J. An Introduction to Programmable Completion (Revision #8)

The programmable completion feature in Bash permits typing a partial command, then pressing the [Tab] key to auto-complete the command sequence.¹ 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.

Example J-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.²

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.³

--

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

Notes

1. ^ This works only from the command line, of course, and not within a script.
2. ^ 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.
3. ^ 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.