}
</ref> after [[Richard Stallman]] became dissatisfied with the lack of progress being made by a prior developer.<ref name = GNUBSD/> Stallman and the [[Free Software Foundation|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.<ref name = GNUBSD/><ref>
{{ cite web
| access-date = 21 March 2011
| archive-date = April 24, 2011
| archive-url = https://web.archive.org/web/20110424064815/https://www.gnu.org/gnu/thegnuproject.html
| author =  [[Richard Stallman]]
| date = 3 October 2010
| publisher =  [[Free Software Foundation]]
| quote = Free Software Foundation employees have written and maintained a number of GNU software packages. Two notable ones are the C library and the shell. ... We funded development of these programs because the GNU Project was not just about tools or a development environment. Our goal was a complete operating system, and these programs were needed for that goal.
| title = About the GNU Project
| url = https://www.gnu.org/gnu/thegnuproject.html
| url-status = live
}}
</ref> Fox released Bash as a beta, version .99, on June 8, 1989,<ref name = BashBeta/> and remained the primary maintainer until sometime between mid-1992<ref>
{{ cite newsgroup
| access-date = October 28, 2010
| archive-date = March 2, 2021
| archive-url = https://web.archive.org/web/20210302213455/https://groups.google.com/forum/
| author = len (g...@prep.ai.mit.edu)
| date = April 20, 1993
| message-id = gnusenet930421bulletin@prep.ai.mit.edu
| newsgroup = gnu.announce
| title = January 1993 GNU's Bulletin
| url = https://groups.google.com/group/gnu.misc.discuss/msg/4f42c739cd7e8bd8
| url-status = live
}}
</ref> and mid-1994,<ref name = "Bourne shell grammar">
{{ cite web
| access-date = November 13, 2008
| archive-date = December 5, 2008
| archive-url = https://web.archive.org/web/20081205082152/http://www.linuxjournal.com/article/2800
| date = August 1, 1994
| first = Chet
| last = Ramey
| title = Bash - the GNU shell (Reflections and Lessons Learned)
| url = http://www.linuxjournal.com/article/2800#N0xa50890.0xb46380
| url-status = live
| work =  [[Linux Journal]]
}}
</ref> when he was laid off from FSF<ref>
{{ citation
| access-date = October 31, 2010
| archive-date = July 20, 2012
| archive-url = https://web.archive.org/web/20120720124341/http://www.scribd.com/doc/40556434/2010-10-31-Chet-Ramey-Early-Bash-Dates
| author = Chet Ramey
| date = October 31, 2010
| title = Dates in your Computerworld interview
| url = https://www.scribd.com/doc/40556434/2010-10-31-Chet-Ramey-Early-Bash-Dates
| url-status = live
}}
</ref> and his responsibility was transitioned to another early contributor, Chet Ramey.<ref name = "computerworld-Hamilton-Ramey">
{{ cite news
| access-date = 1 March 2022
| archive-date = 8 November 2016
| archive-url = https://web.archive.org/web/20161108230747/http://www.computerworld.com.au/article/222764/
| first1 = Naomi
| last1 = Hamilton
| title = The A-Z of Programming Languages: BASH/Bourne-Again Shell
| url = http://www.computerworld.com.au/article/222764/
| website = [[Computerworld]]
| date = 30 March 2008 <!-- https://web.archive.org/web/20081205093826/http://www.computerworld.com.au/index.php/id;1591223321;pp;2;fp;16;fpid;1 -->
}}
</ref><ref name = "tiswww.case.edu-Ramey-BASH">
{{ cite web
| access-date = 1 March 2022
| date = 2021-04-20
| first1 = Chet
| last1 = Ramey
| publisher = [[Case Western Reserve University]]
| title = The GNU Bourne-Again Shell
| url = https://tiswww.case.edu/php/chet/bash/bashtop.html
| website = Technology Infrastructure Services
}}
</ref><ref>
{{ Cite newsgroup
| access-date = November 1, 2010
| archive-date = November 10, 2012
| archive-url = https://web.archive.org/web/20121110194014/http://groups.google.com/group/gnu.bash.bug/msg/1fc7b688f5d44438?hl=en
| author = Chet Ramey
| date = June 12, 1989
| newsgroup = gnu.bash.bug
| title = Bash 0.99 fixes & improvements
| url = https://groups.google.com/group/gnu.bash.bug/msg/1fc7b688f5d44438?hl=en
| url-status = live
}}
</ref><ref>
{{ Cite newsgroup
| access-date = October 30, 2010
| archive-date = November 10, 2012
| archive-url = https://web.archive.org/web/20121110194034/http://groups.google.com/group/gnu.bash.bug/msg/072a03645663caea?hl=en
| author = Chet Ramey
| date = July 24, 1989
| newsgroup = gnu.bash.bug
| title = Some bash-1.02 fixes
| url = https://groups.google.com/group/gnu.bash.bug/msg/072a03645663caea?hl=en
| url-status = live
}}
</ref><ref>
{{ Cite newsgroup
| access-date = October 30, 2010
| archive-date = November 10, 2012
| archive-url = https://web.archive.org/web/20121110194052/http://groups.google.com/group/gnu.bash.bug/msg/e6112ccc8866e2f4?hl=en
| author = Brian Fox
| date = March 2, 1990
| newsgroup = gnu.bash.bug
| title = Availability of bash 1.05
| url = https://groups.google.com/group/gnu.bash.bug/msg/e6112ccc8866e2f4?hl=en
| url-status = live
}}
</ref>
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<ref>
{{ cite book
| access-date = June 6, 2016
| archive-date = March 2, 2021
| archive-url = https://web.archive.org/web/20210302212409/https://books.google.com/books?id=2P3zBgAAQBAJ&q=%22most+popular%22+linux+shell&pg=PA5
| date = April 2015
| edition = 3rd
| first1 = Christine
| first2 = Richard
| isbn = 978-1-119-02122-3
| last1 = Bresnahan
| last2 = Blum
| page = 5
| publisher = John Wiley & Sons, Inc.
| quote = In Linux, most users run bash because it is the most popular shell.
| title = CompTIA Linux+ Powered by Linux Professional Institute Study Guide: Exam LX0-103 and Exam LX0-104
| url = https://books.google.com/books?id=2P3zBgAAQBAJ&q=%22most+popular%22+linux+shell&pg=PA5
| url-status = live
}}
</ref><ref>
{{ cite book
| access-date = June 6, 2016
| archive-date = March 2, 2021
| archive-url = https://web.archive.org/web/20210302212410/https://books.google.com/books?id=tIjrVYbZmUAC&q=bash+most+popular+unix+shell&pg=PA363
| date = February 2006
| first1 = Arman
| first2 = Michael
| isbn = 978-0-7821-5277-7
| last1 = Danesh
| last2 = Jang
| page = 363
| publisher = John Wiley & Sons, Inc.
| quote = The Bourne Again Shell (bash) is the most common shell installed with Linux distributions.
| title = Mastering Linux
| url = https://books.google.com/books?id=tIjrVYbZmUAC&q=bash+most+popular+unix+shell&pg=PA363
| url-status = live
}}
</ref> and on Apple's macOS releases before [[MacOS Catalina|Catalina]] in October 2019.<ref>
{{ cite book
| access-date = June 6, 2016
| archive-date = March 2, 2021
| archive-url = https://web.archive.org/web/20210302212410/https://books.google.com/books?id=dwIRERUpQPEC&q=bash+most+popular+unix+shell&pg=PA6
| date = April 2005
| first1 = Eric
| first2 = John C.
| first3 = Micah
| isbn = 978-0-7645-9791-6
| last1 = Foster-Johnson
| last2 = Welch
| last3 = Anderson
| page = 6
| publisher = John Wiley & Sons, Inc.
| quote = Bash is by far the most popular shell and forms the default shell on Linux and Mac OSX systems.
| title = Beginning Shell Scripting
| url = https://books.google.com/books?id=dwIRERUpQPEC&q=bash+most+popular+unix+shell&pg=PA6
| url-status = live
}}
</ref><ref name="zsh1">
{{ cite web
| access-date = 1 July 2019
| archive-date = December 2, 2019
| archive-url = https://web.archive.org/web/20191202143802/https://support.apple.com/en-us/HT208050
| title = Use zsh as the default shell on your Mac - Apple Support
| url = https://support.apple.com/en-us/HT208050
| url-status = live
}}
</ref><ref name="zsh2">
{{ cite web
| access-date = June 13, 2019
| archive-date = June 10, 2019
| archive-url = https://web.archive.org/web/20190610051842/https://www.theverge.com/2019/6/4/18651872/apple-macos-catalina-zsh-bash-shell-replacement-features
| date = June 4, 2019
| first = Tom
| last = Warren
| title = Apple replaces bash with zsh as the default shell in macOS Catalina
| url = https://www.theverge.com/2019/6/4/18651872/apple-macos-catalina-zsh-bash-shell-replacement-features
| url-status = live
| website = The Verge
}}
</ref> Bash has also been ported to [[Microsoft Windows]]<ref>
{{ cite web
| access-date=July 8, 2016
| archive-date=September 24, 2019
| archive-url=https://web.archive.org/web/20190924163108/https://github.com/microsoft/WSL/issues/107
| title=Missing source code - GPL compliance? · Issue #107 · Microsoft/WSL
| url=https://github.com/Microsoft/BashOnWindows/issues/107
| url-status=live
| website=GitHub
}}
</ref><ref>
{{ cite web
| access-date = 9 April 2016
| archive-date = October 21, 2017
| archive-url = https://web.archive.org/web/20171021004857/http://www.softpedia.com/get/System/System-Miscellaneous/GNU-Bash.shtml
| date = January 23, 2010
| publisher = SoftNews
| title = GNU Bash
| url = http://www.softpedia.com/get/System/System-Miscellaneous/GNU-Bash.shtml
| url-status = live
| website =  [[Softpedia]]
}}
</ref> and distributed with [[Cygwin]] and [[MinGW]], to [[DOS]] by the [[DJGPP]] project, to [[Novell NetWare]], to [[OpenVMS]] by the GNU project,<ref>
{{ cite web
| access-date = 2020-09-04
| archive-date = October 3, 2020
| archive-url = https://web.archive.org/web/20201003182204/https://sourceforge.net/p/gnv/wiki/InstallingGNVPackages/
| title = Installing the new GNU packages
| url = https://sourceforge.net/p/gnv/wiki/InstallingGNVPackages/
| url-status = live
}}
</ref> to [[ArcaOS]],<ref>
{{ cite web
| access-date = 2020-09-04
| archive-date = September 23, 2020
| archive-url = https://web.archive.org/web/20200923001119/https://www.arcanoae.com/wiki/arcaos/compatibility-subsystems/
| title = Compatibility Subsystems
| url = https://www.arcanoae.com/wiki/arcaos/compatibility-subsystems/
| url-status = live
}}
</ref> and to [[Android (operating system)|Android]] via various terminal emulation applications.
In September 2014, Stéphane Chazelas, a Unix/Linux specialist,<ref>
{{ cite news
| access-date = 21 June 2019
| archive-date = June 21, 2019
| archive-url = https://web.archive.org/web/20190621183753/https://www.techolac.com/linux/linux-bash-exit-status-and-how-to-set-exit-status-in-bash/
| date = 10 June 2017
| first1 = Cino
| last1 = Juliana
| title = Linux bash exit status and how to set exit status in bash - Techolac
| url = https://www.techolac.com/linux/linux-bash-exit-status-and-how-to-set-exit-status-in-bash/
| url-status = live
}}
</ref> discovered a  [[security bug]] in the program. The bug, first disclosed on September 24, was named [[Shellshock (software bug)|Shellshock]] and assigned the numbers {{ CVE | 2014-6271 | 2014-6277 | 2014-7169 | leadout=and }}. The bug was regarded as severe, since [[Common Gateway Interface|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 variable]]s.<ref>
{{ cite web
| access-date = September 25, 2014
| archive-date = September 25, 2014
| archive-url = https://web.archive.org/web/20140925035842/https://securityblog.redhat.com/2014/09/24/bash-specially-crafted-environment-variables-code-injection-attack/
| author = Huzaifa Sidhpurwala
| date = 2014-09-24
| publisher = [[Red Hat]]
| title = Bash specially-crafted environment variables code injection attack
| url = https://securityblog.redhat.com/2014/09/24/bash-specially-crafted-environment-variables-code-injection-attack/
| url-status = live
}}</ref>
== Features ==
As a [[Command-line interface|command processor]], Bash operates within a [[Terminal emulator|text window]] where users input commands to execute various tasks. It also supports the execution of commands from files, known as [[shell script]]s, facilitating [[automation]]. In keeping with [[Unix shell]] conventions, Bash incorporates a rich set of features, including:
* [[Function (computer programming)|Invocation]] as a...
** [[Interactivity|Interactive]] shell,
** Non-interactive shell, or
** Login shell;
* A [[command-line interface]];
* [[Exit status]] codes;
* [[Control flow|Control structures]] for
** [[Conditional (computer programming)|Condition testing]],
*** <code>if</code>, <code>case</code>, <code>select</code>,
*** logical AND (<code>&&</code>) and OR (<code>||</code>), and
** [[Iteration]]
*** <code>for</code>, <code>while</code>, <code>until</code> loops, and
*** Arithmetic C-style loop: <code>for ((</code>;
* Syntaxes for file type, string value and integer value testing
** Traditional single bracket test: <code>[</code>,
** Modern double bracket test: <code> &#91;&#91; ... &#93;&#93;</code>, including
*** Extended [[regular expression]] and extglob matching
*** Lexicographic sorting with <code><</code> and <code>></code>;
* [[Unix|UNIX]]-style [[Pipeline (Unix)|pipelines]]: <code>|</code>;
* [[Child process|Subshells]]: <code>( ... )</code>;
* [[Signal (IPC)|Signaling]] as a means of [[inter-process communication]] using the <code>trap</code> builtin;
* Asynchronous execution: <code>job_spec &</code>;
* A shell [[Porting|portability]] mode where commands can be interpreted in conformance with the [[POSIX terminal interface|POSIX]] standard;
* Command parsing:
** Comments are ignored:
*** Bourne-style <code>#</code> hashtag comments, and
*** Thompson-style <code>:</code> colon comments;
** Commands are parsed one line at a time,
*** Control structures are honored, and
*** Backslash <code>\</code> escapes are also honored at the ends of lines,
** Split into words (i.e., [[Text segmentation|word splitting]]) according to [[String literal|quoting]] rules,
*** Including ANSI-C quoting <code>$'...'</code>, and
** Seven kinds of expansions are performed on the resulting string in the following order:
*** (Step 1) Brace expansion <code>kernel{-headers}</code>,
*** (Step 2) Tilde expansion <code>~</code>,
*** (Step 3) In a left-to-right fashion:
**** [[Parameter (computer programming)|Parameter]] and [[Variable (computer science)|variable]] expansion <code>$foo</code> or <code>${bar}</code>, including
***** [[Scope (computer science)#Dynamic scoping|Dynamically scoped]] variables,
***** Indexed [[Array (data type)|arrays]] of unlimited size,
***** [[Associative array]]s via <code>declare -A</code>, and
***** Expansion syntaxes which can perform some tasks more quickly than external utilities, such as
****** Pattern Substitution
******* <code>${foo//x/y}</code> for <code>sed 's/x/y/g'</code>,
****** Remove Matching Prefix or Suffix Pattern
******* <code>${bar##[a-zA-Z0-9]*}</code> for <code>cut -c8-</code>,
****** Print Array Keys
******* <code>${!array[@]}</code>, and
****** Display Error if Null or Unset
******* <code>${var:?error message}</code>, among others,
**** [[Command substitution]]: <code>$( ... )</code>,
**** [[Process substitution]], <code><()</code> or <code>>()</code>, when a system supports it:
**** Arithmetic expansion, <code>(( ... ))</code> or <code>$(( ... ))</code>, including
***** Integer [[arithmetic]] in any [[Radix|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)|<code>bc</code>]] and [[AWK|<code>awk</code>]], among others),
*** (Step 4) [[Text segmentation|Word splitting]] (again),
*** (Step 5) Pathname expansion,
**** Meaning shell-style [[Glob (programming)|globbing]] and [[pattern matching]] using <code>*</code>, <code>?</code>, <code>[...]</code>, and
*** [[String literal|Quote]] removal;
** [[Redirection (computing)|Redirections]] of Standard Input, Standard Output and Standard Error [[Standard streams|data streams]] are performed, including
*** File writing, <code>></code>, and appending, <code>>></code>,
*** [[Here document]]s, <code><<</code>,
*** Here strings, <code><<<</code>, which allow parameters to be used as input, and
*** A redirection operator, <code>>|</code>, which can force overwriting of a file when a shell's "noclobber" setting is enabled;
** Command name lookup is performed, in the following order:
*** [[Command (computing)|Commands]] internal to the [[Shell (computing)|shell]]:
**** [[Alias (command)|Shell aliases]],
**** Shell reserved words,
**** [[Function (computer programming)|Shell functions]], and
**** [[Shell builtin|Shell built-in commands]];
*** Commands external to the shell:
**** Separate [[Unix|UNIX]]-style programs such as [[ls|<code>ls</code>]] or [[Ln (Unix)|<code>ln</code>]], and
**** [[Shell script]]s, which are [[File system|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):<ref>
{{ cite web
| title = Command Execution Environment (Bash Reference Manual)
| url = https://www.gnu.org/software/bash/manual/html_node/Command-Execution-Environment.html
| website = www.gnu.org}}
</ref>
** Shell and [[Session (computer science)|session]] startup files such as <code>~/.bashrc</code> and <code>~/.profile</code> (i.e., [[Hidden file and hidden directory|dotfiles]]);
* Settings (<code>set</code> [[Shell builtin|built-in]]) and shell options (<code>shopt</code> [[Shell builtin|built-in]]) which alter shell behavior;
* With interactive invocation only,
** Unlimited size [[command history]],
** [[Job (computing)|Jobs]] and [[Job control (Unix)|job control]],
** A directory stack (see <code>pushd</code> and <code>popd</code> built-ins),
** [[Command-line completion|Tab completion]],
** Configurable [[Command-line interface#Command prompt|prompts]], and
** Command line editing with [[GNU Readline|GNU readline]];
* Lightweight [[Logging (computing)|logging]] for [[debugging]] purposes (xtrace), and other lightweight debugging options (errexit, noexec, nounset, pipefail, etc.);
* Shell [[Compatibility mode|compatibility modes]]: bash 5.1 can operate as if it were bash 4.2, etc.;
* Documentation:
** A built-in <code>help</code> command,
** A [[man page]], and
** An [[Info (Unix)|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 [[Keyword (computer programming)|keywords]], [[Syntax (programming languages)|syntax]], [[Scope (computer science)#Dynamic scoping|dynamically scoped]] variables and other basic features of the [[formal language|language]] are all copied from [[Bourne shell|sh]]. Other features, e.g., [[C shell#History|history]], are copied from [[C shell|csh]] and [[KornShell|ksh]].
The Bash [[command (computing)|command]] syntax is a [[superset]] of the Bourne shell command syntax. Bash supports [[brace expansion]],<ref>
{{ cite web
| access-date = 2024-01-10
| archive-date = March 15, 2018
| archive-url = https://web.archive.org/web/20180315115230/http://www.gnu.org/software/bash/manual/bash.html#Programmable-Completion
| title = Brace Expansion (Bash Reference Manual)
| url = https://www.gnu.org/software/bash/manual/html_node/Brace-Expansion.html
| website = www.gnu.org
}}
</ref> [[command line completion]] (Programmable Completion),<ref>
{{ cite web
bash.html#Programmable-Completion
| title = Bash Reference Manual
| url = https://www.gnu.org/software/bash/manual/
}}
| website = www.gnu.org
</ref> basic debugging<ref>
{{ cite web
| access-date = November 20, 2018
| archive-date = November 4, 2018
| archive-url = https://web.archive.org/web/20181104185300/http://tldp.org/LDP/Bash-Beginners-Guide/html/sect_02_03.html
| title = Debugging Bash scripts
| url = http://tldp.org/LDP/Bash-Beginners-Guide/html/sect_02_03.html
| url-status = live
| website = tldp.org
}}
</ref><ref>
{{ cite web
| access-date = 2024-01-10
| title = The Set Builtin (Bash Reference Manual)
| url = https://www.gnu.org/software/bash/manual/html_node/The-Set-Builtin.html
| website = www.gnu.org
}}
</ref> and [[Signal (IPC)|signal handling]] (using <code>trap</code>) since bash 2.05a<ref>
{{ cite web
| access-date = September 23, 2019
| archive-date = September 23, 2019
| archive-url = https://web.archive.org/web/20190923203509/https://wiki-dev.bash-hackers.org/scripting/bashchanges
| url = https://wiki-dev.bash-hackers.org/scripting/bashchanges
| url-status = live
| title = Bash changes [Bash Hackers Wiki (DEV 20200708T2203)]
| website = wiki-dev.bash-hackers.org
}}
</ref><ref>
{{ cite web
| access-date = 2024-01-10
| title = Bourne Shell Builtins (Bash Reference Manual)
| url = https://www.gnu.org/software/bash/manual/html_node/Bourne-Shell-Builtins.html
| website = www.gnu.org
}}
</ref> 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 [[KornShell|Korn Shell]] (ksh) and the [[C shell]] (csh) such as command line editing, [[command history]] (<code>history</code> command),<ref>
{{ cite web
| access-date = September 15, 2019
| archive-date = September 15, 2019
| archive-url = https://web.archive.org/web/20190915123105/http://www.gnu.org/savannah-checkouts/gnu/bash/manual/bash.html#Bash-History-Builtins
| title = Bash Reference Manual
| url = https://www.gnu.org/savannah-checkouts/gnu/bash/manual/bash.html#Bash-History-Builtins
| url-status = live
| website = www.gnu.org
}}
</ref> the directory stack, the <code>$RANDOM</code> and <code>$PPID</code> variables, and POSIX [[command substitution]] syntax <code>$(...)</code>.
When a user presses the [[tab key]] within an interactive command-shell, Bash automatically uses [[command line completion]], since beta version 2.04,<ref>
{{ cite web
| access-date = June 21, 2018
| archive-date = June 29, 2018
| archive-url = https://web.archive.org/web/20180629230903/http://www.caliban.org/bash/index.shtml
| title = Working more productively with bash 2.x/3.x
| url = http://www.caliban.org/bash/index.shtml
| url-status = live
| website = www.caliban.org
}}
</ref> 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 <code>((...))</code> command and the <code>$((...))</code> variable syntax for this purpose. Its syntax simplifies [[redirection (computing)|I/O redirection]]. For example, it can redirect [[standard out]]put (stdout) and [[Standard error stream|standard error]] (stderr) at the same time using the <code>&></code> operator. This is simpler to type than the Bourne shell equivalent '<code>command > file 2>&1</code>'. Bash supports [[process substitution]] using the <code><(command)</code> and <code>>(command)</code>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 pipe]]s 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.<ref name="GNUBASHREF">
{{ citation
| access-date = October 26, 2010
| archive-date = December 3, 2010
| archive-url = https://web.archive.org/web/20101203065400/https://www.gnu.org/software/bash/manual/html_node/index.html
| chapter = 6.11 Bash POSIX Mode
| chapter-url = https://www.gnu.org/software/bash/manual/html_node/Bash-POSIX-Mode.html
| date = December 23, 2009
| title = The GNU Bash Reference Manual, for Bash, Version 4.1
| url = https://www.gnu.org/software/bash/manual/html_node/index.html
| url-status = live
}}
</ref>
Bash supports [[here document]]s. Since version 2.05b Bash can redirect [[Stdin|standard input]] (stdin) from a "here string" using the <code><<<</code> operator.
Bash 3.0 supports in-process [[regular expression]] matching using a syntax reminiscent of [[Perl]].<ref>
{{ cite web
| access-date = 2017-03-05
| archive-date = May 5, 2017
| archive-url = https://web.archive.org/web/20170505161903/http://tldp.org/LDP/abs/html/bashver3.html#BASH3REF
| at = Section 37.2 (Bash, version 3)
| title = Advanced Bash-Scripting Guide
| url = http://www.tldp.org/LDP/abs/html/bashver3.html#BASH3REF
| url-status = live
| website = www.tldp.org
}}
</ref>
In February 2009,<ref>
{{ cite web
| access-date = June 25, 2018
| archive-date = July 1, 2018
| archive-url = https://web.archive.org/web/20180701001653/http://www.tldp.org/LDP/abs/html/bashver4.html
| title = Bash, version 4
| url = http://tldp.org/LDP/abs/html/bashver4.html
| url-status = live
| website = tldp.org
}}
</ref> Bash 4.0 introduced support for [[associative arrays]].<ref name="bashfaq061" /> Associative array indices are strings, in a manner similar to [[AWK]] or [[Tcl]].<ref>
{{ cite web
| access-date = July 4, 2018
| archive-date = July 11, 2018
| archive-url = https://web.archive.org/web/20180711142941/http://www.gnu.org/software/bash/manual/html_node/Arrays.html
| title = Arrays (Bash Reference Manual)
| url=https://www.gnu.org/software/bash/manual/html_node/Arrays.html
| url-status = live
| website = www.gnu.org
}}
</ref> They can be used to emulate multidimensional arrays. Bash 4 also switches its license to [[GNU General Public License#Version 3|GPL-3.0-or-later]]; some users suspect this licensing change is why macOS continues to use older versions.<ref>
{{ cite web
| access-date = June 25, 2018
| archive-date = June 25, 2018
| archive-url = https://web.archive.org/web/20180625213216/https://apple.stackexchange.com/questions/193411/update-bash-to-version-4-0-on-osx
| title = macos - Update bash to version 4.0 on OSX
| url = https://apple.stackexchange.com/questions/193411/update-bash-to-version-4-0-on-osx
| url-status = live
| website = Ask Different
}}
</ref> [[Z shell|Zsh]] became the default shell in macOS with the release of macOS Catalina in 2019.<ref name=":0">
{{ cite web
| access-date = 2021-01-12
| archive-date = December 31, 2020
| archive-url = https://web.archive.org/web/20201231231721/https://thenextweb.com/dd/2019/06/04/why-does-macos-catalina-use-zsh-instead-of-bash-licensing/
| date = 2019-06-04
| first = Matthew
| language = en-us
| last = Hughes
| title = Why does macOS Catalina use Zsh instead of Bash? Licensing
| url = https://thenextweb.com/dd/2019/06/04/why-does-macos-catalina-use-zsh-instead-of-bash-licensing/
| url-status = live
| website = The Next Web
}}
</ref>
===Brace expansion===
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:
<syntaxhighlight lang="console">
$ 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.
<syntaxhighlight lang="console">
$ # 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:
<syntaxhighlight lang="bash">
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.
<syntaxhighlight lang="console">
$ 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 <code>eval</code> built-in, thus:
<syntaxhighlight lang="console">
$ 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>
===Startup scripts===
{{ How-to section | date=January 2019 }}
When Bash starts, it executes the commands in a variety of [[Hidden file and hidden directory#Unix and Unix-like environments|dot files]]. Unlike Bash shell scripts, dot files do typically have neither the execute permission enabled nor an [[interpreter directive]] like <code>#!/bin/bash</code>.
==== Legacy-compatible Bash startup example ====
The example <code>~/.bash_profile</code> below is compatible with the Bourne shell and gives semantics similar to csh for the <code>~/.bashrc</code> and <code>~/.bash_login</code>. The <code>[ -r ''filename'' ] && cmd</code> is a [[short-circuit evaluation]] that tests if ''filename'' exists and is readable, skipping the part after the <code>&&</code> if it is not.
<syntaxhighlight lang="bash">
[ -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 <code>/etc</code> 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 <code>~/.xsession</code> or <code>~/.xprofile</code> file to read the <code>~/.profile</code> — which provides the environment variables that Bash shell windows spawned from the window manager need, such as [[xterm]] or [[Gnome Terminal]].
===Portability===
Invoking Bash with the <code>--posix</code> option or stating <code>set -o posix</code> in a script causes Bash to conform very closely with the [[Posix#POSIX.2|POSIX 1003.2 standard]].<ref name=tldp>
{{ cite web
| access-date = January 26, 2012
| archive-date = January 27, 2012
| archive-url = https://web.archive.org/web/20120127105451/http://tldp.org/LDP/abs/html/portabilityissues.html
| author = Mendel Cooper
| publisher = ibiblio.org
| title = Portability Issues
| url = http://tldp.org/LDP/abs/html/portabilityissues.html
| url-status = live
| work = The Linux Documentation Project
}}
</ref> Bash shell scripts intended for [[portability (computer science)|portability]] should take into account at least the POSIX shell standard. Some bash features not found in POSIX are:<ref name=tldp /><ref name=deb>
{{ cite web
| access-date = May 11, 2020
| archive-date = May 12, 2020
| archive-url = https://web.archive.org/web/20200512050159/https://www.debian.org/doc/debian-policy/ch-files.html#scripts
| title = 10. Files
| url = https://www.debian.org/doc/debian-policy/ch-files.html#scripts
| url-status = live
| website = Debian Policy Manual v4.5.0.2
}}
</ref>
{{div col start}}
* Certain extended invocation options
* Brace expansion
* Arrays and associative arrays
* The double bracket {{code| [[...]] }} extended test construct and its regex matching
* The double-parentheses arithmetic-evaluation construct (only {{code|(( ... ))}}; {{code|$(( ... ))}} is POSIX)
* Certain string-manipulation operations in parameter expansion
* {{code|local}} for scoped variables
* Process substitution
* Bash-specific builtins
* [[Coprocess]]es
* $EPOCHSECONDS and $EPOCHREALTIME variables<ref>
{{ cite web
| access-date = June 3, 2020
| archive-date = June 3, 2020
| archive-url = https://web.archive.org/web/20200603230615/https://www.shell-tips.com/linux/how-to-format-date-and-time-in-linux-macos-and-bash/#using-the-gnu-date-command-line
| title = How To Format Date And Time In Linux, MacOS, And Bash?
| url = https://www.shell-tips.com/linux/how-to-format-date-and-time-in-linux-macos-and-bash/#using-the-gnu-date-command-line
| url-status = live
| website = Shell Tips!
}}
</ref>
{{div col end}}
If a piece of code uses such a feature, it is called a "bashism" – a problem for portable use. Debian's {{code|checkbashisms}} and Vidar Holen's {{code|shellcheck}} can be used to make sure that a script does not contain these parts.<ref>{{man|1|checkbashisms|ManKier}}</ref><ref>{{man|1|shellcheck|ManKier}}</ref> The list varies depending on the actual target shell: Debian's policy allows some extensions in their scripts (as they are in the [[Almquist shell#Dash|dash]] shell),<ref name=deb/> while a script intending to support pre-POSIX Bourne shells, like [[autoconf]]'s {{code|configure}}, are even more limited in the features they can use.<ref>
{{ cite web
| access-date = 20 January 2020
| archive-date = March 2, 2021
| archive-url = https://web.archive.org/web/20210302212413/https://www.gnu.org/savannah-checkouts/gnu/autoconf/manual/autoconf-2.70/html_node/Portable-Shell.html
| title = Portable Shell
| url = https://www.gnu.org/software/autoconf/manual/html_node/Portable-Shell.html
| url-status = live
| website = Autoconf
}}
</ref>
===Keyboard shortcuts===
{{
Main article
| GNU Readline
}}
Bash uses  [[GNU Readline]]  to provide keyboard shortcuts for command line editing using the default ( [[Emacs]] ) key bindings.  [[Vi-binding]]s can be enabled by running <code>set -o vi</code>.<ref>
{{ cite web
| access-date = July 21, 2013
| archive-date = March 2, 2021
| archive-url = https://web.archive.org/web/20210302213519/http://www.hypexr.org/bash_tutorial.php#emacs
| date = October 5, 2012
| publisher = Hypexr.org
| title = BASH Help - A Bash Tutorial
| url = http://www.hypexr.org/bash_tutorial.php#emacs
| url-status = live
}}
</ref>
===Process management (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:
<syntaxhighlight lang="bash">
command1; command2
command3
</syntaxhighlight>
In this example, when command1 is finished, command2 is executed, and when command2 has completed, command3 will execute.
A [[Background process|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.
<syntaxhighlight lang="bash">command1 &</syntaxhighlight>
Or to have a concurrent execution of command1 and command2, they must be executed in the Bash shell in the following way:
<syntaxhighlight lang="bash">
command1 & command2
</syntaxhighlight>
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 {{Key press|Ctrl|z}} while the process is running in the foreground.<ref>
{{ cite web
| access-date = March 27, 2018
| archive-date = March 15, 2018
| archive-url = https://web.archive.org/web/20180315115230/http://www.gnu.org/software/bash/manual/bash.html#index-background
| title = Bash Reference Manual
| url = https://www.gnu.org/software/bash/manual/bash.html#index-background
| url-status = live
| website = www.gnu.org
}}
</ref>
A list of all processes, both in the background and stopped, can be achieved by running <code>jobs</code>:
<syntaxhighlight lang="console">
$ jobs
[1]-  Running                  command1 &
[2]+  Stopped                  command2
</syntaxhighlight>
In the output, the number in brackets refers to the job id. The plus sign signifies the default process for <code>bg</code> and <code>fg</code>. 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 <code>fg</code> command brings a process to the foreground, while <code>bg</code> sets a stopped process running in the background. <code>bg</code> and <code>fg</code> 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 <code>jobs</code>. The <code>[[Kill (command)|kill]]</code> command can be used to end a process prematurely, by sending it a [[Signal (IPC)|signal]]. The job id must be specified after a percent sign:
<syntaxhighlight lang="bash">
kill %1
</syntaxhighlight>
===Conditional execution===
Bash supplies "conditional execution" command separators that make execution of a command contingent on the [[Exit status|exit code]] set by a precedent command. For example:
<syntaxhighlight lang="bash">
cd "$SOMEWHERE" && ./do_something || echo "An error occurred" >&2
</syntaxhighlight>
Where ''./do_something'' is only executed if the ''cd'' (change directory) command was "successful" (returned an exit status of zero) and the ''echo'' command would only be executed if either the ''cd'' or the ''./do_something'' command return an "error" (non-zero exit status).
For all commands the exit status is stored in the special variable <code>$?</code>. Bash also supports {{code|2=bash|if ...;then ...;else ...;fi}} and {{code|2=bash|case $VARIABLE in $pattern)...;;$other_pattern)...;; esac}} forms of conditional command evaluation.
===Bug reporting===
{{Redirect|bashbug|the widely reported September 2014 bug found in Bash|Shellshock (software bug)}}
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).<ref>[http://linux.die.net/man/1/bashbug bashbug(1)]
{{ Webarchive
| date = October 2, 2018
| url = https://web.archive.org/web/20181002232146/https://linux.die.net/man/1/bashbug
}}, die.net</ref><ref>[https://developer.apple.com/library/prerelease/mac/documentation/Darwin/Reference/ManPages/man1/bashbug.1.html "Linux / Unix Command: bashbug"]
{{ Webarchive
| date = October 6, 2014
| url = https://web.archive.org/web/20141006131347/https://developer.apple.com/library/prerelease/mac/documentation/Darwin/Reference/ManPages/man1/bashbug.1.html
}}, apple.com</ref>
===Programmable completion===
Bash supports programmable [[Command-line completion|completion]] via built-in <code>complete</code>, {{Code|compopt}}, and <code>compgen</code> commands.<ref>
{{ cite web
| title = Bash Reference Manual
| url = https://tiswww.case.edu/php/chet/bash/bashref.html#Programmable-Completion
| website = tiswww.case.edu
}}
</ref> The feature has been available since the beta version of 2.04 released in 2000.<ref>
{{ cite web
| archive-url = https://web.archive.org/web/20180629230903/http://www.caliban.org/bash/index.shtml
| archive-date = June 29, 2018
| access-date = June 21, 2018
| title = Working more productively with bash 2.x/3.x
| url = http://www.caliban.org/bash/index.shtml
| url-status = live
| website = www.caliban.org
}}
</ref><ref>
{{ cite web
| access-date = September 15, 2019
| archive-date = March 8, 2020
| archive-url = https://web.archive.org/web/20200308104032/http://ftp.swin.edu.au/gnu/bash/
| title = Index of /gnu/bash
| url = https://ftp.swin.edu.au/gnu/bash/
| url-status = live
| website = ftp.swin.edu.au
}}
</ref> These commands enable complex and intelligent completion specification for commands (i.e. installed programs), functions, variables, and filenames.<ref name=":1">
{{ cite web
| access-date = 2022-01-21
| title = An Introduction to Programmable Completion
| url = https://tldp.org/LDP/abs/html/tabexpansion.html
| website = tldp.org
}}
</ref>
The <code>complete</code> and {{Code|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 {{Key|tab}} keystroke after typing its name.<ref name=":1"/>
=== Program name ===
{{Original research|section|date=May 2024|discuss=Talk:Bash (Unix shell)#Possible original research}}
The program's name is a [[figure of speech]] or [[wit]]ticism which begins with an [[Homage (arts)|homage]] to [[Stephen R. Bourne|Stephen Bourne]], the creator of [[Bourne shell|one]] of the shell programs which have sometimes been considered superseded by the bash shell. His name is used as a [[pun]] on the [[Imagery|image]] of childbirth. With that pun, it would seem, is added an [[allusion]]: possibly to the [[Hinduism|Hindu]] or [[Buddhism|Buddhist]] idea of [[reincarnation]]; possibly to the [[Christianity|Christian]] [[idiom]] known as "being [[born again]];" or quite possibly just to the more abstract idea of [[wikt:renewal|renewal]]. While numerous [[English language|English]] [[translation]]s of the Christian [[New Testament]], Book of [[Gospel of John|John]], [https://www.biblegateway.com/passage/?search=John%203&version=NIV chapter] [https://www.biblegateway.com/passage/?search=John%203&version=KJV 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''....<ref>
{{ cite web
| access-date = 2024-01-09
| language = en
| title = Definition of BORN-AGAIN
| url = https://www.merriam-webster.com/dictionary/born-again
| website = www.merriam-webster.com
}}
</ref>" Whatever the original touchstone may have been, in the end the program received the name, "the ''Bourne Again SHell''."<ref name="auto">
{{ cite web
| access-date = 2024-01-10
| title = Bash - GNU Project - Free Software Foundation
| url = https://www.gnu.org/savannah-checkouts/gnu/bash/bash.html
| website = www.gnu.org
}}
</ref><ref>
{{ cite web
| access-date = 2024-01-09
| title = What is Bash? (Bash Reference Manual)
| url = https://www.gnu.org/software/bash/manual/html_node/What-is-Bash_003f.html
| website = www.gnu.org
}}
</ref>
The [[acronym]] of that name then is "bash," a word meaning "to strike violently.<ref>
{{ cite web
| access-date = 2024-01-09
| date = 2024-01-05
| language = en
| title = Definition of BASH
| url = https://www.merriam-webster.com/dictionary/bash
| website = www.merriam-webster.com
}}
</ref>" In the context of computer programming, to "violently hit something," such as a computer keyboard, could be considered a [[Hyperbole|hyperbolic]] [[Imagery|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]]<ref>
{{ cite web
| date = August 31, 2020
| title = Irony Definition, Common Examples, and Significance in Literature
| url = https://literarydevices.net/irony/
}}
</ref> or accidental [[innuendo]].<ref>
{{ cite web
| date = January 2, 2014
| title = Innuendo - Examples and Definition of Innuendo
| url = https://literarydevices.net/innuendo/
}}
</ref> 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,<ref>
{{ cite web
| access-date = 2024-01-10
| title = BashGuide/Arrays - Greg's Wiki
| url = https://mywiki.wooledge.org/BashGuide/Arrays
| website = mywiki.wooledge.org
}}
</ref><ref>
{{ cite web
| access-date = 2024-01-10
| title = CommandSubstitution - Greg's Wiki
| url = https://mywiki.wooledge.org/CommandSubstitution
| website = mywiki.wooledge.org
}}
</ref><ref>
{{ cite web
| access-date = 2024-01-10
| title = BashGuide/TestsAndConditionals - Greg's Wiki
| url = https://mywiki.wooledge.org/BashGuide/TestsAndConditionals#Conditional_Blocks_.28if.2C_test_and_.5B.5B.29
| website = mywiki.wooledge.org
}}
</ref><ref>
{{ cite web
| access-date = 2024-01-10
| title = CHANGES - bash.git - bash
| url = https://git.savannah.gnu.org/cgit/bash.git/tree/CHANGES
| website = git.savannah.gnu.org
}}
</ref><ref>
{{ cite web
| access-date = 2024-01-10
| archive-date = February 14, 2021
| archive-url = https://web.archive.org/web/20210214062716/https://wiki.bash-hackers.org/scripting/bashchanges
| date = 2021-02-14
| title = Bash changes [Bash Hackers Wiki]
| url = https://wiki.bash-hackers.org/scripting/bashchanges
}}
</ref><ref>
{{ cite web
| access-date = 2024-01-10
| archive-date = February 11, 2021
| archive-url = https://web.archive.org/web/20210211050502/https://wiki.bash-hackers.org/scripting/obsolete
| date = 2021-02-11
| title = Obsolete and deprecated syntax [Bash Hackers Wiki]
| url = https://wiki.bash-hackers.org/scripting/obsolete
}}
</ref> 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.
=== Documentation ===
As the standard upon which bash is based, the POSIX, or IEEE Std 1003.1,<ref>
{{ cite web
| title = The Open Group Base Specifications Issue 7, 2018 edition
| url = https://pubs.opengroup.org/onlinepubs/9699919799/
| website = pubs.opengroup.org
}}
</ref> et seq, is informative.
The Linux man page<ref>
{{ cite web
| title = BASH(1) Manual Page
| url = https://tiswww.case.edu/php/chet/bash/bash.html
| website = tiswww.case.edu
}}
</ref><ref>
{{ cite web
| title = bash.0\doc - bash.git - bash
| url = https://git.savannah.gnu.org/cgit/bash.git/tree/doc/bash.0
| website = git.savannah.gnu.org
}}
</ref> is intended to be the authoritative explanatory document for the understanding of how <code>bash</code> operates, while the GNU [https://www.gnu.org/software/bash/manual/index.html manual] is sometimes considered more user-friendly for reading. "You may also find information about Bash by running <kbd>info bash</kbd> ... or by looking at <code>/usr/share/doc/bash/</code>, <code>/usr/local/share/doc/bash/</code>, or similar directories on your system.  A brief summary is available by running <kbd>bash --help</kbd>.<ref name="auto"/>"
On modern Linuxes, information on shell built-in commands can be found by executing <code>help,</code> <code>help [built-in name]</code>or <code>man builtins</code> at a terminal prompt where bash is installed. Some commands, such as <code>echo</code>, <code>false</code>, <code>kill</code>, <code>printf</code>, <code>test</code> or <code>true</code>, 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., <code>/bin/printf</code>) is one way of ensuring that the shell uses a system binary. This name collision issue also effects any "help summaries" viewed with <code>kill --help</code> and <code>/bin/kill --help</code>. 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",<ref>
{{ cite web
| title = The GNU Bourne-Again Shell
| url = https://tiswww.case.edu/php/chet/bash/bashtop.html
| website = tiswww.case.edu
}}
</ref><ref>
{{ cite web
| title = Frequently Asked Questions
| url = https://tiswww.case.edu/php/chet/bash/FAQ
}}
</ref><ref name="auto"/> this FAQ is current as of bash version 5.1 and is no longer updated.
== Vulnerabilities ==
A [[Security bug|security hole]] in Bash dating from version 1.03 (August 1989),<ref name="Seclists-20141004">
{{ cite web
| access-date = 4 October 2014
| archive-date = October 6, 2014
| archive-url = https://web.archive.org/web/20141006093452/http://seclists.org/oss-sec/2014/q4/102
| date = 4 October 2014
| first = Stephane
| last = Chazelas
| title = oss-sec mailing list archives
| url = http://seclists.org/oss-sec/2014/q4/102
| url-status = live
| work = Seclists.org
}}
</ref> dubbed [[Shellshock (software bug)|Shellshock]], was discovered in early September 2014 and quickly led to a range of [[Attack (computing)|attacks]] across the [[Internet]].<ref name="TR-20140924">
{{ cite web
| access-date = September 25, 2014
| archive-date = October 16, 2014
| archive-url = https://web.archive.org/web/20141016214010/http://www.theregister.co.uk/2014/09/24/bash_shell_vuln
| date = September 24, 2014
| first = John
| last = Leyden
| title = Patch Bash NOW: 'Shell Shock' bug blasts OS X, Linux systems wide open
| url = https://www.theregister.co.uk/2014/09/24/bash_shell_vuln/
| url-status = live
| work = [[The Register]]
}}
</ref><ref name="NYT-20140925-NP">
{{ cite news
| access-date = September 25, 2014
| archive-date = April 5, 2019
| archive-url = https://web.archive.org/web/20190405000511/https://www.nytimes.com/2014/09/26/technology/security-experts-expect-shellshock-software-bug-to-be-significant.html
| date = September 25, 2014
| first = Nicole
| last = Perlroth
| title = Security Experts Expect 'Shellshock' Software Bug in Bash to Be Significant
| url = https://www.nytimes.com/2014/09/26/technology/security-experts-expect-shellshock-software-bug-to-be-significant.html
| url-status = live
| work = [[The New York Times]]
}}
</ref><ref name="ZDN-20140929">
{{ cite web
| archive-date = May 14, 2016
| archive-url = https://web.archive.org/web/20160514191755/http://www.zdnet.com/article/hackers-jump-on-the-shellshock-bash-bandwagon/
| date = 29 September 2014
| first = Larry
| last = Seltzer
| title = Shellshock makes Heartbleed look insignificant
| url = https://www.zdnet.com/article/shellshock-makes-heartbleed-look-insignificant/
| url-status = live
| work = [[ZDNet]]
}}
</ref> Patches to fix the bugs were made available soon after the bugs were identified.
== Licensing ==
Since 4.0: [[GNU General Public License#Version 3|GPL-3.0-or-later]]
<ref name="license">
{{ cite web
| access-date = April 16, 2014
| archive-date = April 26, 2019
| archive-url = https://web.archive.org/web/20190426210620/http://www.gnu.org/software/bash/
| author = GNU Project
| author-link = GNU Project
| quote = 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).
| title = README file
| url = https://www.gnu.org/software/bash/
| url-status = live
}}
</ref>
1.11? to 3.2: [[GNU General Public License#Version 2|GPL-2.0-or-later]]
<ref>
{{ cite web
| access-date = June 9, 2021
| archive-date = October 15, 2021
| archive-url = https://web.archive.org/web/20211015233550/http://www.oldlinux.org/Linux.old/bin/old/bash-1.11/
| quote = See test.c for GPL-2.0-or-later
| title = bash-1.11
| url = http://www.oldlinux.org/Linux.old/bin/old/bash-1.11/
| url-status = dead
| website = oldlinux.org
}}
</ref>
0.99? to 1.05?: [[GNU General Public License#Version 1|GPL-1.0-or-later]]
<ref>
{{ cite web
| title = bash-1.05.tar
| url = http://www.oldlinux.org/Linux.old/bin/old/bash-1.05.tar
| website = oldlinux.org
}}
</ref>
<ref name="bashfaq061">
{{ Cite web
| access-date = 2021-03-01
| archive-date = March 2, 2021
| archive-url = https://web.archive.org/web/20210302212811/https://mywiki.wooledge.org/BashFAQ/061
| title = BashFAQ/061 - Greg's Wiki
| url = https://mywiki.wooledge.org/BashFAQ/061
| url-status = live
| website = mywiki.wooledge.org
}}
</ref>
<ref>
{{ cite web
| title = Is there a way to download the presumably initial bash source bash-0.99?
| url = https://unix.stackexchange.com/questions/243375/is-there-a-way-to-download-the-presumably-initial-bash-source-bash-0-99
| website = unix.stackexchange.com
}}
</ref>
== Versions ==
A version is also available for [[Windows 10]] and [[Windows 11]] via the [[Windows Subsystem for Linux]].<ref>
{{ cite web
| access-date = November 20, 2016
| archive-date = November 20, 2016
| archive-url = https://web.archive.org/web/20161120213017/http://www.windowscentral.com/how-install-bash-shell-command-line-windows-10
| date = September 28, 2016
| title = How to install Bash shell command-line tool on Windows 10
| url = http://www.windowscentral.com/how-install-bash-shell-command-line-windows-10
| url-status = live
}}
</ref><ref>
{{ cite web
| access-date = 2022-10-12
| date = July 30, 2021
| first = Chris
| language = en-US
| last = Hoffman
| title = How to Install the Windows Subsystem for Linux on Windows 11
| url = https://www.howtogeek.com/744328/how-to-install-the-windows-subsystem-for-linux-on-windows-11/
| website = How-To Geek
}}
</ref> It is also the default user shell in [[Solaris (operating system)|Solaris]] 11.<ref>
{{ cite web
| access-date = June 8, 2018
| archive-date = June 12, 2018
| archive-url = https://web.archive.org/web/20180612144748/https://docs.oracle.com/cd/E23824_01/html/E24456/userenv-1.html
| publisher = Oracle
| title = User Environment Feature Changes
| url = https://docs.oracle.com/cd/E23824_01/html/E24456/userenv-1.html
| url-status = live
}}
</ref> Bash was also the default shell in [[BeOS]],<ref name="forb" /> and in versions of [[MacOS|Apple macOS]] from 10.3 (originally, the default shell was [[tcsh]]) to 10.15 ([[macOS Catalina]]), which changed the default shell to [[zsh]],<ref name="zsh2" /> although Bash remains available as an alternative shell.<ref name=":0" />
=== Release history ===
{| class="wikitable"
|-
! Version
! Release date
!Release notes
|-
| bash-5.2.15
| 2022-12-13
|[https://git.savannah.gnu.org/cgit/bash.git/tree/NEWS?h=bash-5.2 NEWS]
|-
| bash-5.2
| 2022-09-26
|
|-
| bash-5.1
| 2020-12-07
|[https://github.com/bminor/bash/blob/8868edaf2250e09c4e9a1c75ffe3274f28f38581/NEWS github version history NEWS] <ref name="bashfaq061" />
|-
| bash-5.0
| 2019-01-07
|<ref>
{{ Cite web
| access-date = 2021-03-01
| archive-date = May 4, 2022
| archive-url = https://web.archive.org/web/20220504233136/https://github.com/bminor/bash/blob/bash-5.0/NEWS
| language = en
| title = github version history NEWS v5.0
| url = https://github.com/bminor/bash/blob/bash-5.0/NEWS
| url-status = live
| website = GitHub
}}
</ref><ref>
{{ cite web
| access-date = November 25, 2019
| archive-date = March 18, 2020
| archive-url = https://web.archive.org/web/20200318131209/https://wiki.bash-hackers.org/scripting/bashchanges
| title = Bash changes &#91;Bash Hackers Wiki&#93;
| url = https://wiki.bash-hackers.org/scripting/bashchanges
| url-status = live
}}
</ref><ref>
{{ cite web
| access-date = 2021-03-01
| archive-date = November 8, 2020
| archive-url = https://web.archive.org/web/20201108001733/https://lists.gnu.org/archive/html/bug-bash/2019-01/msg00063.html
| title = Bash-5.0 release available
| url = https://lists.gnu.org/archive/html/bug-bash/2019-01/msg00063.html
| url-status = live
| website = lists.gnu.org
}}
</ref>
|-
| bash-5.0-rc1
| 2018-12-20
|
|-
| bash-5.0-beta2
| 2018-11-28
|
|-
| bash-5.0-beta
| 2018-09-17
|
|-
| bash-5.0-alpha
| 2018-05-22
|
|-
| bash-4.4
| 2016-09-15
|[https://github.com/bminor/bash/blob/a0c0a00fc419b7bc08202a79134fcd5bc0427071/NEWS github version history NEWS v4.4]
|-
| bash-4.4-rc2
| 2016-08-22
|
|-
| bash-4.4-rc1
| 2016-02-24
|
|-
| bash-4.4-beta2
| 2016-07-11
|
|-
| bash-4.4-beta
| 2015-10-12
|
|-
| bash-4.3
| 2014-02-26
|
|-
| bash-4.2
| 2011-02-13
|
|-
| bash-4.1
| 2009-12-31
|
|-
| bash-4.0
| 2009-02-20
|
|-
| bash-4.0-rc1
| 2009-01-12
|
|-
| bash-3.2
| 2006-10-11
|
|-
| bash-3.1
| 2005-12-08
|
|-
| bash-3.0
| 2004-08-03
|
|-
| bash-2.05b
| 2002-07-17
|
|-
| bash-2.05a
| 2001-11-16
|
|-
| bash-2.05
| 2001-04-09
|
|-
| bash-2.04
| 2000-03-21
|
|-
| bash-2.03
| 1999-02-19
|
|-
| bash-2.02
| 1998-04-18
|
|-
| bash-2.01
| 1997-06-05
|
|-
| bash-2.0
| 1996-12-31
|
|}
==See also==
{{Portal|Free and open-source software}}
* [[Comparison of command shells]]
{{Clear}}
==References==
{{Reflist}}
== Further reading ==
* {{cite book|title=[[In the Beginning... Was the Command Line]]|isbn=978-0380815937|first=Neal |last=Stephenson|date=2003 |publisher=HarperCollins }}
* {{ cite web
| access-date = 19 May 2024
| title = Evolution of shells in Linux
| url = https://developer.ibm.com/tutorials/l-linux-shells/
| website = ibm.com
}}
* {{ cite web
| access-date = 19 May 2024
| title = Scripting Reference :: Scripting with the Bourne-Again Shell (Bash)
| url = https://decal.ocf.berkeley.edu/labs/scripting/
| website = berkeley.edu
}}
* {{ cite web
| access-date = 19 May 2024
| title = IRIS :: Instructional & Research Information Systems :: FAQ: Unix :: About UNIX Shells
| url = https://iris.eecs.berkeley.edu/faq/unix/
| website = berkeley.edu
}}
{{GNU}}
{{Unix shells}}{{Programming languages}}
{{Authority control}}
{{DEFAULTSORT:Bash (Unix Shell)}}
[[Category:1989 software]]
[[Category:Cross-platform free software]]
[[Category:Domain-specific programming languages]]
[[Category:Dynamically scoped programming languages]]
[[Category:Free software programmed in C]]
[[Category:GNU Project software]]
[[Category:Scripting languages]]
[[Category:Text-oriented programming languages]]
[[Category:Unix shells]]