Bash

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Bash (Bourne-again Shell) is a command-line shell/programming language by the GNU Project. Its name is a homaging reference to its predecessor: the long-deprecated Bourne shell. Bash can be run on most UNIX-like operating systems, including GNU/Linux.

Invocation

Bash behaviour can be altered depending on how it is invoked. Some descriptions of different modes follow.

If Bash is spawned by login in a TTY, by an SSH daemon, or similar means, it is considered a login shell. This mode can also be engaged using the -l/--login command line option.

Bash is considered an interactive shell when its standard input and error are connected to a terminal (for example, when run in a terminal emulator), and it is not started with the -c option or non-option arguments (for example, bash script). All interactive shells source /etc/bash.bashrc and ~/.bashrc, while interactive login shells also source /etc/profile and ~/.bash_profile.

Note: In Arch /bin/sh (which used to be the Bourne shell executable) is symlinked to /bin/bash. If Bash is invoked with the name sh, it tries to mimic the startup behavior of historical versions of sh, including POSIX compability.

Configuration files

See 6.2 Bash Startup Files and DotFiles for a complete description.

File Description Login shells (see note) Interactive, non-login shells
/etc/profile Sources application settings in /etc/profile.d/*.sh and /etc/bash.bashrc. Yes No
~/.bash_profile Per-user, after /etc/profile. If this file does not exist, ~/.bash_login and ~/.profile are checked in that order. The skeleton file /etc/skel/.bash_profile also sources ~/.bashrc. Yes No
~/.bash_logout After exit of a login shell. Yes No
/etc/bash.bashrc Depends on the -DSYS_BASHRC="/etc/bash.bashrc" compilation flag. Sources /usr/share/bash-completion/bash_completion. No Yes
~/.bashrc Per-user, after /etc/bash.bashrc. No Yes
Note:
  • Login shells can be non-interactive when called with the --login argument.
  • While interactive, non-login shells do not source ~/.bash_profile, they still inherit the environment from their parent process (which may be a login shell). See On processes, environments and inheritance for details.

Shell and environment variables

The behavior of Bash and programs run by it can be influenced by a number of environment variables. Environment variables are used to store useful values such as command search directories, or which browser to use. When a new shell or script is launched it inherits its parent's variables, thus starting with an internal set of shell variables[1].

These shell variables in Bash can be exported in order to become environment variables:

VARIABLE=content
export VARIABLE

or with a shortcut

export VARIABLE=content

Environment variables are conventionally placed in ~/.profile or /etc/profile so that all bourne-compatible shells can use them.

See Environment variables for more general information.

Command line

Bash command line is managed by the separate library called Readline. Readline provides a lot of shortcuts for interacting with the command line i.e. moving back and forth on the word basis, deleting words etc. It is also Readline's responsibility to manage history of input commands. Last, but not least, it allows you to create macros.

Tab completion

Tab completion is the option to auto-complete typed commands by pressing Tab (enabled by default).

Single-tab

It may require up to three tab-presses to show all possible completions for a command. To reduce the needed number of tab-presses, see Readline#Faster completion.

Common programs and options

By default, Bash only tab-completes commands, filenames, and variables. The package bash-completion extends this by adding more specialized tab completions for common commands and their options. With bash-completion, normal completions (such as $ ls file.*<tab><tab>) will behave differently; however, they can be re-enabled with $ compopt -o bashdefault program (see [2] and [3] for more detail).

Customize per-command

Note: Using the complete builtin may cause conflicts with bash-completion.

By default Bash only tab-completes file names following a command. You can change it to complete command names using complete -c:

~/.bashrc
complete -c man which

or complete command names and file names with -cf:

complete -cf sudo

See the Bash man page for more completion options.

History

History completion

You can bind the up and down arrow keys to search through Bash's history (see: Readline#History and Readline Init File Syntax):

~/.bashrc
 bind '"\e[A": history-search-backward'
 bind '"\e[B": history-search-forward'

or to affect all readline programs:

~/.inputrc
"\e[A": history-search-backward
"\e[B": history-search-forward

Shorter history

The HISTCONTROL variable can prevent certain commands from being logged to the history. For example, to stop logging of repeated identical commands

~/.bashrc
export HISTCONTROL=ignoredups

or set it to erasedups to ensure that Bash's history will only contain one copy of each command (regardless of order). See the Bash man page for more options.

Mimic Zsh run-help ability

Zsh can invoke the manual for the command preceding the cursor by pressing Alt+h. A similar behaviour is obtained in Bash using this Readline bind:

~/.bashrc
bind '"\eh": "\C-a\eb\ed\C-y\e#man \C-y\C-m\C-p\C-p\C-a\C-d\C-e"'

This assumes are you using the (default) Emacs editing mode.

Aliases

alias is a command, which enables a replacement of a word with another string. It is often used for abbreviating a system command, or for adding default arguments to a regularly used command.

Personal aliases are preferably stored in ~/.bashrc, and system-wide aliases (which affect all users) belong in /etc/bash.bashrc. See [4] for example aliases.

For functions, see Bash/Functions.

Tips and tricks

Prompt customization

See Bash/Prompt customization.

Command-not-found (AUR)

pkgfile includes a "command not found" hook that will automatically search the official repositories, when entering an unrecognized command. An alternative "command not found" hook is provided by command-not-foundAUR. Usage example:

$ abiword
The command 'abiword' is been provided by the following packages:
abiword (2.8.6-7) from extra
	[ abiword ]
abiword (2.8.6-7) from staging
	[ abiword ]
abiword (2.8.6-7) from testing
	[ abiword ]

To load it automatically:

~/.bashrc or ~/.zshrc
[ -r /etc/profile.d/cnf.sh ] && . /etc/profile.d/cnf.sh

Disable Ctrl+z in terminal

You can disable the Ctrl+z feature (pauses/closes your application) by wrapping your command like this:

#!/bin/bash
trap "" 20
adom

Now when you accidentally press Ctrl+z in adomAUR instead of Shift+z nothing will happen because Ctrl+z will be ignored.

Clear the screen after logging out

To clear the screen after logging out on a virtual terminal:

~/.bash_logout
clear
reset

Auto "cd" when entering just a path

Bash can automatically prepend cd when entering just a path in the shell. For example:

$ /etc
bash: /etc: Is a directory

But after adding one line into .bashrc file:

~/.bashrc
...
shopt -s autocd
...

You get:

[user@host ~]$ /etc
cd /etc
[user@host etc]$

Autojump

autojump allows navigating the file system by searching for strings in a database with the user's most-visited paths.

After installation, /etc/profile.d/autojump.bash must be sourced in order to start using the application.

Troubleshooting

Line wrap on window resize

When resizing a terminal emulator, Bash may not receive the resize signal. This will cause typed text to not wrap correctly and overlap the prompt. The checkwinsize shell option checks the window size after each command and, if necessary, updates the values of LINES and COLUMNS.

~/.bashrc
shopt -s checkwinsize

Shell exits even if ignoreeof set

If you have set the ignoreeof option and you find that repeatedly hitting ctrl-d causes the shell to exit, it is because this option only allows 10 consecutive invocations of this keybinding (or 10 consecutive EOF characters, to be precise), before exiting the shell.

To allow higher values, you have to use the IGNOREEOF variable.

For example:

 export IGNOREEOF=100

See also

Tutorials

Community

Examples