Difference between revisions of "Bash"

Revision as of 04:12, 16 April 2013

zh-CN:Bash 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.

Invocation

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.

Configuration

The following files can be used to configure bash:

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

These files are commonly used:

• /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.
• ~/.profile is read and sourced by bash when an interactive login shell is started.
• ~/.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

And by default in Arch:

• /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.

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:

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.

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.

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

~/.bashrc

# 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'      # requires an argument
alias openports='ss --all --numeric --processes --ipv4 --ipv6'
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'
fi

# 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 necessary, replace 'pacman' with your favorite AUR helper and adapt the commands accordingly)
alias pac="sudo /usr/bin/pacman -S"		# default action	- install one or more packages
alias pacu="/usr/bin/pacman -Syu"		# '[u]pdate'		- upgrade all packages to their newest version
alias pacr="sudo /usr/bin/pacman -Rs"		# '[r]emove'		- uninstall one or more packages
alias pacs="/usr/bin/pacman -Ss"		# '[s]earch'		- search for a package using one or more keywords
alias paci="/usr/bin/pacman -Si"		# '[i]nfo'		- show information about a package
alias paclo="/usr/bin/pacman -Qdt"		# '[l]ist [o]rphans'	- list all packages which are orphaned
alias pacc="sudo /usr/bin/pacman -Scc"		# '[c]lean cache'	- delete all not currently installed package files
alias paclf="/usr/bin/pacman -Ql"		# '[l]ist [f]iles'	- list all files installed by a given package
alias pacexpl="/usr/bin/pacman -D --asexp"	# 'mark as [expl]icit'	- mark one or more packages as explicitly installed 
alias pacimpl="/usr/bin/pacman -D --asdep"	# 'mark as [impl]icit'	- mark one or more packages as non explicitly installed

# '[r]emove [o]rphans' - recursively remove ALL orphaned packages
alias pacro="/usr/bin/pacman -Qtdq > /dev/null && sudo /usr/bin/pacman -Rs \$(/usr/bin/pacman -Qtdq | sed -e ':a;N;$!ba;s/\n/ /g')"

Functions

Bash also supports 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> ...

~/.bashrc

extract() {
    local c e i

    (($#)) || return

    for i; do
        c=''
        e=1

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

        case $i in
        *.t@(gz|lz|xz|b@(2|z?(2))|a@(z|r?(.@(Z|bz?(2)|gz|lzma|xz)))))
               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
               continue;;
        esac

        command $c "$i"
        e=$?
    done

    return $e
}

Another way to do this is to install the unp^AUR package from the AUR which contains a Perl script.

Very often changing to a directory is followed by the ls command to list its contents. Therefore it is helpful to have a second function doing both at once. In this example we will name it cl and show an error message if the specified directory does not exist.

~/.bashrc

# cd and ls in one
cl() {
if [ -d "$1" ]; then
	cd "$1"
	ls
	else
	echo "bash: cl: '$1': Directory not found"
fi
}

Of course the ls command can be altered to fit your needs, for example $ ls -hall --color=auto.

More Bash function examples can be found in BBS#30155.

Tips and tricks

Prompt customization

The bash prompt is governed by the variable $PS1. To colorize the bash prompt, use:

~/.bashrc

#PS1='[\u@\h \W]\$ '  # To leave the default one
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 more info, see: Color Bash Prompt.

Tab completion

Tab completion allows for completing partially typed commands by pressing Template:Keypress twice.

Despite Bash's native support for basic file name, command, and variable tab completion the package bash-completion (available in the Official repositories) extends functionality by adding it to a wide range of commands and their options.

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

Faster completion

For a single press of Template:Keypress to produce a list of all possible completions (both when a partial or no completion is possible):

~/.inputrc

set show-all-if-ambiguous on

Alternatively, to produce such a list only when no completion is possible:

~/.inputrc

set show-all-if-ambiguous on
set show-all-if-unmodified on

Manually

For basic completion use lines in the form of complete -cf your_command:

~/.bashrc

complete -cf sudo
complete -cf man

The "command not found" hook

The pkgfile package includes a "command not found" hook that will automatically search the official repositories when you enter an unrecognized command. Then it will display something like this:

An alternative "command not found" hook is also provided by the AUR package command-not-found, which will generate an output like the following:

Display error codes

To set trap to intercept the non-zero return code of last program:

~/.bashrc

EC() { echo -e '\e[1;33m'code $?'\e[m\n'; }
trap EC ERR

Disable Ctrl+z in terminal

You can disable the Template:Keypress feature (pauses/closes your application) by wrapping your command like this:

#!/bin/bash
trap "" 20
adom

Now when you accidentally press Template:Keypress in adom^AUR instead of Template:Keypress 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:

~/.bash_logout

clear
reset

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, use:

~/.bashrc

cat /path/to/text/file

Random poignant, inspirational, silly or snide phrases can be found in fortune-mod.

$ 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, use:

~/.bashrc

command fortune

These two features can be combined, using the program cowsay:

command cowsay $(fortune)

The earth is like a tiny grain of sand, 
only much, much heavier.                
----------------------------------------- 
       \   ^__^
        \  (oo)\_______
           (__)\       )\/\
               ||----w |
               ||     ||
(user@host)-(10:10 AM Wed Dec 22)
--(~))--->

command cowthink $(fortune)

( The best cure for insomnia is to get a )
( lot of sleep. -W.C. Fields             )
 ---------------------------------------- 
        o   ^__^
         o  (oo)\_______
            (__)\       )\/\
                ||----w |
                ||     ||
(user@host)-(10:10 AM Wed Dec 22)
--(~))--->

The ASCII images are generated by .cow text files located in /usr/share/cows, and all themes can be listed with the 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 is to use an existing one as a template. To test the custom file:

$ cowsay -f /path/to/file $(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:

command fortune -a | fmt -80 -s | $(shuf -n 1 -e cowsay cowthink) -$(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?         )
---------------------------------------- 
       o
         o
           o  
              ,'``.._   ,'``.
             :,--._:)\,:,._,.:
             :`--,''@@@:`...';\        
              `,'@@@@@@@`---'@@`.     
              /@@@@@@@@@@@@@@@@@:
             /@@@@@@@@@@@@@@@@@@@\
           ,'@@@@@@@@@@@@@@@@@@@@@:\.___,-.
          `...,---'``````-..._@@@@|:@@@@@@@\
            (                 )@@@;:@@@@)@@@\  _,-.
             `.              (@@@//@@@@@@@@@@`'@@@@\
              :               `.//@@)@@@@@@)@@@@@,@;
              |`.            _,'/@@@@@@@)@@@@)@,'@,'
              :`.`-..____..=:.-':@@@@@.@@@@@_,@@,'
             ,'\ ``--....-)='    `._,@@\    )@@@'``._
            /@_@`.       (@)      /@@@@@)  ; / \ \`-.'
           (@@@`-:`.     `' ___..'@@_,-'   |/   `.)
            `-. `.`.``-----``--,@@.'
              |/`.\`'        ,',');
                  `         (/  (/
(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:

~/.bashrc

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:

~/.bashrc

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.

~/.bashrc

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

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

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:

~/.bashrc

shopt -s autocd

You get:

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

See also

• Advanced Bash Scripting Guide - Very good resource regarding shell scripting using bash
• Bash Reference Manual - Official reference (654K)
• Bash Hackers Wiki - Excellent Bash Wiki
• Bashscripts.org - Forum for bash coders.
• Bash Scripting by Example
• Completion Guide
• Greg's Wiki - Highly recommended
• man page
• Quote Tutorial
• irc://irc.freenode.net#bash - Active and friendly Internet Relay Chat channel for Bash.
• http://chakra-project.org/wiki/index.php/Startup_files
• The Bourne-Again Shell - The third chapter of The Architecture of Open Source Applications
• How to change the title of an xterm
• Readline Init File Syntax
• Custom Bash Commands & Functions