Bash (Unix shell)

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"Bash (software)" redirects here. For other software, see Bash (disambiguation).
Bash screenshot.png
Screenshot of Bash and sh sessions demonstrating some features
Original author(s) Brian Fox
Initial release June 7, 1989; 25 years ago (1989-06-07)
Stable release 4.3.33 / December 29, 2014; 2 months ago (2014-12-29)[1]
Written in C
Operating system Cross-platform
Platform GNU
Available in English, multilingual (gettext)
Type Unix shell
License GNU GPL v3+[2]

Bash is a Unix shell written by Brian Fox for the GNU Project as a free software replacement for the Bourne shell.[3][4] Released in 1989,[5] it has been distributed widely as the shell for the GNU operating system and as a default shell on Linux and OS X. It has been ported to Microsoft Windows and distributed with Cygwin and MinGW, to DOS by the DJGPP project, to Novell NetWare and to Android via various terminal emulation applications. In the late 1990s, Bash was a minor player among multiple commonly used shells; at present Bash has overwhelming favor.

Bash is a command processor that typically runs in a text window, where the user types commands that cause actions. Bash can also read commands from a file, called a script. Like all Unix shells, it supports filename wildcarding, piping, here documents, command substitution, variables and control structures for condition-testing and iteration. The keywords, syntax and other basic features of the language were all copied from sh. Other features, e.g., history, were copied from csh and ksh. Bash is a POSIX shell, but with a number of extensions.

The name itself is an acronym, a pun, and a description. As an acronym, it stands for Bourne-again shell, referring to its objective as a free replacement for the Bourne shell.[6] As a pun, it expressed that objective in a phrase that sounds similar to born again, a term for spiritual rebirth.[7][8] The name is also descriptive of what it did, bashing together the features of sh, csh, and ksh.[9]

A security hole in Bash dubbed Shellshock, dating from version 1.03,[10] was discovered in early September 2014.[11][12]


Brian Fox began coding Bash on January 10, 1988[13] after Richard Stallman became dissatisfied with the lack of progress being made by a prior developer.[3] Stallman and the Free Software Foundation (FSF) considered a free shell that could run existing sh 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.[3][14] Fox released Bash as a beta, version .99, on June 7, 1989[5] and remained the primary maintainer until sometime between mid-1992[15] and mid-1994,[16] when he was laid off from FSF[17] and his responsibility was transitioned to another early contributor, Chet Ramey.[18][19][20]

In September 2014, Stéphane Chazelas, a Unix/Linux, network and telecom specialist working in the UK,[21] 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[22] and CVE-2014-7169. The bug was regarded as severe, since CGI scripts using Bash could be vulnerable, enabling arbitrary code execution. The bug is related to how Bash passes function definitions to subshells through environment variables.[23]


The Bash command syntax is a superset of the Bourne shell command syntax. The vast majority of Bourne shell scripts can be executed by Bash without modification, with the exception of Bourne shell scripts stumbling into fringe syntax behavior interpreted differently in Bash or attempting to run a system command matching a newer Bash builtin, etc. Bash command syntax includes ideas drawn from the Korn shell (ksh) and the C shell (csh) such as command line editing, command history, the directory stack, the $RANDOM and $PPID variables, and POSIX command substitution syntax $(…). When used as an interactive command shell and pressing the tab key, Bash automatically uses command line completion 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 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) syntax, which substitutes the output of (or input to) a command where a filename is normally used.

When using the 'function' keyword, Bash function declarations are not compatible with Bourne/Korn/POSIX scripts (the Korn shell 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,[24] Bash conformance with POSIX is nearly perfect.[citation needed]

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.[25]

Bash 4.0 introduced support for associative arrays.[24][26] Associative arrays allow a fake support for multi-dimensional (indexed) arrays, in a similar way to AWK:

declare -A a         # declare an associative array 'a' faking a bi-dimensional indexed array
i=1; j=2             # initialize some indices
a[$i,$j]=5           # associate value "5" to key "$i,$j" (i.e. "1,2")
echo ${a[$i,$j]}     # print the stored value at key "$i,$j"

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
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}    # expands to 1 2 3 4 5 6 7 8 9 10
echo {a..e}     # expands to a b c d e
echo {1..10..3} # expands to 1 4 7 10
echo {a..j..3}  # expands to a d g j

Startup scripts[edit]

When Bash starts it executes the commands in a variety of dot files. Though similar to Bash shell script commands, which have execute permission enabled and an interpreter directive like #!/bin/bash, the initialization files used by Bash require neither.

Execution order of startup files[edit]

When started as an interactive login shell[edit]

Bash reads and executes /etc/profile (if it exists). (Often this file calls /etc/bash.bashrc.)

After reading that file, it looks for ~/.bash_profile, ~/.bash_login, and ~/.profile in that order, and reads and executes the first one that exists and is readable.

When a login shell exits[edit]

Bash reads and executes ~/.bash_logout (if it exists).

When started as an interactive shell (but not a login shell)[edit]

Bash reads and executes ~/.bashrc (if it exists). This may be inhibited by using the --norc option. The --rcfile file option forces Bash to read and execute commands from file instead of ~/.bashrc.

Comparison with the Bourne shell and csh startup sequences[edit]

Elements of Bash were derived from the Bourne shell and csh, and allow limited startup file sharing with the Bourne shell and provide some startup features familiar to users of the csh.

Setting inheritable environment variables[edit]

The Bourne shell uses the ~/.profile at login to set environment variables that subprocesses then inherit. Bash can use the ~/.profile in a compatible way, by executing it explicitly from the Bash-specific ~/.bash_profile or ~/.bash_login with the line below. Bash-specific syntax can be kept out of the ~/.profile to keep the latter compatible with the Bourne shell.

. ~/.profile
Aliases and Functions[edit]

These two facilities, aliases from csh and the more general functions that largely supersede them from Bourne shell, were not typically inheritable from the login shell, and had to be redefined in each subshell spawned from the login shell. Although there is an ENV environment variable that could be applied to the problem, both csh and Bash support per-subshell startup files that address it directly. In Bash, the ~/.bashrc is called for interactive subshells. If user-defined functions from the ~/.bashrc are desired in the login shell as well, the ~/.bash_login can include the line below after any setting up of environment variables:

. ~/.bashrc
Commands performed only at login and logout[edit]

The csh supports a ~/.login file for purposes of tasks performed only during initial login, such as displaying system load, disk status, whether email has come in, logging the login time, etc. The Bourne shell can emulate this in the ~/.profile, but doesn't predefine a file name. To achieve similar semantics to the csh model, the ~/.bash_profile can contain the line below, after the environment setup and function setup:

. ~/.bash_login

Likewise, the csh has a ~/.logout file run only when the login shell exits. The Bash equivalent is ~/.bash_logout, and requires no special setup. In the Bourne shell, the trap built-in can be used to achieve a similar effect.

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 ] are tests to see if the filename exists and is readable, simply skipping the part after the && if it's 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.

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 the Bash shell windows spawned from the window manager needs, such as xterm or Gnome Terminal.


Invoking Bash with the --posix option or stating set -o posix in a script causes Bash to conform very closely to the POSIX 1003.2 standard.[27] Bash shell scripts intended for portability should at least take into account the Bourne shell it intends to replace. Bash has certain features that the traditional Bourne shell lacks. Among these are:[27]

  • Certain extended invocation options
  • Command substitution using $( ) notation (this feature is part of the POSIX 1003.2 standard though)
  • Brace expansion
  • Certain array operations, and associative arrays
  • The double brackets extended test construct
  • The double-parentheses arithmetic-evaluation construct
  • Certain string manipulation operations
  • Process substitution
  • A Regular Expression matching operator
  • Bash-specific builtins
  • Coprocesses

Keyboard shortcuts[edit]

The following shortcuts work when using default (Emacs) key bindings. Vi-bindings can be enabled by running set -o vi.[28]

Note: For shortcuts involving Alt, you may be able to use Esc instead.

Note: Sometimes, you must use Esc instead of Alt, because the Alt shortcut conflicts with another shortcut. For example, in Trisquel 5.0 (a distribution of Linux), pressing Alt+f does not move the cursor forward one word, but activates "File" in the menu of the terminal window.

See also: Readline
  • Tab  : Autocompletes from the cursor position.
  • Ctrl+a : Moves the cursor to the line start (equivalent to the key Home).
  • Ctrl+b : Moves the cursor back one character (equivalent to the key ).
  • Ctrl+c : Sends the signal SIGINT to the current task, which aborts and closes it.
  • Ctrl+d
    • Sends an EOF marker, which (unless disabled by an option) closes the current shell (equivalent to the command exit). (Only if there is no text on the current line)
    • If there is text on the current line, deletes the current character (then equivalent to the key Delete).
  • Ctrl+e : (end) moves the cursor to the line end (equivalent to the key End).
  • Ctrl+f : Moves the cursor forward one character (equivalent to the key ).
  • Ctrl+g : Abort the research and restore the original line.
  • Ctrl+h : Deletes the previous character (same as backspace).
  • Ctrl+i : Equivalent to the tab key.
  • Ctrl+j : Equivalent to the enter key.
  • Ctrl+k : Clears the line content after the cursor and copies it into the clipboard.
  • Ctrl+l : Clears the screen content (equivalent to the command clear).
  • Ctrl+n : (next) recalls the next command (equivalent to the key ).
  • Ctrl+o : Executes the found command from history, and fetch the next line relative to the current line from the history for editing.
  • Ctrl+p : (previous) recalls the prior command (equivalent to the key ).
  • Ctrl+r : (reverse search) recalls the last command including the specified character(s). A second Ctrl+r recalls the next anterior command that corresponds to the search
  • Ctrl+s : Go back to the next more recent command of the research (beware to not execute it from a terminal because this command also launches its XOFF). If you changed that XOFF setting, use Ctrl+q to return.
  • Ctrl+t : Transpose the previous two characters.
  • Ctrl+u : Clears the line content before the cursor and copies it into the clipboard.
  • Ctrl+v : If the next input is also a control sequence, type it literally (e. g. * Ctrl+v Ctrl+h types "^H", a literal backspace.)
  • Ctrl+w : Clears the word before the cursor and copies it into the clipboard.
  • Ctrl+x Ctrl+e : Edits the current line in the $EDITOR program, or vi if undefined.
  • Ctrl+x Ctrl+r : Read in the contents of the inputrc file, and incorporate any bindings or variable assignments found there.
  • Ctrl+x Ctrl+u : Incremental undo, separately remembered for each line.
  • Ctrl+x Ctrl+v : Display version information about the current instance of Bash.
  • Ctrl+x Ctrl+x : Alternates the cursor with its old position. (C-x, because x has a crossing shape).
  • Ctrl+y : (yank) adds the clipboard content from the cursor position.
  • Ctrl+z : Sends the signal SIGTSTP to the current task, which suspends it. To execute it in background one can enter bg. To bring it back from background or suspension fg ['process name or job id'] (foreground) can be issued.
  • Ctrl+_ : Incremental undo, separately remembered for each line.
  • Alt+b : (backward) moves the cursor backward one word.
  • Alt+c : Capitalizes the character under the cursor and moves to the end of the word.
  • Alt+d : Cuts the word after the cursor.
  • Alt+f : (forward) moves the cursor forward one word.
  • Alt+l : Lowers the case of every character from the cursor's position to the end of the current word.
  • Alt+r : Cancels the changes and puts back the line as it was in the history.
  • Alt+u : Capitalizes every character from the cursor's position to the end of the current word.
  • Alt+. : Insert the last argument to the previous command (the last word of the previous history entry).

Process management[edit]

The Bash shell has two modes of execution for commands: batch (Unix), and concurrent mode.

To execute commands in batch (i.e., in sequence) they must be separated by the character ";":

command1; command2

in this example, when command1 is finished, command2 is executed.

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

command1 & command2

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


  • Normally a command is executed in foreground (fg). The control of the shell returns to the user after the command finishes its execution.
  • with the symbol & after the command, it can be executed in background (bg). The shell is ready to execute other commands, concurrently to the first command.
  • A program in the running state and in foreground (fg) can be suspended pressing CTRL-Z
  • A suspended program can be restarted in foreground using the command fg or background using the command bg.

Bug reporting[edit]

"bashbug" redirects here. For the widely reported September 2014 bug found in Bash, see Shellshock (software bug).

An external command called bashbug reports Bash shell bugs.[29][30] 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.

See also[edit]


  1. ^ "Bash-4.3 patch list". December 29, 2014. Retrieved January 19, 2015. 
  2. ^ 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). 
  3. ^ a b c Richard Stallman (forwarded with comments by Chet Ramey) (February 10, 1988). "GNU + BSD = ?". Newsgroupcomp.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. 
  4. ^ 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. 
  5. ^ a b Brian Fox (forwarded by Leonard H. Tower Jr.) (June 8, 1989). "Bash is in beta release!". Newsgroupgnu.announce. Retrieved October 28, 2010. 
  6. ^ C Programming by Al Stevens, Dr. Dobb's Journal, July 1, 2001
  7. ^ 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. 
  8. ^ Gattol, Markus (March 13, 2011), Bourne-again Shell, 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". 
  9. ^ Ian Darwin (June 13, 1989). "at&t-free ksh (was: job control is a bug, not a feature)". Newsgroupcomp.os.minix. Retrieved March 21, 2011. Yup, the gnu project's Born Again Shell ("bash") is an attempt at bashing all the features of sh together with many of those from both csh and ksh. 
  10. ^ Chazelas, Stephane (4 October 2014). "oss-sec mailing list archives". Retrieved 4 October 2014. 
  11. ^ Leyden, John (September 24, 2014). "Patch Bash NOW: 'Shell Shock' bug blasts OS X, Linux systems wide open". The Register. Retrieved September 25, 2014. 
  12. ^ Perlroth, Nicole (September 25, 2014). "Security Experts Expect ‘Shellshock’ Software Bug in Bash to Be Significant". New York Times. Retrieved September 25, 2014. 
  13. ^ Brian Fox (August 29, 1996), shell.c, Free Software Foundation, Birthdate: Sunday, January 10th, 1988. Initial author: Brian Fox 
  14. ^ 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. 
  15. ^ len ( (April 20, 1993). "January 1993 GNU's Bulletin". Newsgroupgnu.announce. Usenet: Retrieved October 28, 2010. 
  16. ^ 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. 
  17. ^ Chet Ramey (October 31, 2010), Dates in your Computerworld interview, retrieved October 31, 2010 
  18. ^ Chet Ramey (June 12, 1989). "Bash 0.99 fixes & improvements". Newsgroupgnu.bash.bug. Retrieved November 1, 2010. 
  19. ^ Chet Ramey (July 24, 1989). "Some bash-1.02 fixes". Newsgroupgnu.bash.bug. Retrieved October 30, 2010. 
  20. ^ Brian Fox (March 2, 1990). "Availability of bash 1.05". Newsgroupgnu.bash.bug. Retrieved October 30, 2010. 
  21. ^
  22. ^
  23. ^ Huzaifa Sidhpurwala (2014-09-24). "Bash specially-crafted environment variables code injection attack". Redhat. 
  24. ^ a b "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 
  25. ^ The syntax matches that shown on the regex(7) man page.
  26. ^ "The shell provides associative array variables, with the appropriate support to create, delete, assign values to, and expand them."
  27. ^ a b Mendel Cooper. "Portability Issues". The Linux Documentation Project. 
  28. ^ "BASH Help - A Bash Tutorial". October 5, 2012. Retrieved July 21, 2013. 
  29. ^ bashbug(1),
  30. ^ "Linux / Unix Command: bashbug",

External links[edit]