Bash (Unix shell)

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.^[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 globbing (wildcard matching), 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 dating from version 1.03 (August 1989),^[10] dubbed Shellshock, was discovered in early September 2014.^[11][12]

History

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 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.^[3][14] Fox released Bash as a beta, version .99, on June 8, 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]

Features

The Bash command syntax is a superset of the Bourne shell command syntax. 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, 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 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 which 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 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.

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"
5

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:

$ 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.

#!/bin/sh

# 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}
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 built-in eval function 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

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

Comparison with the Bourne shell and csh startup sequences

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.

. ~/.profile

. ~/.bashrc

. ~/.bash_login

Legacy-compatible Bash startup example

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.
fi

Operating system issues in Bash startup

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

Portability

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. They include:^[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

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.^[28]

Process management

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.

Summarizing:

• Normally a command is executed in foreground (fg). The control of the shell returns to the user after the command finishes its execution and the special variable $! is set to the process ID of the newly spawned background job.
• 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 resumed in foreground using the command fg or background using the command bg.
• A background process may become blocked on attempts to write to the terminal device (as distinct from simple writes to its standard output).
• The shell can synchronize with the completion of specific background jobs using the wait command by supplying a process ID or job number, or await completion of all background jobs by calling wait with no arguments.

Conditional Execution

Bash supplies "conditional execution" command separators which allow for the execution of a command to be 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 support 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.^[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

• Comparison of command shells

References

External links

Wikimedia Commons has media related to Bash.

Wikibooks has a book on the topic of: Bash Shell Scripting

• Official website
• Bash Reference Manual  (HTML PS PDF)
• Bash Guide for Beginners article at The Linux Documentation Project
• Advanced Bash Scripting Guide article at The Linux Documentation Project
• Linux Shell Scripting Tutorial (LSST) v2.0 wiki
• "The Comprehensive List of bash Reference Documentation and Examples"
• Useful Bash History Tips and Tricks
• 2008 interview with GNU Bash's maintainer, Chet Ramey
• Working with BASH environment variables
• Video Tutorial for Creating a Bash Script
• Bash commands and examples
• Colorized Bash prompt - how to set up a colorized Bash prompt based on the current connection method (SSH, telnet, etc.).
• jBash Project is a Java Parser for the Bourne Again Shell
• The 'official' channel FAQ for freenode's #bash channel is BashFAQ.
• BASHDB - Bash with a built-in debugger.
• Bash Quick Reference card <-- Dates from 1999 and version 2.02.0 of BASH. Very old. -->
• Beginner Linux Command Line Tutorial - Covers the basics, all the way up to scripting.
• Bash tutorial - A quick tutorial about Bash shell, with the explanation of the most useful commands. Slides provided.

GNU Project History GNU Manifesto Free Software Foundation Europe India Latin America History of free software Licenses GNU General Public License GNU Lesser General Public License Affero General Public License GNU Free Documentation License GPL linking exception Software GNU (variants) Hurd Linux-libre glibc Bash coreutils findutils Build system GCC binutils GDB GRUB GNOME GNUstep GIMP GNU Emacs GNU TeXmacs GNU Octave GNU R GSL GMP GNU Electric GNU Archimedes GNUnet GNU Privacy Guard Gnuzilla (IceCat) GNU Health GNUmed GNU LilyPond GNU Go GNU Chess Gnash more... Public speakers Alexandre Oliva Benjamin Mako Hill Bradley M. Kuhn Eben Moglen Federico Heinz Georg C. F. Greve John Sullivan Loïc Dachary Nagarjuna G. Ricardo Galli Richard Stallman Robert J. Chassell Other topics GNU/Linux naming controversy Revolution OS Free Software Foundation anti-Windows campaigns Defective by Design