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.
Configuration files
See section “6.2 Bash Startup Files” in /usr/share/doc/bash/bashref.html (online link) and GregsWiki: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
--loginargument. - 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.
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 the Bash man page for more completion options.
