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  xtrapstatsbash$ xtrac[Tab]xtraceroute   xtraceroute.realbash$ 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 $BASH_SOURCEbash$ 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_commandbash$ touch file1.txt file2.txt file2.doc file30.txt file4.zzzbash$ chmod +x sample_commandbash$ complete -f -X '!*.txt' sample_commandbash$ ./sample[Tab][Tab]sample_commandfile1.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.

# 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-2bash$ ./UseGetOpt-2.sh -[Tab]-- --aoption  --debug --file --help --log --test -a -d -f -h -l -tbash$ ./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]

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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