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.