Original author(s) | Brian Fox |
---|---|
Developer(s) | Chet Ramey [1] [2] |
Initial release | June 8, 1989 |
Stable release | |
Repository | |
Written in | C |
Operating system | |
Platform | GNU |
Available in | Multilingual (gettext) |
Type | Unix shell, command language |
License | Since 4.0: GPL-3.0-or-later [10] 1.11? to 3.2: GPL-2.0-or-later [11] 0.99? to 1.05?: GPL-1.0-or-later [12] [13] [14] |
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. [15] [16] The shell's name is an acronym for Bourne-Again SHell, a pun on the name of the Bourne shell that it replaces [17] and the notion of being "born again". [18] [19] First released in 1989, [20] it has been used as the default login shell for most Linux distributions and it was one of the first programs Linus Torvalds ported to Linux, alongside GCC. [21] It is available on nearly all modern operating systems.
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 most 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.
A version is also available for Windows 10 and Windows 11 via the Windows Subsystem for Linux. [22] [23] It is also the default user shell in Solaris 11. [24] Bash was also the default shell in BeOS, [7] and in versions of Apple macOS from 10.3 (originally, the default shell was tcsh) to 10.15 (macOS Catalina), which changed the default shell to zsh, [25] although Bash remains available as an alternative shell. [26]
A security hole in Bash dating from version 1.03 (August 1989), [27] dubbed Shellshock, was discovered in early September 2014 and quickly led to a range of attacks across the Internet. [28] [29] [30] Patches to fix the bugs were made available soon after the bugs were identified.
Brian Fox began coding Bash on January 10, 1988, [31] after Richard Stallman became dissatisfied with the lack of progress being made by a prior developer. [15] 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. [15] [32] Fox released Bash as a beta, version .99, on June 8, 1989, [20] and remained the primary maintainer until sometime between mid-1992 [33] and mid-1994, [34] when he was laid off from FSF [35] and his responsibility was transitioned to another early contributor, Chet Ramey. [36] [37] [38]
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 [39] [40] (although Almquist shell may be the default scripting shell) and on Apple's macOS releases before Catalina in October 2019. [41] [42] [25] 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, [43] to ArcaOS, [44] and to Android via various terminal emulation applications.
In September 2014, Stéphane Chazelas, a Unix/Linux specialist, [45] 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 andCVE- 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. [46]
The Bash command syntax is a superset of the Bourne shell command syntax. Bash supports brace expansion, [47] command line completion (Programmable Completion), [48] basic debugging [49] [50] and signal handling (using trap
) since bash 2.05a [51] [52] among other features. Bash can execute the vast majority of Bourne shell scripts without modification, with the exception of Bourne shell scripts stumbling into fringe syntax behavior interpreted differently in Bash or attempting to run a system command matching a newer Bash builtin, etc. Bash command syntax includes ideas drawn from the Korn Shell (ksh) and the C shell (csh) such as command line editing, command history (history
command), [53] 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, [54] 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. [55]
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. [56]
In February 2009, [57] Bash 4.0 introduced support for associative arrays. [13] Associative array indices are strings, in a manner similar to AWK or Tcl. [58] They can be used to emulate multidimensional arrays. Bash 4 also switches its license to GPL-3.0-or-later; some users suspect this licensing change is why MacOS continues to use older versions. [59] Apple finally stopped using Bash in its operating systems as default shell with the release of MacOS Catalina in 2019. [26]
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:
$ echoa{p,c,d,b}e ape ace ade abe$ echo{a,b,c}{d,e,f}ad ae af bd be bf cd ce cf
Users should not use brace expansions in portable shell scripts, because the Bourne shell does not produce the same output.
$ # bash shell$/bin/bash-c'echo a{p,c,d,b}e'ape ace ade abe$ # A traditional shell does not produce the same output$ /bin/sh-c'echo a{p,c,d,b}e'a{p,c,d,b}e
When brace expansion is combined with wildcards, the braces are expanded first, and then the resulting wildcards are substituted normally. Hence, a listing of JPEG and PNG images in the current directory could be obtained using:
ls*.{jpg,jpeg,png}# expands to *.jpg *.jpeg *.png - after which,# the wildcards are processedecho*.{png,jp{e,}g}# echo just shows the expansions -# and braces in braces are possible.
In addition to alternation, brace expansion can be used for sequential ranges between two integers or characters separated by double dots. Newer versions of Bash allow a third integer to specify the increment.
$ echo{1..10}1 2 3 4 5 6 7 8 9 10$ echo{01..10}01 02 03 04 05 06 07 08 09 10$ echofile{1..4}.txt file1.txt file2.txt file3.txt file4.txt$ echo{a..e}a b c d e$ echo{1..10..3}1 4 7 10$ echo{a..j..3}a d g j
When brace expansion is combined with variable expansion (A.K.A. parameter expansion and parameter substitution) the variable expansion is performed after the brace expansion, which in some cases may necessitate the use of the eval
built-in, thus:
$ start=1;end=10$ echo{$start..$end}# fails to expand due to the evaluation order{1..10}$ evalecho{$start..$end}# variable expansion occurs then resulting string is evaluated1 2 3 4 5 6 7 8 9 10
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 typically have neither the execute permission enabled nor an interpreter directive like #!/bin/bash
.
The example ~/.bash_profile
below is compatible with the Bourne shell and gives semantics similar to csh for the ~/.bashrc
and ~/.bash_login
. The [ -r filename ] && cmd
is a short-circuit evaluation that tests if filename exists and is readable, skipping the part after the &&
if it is not.
[-r~/.profile]&&.~/.profile# set up environment, once, Bourne-sh syntax onlyif[-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 onlyfi# End of "if" block
Some versions of Unix and Linux contain Bash system startup scripts, generally under the /etc
directory. Bash executes these files as part of its standard initialization, but other startup files can read them in a different order than the documented Bash startup sequence. The default content of the root user's files may also have issues, as well as the skeleton files the system provides to new user accounts upon setup. The startup scripts that launch the X window system may also do surprising things with the user's Bash startup scripts in an attempt to set up user-environment variables before launching the window manager. These issues can often be addressed using a ~/.xsession
or ~/.xprofile
file to read the ~/.profile
— which provides the environment variables that Bash shell windows spawned from the window manager need, such as xterm or Gnome Terminal.
Invoking Bash with the --posix
option or stating set -o posix
in a script causes Bash to conform very closely to the POSIX 1003.2 standard. [60] Bash shell scripts intended for portability should take into account at least the POSIX shell standard. Some bash features not found in POSIX are: [60] [61]
[[ ... ]]
extended test construct and its regex matching(( ... ))
; $(( ... ))
is POSIX)local
for scoped variablesIf 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. [63] [64] The list varies depending on the actual target shell: Debian's policy allows some extensions in their scripts (as they are in the dash shell), [61] while a script intending to support pre-POSIX Bourne shells, like autoconf's configure
, are even more limited in the features they can use. [65]
Bash uses GNU Readline to provide keyboard shortcuts for command line editing using the default (Emacs) key bindings. Vi-bindings can be enabled by running set -o vi
. [66]
The Bash shell has two modes of execution for commands: batch (asynchronous), and concurrent (synchronous).
To execute commands in batch mode (i.e., in sequence) they must be separated by the character ";", or on separate lines:
command1;command2 command3
In this example, when command1 is finished, command2 is executed, and when command2 has completed, command3 will execute.
A background execution of command1 can occur using (symbol &) at the end of an execution command, and process will be executed in background while 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. [67]
A list of all processes, both in the background and stopped, can be achieved by running jobs
:
$ jobs[1]- Running command1 &[2]+ Stopped command2
In the output, the number in brackets refers to the job id. The plus sign signifies the default process for bg
and fg
. The text "Running" and "Stopped" refer to the process state. The last string is the command that started the process.
The state of a process can be changed using various commands. The fg
command brings a process to the foreground, while bg
sets a stopped process running in the background. bg
and fg
can take a job id as their first argument, to specify the process to act on. Without one, they use the default process, identified by a plus sign in the output of jobs
. The kill
command can be used to end a process prematurely, by sending it a signal. The job id must be specified after a percent sign:
kill%1
Bash supplies "conditional execution" command separators that make execution of a command contingent on the exit code set by a precedent command. For example:
cd"$SOMEWHERE"&&./do_something||echo"An error occurred">&2
Where ./do_something is only executed if the cd (change directory) command was "successful" (returned an exit status of zero) and the echo command would only be executed if either the cd or the ./do_something command return an "error" (non-zero exit status).
For all commands the exit status is stored in the special variable $?
. Bash also supports if...;then...;else...;fi
and case$VARIABLEin$pattern)...;;$other_pattern)...;;esac
forms of conditional command evaluation.
An external command called bashbug reports Bash shell bugs. When the command is invoked, it brings up the user's default editor with a form to fill in. The form is mailed to the Bash maintainers (or optionally to other email addresses). [68] [69]
Bash supports programmable completion via built-in complete
, compopt
, and compgen
commands. [70] The feature has been available since the beta version of 2.04 released in 2000. [71] [72] These commands enable complex and intelligent completion specification for commands (i.e. installed programs), functions, variables, and filenames. [73]
The complete
and compopt
two commands specify how arguments of some available commands or options are going to be listed in the readline input. As of version 5.1 completion of the command or the option is usually activated by the Tab ↹ keystroke after typing its name. [73]
The program's name is a figure of speech or witticism which begins with an homage to Stephen Bourne, the creator of one of the shell programs which have sometimes been considered superseded by the bash shell. His name is used as a pun on the image of childbirth, and with that pun is added an allusion to the Christian idiom of being "born again," or possibly the Buddhist idea of reincarnation. Sometimes considered a reference to John 3 of the Christian New Testament, Merriam-Webster has "born-again" defined as "person who has made a renewed ... commitment of faith. [74] " This idiom is then used to name the program: the Bourne Again SHell. [75] [76]
The acronym of that name then is "bash," a word meaning "to strike violently. [77] " In the context of computer programming, to "violently hit something," such as a computer keyboard, could be considered a hyperbolic image of some frustration. Such imagery of negative emotionality could be seen as standing in direct juxtaposition to the idea of becoming "born again."
The naming could be considered an instance of verbal irony or accidental innuendo. 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, [78] [79] [80] [81] [82] [83] so much so that it seems likely that the bash project has been committed to improving its usablilty. Since then, bash has become the de facto default shell program in most Linux and Unix operating systems.
As the standard upon which bash is based, the POSIX, or IEEE Std 1003.1™, et seq, is informative.
The Linux man page is intended to be the authoritative explanatory document for the understanding of how `bash` operates, while the GNU manual is sometimes considered more user-friendly for reading. "You may also find information about Bash by running info bash ... or by looking at /usr/share/doc/bash/
, /usr/local/share/doc/bash/
, or similar directories on your system. A brief summary is available by running bash --help. [84] "
On modern Linuxes, information on shell built-in commands can be found by executing help,
help [built-in name]
or `man builtins` at a terminal prompt where bash is installed. Some commands, such as `echo`, `false`, `kill`, `printf`, `test` or `true`, depending on your system and on your locally installed version of bash, can refer to either a shell built-in or a system binary executable file. When one of these command name collisions occurs, bash will by default execute a given command line using the shell built-in. Specifying a binary executable's absolute path (i.e., `/bin/printf`) is one way of ensuring that the shell uses a system binary. This name collision issue also effects any "help summaries" viewed with `kill --help` and `/bin/kill --help`. Shell built-ins and system binary executable files of the same name often have differing options.
"The project maintainer also has a Bash page which includes Frequently Asked Questions, [85] " this FAQ is current as of bash version 5.1 and is no longer updated.
Version | Release date | Release notes |
---|---|---|
bash-5.2.15 | 2022-12-13 | NEWS |
bash-5.2 | 2022-09-26 | |
bash-5.1 | 2020-12-07 | github version history NEWS [13] |
bash-5.0 | 2019-01-07 | [86] [87] [88] |
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 | 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 |
Cygwin is a Unix-like environment and command-line interface for Microsoft Windows.
A shell script is a computer program designed to be run by a Unix shell, a command-line interpreter. The various dialects of shell scripts are considered to be scripting languages. Typical operations performed by shell scripts include file manipulation, program execution, and printing text. A script which sets up the environment, runs the program, and does any necessary cleanup or logging, is called a wrapper.
A Unix shell is a command-line interpreter or shell that provides a command line user interface for Unix-like operating systems. The shell is both an interactive command language and a scripting language, and is used by the operating system to control the execution of the system using shell scripts.
The Bourne shell (sh
) is a shell command-line interpreter for computer operating systems.
The Z shell (Zsh) is a Unix shell that can be used as an interactive login shell and as a command interpreter for shell scripting. Zsh is an extended Bourne shell with many improvements, including some features of Bash, ksh, and tcsh.
rc is the command line interpreter for Version 10 Unix and Plan 9 from Bell Labs operating systems. It resembles the Bourne shell, but its syntax is somewhat simpler. It was created by Tom Duff, who is better known for an unusual C programming language construct.
Almquist shell is a lightweight Unix shell originally written by Kenneth Almquist in the late 1980s. Initially a clone of the System V.4 variant of the Bourne shell, it replaced the original Bourne shell in the BSD versions of Unix released in the early 1990s.
In software development, Make is a build automation tool that builds executable programs and libraries from source code by reading files called makefiles which specify how to derive the target program. Though integrated development environments and language-specific compiler features can also be used to manage a build process, Make remains widely used, especially in Unix and Unix-like operating systems.
In Unix-like and some other operating systems, the pwd
command writes the full pathname of the current working directory to the standard output.
In the Unix operating system, shar is an archive format created with the Unix shar
utility. A shar file is a type of self-extracting archive, because it is a valid shell script, and executing it will recreate the files. To extract the files, only the standard Unix Bourne shell sh is usually required.
MinGW, formerly mingw32, is a free and open source software development environment to create Microsoft Windows applications.
In computer programming, glob patterns specify sets of filenames with wildcard characters. For example, the Unix Bash shell command mv *.txttextfiles/
moves all files with names ending in .txt
from the current directory to the directory textfiles
. Here, *
is a wildcard and *.txt
is a glob pattern. The wildcard *
stands for "any string of any length including empty, but excluding the path separator characters ".
In computing, time
is a command in Unix and Unix-like operating systems. It is used to determine the duration of execution of a particular command.
In computing, echo
is a command that outputs the strings that are passed to it as arguments. It is a command available in various operating system shells and typically used in shell scripts and batch files to output status text to the screen or a computer file, or as a source part of a pipeline.
The Thompson shell was the first Unix shell, introduced in the first version of Unix in 1971, and was written by Ken Thompson. It was a simple command interpreter, not designed for scripting, but nonetheless introduced several innovative features to the command-line interface and led to the development of the later Unix shells.
A command shell is a command-line interface to interact with and manipulate a computer's operating system.
In computing, a shebang is the character sequence consisting of the characters number sign and exclamation mark at the beginning of a script. It is also called sharp-exclamation, sha-bang, hashbang, pound-bang, or hash-pling.
The restricted shell is a Unix shell that restricts some of the capabilities available to an interactive user session, or to a shell script, running within it. It is intended to provide an additional layer of security, but is insufficient to allow execution of entirely untrusted software. A restricted mode operation is found in the original Bourne shell and its later counterpart Bash, and in the KornShell. In some cases a restricted shell is used in conjunction with a chroot jail, in a further attempt to limit access to the system as a whole.
A command-line interface (CLI) is a means of interacting with a computer program by inputting lines of text called command-lines. Command-line interfaces emerged in the mid-1960s, on computer terminals, as an interactive and more user-friendly alternative to the non-interactive interface available with punched cards.
Shellshock, also known as Bashdoor, is a family of security bugs in the Unix Bash shell, the first of which was disclosed on 24 September 2014. Shellshock could enable an attacker to cause Bash to execute arbitrary commands and gain unauthorized access to many Internet-facing services, such as web servers, that use Bash to process requests.
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).
See test.c for GPL-2.0-or-later
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.
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.Original computerworld.com.au link is dead: see also copies of original material at readthedocs.io, computerworld.com.au and the University of South Carolina.
"Bourne Again Shell" is a play on the name Bourne Shell, which was the usual shell on Unix.
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".
I've currently ported bash(1.08) and gcc(1.40), and things seem to work.
Birthdate: Sunday, January 10th, 1988. Initial author: Brian Fox
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.
In Linux, most users run bash because it is the most popular shell.
The Bourne Again Shell (bash) is the most common shell installed with Linux distributions.
Bash is by far the most popular shell and forms the default shell on Linux and Mac OSX systems.