Appendix J. 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.

   1 # file: UseGetOpt-2
   2 # UseGetOpt-2.sh parameter-completion
   3 
   4 _UseGetOpt-2 ()   #  By convention, the function name
   5 {                 #+ starts with an underscore.
   6   local cur
   7   # Pointer to current completion word.
   8   # By convention, it's named "cur" but this isn't strictly necessary.
   9 
  10   COMPREPLY=()   # Array variable storing the possible completions.
  11   cur=${COMP_WORDS[COMP_CWORD]}
  12 
  13   case "$cur" in
  14     -*)
  15     COMPREPLY=( $( compgen -W '-a -d -f -l -t -h --aoption --debug \
  16                                --file --log --test --help --' -- $cur ) );;
  17 #   Generate the completion matches and load them into $COMPREPLY array.
  18 #   xx) May add more cases here.
  19 #   yy)
  20 #   zz)
  21   esac
  22 
  23   return 0
  24 }
  25 
  26 complete -F _UseGetOpt-2 -o filenames ./UseGetOpt-2.sh
  27 #        ^^ ^^^^^^^^^^^^  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