Bash (Unix shell)
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![]() Screenshot of a Bash session | |
Original author(s) | Brian Fox |
---|---|
Initial release | June 8, 1989 |
Stable release | 5.0 (January 7, 2019[±][1][2] | )
Repository | ![]() |
Written in | C |
Operating system | |
Platform | GNU |
Available in | Multilingual (gettext) |
Type | Unix shell, command language |
License | GPLv3+[6] |
Website | www |
Bash is a Unix shell and command language written by Brian Fox for the GNU Project as a free software replacement for the Bourne shell.[7][8] First released in 1989,[9] it has been used as the default login shell for most Linux distributions and all releases of Apple's macOS prior to macOS Catalina.[10] A version is also available for Windows 10 via the Windows Subsystem for Linux.[11] It is also the default user shell in Solaris 11.[12]
Bash is a command processor that typically runs in a text window where the user types commands that cause actions. Bash can also read and execute commands from a file, called a shell script. Like all Unix shells, it supports filename globbing (wildcard matching), piping, here documents, command substitution, variables, and control structures for condition-testing and iteration. The keywords, syntax, dynamically scoped variables and other basic features of the language are all copied from sh. Other features, e.g., history, are copied from csh and ksh. Bash is a POSIX-compliant shell, but with a number of extensions.
The shell's name is an acronym for Bourne Again Shell, a pun on the name of the Bourne shell that it replaces[13] and the notion of being "born again".[14][15]
A security hole in Bash dating from version 1.03 (August 1989),[16] dubbed Shellshock, was discovered in early September 2014 and quickly led to a range of attacks across the Internet.[17][18][19] Patches to fix the bugs were made available soon after the bugs were identified.
History[edit]
Brian Fox began coding Bash on January 10, 1988,[20] after Richard Stallman became dissatisfied with the lack of progress being made by a prior developer.[7] 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.[7][21] Fox released Bash as a beta, version .99, on June 8, 1989,[9] and remained the primary maintainer until sometime between mid-1992[22] and mid-1994,[23] when he was laid off from FSF[24] and his responsibility was transitioned to another early contributor, Chet Ramey.[25][26][27]
Since then, Bash has become by far the most popular shell among users of Linux, becoming the default interactive shell on that operating system's various distributions[28][29] (although Almquist shell may be the default scripting shell) and on Apple's macOS releases before Catalina in October 2019.[30][31][10] Bash has also been ported to Microsoft Windows and distributed with Cygwin and MinGW, to DOS by the DJGPP project, to Novell NetWare, to OpenVMS by the GNV project,[32] to ArcaOS,[33] and to Android via various terminal emulation applications.
In September 2014, Stéphane Chazelas, a Unix/Linux specialist,[34] discovered a security bug in the program. The bug, first disclosed on September 24, was named Shellshock and assigned the numbers CVE-2014-6271, CVE-2014-6277 and CVE-2014-7169. The bug was regarded as severe, since CGI scripts using Bash could be vulnerable, enabling arbitrary code execution. The bug was related to how Bash passes function definitions to subshells through environment variables.[35]
Features[edit]
The Bash command syntax is a superset of the Bourne shell command syntax. Bash supports brace expansion, command line completion (Programmable Completion),[36] basic debugging[37] and signal handling (using trap
) since bash 2.05a[38] 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 (ksh) and the C shell (csh) such as command line editing, command history (history
command),[39] the directory stack, the $RANDOM
and $PPID
variables, and POSIX command substitution syntax $(…)
.
When a user presses the tab key within an interactive command-shell, Bash automatically uses command line completion, since beta version 2.04,[40] to match partly typed program names, filenames and variable names. The Bash command-line completion system is very flexible and customizable, and is often packaged with functions that complete arguments and filenames for specific programs and tasks.
Bash's syntax has many extensions lacking in the Bourne shell. Bash can perform integer calculations ("arithmetic evaluation") without spawning external processes. It uses the ((…))
command and the $((…))
variable syntax for this purpose. Its syntax simplifies I/O redirection. For example, it can redirect standard output (stdout) and standard error (stderr) at the same time using the &>
operator. This is simpler to type than the Bourne shell equivalent 'command > file 2>&1
'. Bash supports process substitution using the <(command)
and >(command)
syntax, which substitutes the output of (or input to) a command where a filename is normally used. (This is implemented through /proc/fd/ unnamed pipes on systems that support that, or via temporary named pipes where necessary).
When using the 'function' keyword, Bash function declarations are not compatible with Bourne/Korn/POSIX scripts (the KornShell has the same problem when using 'function'), but Bash accepts the same function declaration syntax as the Bourne and Korn shells, and is POSIX-conformant. Because of these and other differences, Bash shell scripts are rarely runnable under the Bourne or Korn shell interpreters unless deliberately written with that compatibility in mind, which is becoming less common as Linux becomes more widespread. But in POSIX mode, Bash conforms with POSIX more closely.[41]
Bash supports here documents. Since version 2.05b Bash can redirect standard input (stdin) from a "here string" using the <<<
operator.
Bash 3.0 supports in-process regular expression matching using a syntax reminiscent of Perl.[42][43]
In February 2009,[44] Bash 4.0 introduced support for associative arrays.[45] Associative array indices are strings, in a manner similar to AWK or Tcl.[46] They can be used to emulate multidimensional arrays. Bash 4 also switches its license to GPLv3; some users suspect this licensing change is why MacOS continues to use older versions.[47]
Brace expansion[edit]
Brace expansion, also called alternation, is a feature copied from the C shell. It generates a set of alternative combinations. Generated results need not exist as files. The results of each expanded string are not sorted and left to right order is preserved:
$ echo a{p,c,d,b}e
ape ace ade abe
$ echo {a,b,c}{d,e,f}
ad ae af bd be bf cd ce cf
Users should not use brace expansions in portable shell scripts, because the Bourne shell does not produce the same output.
$ # 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 show 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 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 the variable expansion is performed after the brace expansion, which in some cases may necessitate the use of the eval
built-in, thus:
$ start=1; end=10
$ echo {$start..$end} # fails to expand due to the evaluation order
{1..10}
$ eval echo {$start..$end} # variable expansion occurs then resulting string is evaluated
1 2 3 4 5 6 7 8 9 10
Startup scripts[edit]
![]() | This section contains instructions, advice, or how-to content. (January 2019) |
When Bash starts, it executes the commands in a variety of dot files. Unlike Bash shell scripts, dot files do not typically have execute permission enabled nor an interpreter directive like #!/bin/bash
.
Legacy-compatible Bash startup example[edit]
The skeleton ~/.bash_profile
below is compatible with the Bourne shell and gives semantics similar to csh for the ~/.bashrc
and ~/.bash_login
. The [ -r filename ] && cmd
is a short-circuit evaluation that tests if filename exists and is readable, skipping the part after the &&
if it is not.
[ -r ~/.profile ] && . ~/.profile # set up environment, once, Bourne-sh syntax only
if [ -n "$PS1" ] ; then # are we interactive?
[ -r ~/.bashrc ] && . ~/.bashrc # tty/prompt/function setup for interactive shells
[ -r ~/.bash_login ] && . ~/.bash_login # any at-login tasks for login shell only
fi # End of "if" block
Operating system issues in Bash startup[edit]
Some versions of Unix and Linux contain Bash system startup scripts, generally under the /etc
directories. Bash calls these as part of its standard initialization, but other startup files can read them in a different order than the documented Bash startup sequence. The default content of the root user's files may also have issues, as well as the skeleton files the system provides to new user accounts upon setup. The startup scripts that launch the X window system may also do surprising things with the user's Bash startup scripts in an attempt to set up user-environment variables before launching the window manager. These issues can often be addressed using a ~/.xsession
or ~/.xprofile
file to read the ~/.profile
— which provides the environment variables that Bash shell windows spawned from the window manager need, such as xterm or Gnome Terminal.
Portability[edit]
Invoking Bash with the --posix
option or stating set -o posix
in a script causes Bash to conform very closely to the POSIX 1003.2 standard.[48] Bash shell scripts intended for portability should take into account at least the POSIX shell standard. Some bash features not found in POSIX are:[48][49]
- Certain extended invocation options
- Brace expansion
- Arrays and associative arrays
- The double bracket
[[ ... ]]
extended test construct and its regex matching - The double-parentheses arithmetic-evaluation construct (only
(( ... ))
;$(( ... ))
is POSIX) - Certain string-manipulation operations in parameter expansion
local
for scoped variables- Process substitution
- Bash-specific builtins
- Coprocesses
- $EPOCHSECONDS and $EPOCHREALTIME variables [50]
If a piece of code uses such a feature, it is called a "bashism" – a problem for portable use. Debian's checkbashisms
and Vidar Holen's shellcheck
can be used to make sure that a script does not contain these parts.[51][52] The list varies depending on the actual target shell: Debian's policy allows some extensions in their scripts (as they are in dash
),[49] while a script intending to support pre-POSIX Bourne shells, like autoconf's configure
, are even more limited in the features they can use.[53]
Keyboard shortcuts[edit]
Bash uses readline to provide keyboard shortcuts for command line editing using the default (Emacs) key bindings. Vi-bindings can be enabled by running set -o vi
.[54]
Process management[edit]
The Bash shell has two modes of execution for commands: batch, and concurrent mode.
To execute commands in batch (i.e., in sequence) they must be separated by the character ";", or on separate lines:
command1; command2
in this example, when command1 is finished, command2 is executed.
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 returning immediately control to the shell and allowing continued execution of commands.
command1 &
Or to have a concurrent execution of two 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.[55]
A list of all processes, both in the background and stopped, can be achieved by running jobs
:
$ jobs
[1]- Running command1 &
[2]+ Stopped command2
In the output, the number in brackets refers to the job id. The plus sign signifies the default process for bg
and fg
. The text "Running" and "Stopped" refer to the Process state. The last string is the command that started the process.
The state of a process can be changed using various commands. The fg
command brings a process to the foreground, while bg
sets a stopped process running in the background. bg
and fg
can take a job id as their first argument, to specify the process to act on. Without one, they use the default process, identified by a plus sign in the output of jobs
. The kill
command can be used to end a process prematurely, by sending it a signal. The job id must be specified after a percent sign:
kill %1
Conditional execution[edit]
Bash supplies "conditional execution" command separators that make execution of a command contingent on the exit code set by a precedent command. For example:
cd "$SOMEWHERE" && ./do_something || echo "An error occurred" >&2
Where ./do_something is only executed if the cd (change directory) command was "successful" (returned an exit status of zero) and the echo command would only be executed if either the cd or the ./do_something command return an "error" (non-zero exit status).
For all commands the exit status is stored in the special variable $?
. Bash also supports if ...;then ...;else ...;fi
and case $VARIABLE in $pattern)...;;$other_pattern)...;; esac
forms of conditional command evaluation.
Bug reporting[edit]
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).[56][57]
Programmable completion[edit]
Bash programmable completion, complete
and compgen
commands[58] have been available since the beta version of 2.04[59] in 2000.[60] These facilities allow complex intelligent completion, such as offering to tab-complete available program options and then, after the user selects an option that requires a filename as its next input, only auto-completing file paths (and not other options) for the next token.
Release history[edit]
VERSION | RELEASE DATE |
---|---|
bash-5.0 | 2019-01-07 |
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 |
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[edit]
References[edit]
- ^ "Index of /gnu/bash". ftp.gnu.org. Retrieved March 19, 2019.
- ^ Ramey, Chet (January 7, 2019). "Bash-5.0 release available". lists.gnu.org. Archived from the original on January 8, 2019. Retrieved March 19, 2019.
- ^ "Bash FAQ, version 4.14". Archived from the original on September 1, 2018. Retrieved April 9, 2016.
- ^ "Missing source code - GPL compliance? · Issue #107 · Microsoft/WSL". GitHub.
- ^ "GNU Bash". Softpedia. SoftNews. Retrieved April 9, 2016.
- ^ GNU Project. "README file".
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).
- ^ a b c
Richard Stallman (forwarded with comments by Chet Ramey) (February 10, 1988). "GNU + BSD = ?". Newsgroup: comp.unix.questions. Usenet: 2362@mandrill.CWRU.Edu. Retrieved March 22, 2011.
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.
- ^
Hamilton, Naomi (May 30, 2008), "The A-Z of Programming Languages: BASH/Bourne-Again Shell", Computerworld: 2, retrieved March 21, 2011,
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.
- ^ a b Brian Fox (forwarded by Leonard H. Tower Jr.) (June 8, 1989). "Bash is in beta release!". Newsgroup: gnu.announce. Retrieved October 28, 2010.
- ^ a b Warren, Tom (June 4, 2019). "Apple replaces bash with zsh as the default shell in macOS Catalina". The Verge. Retrieved June 13, 2019.
- ^ "How to install Bash shell command-line tool on Windows 10". September 28, 2016.
- ^ "User Environment Feature Changes". Oracle.
- ^ "I Almost Get a Linux Editor and Compiler". Dr. Dobb's. Retrieved September 12, 2020.
- ^
Richard Stallman (November 12, 2010). "About the GNU Project". Free Software Foundation. Archived from the original on April 24, 2011. Retrieved March 13, 2011.
"Bourne Again Shell" is a play on the name Bourne Shell, which was the usual shell on Unix.
- ^
Gattol, Markus (March 13, 2011), Bourne-again Shell, archived from the original on March 9, 2011, retrieved March 13, 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".
- ^ Chazelas, Stephane (October 4, 2014). "oss-sec mailing list archives". Seclists.org. Retrieved October 4, 2014.
- ^ Leyden, John (September 24, 2014). "Patch Bash NOW: 'Shell Shock' bug blasts OS X, Linux systems wide open". The Register. Retrieved September 25, 2014.
- ^ Perlroth, Nicole (September 25, 2014). "Security Experts Expect 'Shellshock' Software Bug in Bash to Be Significant". The New York Times. Retrieved September 25, 2014.
- ^ Seltzer, Larry (September 29, 2014). "Shellshock makes Heartbleed look insignificant". ZDNet. Archived from the original on May 14, 2016.
- ^
Brian Fox (August 29, 1996), shell.c, Free Software Foundation,
Birthdate: Sunday, January 10th, 1988. Initial author: Brian Fox
- ^
Richard Stallman (October 3, 2010). "About the GNU Project". Free Software Foundation. Archived from the original on April 24, 2011. Retrieved March 21, 2011.
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.
- ^ len (g...@prep.ai.mit.edu) (April 20, 1993). "January 1993 GNU's Bulletin". Newsgroup: gnu.announce. Usenet: gnusenet930421bulletin@prep.ai.mit.edu. Retrieved October 28, 2010.
- ^ Ramey, Chet (August 1, 1994). "Bash - the GNU shell (Reflections and Lessons Learned)". Linux Journal. Archived from the original on December 5, 2008. Retrieved November 13, 2008.
- ^ Chet Ramey (October 31, 2010), Dates in your Computerworld interview, retrieved October 31, 2010
- ^ Chet Ramey (June 12, 1989). "Bash 0.99 fixes & improvements". Newsgroup: gnu.bash.bug. Retrieved November 1, 2010.
- ^ Chet Ramey (July 24, 1989). "Some bash-1.02 fixes". Newsgroup: gnu.bash.bug. Retrieved October 30, 2010.
- ^ Brian Fox (March 2, 1990). "Availability of bash 1.05". Newsgroup: gnu.bash.bug. Retrieved October 30, 2010.
- ^
Bresnahan, Christine; Blum, Richard (April 2015). CompTIA Linux+ Powered by Linux Professional Institute Study Guide: Exam LX0-103 and Exam LX0-104 (3rd ed.). John Wiley & Sons, Inc. p. 5. ISBN 978-1-119-02122-3. Retrieved June 6, 2016.
In Linux, most users run bash because it is the most popular shell.
- ^
Danesh, Arman; Jang, Michael (February 2006). Mastering Linux. John Wiley & Sons, Inc. p. 363. ISBN 978-0-7821-5277-7. Retrieved June 6, 2016.
The Bourne Again Shell (bash) is the most common shell installed with Linux distributions.
- ^
Foster-Johnson, Eric; Welch, John C.; Anderson, Micah (April 2005). Beginning Shell Scripting. John Wiley & Sons, Inc. p. 6. ISBN 978-0-7645-9791-6. Retrieved June 6, 2016.
Bash is by far the most popular shell and forms the default shell on Linux and Mac OSX systems.
- ^ "Use zsh as the default shell on your Mac - Apple Support". Retrieved July 1, 2019.
- ^ "Installing the new GNV packages". Retrieved September 4, 2020.
- ^ "Compatibility Subsystems". Retrieved September 4, 2020.
- ^ Juliana, Cino (June 10, 2017). "Linux bash exit status and how to set exit status in bash - Techolac". Retrieved June 21, 2019.
- ^ Huzaifa Sidhpurwala (September 24, 2014). "Bash specially-crafted environment variables code injection attack". Red Hat.
- ^ "Bash Reference Manual". www.gnu.org.
- ^ "Debugging Bash scripts". tldp.org.
- ^ "Bash changes [Bash Hackers Wiki (DEV 20200708T2203)]". wiki-dev.bash-hackers.org.
- ^ "Bash Reference Manual". www.gnu.org.
- ^ "Working more productively with bash 2.x/3.x". www.caliban.org.
- ^ "6.11 Bash POSIX Mode", The GNU Bash Reference Manual, for Bash, Version 4.1, December 23, 2009, archived from the original on December 3, 2010, retrieved October 26, 2010
- ^ "Advanced Bash-Scripting Guide". www.tldp.org. Section 37.2 (Bash, version 3). Retrieved March 5, 2017.
- ^ Commission on the Bicentennial of the United States Constitution. Frank and Virginia Williams Collection of Lincolniana (Mississippi State University. Libraries). The Supreme Court of the United States : its beginnings & its justices, 1790-1991. OCLC 25546099.
- ^ "Bash, version 4". tldp.org.
- ^ "References", Diagnostic Reference Index of Clinical Neurology, Elsevier, pp. Ref–1a-Ref-70, 1986, doi:10.1016/b978-0-409-90016-3.50033-9, ISBN 978-0-409-90016-3, retrieved September 12, 2020
- ^ "Arrays (Bash Reference Manual)". www.gnu.org.
- ^ "macos - Update bash to version 4.0 on OSX". Ask Different.
- ^ a b Mendel Cooper. "Portability Issues". The Linux Documentation Project. ibiblio.org.
- ^ a b "10. Files". Debian Policy Manual v4.5.0.2.
- ^ "How To Format Date And Time In Linux, MacOS, And Bash?". Shell Tips!. Retrieved June 3, 2020.
- ^ Linux General Commands Manual –
- ^ Linux General Commands Manual –
- ^ "Portable Shell". Autoconf. Retrieved January 20, 2020.
- ^ "BASH Help - A Bash Tutorial". Hypexr.org. October 5, 2012. Retrieved July 21, 2013.
- ^ "Bash Reference Manual". www.gnu.org.
- ^ bashbug(1), die.net
- ^ "Linux / Unix Command: bashbug", apple.com
- ^ "Bash Reference Manual". tiswww.case.edu.
- ^ "Working more productively with bash 2.x/3.x". www.caliban.org.
- ^ "Index of /gnu/bash". ftp.swin.edu.au.
External links[edit]
![]() |
Wikimedia Commons has media related to Bash. |
![]() |
Wikibooks has a book on the topic of: Bash Shell Scripting |
- Official website
- 2008 interview with GNU Bash's maintainer, Chet Ramey
- MPI-Bash: A MPI-enabled plugin for the Bourne-Again Shell by Scott Pakin
- List of bash changes
- The Bash Acedemy