Getopt modules 23: Getopt::Complete

About this mini-article series. Each day for 24 days, I will be reviewing a module that parses command-line options (such module is usually under the Getopt::* namespace). First article is here.

Getopt::Complete (GC) is a module written by Scott Smith (SAKOHT) in 2009 and also co-maintained by Nathan Nutter (NNUTTER). Last release is in 2011. So far it registers one CPAN distribution depending on it, although it's written by Scott himself.

Shell tab completion is a topic which I have been interested in since around 2012. I've released numerous modules related to completion, including two option parsing modules Getopt::Long::Complete (GLC) and Getopt::Long::More (GLM) which sports completion as (one of) its selling point, so it's natural that I want to compare them to Getopt::Complete. Throughout the article I'll be repeatedly doing those comparisons, and I hope it's not becoming too annoying.

Interface

GC, like GLC and GLM, is a Getopt::Long (GL) wrapper that adds tab completion feature. To let the module detect tab completion mode and return completion answer as soon as possible, GC offers this interface:

use Getopt::Complete (
    'frog'        => ['ribbit','urp','ugh'],
    'fraggle'     => sub { return ['rock','roll'] },
    'quiet!'      => undef,
    'name'        => undef,
    'age=n'       => undef,
    'outfile=s@'  => 'files',
    'outdir'      => 'directories',
    'runthis'     => 'commands',
    'username'    => 'users',
    ''          => 'directories',
);

That is, it accepts the options specification as import arguments. This looks simple but presents its own inconveniences.

The second thing you'll notice that the options specification are different than GL. While GLC and GLM choose to use an interface that is backward-compatible with GL, GC focuses on tab completion. The values of the pairs in the options specification is not a variable reference/coderef as you would expect in GL, but solely completion specification: it's either undef (meaning the option does not require argument), a string (meaning a completion type/routine to use, e.g. files to complete from filenames, commands to complete from program names in PATH, and so on. The options values themselves are collected in %ARGS.

Thus, compared to GLC and GLM, specifying completion routines is simpler in GC (but I also wrote Shell::Completer to provide the same level of convenience with more flexibility).

Activating Completion

To activate completion in bash, you need to declare this shell function first:

function _getopt_complete () {
COMPREPLY=($( COMP_CWORD=$COMP_CWORD perl `which ${COMP_WORDS[0]}` ${COMP_WORDS[@]:0} ));
}

then for each CLI application you also need to do:

% complete -F _getopt_complete myapp

This is different than the way you activate completion for GLC- or GLM-based scripts:

% complete -C myapp myapp

External programs receive raw COMP_LINE and COMP_POINT environment variables from bash when doing tab completion, while shell functions are provided with the already-parsed command-line COMP_WORDS array variable and COMP_CWORD. GC wants to avoid parsing the command-line on its own, so the _getopt_complete function is used to give the Perl program parsed command-line arguments in @ARGV, and COMP_CWORD in another environment variable.

Using command-line that is already parsed by bash in COMP_WORDS has its pros as well as cons, due to the way that bash parses command-line for COMP_WORDS. So I cannot say which way is better, but what I can say is parsing COMP_LINE ourselves is more flexible.

Completion behavior and bugs

When you press tab after the command:

% myapp <tab>

GC offers only completion from the <> specification. In the above example, it only offers list of directories as answer. On the other hand, GLC and GLM also shows the list of available option names. With GC, to list the available options, you have to do:

% myapp -<tab>

I also cannot say that GLC's and GLM's way is better, but it certainly makes the CLI program more discoverable. By just pressing Tab, a user (especially a new user) can know more about what's possible.

GC has still a few problems. First of all, it cannot complete "–opt=" when COMP_WORDBREAKS contains "=". I have put workarounds for this issue in GLC and GLM. Second, it cannot handle filenames/directory names with spaces, or quotes, and probably other special characters too.

Third, GLC and GLM through Complete::Util offers some matching algorithms aside from simple prefix matching, for extra convenience. This is not offered by GC.

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