App::Cmd::Tutorial(3)

NAME
App::Cmd::Tutorial - getting started with App::Cmd

VERSION
version 0.318

DESCRIPTION
App::Cmd is a set of tools designed to make it simple to write sophisticated command line programs. It handles commands with multiple subcommands, generates usage text, validates options, and lets you write your program as easy-to-test classes.

An App::Cmd-based application is made up of three main parts: the script, the application class, and the command classes.

The script is the actual executable file run at the command line. It can generally consist of just a few lines:

  #!/usr/bin/perl
  use YourApp;
  YourApp->run;

All the work of argument parsing, validation, and dispatch is taken care of by your application class. The application class can also be pretty simple, and might look like this:

  package YourApp;
  use App::Cmd::Setup -app;
  1;

When a new application instance is created, it loads all of the command classes it can find, looking for modules under the Command namespace under its own name. In the above snippet, for example, YourApp will look for any module with a name starting with "YourApp::Command::".

We can set up a simple command class like this:

  package YourApp::Command::initialize;
  use YourApp -command;
  1;

Now, a user can run this command, but he'll get an error:

  $ yourcmd initialize
  YourApp::Command::initialize does not implement mandatory method 'execute'

Oops! This dies because we haven't told the command class what it should do when executed. This is easy, we just add some code:

  sub execute {
    my ($self, $opt, $args) = @_;

    print "Everything has been initialized.  (Not really.)\n";
  }

Now it works:

  $ yourcmd initialize
  Everything has been initialized.  (Not really.)

The arguments to the execute method are the parsed options from the command line (that is, the switches) and the remaining arguments. With a properly configured command class, the following invocation:

  $ yourcmd reset -zB --new-seed xyzxy foo.db bar.db

might result in the following data:

  $opt = {
    zero      => 1,
    no_backup => 1,
    new_seed  => 'xyzzy',
  };

  $args = [ qw(foo.db bar.db) ];

Arguments are processed by Getopt::Long::Descriptive (GLD). To customize its argument processing, a command class can implement a few methods: "usage_desc" provides the usage format string; "opt_spec" provides the option specification list; "validate_args" is run after Getopt::Long::Descriptive, and is meant to validate the $args, which GLD ignores.

The first two methods provide configuration passed to GLD's "describe_options" routine. To improve our command class, we might add the following code:

  sub usage_desc { "yourcmd %o [dbfile ...]" }

  sub opt_spec {
    return (
      [ "skip-refs|R",  "skip reference checks during init", ],
      [ "values|v=s@",  "starting values", { default => [ 0, 1, 3 ] } ],
    );
  }

  sub validate_args {
    my ($self, $opt, $args) = @_;

    # we need at least one argument beyond the options; die with that message
    # and the complete "usage" text describing switches, etc
    $self->usage_error("too few arguments") unless @$args;
  }

TIPS

•   Delay using large modules using autouse, Class::Autouse or "require" in your commands to save memory and make startup faster. Since only one of these commands will be run anyway, there's no need to preload the requirements for all of them.

•   To add a "--help" option to all your commands create a base class like:

      package MyApp::Command;
      use App::Cmd::Setup -command;

      sub opt_spec {
        my ( $class, $app ) = @_;
        return (
          [ 'help' => "This usage screen" ],
          $class->options($app),
        )
      }

      sub validate_args {
        my ( $self, $opt, $args ) = @_;
        if ( $opt->{help} ) {
          my ($command) = $self->command_names;
          $self->app->execute_command(
            $self->app->prepare_command("help", $command)
          );
          exit;
        }
        $self->validate( $opt, $args );
      }

    Where "options" and "validate" are "inner" methods which your command subclasses implement to provide command-specific options and validation.

•   Add a "description" method to your commands for more verbose output from the built-in "App::Cmd::Command::help|help" command.

      sub description {
        return "The initialize command prepares ...";
      }

•   To let your users configure default values for options, put a sub like

      sub config {
        my $app = shift;
        $app->{config} ||= TheLovelyConfigModule->load_config_file();
      }

    in your main app file, and then do something like:

      sub opt_spec {
        my ( $class, $app ) = @_;
        my ( $name ) = $class->command_names;
        return (
          [ 'blort=s' => "That special option",
            { default => $app->config->{$name}{blort} || $fallback_default },
          ],
        );
      }

    Or better yet, put this logic in a superclass and process the return value from an "inner" method (see previous tip for an example).

AUTHOR
Ricardo Signes <rjbs@cpan.org>

COPYRIGHT AND LICENSE
This software is copyright (c) 2012 by Ricardo Signes.

This is free software; you can redistribute it and/or modify it under the same terms as the Perl 5 programming language system itself.