The programmable completion feature in Bash permits typing a partial command, then pressing the [Tab] key to auto-complete the command sequence. [146] If multiple completions are possible, then [Tab] lists them all. Let's see how it works.
bash$ xtra[Tab]
xtraceroute xtrapin xtrapproto
xtraceroute.real xtrapinfo xtrapreset
xtrapchar xtrapout xtrapstats
bash$ xtrac[Tab]
xtraceroute xtraceroute.real
bash$ xtraceroute.r[Tab]
xtraceroute.real
Tab completion also works for variables and path names.
bash$ echo $BASH[Tab]
$BASH $BASH_COMPLETION $BASH_SUBSHELL
$BASH_ARGC $BASH_COMPLETION_DIR $BASH_VERSINFO
$BASH_ARGV $BASH_LINENO $BASH_VERSION
$BASH_COMMAND $BASH_SOURCE
bash$ echo /usr/local/[Tab]
bin/ etc/ include/ libexec/ sbin/ src/
doc/ games/ lib/ man/ share/
The Bash complete and compgen builtins make it possible for tab completion to recognize partial parameters and options to commands. In a very simple case, we can use complete from the command-line to specify a short list of acceptable parameters.
bash$ touch sample_command bash$ touch file1.txt file2.txt file2.doc file30.txt file4.zzz bash$ chmod +x sample_command bash$ complete -f -X '!*.txt' sample_command bash$ ./sample[Tab][Tab] sample_command file1.txt file2.txt file30.txt
The -f option to complete specifies filenames, and -X the filter pattern.
For anything more complex, we could write a script that specifies a list of acceptable command-line parameters. The compgen builtin expands a list of arguments to generate completion matches.
Let us take a modified version of the UseGetOpt.sh script as an example command. This script accepts a number of command-line parameters, preceded by either a single or double dash. And here is the corresponding completion script, by convention given a filename corresponding to its associated command.
Exemple I.1. Completion script for UseGetOpt.sh
# file: UseGetOpt-2
# UseGetOpt-2.sh parameter-completion
_UseGetOpt-2 () # By convention, the function name
{ #+ starts with an underscore.
local cur
# Pointer to current completion word.
# By convention, it's named "cur" but this isn't strictly necessary.
COMPREPLY=() # Array variable storing the possible completions.
cur=${COMP_WORDS[COMP_CWORD]}
case "$cur" in
-*)
COMPREPLY=( $( compgen -W '-a -d -f -l -t -h --aoption --debug \
--file --log --test --help --' -- $cur ) );;
# Generate the completion matches and load them into $COMPREPLY array.
# xx) May add more cases here.
# yy)
# zz)
esac
return 0
}
complete -F _UseGetOpt-2 -o filenames ./UseGetOpt-2.sh
# ^^ ^^^^^^^^^^^^ Invokes the function _UseGetOpt-2.
Now, let's try it.
bash$ source UseGetOpt-2 bash$ ./UseGetOpt-2.sh -[Tab] -- --aoption --debug --file --help --log --test -a -d -f -h -l -t bash$ ./UseGetOpt-2.sh --[Tab] -- --aoption --debug --file --help --log --test
We begin by sourcing the « completion script. » This sets the command-line parameters. [147]
In the first instance, hitting [Tab] after a single dash, the output is all the possible parameters preceded by one or more dashes. Hitting [Tab] after two dashes gives the possible parameters preceded by two or more dashes.
Now, just what is the point of having to jump through flaming hoops to enable command-line tab completion? It saves keystrokes. [148]
--
Resources:
Bash programmable completion project
Mitch Frazier's Linux Journal article, More on Using the Bash Complete Command
Steve's excellent two-part article, « An Introduction to Bash Completion »: Part 1 and Part 2
[146] This works only from the command line, of course, and not within a script.
[147] Normally the default parameter completion files reside in either the /etc/profile.d directory or in /etc/bash_completion. These autoload on system startup. So, after writing a useful completion script, you might wish to move it (as root, of course) to one of these directories.
[148] It has been extensively documented that programmers are willing to put in long hours of effort in order to save ten minutes of « unnecessary » labor. This is known as optimization.