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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.

This article was taken partly or wholly, with or without modification from: ArchWiki

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 Void Linux /bin/sh is symlinked to /bin/dash, not /bin/bash, unlike many distributions.

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 partial typed commands by pressing Tab twice (enabled by default).

Single-tab ability

For single press Tab results for when a partial or no completion is possible:

~/.inputrc
set show-all-if-ambiguous on

Alternatively, for results when no completion is possible:

~/.inputrc
set show-all-if-unmodified on

Additional programs and options

Bash has native support for tab completion of: commands, filenames, and variables. This functionality can be extended with the package bash-completion; it extends its functionality by adding a subset of tab completions to popular commands and their options. With bash-completion know that normal completions (such as $ ls file.*<tab><tab>) will behave different; however, they can be re-enabled with $ compopt -o bashdefault <prog> (see [2] and [3] for more detail). Also for older systems bash-completion may not be resourcefully convenient.

Additional programs and options manually

For basic completion use lines in the form of complete -cf your_command (these will conflict with the bash-completion settings):

~/.bashrc
complete -cf sudo
complete -cf man

History completion

History completion bound to arrow keys (down, up) (see: Readline#History and Readline Init File Syntax):

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

or:

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

Fast word movement with Ctrl

Xterm supports moving between words with Ctrl+Left and Ctrl+Right by default. To achieve this effect with other terminal emulators, find the correct terminal codes, and bind them to backward-word and forward-word in ~/.inputrc. The codes can be made visible by first issuing the cat command.

For example, for urxvt:

~/.inputrc
"\eOd": backward-word
"\eOc": forward-word

Mimic Zsh run-help ability

Zsh can invoke the manual for the written command pushing Alt+h. A similar behaviour is obtained in Bash by appending this line in your inputrc file:

/etc/inputrc
"\eh": "\C-a\eb\ed\C-y\e#man \C-y\C-m\C-p\C-p\C-a\C-d\C-e"

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.

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]$

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