Difference between revisions of "Bash"

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m (Clarified when sourcing takes place, may not be obvious for folks)
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The file {{ic|~/.bashrc}} is read and sourced by bash when a non-login interactive shell is started, for example, when you open a virtual console from the desktop environment. This file is useful for setting up a user-specific shell environment.
The file {{ic|~/.bashrc}} is read and sourced by bash when a non-login interactive shell is started, for example, when you open a virtual console from the desktop environment. This file is useful for setting up a user-specific shell environment.
===Configuration file sourcing order===
===Configuration file sourcing order at startup===
These files are sourced by bash in different circumstances.  
These files are sourced by bash in different circumstances.  
* if interactive + login shell → {{ic|/etc/profile}} then the first readable of {{ic|~/.bash_profile}}, {{ic|~/.bash_login}}, and {{ic|~/.profile}}
* if interactive + login shell → {{ic|/etc/profile}} then the first readable of {{ic|~/.bash_profile}}, {{ic|~/.bash_login}}, and {{ic|~/.profile}}

Revision as of 21:07, 9 June 2012

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Template:Article summary start Template:Article summary text Template:Article summary heading Template:Article summary wiki Template:Article summary wiki Template:Article summary wiki Template:Article summary end Bash (Bourne-again Shell) is a 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. Arch copiously uses Bash throughout its init-scripts.


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

Login shell

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 or --login command line options.

Interactive shell

Bash is considered an interactive shell if it is started neither with the -c option nor any non-option arguments, and whose standard input and error are connected to terminals.

POSIX compliance

Bash can be run with enhanced POSIX compliance by starting Bash with the --posix command-line option or executing ‘set -o posix’ while Bash is running.

Legacy mode

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 as closely as possible, while conforming to the POSIX standard as well.

In this mode, Bash sources the startup files and then enters POSIX compliance.


Tango-edit-clear.pngThis article or section needs language, wiki syntax or style improvements. See Help:Style for reference.Tango-edit-clear.png

Reason: please use the first argument of the template to provide a brief explanation. (Discuss in Talk:Bash#)

Configuration file overview

  • /etc/profile
  • ~/.bash_profile
  • ~/.bash_login
  • ~/.profile
  • /etc/bash.bashrc (Non-standard: only some distros, Arch included)
  • ~/.bashrc
  • ~/.bash_logout

An overview of the commonly used configuration files:


/etc/profile is sourced by all Bourne-compatible shells upon login. It sets up an environment upon login and loads application-specific (/etc/profile.d/*.sh) settings.


This file is read and sourced by bash when an interactive login shell is started.


The file ~/.bashrc is read and sourced by bash when a non-login interactive shell is started, for example, when you open a virtual console from the desktop environment. This file is useful for setting up a user-specific shell environment.

Configuration file sourcing order at startup

These files are sourced by bash in different circumstances.

  • if interactive + login shell → /etc/profile then the first readable of ~/.bash_profile, ~/.bash_login, and ~/.profile
    • Bash will source ~/.bash_logout upon exit.
  • if interactive + non-login shell → /etc/bash.bashrc then ~/.bashrc
  • if login shell + legacy mode → /etc/profile then ~/.profile

But, in Arch, by default:

  • /etc/profile (indirectly) sources /etc/bash.bashrc
  • /etc/skel/.bash_profile which users are encouraged to copy to ~/.bash_profile, sources ~/.bashrc

which means that /etc/bash.bashrc and ~/.bashrc will be executed for all interactive shells, whether they are login shells or not.

Examples of the user dotfiles can be found in /etc/skel/.

Note: legacy mode is when invoked with the name sh

Shell and environment variables

The behavior of bash and programs run by it can be influenced by a number of environment variable. 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:


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.


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.

An example excerpt from ~/.bashrc covering several time-saving aliases:

# modified commands
alias diff='colordiff'              # requires colordiff package
alias grep='grep --color=auto'
alias more='less'
alias df='df -h'
alias du='du -c -h'
alias mkdir='mkdir -p -v'
alias nano='nano -w'
alias ping='ping -c 5'
alias ..='cd ..'

# new commands
alias da='date "+%A, %B %d, %Y [%T]"'
alias du1='du --max-depth=1'
alias hist='history | grep $1'      # requires an argument
alias openports='netstat --all --numeric --programs --inet --inet6'
alias pg='ps -Af | grep $1'         # requires an argument (note: /usr/bin/pg is installed by the util-linux package; maybe a different alias name should be used)

# privileged access
if [ $UID -ne 0 ]; then
    alias sudo='sudo '
    alias scat='sudo cat'
    alias svim='sudo vim'
    alias root='sudo su'
    alias reboot='sudo reboot'
    alias halt='sudo halt'
    alias update='sudo pacman -Su'
    alias netcfg='sudo netcfg2'

# ls
alias ls='ls -hF --color=auto'
alias lr='ls -R'                    # recursive ls
alias ll='ls -l'
alias la='ll -A'
alias lx='ll -BX'                   # sort by extension
alias lz='ll -rS'                   # sort by size
alias lt='ll -rt'                   # sort by date
alias lm='la | more'

# safety features
alias cp='cp -i'
alias mv='mv -i'
alias rm='rm -I'                    # 'rm -i' prompts for every file
alias ln='ln -i'
alias chown='chown --preserve-root'
alias chmod='chmod --preserve-root'
alias chgrp='chgrp --preserve-root'

# pacman aliases (if applicable, replace 'pacman' with your favorite AUR helper)
alias pac="pacman -S"      # default action     - install one or more packages
alias pacu="pacman -Syu"   # '[u]pdate'         - upgrade all packages to their newest version
alias pacs="pacman -Ss"    # '[s]earch'         - search for a package using one or more keywords
alias paci="pacman -Si"    # '[i]nfo'           - show information about a package
alias pacr="pacman -R"     # '[r]emove'         - uninstall one or more packages
alias pacl="pacman -Sl"    # '[l]ist'           - list all packages of a repository
alias pacll="pacman -Qqm"  # '[l]ist [l]ocal'   - list all packages which were locally installed (e.g. AUR packages)
alias paclo="pacman -Qdt"  # '[l]ist [o]rphans' - list all packages which are orphaned
alias paco="pacman -Qo"    # '[o]wner'          - determine which package owns a given file
alias pacf="pacman -Ql"    # '[f]iles'          - list all files installed by a given package
alias pacc="pacman -Sc"    # '[c]lean cache'    - delete all not currently installed package files
alias pacm="makepkg -fci"  # '[m]ake'           - make package from PKGBUILD file in current directory


Bash also support functions. The following function will extract a wide range of compressed file types. Add the function to ~/.bashrc and use it with the syntax extract <file1> <file2> ...

extract() {
    local c e i

    (($#)) || return

    for i; do

        if [[ ! -r $i ]]; then
            echo "$0: file is unreadable: \`$i'" >&2

        case $i in
               c='bsdtar xvf';;
        *.7z)  c='7z x';;
        *.Z)   c='uncompress';;
        *.bz2) c='bunzip2';;
        *.exe) c='cabextract';;
        *.gz)  c='gunzip';;
        *.rar) c='unrar x';;
        *.xz)  c='unxz';;
        *.zip) c='unzip';;
        *)     echo "$0: unrecognized file extension: \`$i'" >&2

        command $c "$i"

    return $e
Note: Bash users should make sure extglob is enabled: shopt -s extglob, for example by adding it to the .bashrc. It is enabled by default if using Bash completion. Zsh users should do: setopt kshglob instead.

Another way to do this is to install unp package.

More bash function examples can be found here.

Tips and tricks

Prompt customization

The bash prompt is governed by the variable $PS1. To colorize the bash prompt, first comment out the default $PS1:

#PS1='[\u@\h \W]\$ '

Then add the following line:

PS1='\[\e[0;31m\]\u\[\e[m\] \[\e[1;34m\]\w\[\e[m\] \[\e[0;31m\]\$ \[\e[m\]\[\e[0;32m\] '

This $PS1 is useful for a root bash prompt, with red designation and green console text. For details on customizing your bash prompt, see Color Bash Prompt.


It is useful to have the auto-complete feature (pressing Template:Keypress key twice on the keyboard) after you type some command like sudo.

To do this add a line in this format to your ~/.bashrc file:

complete -cf your_command

For example, to enable auto-complete after sudo and man:

complete -cf sudo
complete -cf man

Advanced completion

Despite Bash's native support for basic file name, command, and variable auto-completion, there are ways of improving and extending its reach.

The bash-completion package extends functionality by adding auto-completion to a wide range of commands and their options. Enabling advanced bash completion is quite simple, just install the following package:

# pacman -S bash-completion

Start a new shell and it will be automatically enabled thanks to /etc/bash.bashrc.

Note: If you added any lines similar to "complete -cf sudo" as mentioned in the previous settings and have problems with bash-completion, try removing those lines.

Faster completion

By appending the following into the readline initialization file (~/.inputrc or /etc/inputrc by default):

set show-all-if-ambiguous on

it is no longer necessary to hit Template:Keypress (default binding) twice to produce a list of all possible completions (both when a partial completion is possible and when no completion is possible), as a single key-press will suffice. Alternatively, to produce such a list only when no completion is possible (i.e., not when a partial completion is possible), append the following command in lieu of the previous one:

set show-all-if-unmodified on

Disable Ctrl+z in terminal

You can disable Template:Keypress (pauses/closes your CLI application) feature for you CLI by wrapping your command in this script

trap "" 20


trap "" 20

With this example script, when you accidentally press Template:Keypress instead of Template:Keypress or some other key combination while playing Adom(game) your game will not end. Nothing will happen because Template:Keypress will be ignored.

Clear the screen after logging out

To clear the screen after logging out on a virtual terminal, append the following lines to ~/.bash_logout:


ASCII art, fortunes and cowsay

Along with colors, system info and ASCII symbols, Bash can be made to display a piece of ASCII art on login. ASCII images can be found online and pasted into a text file, or generated from scratch. To set the image to display in a terminal on login, place the string

cat /path/to/text/file

at the top of ~/.bashrc.

Random poignant, inspirational, silly or snide phrases can also be shown. To see an example, install the fortune-mod package from the extra repository.

(user@host)-(10:10 AM Wed Dec 22)
--(~)--->  fortune

It is Texas law that when two trains meet each other at a railroad crossing, each shall come to a full stop, and neither shall proceed until the other has gone.

To have a random phrase displayed when logging into a terminal, just set

command fortune

as the top line in ~/.bashrc.

Note: By default, fortune displays quotes and phrases that are rather inoccuous. However, the package does contain a set of comments some will find offensive, located in /usr/share/fortune/off. See the man page for more info on these.

These two features can be combined, using the program cowsay. Modify the line at the top of ~/.bashrc to read

command cowsay $(fortune)


command cowthink $(fortune)

The earth is like a tiny grain of sand, 
only much, much heavier.                
       \   ^__^
        \  (oo)\_______
           (__)\       )\/\
               ||----w |
               ||     ||

The ASCII images are generated by .cow text files located in /usr/share/cows, and all themes can be listed with the command cowsay -l These files can be edited to the user's liking; custom images can also be created from scratch or found on the net. The easiest way create a custom cow file from an image found online would be to open an existing .cow file in a text editor, copy-and-paste the image from a browser and save the file as a different name. Test the custom file using

# cowsay -f cowfile $(fortune)

This can produce some nice eye candy, and the commands used can be more complex. For a specialized example, take a look here. Another example, to use a random cow, random facial expression, and nicely wrap the text of long fortunes.

fortune -a | fmt -80 -s | cowsay -$(shuf -n 1 -e b d g p s t w y) -f $(shuf -n 1 -e $(cowsay -l | tail -n +2)) -n
( Fry: I must be a robot. Why else would )
( human women refuse to date me?         )
              ,'``.._   ,'``.
            (                 )@@@;:@@@@)@@@\  _,-.
             `.              (@@@//@@@@@@@@@@`'@@@@\
              :               `.//@@)@@@@@@)@@@@@,@;
              |`.            _,'/@@@@@@@)@@@@)@,'@,'
             ,'\ ``--....-)='    `._,@@\    )@@@'``._
            /@_@`.       (@)      /@@@@@)  ; / \ \`-.'
           (@@@`-:`.     `' ___..'@@_,-'   |/   `.)
            `-. `.`.``-----``--,@@.'
              |/`.\`'        ,',');
                  `         (/  (/
(user@host)-(10:10 AM Wed Dec 22)--(~))--->

ASCII Historical Calendar

To install calendar files in your ~/.calendar directory you will need the rpmextract package installed. Then from your home directory, run the following:

$ mkdir -p ~/.calendar
$ curl -o calendar.rpm http://download.fedora.redhat.com/pub/epel/5/x86_64/calendar-1.25-4.el5.x86_64.rpm
$ rpm2cpio calendar.rpm | bsdtar -C ~/.calendar --strip-components=4 -xf - ./usr/share/c*

This will then print out the calendar items

$ sed -n "/$(date +%m\\/%d\\\|%b\*\ %d)/p" $(find ~/.calendar /usr/share/calendar -maxdepth 1 -type f -name 'c*' 2>/dev/null);

Customise Title

The $PROMPT_COMMAND variable allows you to execute a command before the prompt. For example, this will change the title to your full current working directory:

export PROMPT_COMMAND='echo -ne "\033]0;$PWD\007"'

This will change your title to the last command run, and make sure your history file is always up-to-date:

export HISTCONTROL=ignoreboth
export HISTIGNORE='history*'
export PROMPT_COMMAND='history -a;echo -en "\e]2;";history 1|sed "s/^[ \t]*[0-9]\{1,\}  //g";echo -en "\e\\";

Fix line wrap on window resize

When you resize your xterm in vi for example, Bash will not get the resize signal, and the text you type will not wrap correctly, overlapping the prompt.

Use the following in your /etc/bash.bashrc (from Debian) :

# check the window size after each command and, if necessary,
# update the values of LINES and COLUMNS.
shopt -s checkwinsize

Bash history completion

Bash history completion bound to arrow keys (down, up):

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

or equivalently in ~/.inputrc:

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

More info at Readline#History and http://www.gnu.org/software/bash/manual/html_node/Readline-Init-File-Syntax.html

See also