App::Cmd::Tutorial - getting started with App::Cmd
version 0.319
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::.
YourApp::Command::
We can set up a simple command class like this:
# ABSTRACT: set up YourApp 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.)
By default applications made with App::Cmd know two commands: commands and help.
commands
help
lists available commands.
$yourcmd commands Available commands: commands: list the application's commands help: display a command's help screen init: set up YourApp
Note that by default the commands receive a description from the # ABSTRACT comment in the respective command's module, or from the =head1 NAME Pod section.
# ABSTRACT
=head1 NAME
allows to query for details on command's specifics.
$yourcmd help initialize yourcmd initialize [-z] [long options...] -z --zero ignore zeros
Of course, it's possible to disable or change the default commands, see App::Cmd.
In this example
$ yourcmd reset -zB --new-seed xyzxy foo.db bar.db
-zB and --new-seed xyzxy are "options" and foo.db and bar.db are "arguments."
-zB
--new-seed xyzxy
foo.db
bar.db
With a properly configured command class, the above invocation results in nicely formated data:
$opt = { zero => 1, no_backup => 1, #default value 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. See Getopt::Long for format specifications.
usage_desc
opt_spec
validate_args
$args
The first two methods provide configuration passed to GLD's describe_options routine. To improve our command class, we might add the following code:
describe_options
sub usage_desc { "yourcmd %o [dbfile ...]" } sub opt_spec { return ( [ "skip-refs|R", "skip reference checks during init", ], [ "values|v=s@", "starting values", { default => [ 0, 1, 3 ] } ], ); } sub validate_args { my ($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; }
There are several ways of making options available everywhere (globally). This recipe makes local options accessible in all commands.
To add a --help option to all your commands create a base class like:
--help
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.
options
validate
Note: this is a new file, previously not mentioned in this tutorial and this tip does not recommend the use of global_opt_spec which offers an alternative way of specifying global options.
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.
require
Add a description method to your commands for more verbose output from the built-in App::Cmd::Command::help|help command.
description
App::Cmd::Command::help|help
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:
package YourApp; 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:
package YourApp::Command; sub opt_spec { my ( $class, $app ) = @_; return ( [ 'help' => "this usage screen" ], $class->options($app), ) }
You need to activate strict and warnings as usual if you want them. App::Cmd doesn't do that for you.
strict
warnings
Some people find that for whatever reason, they wish to put Modules in their MyApp::Command:: namespace which are not commands, or not commands intended for use by MyApp.
MyApp::Command::
MyApp
Good examples include, but are not limited to, things like MyApp::Command::frobrinate::Plugin::Quietly, where ::Quietly is only useful for the frobrinate command.
MyApp::Command::frobrinate::Plugin::Quietly
::Quietly
frobrinate
The default behaviour is to treat such packages as errors, as for the majority of use cases, things in ::Command are expected to only be commands, and thus, anything that, by our heuristics, is not a command, is highly likely to be a mistake.
::Command
And as all commands are loaded simultaneously, an error in any one of these commands will yeild a fatal error.
There are a few ways to specify that you are sure you want to do this, with varying ranges of scope and complexity.
This is the simplest approach, and most useful for one-offs.
package YourApp::Command::foo::NotACommand; use YourApp -ignore; <whatever you want here>
This will register this packages namespace with YourApp to be excluded from its plugin validation magic. It otherwise makes no changes to ::NotACommand's namespace, does nothing magical with @ISA, and doesn't bolt any hidden functions on.
::NotACommand
@ISA
Its also probably good to notice that it is ignored only by YourApp. If for whatever reason you have two different App::Cmd systems under which ::NotACommand is visible, you'll need to set it ignored to both.
YourApp
App::Cmd
This is probably a big big warning NOT to do that.
If you really fancy it, you can override the should_ignore method provided by App::Cmd to tweak its ignore logic. The most useful example of this is as follows:
should_ignore
sub should_ignore { my ( $self, $command_class ) = @_; return 1 if not $command_class->isa( 'App::Cmd::Command' ); return; }
This will prematurely mark for ignoring all packages that don't subclass App::Cmd::Command, which causes non-commands ( or perhaps commands that are coded wrongly / broken ) to be silently skipped.
App::Cmd::Command
Note that by overriding this method, you will lose the effect of any of the other ignore mechanisms completely. If you want to combine the original should_ignore method with your own logic, you'll want to steal Moose's around method modifier.
Moose
around
use Moose::Util; Moose::Util::add_method_modifier( __PACKAGE__, 'around', [ should_ignore => sub { my $orig = shift; my $self = shift; return 1 if not $command_class->isa( 'App::Cmd::Command' ); return $self->$orig( @_ ); }]);
CPAN modules using App::Cmd
Ricardo Signes <rjbs@cpan.org>
This software is copyright (c) 2013 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.
To install App::Cmd, copy and paste the appropriate command in to your terminal.
cpanm
cpanm App::Cmd
CPAN shell
perl -MCPAN -e shell install App::Cmd
For more information on module installation, please visit the detailed CPAN module installation guide.