Bash autocomplete

Motivation

Recently I worked with Git in my bash shell and when I typed git stat<TAB> and magically the words git status appeared I thought to myself: “How does this actually work?”

Autocomplete with Bash

One of the very handy features of Bash is its autocompletion feature. Pressing tab at the prompt will give suggestions depending on the current context. Bash uses the readline library to provide the prompt editing features. But I don’t want to speak about the internals and the readline implementation, because Bash actually provides a more controlled configuration interface for its autocompletion features.

Programmable Completion

The Bash documentation refers to this feature as Programmable Completion. Bash provides three functions to handle most of the bulk work to make autocompletion available: compgen, complete and compopt.

The complete function registers completer functions (often using compgen) for every command that should have autocompletion functionality. In order to register the function foo_completer to assist in autocompleting the command foo we would type

~$ complete -F foo_completer foo

Part 1 and Part 2 of this article give a very nice and concise explanation of the ongoings.

Programmable Completion for Git

I just want to give the concrete example in case of Git on my PC. Most likely the mentioned paths differ for other distros. On my PC this is the default installation and location behaviour.

The Bash completion functions are searched in the file /etc/bash_completion. In my case this file is only sourcing the file /usr/share/bash_completion. In this file a lot of autocompletion for common command line commands like cd are initialized.

The completion for Git are located under /usr/share/bash-completion/completions/git. This file is loaded with helper functions, but almost at the bottom comes the crucial part.

# Setup completion for certain functions defined above by setting common
# variables and workarounds.
# This is NOT a public function; use at your own risk.
__git_complete ()
{

local wrapper=“__git_wrap${2}” eval “$wrapper () { __git_func_wrap $2 ; }” complete -o bashdefault -o default -o nospace -F $wrapper $1 2>/dev/null

|| complete -o default -o nospace -F $wrapper $1

}

# wrapper for backwards compatibility _git () {

__git_wrap__git_main

}

# wrapper for backwards compatibility _gitk () {

__git_wrap__gitk_main

}

__git_complete git __git_main __git_complete gitk __gitk_main

Line 2749 does the actual registration for the autocompletion of the git command.

Location best practice

Above mentioned locations are the locations for system installation. Custom user completion files should be saved under /etc/bash_completion.d/. Every file in this directory will be sourced (loaded) by Bash and thus made available.