Shell Completion

(Up-to-date source of this post. First version created 2013-03-12)

Bash (one of the most popular shells) offers a great feature that makes many people's Tab key pretty worn. It completes the names of commands, directories and files you start to write. The complete command (man bash => "Programmable Completion") lets users extend the standard completion fucntion.

Bash Completion

The Bash Completion project offers many many completion rules that you can add to ~/.bashrc. If not already installed:

aptitude install bash-completion

Then you may need to source it from /etc/bashrc or ~/.bashrc:

# Use bash-completion, if available
[[ $PS1 && -f /usr/share/bash-completion/bash_completion ]] && \
    . /usr/share/bash-completion/bash_completion

Debian does this for you via /etc/bash.bashrc.

Try it out by typing:

ssh [TAB]

If you don't get any meaningful results add some of your hosts into ~/.ssh/config and try again:

host login.example.org
host bigserver.example.net

If you have ssh keys deployed one the remote hosts, try out:

scp bigserver.example.net:[TAB]

Bash Completion with Perl

Bash completion project builds on shell scripting which is easy to write but limited. You might prefer to use a more complete language, like Perl.

Perl Programs

If you want your Perl applications to complete their options use Getopt::Complete module:

#!/usr/bin/perl -w
# getopt-complete -- sample self-completing script
use strict;

use Getopt::Complete(
    'tmpdir' => [ "/tmp",     "$ENV{HOME}/temp", "/var/tmp" ],
    'user'   => [ $ENV{USER}, "root" ],
);

Add the following to ~/.bashrc:

function _getopt_complete () {
    COMPREPLY=($( COMP_CWORD=$COMP_CWORD perl `which ${COMP_WORDS[0]}` ${COMP_WORDS[@]:0} ));
}
complete -F _getopt_complete getopt-complete

Then source ~/.bashrc or relogin and run:

./getopt-complete <TAB>

Compiled Programs

In case you want to add the completion functionality to a compiled program you can't rewrite, you have to wrap it into an external helper, like Mike Schilli did in github-helper. If you want to use this script, put it into your PATH and add following to ~/.bashrc:

complete -C github-helper -o default git

The -o default option reverts to shell's completion mechanism if github-helper has nothing to offer.