Bash
Bash (Bourne-again Shell) is a command-line shell/programming language by the GNU Project. Its name alludes to its predecessor, the long-deprecated Bourne shell. Bash can be run on most UNIX-like operating systems, including GNU/Linux.
Bash is the default command-line shell on Arch 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, output 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
.
/bin/sh
(which used to be the Bourne shell executable) is symlinked to bash
. If Bash is invoked with the name sh
, it tries to mimic the startup behavior of historical versions of sh
, including POSIX compatibility.Configuration files
Bash will attempt to execute a set of startup files depending on how it was invoked. See the Bash Startup Files section of the GNU Bash manual 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
|
Per-user, after exit of a login shell. | Yes | No |
/etc/bash.bash_logout
|
Depends on the -DSYS_BASH_LOGOUT="/etc/bash.bash_logout" compilation flag. 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 |
- 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 GregsWiki:ProcessManagement#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 other 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 emacs and vi styles 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, which can be enabled by sourcing /usr/share/bash-completion/bash_completion
(which has been already sourced in Arch's /etc/bash.bashrc
). 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
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 bash(1) § Programmable Completion 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
History customization
The HISTCONTROL
variable can prevent certain commands from being logged to the history.
To stop logging of consecutive identical commands:
~/.bashrc
export HISTCONTROL=ignoredups
To remove all but the last identical command:
~/.bashrc
export HISTCONTROL=erasedups
To avoid saving commands that start with a space:
~/.bashrc
export HISTCONTROL=ignorespace
To avoid saving consecutive identical commands, and commands that start with a space:
~/.bashrc
export HISTCONTROL=ignoreboth
To remove all but the last identical command, and commands that start with a space:
~/.bashrc
export HISTCONTROL="erasedups:ignorespace"
See bash(1) § HISTCONTROL for details.
Disable history
To disable the bash history only temporarily:
$ set +o history
The commands entered now are not logged to the $HISTFILE
.
For example, now you can hash passwords with printf secret | sha256sum
, or hide GPG usage like gpg -eaF secret-pubkey.asc
and your secret is not written to disk.
To enable history:
$ set -o history
To disable all bash history:
~/.bashrc or /etc/profile
export HISTSIZE=0
... and just to make sure, destroy your old histfile forever:
$ wipe -i -l2 -x4 -p4 "$HISTFILE" $ ln -sv /dev/null "$HISTFILE"
Mimic Zsh run-help ability
Zsh can invoke the manual for the command preceding the cursor by pressing Alt+h
.
A similar behaviour is obtained in Bash using this Readline bind:
~/.bashrc
run-help() { help "$READLINE_LINE" 2>/dev/null || man "$READLINE_LINE"; } bind -m vi-insert -x '"\eh": run-help' bind -m emacs -x '"\eh": run-help'
This assumes are you using the (default) Emacs editing mode.
atuin replaces your existing shell history with an SQLite database, and records additional context for your commands. Additionally, it provides optional and fully encrypted synchronization of your history between machines, via an Atuin server.
Enable bash history timestamps (export HISTTIMEFORMAT="%F %T "
) before syncing. Atuin works well with tools like blesh-gitAUR and cmd-wrapped to provide an enhanced terminal experience across machines.
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 can be stored in ~/.bashrc
or any separate file sourced from ~/.bashrc
. 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.
Syntax highlighting and autosuggestions
ble.sh (Bash Line Editor), packed as blesh-gitAUR, is a command line editor written in pure Bash, which is an alternative to GNU Readline. It has many enhanced features like syntax highlighting, autosuggestions, menu-completion, abbreviations, Vim editing mode, and hook functions. Other interesting features include status line, history share, right prompt, transient prompt, and xterm title.
After installing it, source it in an interactive session.
~/.bashrc
source /usr/share/blesh/ble.sh
Configurations are explained in depth in the ~/.blerc file and at the wiki. The stable bleshAUR package is also available.
Command not found
pkgfile includes a "command not found" hook that will automatically search the official repositories, when entering an unrecognized command.
You need to source the hook to enable it, for example:
~/.bashrc
source /usr/share/doc/pkgfile/command-not-found.bash
Then attempting to run an unavailable command will show the following info:
$ abiword
abiword may be found in the following packages: extra/abiword 3.0.1-2 /usr/bin/abiword
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-gitAUR is a python script which allows navigating the file system by searching for strings in a database with the user's most-visited paths.
zoxide is an alternative which has additional features and performance improvements compared to the original autojump
and can serve as a drop-in replacement for autojump.
Prevent overwrite of files
For the current session, to disallow existing regular files to be overwritten by redirection of shell output:
$ set -o noclobber
This is identical to set -C
.
To make the changes persistent for your user:
~/.bashrc
... set -o noclobber
To manually overwrite a file while noclobber
is set:
$ echo "output" >| file.txt
pushd
and popd
can be used to push or pop directories to a stack while switching to them. This can be useful for "replaying" your navigation history.
[user@host ~] pushd /tmp/dir1 [user@host /tmp/dir1] pushd /var/lib [user@host /var/lib] popd [user@host /tmp/dir1] popd [user@host ~]
See bash(1) § DIRSTACK.
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
Checking errors by analyzing scripts
The package shellcheck analyzes bash (and other shell) scripts, prints possible errors, and suggests better coding.
There is also the web site shellcheck.net of the same purpose, based on this program.
See also
- Wikipedia:Bash (Unix shell)
- Bash Reference Manual, or
/usr/share/doc/bash/bashref.html
- Readline Init File Syntax
- The Bourne-Again Shell - The third chapter of The Architecture of Open Source Applications
- PS1 generator - generate your .bashrc/PS1 bash prompt using an intuitive UI
- Even more useful .bashrc commands