NAME¶

CLI::Framework::Tutorial - "HOWTO" develop CLIF applications using best practices

CLIF DOCUMENTATION¶

This is a guide to developing CLIF applications. It is a supplement to the documentation in CLI::Framework, CLI::Framework::Application and CLI::Framework::Command, which have more thorough coverage of some finer points.

It is suggested that new users start by reading this document, then use the other documentation for reference as necessary.

INTRODUCTION¶

Developers have been reluctantly writing ad-hoc, disposable scripts for too long or struggling to decide how not to do so. There is a better alternative.

The CLI::Framework documentation enumerates many advantages to using CLIF instead of writing yet-another-getopt-based-script. CLIF comes with a lot of documentation, but don't take that to mean that using CLIF is complicated. CLIF apps with simple needs are very easy to build. Apps with complex needs are a bit more work, but much easier to build (and far easier to test and maintain) than doing that work from scratch.

This document will first demonstrate a very simple CLIF application. Next, a complete application will be shown to demonstrate more advanced CLIF features.

Think of a typical command-line script. It needs to parse command-line options and arguments, check that any required external resources (files, databases, etc.) are available, fail nicely if something is missing or inconsistent, then do something application-specific that depends on the options, arguments, and external resources.

What happens when new scripts are created to do something similar? All too often, they end up with different option names for conceptually the same purpose. It is common for functionality needed by several scripts to be duplicated in each similar script. This rapidly gets out of hand, becoming a maintenance frustration. Your team members are not "on the same page" and new people learning your tools must have lengthy, verbal, one-on-one code tours.

Instead, a set of related scripts could be combined into a CLIF application. Consistent naming conventions and sharing of common code is naturally encouraged. The commands are easy to test. New commands can be added with ease.

FROM P.O.S. TO CLIF IN A FEW EASY STEPS¶

A "P.O.S." is a "Plain Old Script." This section shows you how to reform an old P.O.S., creating a shiny new CLIF application!

Please see working code for this example included with the "CLI::Framework" distribution (examples/demo-simple.pl).

This example demonstrates the following features:

• inline application definition
• basics (app, commands, command options and args)
• the relationship between plain scripts and CLIF applications (including how to convert between them)

To understand CLIF commands, imagine converting a legacy script to a CLIF application. First, create a Perl class that inherits from CLI::Framework::Command. Place the main body of the script in a "run()" method. Add the functions that the script defines, if any.

    # Your Command subclass...
    package Converted::Script::Command::LegacyScript;
    use base qw( CLI::Framework::Command );
    # main body of former script goes inside run():
    sub run { ... }

Next, create a Perl class (creating a separate package file for the class is totally optional) that inherits from CLI::Framework::Application (or you can use "CLI::Framework" as a shorthand) and define a method, "command_map()", that links command names with classes that implement the commands:

    # Your Application class...
    package Converted::Script;
    use base qw( CLI::Framework );
    sub command_map {
        'legacy-script' => 'Converted::Script::Command::LegacyScript',
    }

The code that provides a friendly usage message (if the legacy script provided one) can be replaced by defining the "usage_text" method:

    sub usage_text {
        qq{
        $0 [--verbose|v] [--help|h]: how to use this application...
        }
    }

Back in your Command subclass, the option/argument processing code will be replaced with a method defining what options will be recognized (the data structure to be returned is exactly as documented in Getopt::Long::Descriptive):

    sub option_spec {
        [ 'help|h'      => 'show help' ],
        [ 'verbose|v'   => 'be verbose' ],
    }

...and that's all it takes to convert a simple script to a CLIF app. This contrived example demonstrates the mechanics, but let me point out a few advantages (see DESIGN GOALS AND FEATURES for the long list):

Clear division of responsibilities

Using packages, subroutines, and separate files (if desired), CLIF apps follow established convention and provide a new pattern for creating tools.

Easy to test

Now that functional units of code are subroutines in packages, you can unit test each component independently.

Easy to maintain

Instead of puzzling over a several-thousand-line script, maintaining a CLIF application is like maintaining any other well-engineered application code.

Easy to extend

Related tools frequently occur in groups. Instead of awkwardly forcing loosely-related behaviors into the same script, CLIF makes it easy to add additional commands in a modular way.

WHEN NOT TO USE CLIF¶

CLIF could be used for the simplest of needs, but it may be overkill in very simple situations.

You may want to avoid CLIF for very basic scripts that have a single behavior and are completely independent from other such tools. However, if there's a chance that the scripts might grow to become more complex or if you would simply like a pattern to follow, it may still be worth considering.

CONCEPTS AND DEFINITIONS¶

See CONCEPTS AND DEFINITIONS.

UNDERSTANDING THE APPLICATION RUN SEQUENCE¶

See APPLICATION RUN SEQUENCE.

Understanding this is important to building more complex apps. You need, at the least, to understand how CLIF differentiates between options and arguments that are meant for the application itself and those options and arguments that are meant for individual commands.

The following examples demonstrate the alternative command request forms. Note that in all cases, any number of (sub)command options and arguments can be passed (these examples show only one of each for brevity).

FORM #1 (without subcommands) -- command requests that involve NO subcommands take the following form:

    <app> [--app-opt] <cmd> [--cmd-opt] [cmd-arg] ...

(notice how the position of options and arguments determines whether they are meant for the application as a whole or for the specific command).

FORM #2 (with subcommands) -- Command requests that involve A SINGLE subcommand take this form:

    <app> [--app-opt] <cmd> [--cmd-opt] <subcmd> [--subcmd-opt] [subcmd-arg] ...

Command requests that involve MULTIPLE subcommands follow the same form:

    <app> [--app-opt] <cmd> [--cmd-opt] <subcmd1> [--subcmd1-opt] <subcmd2> [--subcmd2-opt] [subcmd2-arg] ...

(notice that the final arguments apply to the final subcommand. The only command that can receive arguments is the final subcommand).

A MORE INVOLVED EXAMPLE¶

Please see working code for this example included with the "CLI::Framework" distribution (examples/queue).

The next example demonstrates the following features:

• inline application definition
• basics (app, commands, command options and args)
• subcommands
• validation of application and command arguments
• interactive mode and non-interactive mode

Suppose we need to write a command-line application that provides an interface to a queue. Strings can be added to or removed from the queue, queue contents can be displayed, and queue "properties" can be set to restrict the contents added to the queue. The interface should work interactively.

The following usage demonstrates the desired behavior:

    [somebody@somewhere]$ examples/queue --qin=/tmp/qfile --qout=/tmp/qfile console
    # ---- interactive mode ----
    1) dequeue
    2) cmd-list
    3) enqueue
    4) print
    5) alias
    6) property
    > help enqueue
    enqueue [--tag=<tag1> [--tag=<tag2> [...] ] ] <item1> [<item2> ...  <itemN>]: add item(s) to queue
    > enqueue --tag=x "something"
    > property set --evens
    > e 1 21 514 937 18

The working example in examples/queue accomplishes this goal in a single inline application containing the Application class and multiple Command Classes.

This application is created in fundamentally the same way as the simple one presented earlier. It uses more commands, more Application class/Command Class hooks, and subcommands. The code is much longer but almost all of it is for business logic -- very little additional CLIF-specific code is needed.

The example code shows how various commands can be managed by an Application subclass. The code is commented thoroughly to explain the various hooks that are available for Application class and Command Classes.

Of course, CLIF applications can always be used in non-interactive mode:

    # ---- non-interactive mode ----
    $ examples/queue --qout=/tmp/qfile enqueue 'first'
    $ examples/queue --qin=/tmp/qfile --qout=/tmp/qfile enqueue --tag=x --tag=y 'second'
    $ examples/queue --qin=/tmp/qfile --qout=/tmp/qfile property list
    $ examples/queue --qin=/tmp/qfile --qout=/tmp/qfile property set --evens
    $ examples/queue --qin=/tmp/qfile --qout=/tmp/qfile property list
    $ examples/queue --qin=/tmp/qfile --qout=/tmp/qfile enqueue 17
    $ examples/queue --qin=/tmp/qfile --qout=/tmp/qfile enqueue 4
    $ examples/queue --qin=/tmp/qfile --qout=/tmp/qfile enqueue 2
    $ examples/queue --qin=/tmp/qfile --qout=/tmp/qfile dequeue
    $ examples/queue --qin=/tmp/qfile --qout=/tmp/qfile dequeue
    $ examples/queue --qin=/tmp/qfile --qout=/tmp/qfile dequeue
    $ examples/queue --qin=/tmp/qfile --qout=/tmp/qfile enqueue 3
    $ examples/queue --qin=/tmp/qfile print

PLANNING A COMPLEX CLIF APPLICATION¶

Little additional thought (beyond that needed for business logic) is required to create a basic CLIF app -- the strategy explained in "FROM P.O.S. TO CLIF IN A FEW EASY STEPS" demonstrates how CLIF differs from a "Plain Old Script".

A more sophisticated command line application will benefit from a wider variety of the features CLIF provides. The extra features are easy to use, but the additional complexity warrants careful planning.

After the initial learning curve, applying interface design principles and implementing business rules will become the only challenging aspects to developing your CLIF applications. This is as it should be -- the framework handles application-independent aspects, leaving you to focus on the unique features of your application.

Here are some considerations:

Basic interface

High-level code layout

Which components of the application will be defined in their own package files? Which will be defined inline?

Separation of concerns using MVC strategy

How will the model be separated from the rest of the application? What about the view?

Data sharing between application and commands

What data will data be shared between the application and the commands? Will this be arranged by using the cache, using a Command superclass (a generic command class that all of your commands inherit from), or by some other means?

Read on for possible answers to some of these questions.

HOW CAN I ...?¶

This section briefly highlights how CLIF could be used to support various common goals. Even if your particular situation does not appear here, reading this short section will give you an understanding of how CLIF could be set up to support novel cases.

    sub init {
        my ($app, $opts) = @_;
        # imagine fancy logic to determine whether or not to run interactively...
        if( $opts->{interactive} ) {
            $app->set_current_command('console');
        }
        return 1;
    }

TROUBLESHOOTING¶

The following solutions may be helpful when working with CLIF.

• Don't forget to inherit from CLI::Framework::Application in your Application class and CLI::Framework::Command in your command class
• Don't forget to override command_map() in your Application class
• Don't forget to override run() in your Command class
• If in doubt, run "perl -wc <your command class file>"

  If a user-defined command class does not compile, your CLIF application will fail silently. Running "perl -wc Class.pm" will report compilation problems for Class.pm.

LICENSE AND COPYRIGHT¶

Copyright (c) 2009 Karl Erisman (kerisman@cpan.org). All rights reserved.

This is free software; you can redistribute it and/or modify it under the same terms as Perl itself. See perlartistic.

AUTHOR¶

Karl Erisman (kerisman@cpan.org)