App::Cmd::Tutorial - getting started with App::Cmd |
App::Cmd::Tutorial - getting started with App::Cmd
version 0.327
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:
# 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
.
$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.
$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 the App::Cmd manpage.
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.''
With a properly configured command class, the above invocation results in nicely formatted data:
$opt = { zero => 1, no_backup => 1, #default value new_seed => 'xyzzy', };
$args = [ qw(foo.db bar.db) ];
Arguments are processed by the Getopt::Long::Descriptive manpage (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 $args
, which GLD ignores. See the Getopt::Long manpage
for format specifications.
The first two methods provide configuration passed to GLD's describe_options
routine. To improve our command class, we might add the following code:
sub usage_desc { "yourcmd %o [dbfile ...]" }
sub opt_spec { return ( [ "skip-refs|R", "skip reference checks during init", ], [ "values|v=s@", "starting values", { default => [ 0, 1, 3 ] } ], ); }
sub validate_args { my ($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; }
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:
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.
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.
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.
Add a description
method to your commands for more verbose output
from the built-in 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:
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.
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
.
Good examples include, but are not limited to, things like
MyApp::Command::frobrinate::Plugin::Quietly
, where ::Quietly
is only
useful for the frobrinate
command.
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.
And as all commands are loaded simultaneously, an error in any one of these commands will yield 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 package's 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.
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.
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:
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.
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.
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) 2015 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.
App::Cmd::Tutorial - getting started with App::Cmd |