Bash (Unix shell)

Bash

Screenshot of bash and sh sessions demonstrating some features
Developer(s)	Chet Ramey
Development status	Active
Written in	C
Operating system	Cross-platform
Platform	GNU
Available in	English, multilingual (gettext)
Type	Unix shell
License	GNU General Public License version 3+[1]
Website	Home page

Bash is a free software Unix shell written for the GNU Project. Its name is an acronym which stands for Bourne-again shell.^[2] 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,^[3] and the phrase born again. Bash was created in 1987 by Brian Fox. In 1990 Chet Ramey became the primary maintainer.^[4]

Bash is a POSIX shell with a number of extensions. It is the shell for the GNU operating system from the GNU Project. It can be run on most Unix-like operating systems. It is the default shell on most systems built on top of the Linux kernel as well as on Mac OS X and Darwin. It has also been ported to Microsoft Windows using Subsystem for UNIX-based Applications (SUA), or POSIX emulation provided by Cygwin and MSYS. It has been ported to MS-DOS by the DJGPP project and to Novell NetWare.

Features

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 (nested parentheses broke under Bash, for example, in the Mozilla startup script some years back ), 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.

Bash's syntax has many extensions which the Bourne shell lacks. Bash can perform integer calculations without spawning external processes, unlike the Bourne shell. Bash uses the ((…)) command and the $((…)) variable syntax for this purpose. Bash syntax simplifies I/O redirection in ways that are not possible in the traditional Bourne shell. For example, Bash 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 function declarations (using the key word 'function') are not compatible with Bourne/Korn/POSIX/C-shell scripts. Due to 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.

Bash supports here documents just as the Bourne shell always has. However, 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.^[5]

Brace expansion

Brace expansion is a feature, originating in csh, that allows arbitrary strings to be generated using a similar technique to filename expansion. However the generated names 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

Brace expansions should not be used in portable shell scripts, because the Bourne shell will 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, then the resulting wildcards are substituted normally. Hence, a listing of JPEG and PNG images in the current directory could be obtained with:

ls *.{jpg,jpeg,png}    # expands to *.jpg *.jpeg *.png - after which,
                       # the wildcards are processed

Startup scripts

When Bash starts, it executes the commands in a variety of different scripts.

When Bash is invoked as an interactive login shell, it first reads and executes commands from the file /etc/profile, if that file exists. After reading that file, it looks for ~/.bash_profile, ~/.bash_login, and ~/.profile, in that order, and reads and executes commands from the first one that exists and is readable.

When a login shell exits, Bash reads and executes commands from the file ~/.bash_logout, if it exists.

When an interactive shell that is not a login shell is started, Bash reads and executes commands from ~/.bashrc, if that file exists. This may be inhibited by using the --norc option. The --rcfile file option will force Bash to read and execute commands from file instead of ~/.bashrc.

Some versions of Unix have especially contorted system scripts for Bash which will effectively violate the documented script load order by loading scripts too early or attempting to combine Bash startup with the startup scripts for other shells in various ways.

Portability

Shell scripts written with Bash-specific features (bashisms) will not function on a system using the Bourne shell or one of its replacements, unless Bash is also installed and the script begins with a "shebang line" of #!/bin/bash interpreter directive instead of #!/bin/sh.

Keyboard shortcuts

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

See also: Readline

• Tab ⇆ : Autocompletes from the cursor position.
• Ctrl+a : moves the cursor at the beginning of the line (equivalent to the key Home).
• Ctrl+e : (end) moves the cursor at the line end (equivalent to the key End).
• Ctrl+p : (previous) recalls the previous command (equivalent to the key ↑).
• Ctrl+n : (next) recalls the next command (equivalent to the key ↓).
• Ctrl+r : (research) recalls the last command including the specified character(s). A second Ctrl+r recalls the next anterior command which corresponds to the research
• 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+o : executes the found command from research.
• Ctrl+l : clears the screen content (equivalent to the command clear).
• Ctrl+u : clears the line content before the cursor and copy it in the clipboard.
• Ctrl+k : clears the line content after the cursor and copy it in the clipboard.
• Ctrl+w : clears the word before the cursor and copy it in the clipboard.
• Ctrl+y : (yank) adds the clipboard content from the cursor position.
• 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)
• Ctrl+c : sends the signal SIGINT to the current task, which aborts and closes it.
• Ctrl+z : sends the signal SIGTSTP to the current task, which suspends it. To return to it later one can enter fg ['process name or job id'] (foreground).
• Ctrl+x Ctrl+x : (because x has a crossing shape) alternates the cursor with its old position.
• Ctrl+x Ctrl+e : edits the current line in the $EDITOR program, or vi if undefined.
• Alt+f : (forward) moves forward the cursor of one word.
• Alt+b : (backward) moves backward the cursor of one word.
• Alt+Del : cuts the word before the cursor.
• Alt+d : cuts the word after the cursor.
• Alt+u : capitalizes every character from the cursor's position to the end of the current word.
• Alt+l : lowers the case of every character from the cursor's position to the end of the current word.
• Alt+c : capitalizes the character under the cursor and moves to the end of the word.
• Alt+r : cancels the changes and put back the line as it was in the history.

See also

• Comparison of command shells

References

External links

• Bash home page
• Bash Guide for Beginners article at The Linux Documentation Project
• Advanced Bash Scripting Guide article at The Linux Documentation Project
• "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.

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