Bash (Unix shell)

Bash, short for Bourne-Again SHell, is a shell program and command language supported by the Free Software Foundation and first developed for the GNU Project by Brian Fox. Designed as a 100% free software alternative for the Bourne shell, it was initially released in 1989. Its moniker is a play on words, referencing both its predecessor, the Bourne shell, and the concept of rebirth.

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 (GCC). It is available on nearly all modern operating systems, making it a versatile tool in various computing environments.

As a command processor, Bash operates within a text window where users input commands to execute various tasks. It also supports the execution of commands from files, known as shell scripts, facilitating automation. In keeping with Unix shell conventions, Bash incorporates a rich set of features. The keywords, syntax, dynamically scoped variables and other basic features of the language are all copied from the Bourne shell (`sh`). Other features, e.g., history, are copied from the C shell (`csh`) and the Korn Shell (`ksh`). Bash is a POSIX-compliant shell, but with a number of extensions.

History
While bash is considered Unix-like, it's also available on MacOS, Windows, BeOS, and Haiku.

Brian Fox began coding Bash on January 10, 1988, after Richard Stallman became dissatisfied with the lack of progress being made by a prior developer. Stallman and the 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. Fox released Bash as a beta, version .99, on June 8, 1989, and remained the primary maintainer until sometime between mid-1992 and mid-1994, when he was laid off from FSF and his responsibility was transitioned to another early contributor, Chet Ramey.

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 and on Apple's macOS releases before Catalina in October 2019. 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 GNU project, to ArcaOS, and to Android via various terminal emulation applications.

In September 2014, Stéphane Chazelas, a Unix/Linux specialist, discovered a security bug in the program. The bug, first disclosed on September 24, was named Shellshock and assigned the numbers. 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.

Features
As a command processor, Bash operates within a text window where users input commands to execute various tasks. It also supports the execution of commands from files, known as shell scripts, facilitating automation. In keeping with Unix shell conventions, Bash incorporates a rich set of features, including:
 * Interactive and non-interactive invocation
 * Execution of:
 * External (to the shell) UNIX-style commands, such as 'ls' or 'ln', which are actually separate programs
 * Internal "commands" such as aliases, built-ins or functions, and
 * Files containing such commands, i.e., "shell scripts," which do not require compilation before execution and which can be invoked as commands by using their filename.
 * Command history, jobs / job control and tab completion in interactive mode
 * A built-in "help" command, a man page, and an info page
 * Shell compatibility and portability (i.e., POSIX) modes
 * Control structures for condition-testing and iteration
 * UNIX-style pipelines
 * Configurable execution environment(s)
 * Command line parsing
 * Quoting rules
 * Parameters (or "variables"), arrays and associative arrays
 * "Command," arithmetic and "process" substitutions
 * "Word splitting"
 * Filename expansion (globbing) and pattern matching
 * Quote removal
 * Command name lookup, including "aliases," "functions," shell built-ins and shell keywords
 * Redirections of inputs, outputs and error data streams, including "here documents" and "here strings"
 * Subshells
 * Exit status codes
 * Signaling as a means of inter-process communication
 * Asynchronous execution
 * ANSI-C quoting
 * Brace and tilde expansions
 * Configurable prompt and command line editing with GNU readline in interactive mode
 * Shell and session startup files (dotfiles) such as '~/.bashrc'
 * Various settings and shell options with the 'set' and 'shopt' built-ins
 * Execution traces ("xtrace"): lightweight logging for debugging purposes

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 Bash command syntax is a superset of the Bourne shell command syntax. Bash supports brace expansion, command line completion (Programmable Completion), basic debugging and signal handling (using  ) since bash 2.05a  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 ( command), the directory stack, the   and   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, 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 ' '. Bash supports process substitution using the  and  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.

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.

In February 2009, Bash 4.0 introduced support for associative arrays. Associative array indices are strings, in a manner similar to AWK or Tcl. 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. Apple finally stopped using Bash in its operating systems as default shell with the release of MacOS Catalina in 2019.

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:

Users should not use brace expansions in portable shell scripts, because the Bourne shell does not produce the same output.

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:

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.

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  built-in, thus:

Startup scripts
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.

Legacy-compatible Bash startup example
The example  below is compatible with the Bourne shell and gives semantics similar to csh for the   and. The  is a short-circuit evaluation that tests if filename exists and is readable, skipping the part after the   if it is not.

Operating system issues in Bash startup
Some versions of Unix and Linux contain Bash system startup scripts, generally under the  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  or   file to read the   — 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  option or stating   in a script causes Bash to conform very closely to the POSIX 1003.2 standard. Bash shell scripts intended for portability should take into account at least the POSIX shell standard. Some bash features not found in POSIX are:


 * 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

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. 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), while a script intending to support pre-POSIX Bourne shells, like autoconf's configure, are even more limited in the features they can use.

Keyboard shortcuts
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.

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:

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.

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

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 while the process is running in the foreground.

A list of all processes, both in the background and stopped, can be achieved by running :

In the output, the number in brackets refers to the job id. The plus sign signifies the default process for  and. 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  command brings a process to the foreground, while   sets a stopped process running in the background. and  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. The  command can be used to end a process prematurely, by sending it a signal. The job id must be specified after a percent sign:

Conditional execution
Bash supplies "conditional execution" command separators that make execution of a command contingent on the exit code set by a precedent command. For example:

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
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).

Programmable completion
Bash supports programmable completion via built-in, compopt, and   commands. The feature has been available since the beta version of 2.04 released in 2000. These commands enable complex and intelligent completion specification for commands (i.e. installed programs), functions, variables, and filenames.

The  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 keystroke after typing its name.

Program name
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. With that pun, it would seem, is added an allusion: possibly to the Hindu or Buddhist idea of reincarnation; possibly to the Christian idiom known as "being born again;" or quite possibly just to the more abstract idea of renewal. While numerous English translations of the Christian New Testament, Book of John, chapter 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.... " Whatever the original touchstone may have been, in the end the program received the name, "the Bourne Again SHell."

The acronym of that name then is "bash," a word meaning "to strike violently. " 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,     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.

Documentation
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,  , or similar directories on your system.  A brief summary is available by running bash --help . "

On modern Linuxes, information on shell built-in commands can be found by executing   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", this FAQ is current as of bash version 5.1 and is no longer updated.

Vulnerabilities
A security hole in Bash dating from version 1.03 (August 1989), dubbed Shellshock, was discovered in early September 2014 and quickly led to a range of attacks across the Internet. Patches to fix the bugs were made available soon after the bugs were identified.

Licensing
Since 4.0: GPL-3.0-or-later

1.11? to 3.2: GPL-2.0-or-later

0.99? to 1.05?: GPL-1.0-or-later

Versions
A version is also available for Windows 10 and Windows 11 via the Windows Subsystem for Linux. It is also the default user shell in Solaris 11. Bash was also the default shell in BeOS, 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, although Bash remains available as an alternative shell.