Advanced Bash-Scripting Guide:
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# Appendix I. An Introduction to Programmable Completion

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 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. [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.

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