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
- What commands and subcommands should be available?
- What options and arguments will they support?
- What kind of validation should be done on the provided command
requests?
- Which built-in commands will be used?
- Will an interactive mode be provided?
- If so, will a custom menu be created?
- Do any commands need to directly access or modify the application itself
or the other commands (these will be metacommands)?
- 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.
How can I quickly create a very simple application?¶
For a demonstration of how to create a very simple CLIF app, see "FROM
P.O.S. TO CLIF IN A FEW EASY STEPS". CLIF applications require, at the
minimum:
- •
- An Application class that inherits from CLI::Framework::Application (or
"CLI::Framework"). For anything useful to happen, it should
override the "command_map()" hook and include a new
command.
- •
- A Command Class that inherits from CLI::Framework::Command. It should
override the "run()" hook (or have a subcommand that overrides
"run()").
- •
- An Application Script that calls the "run()" method in your
application.
These can all be defined in one file or each class can be placed in a separate
file. Do whatever works best for your particular needs.
How can I add an interactive mode to my application?¶
The built-in console command can be used to enable your application to run
interactively. To do this, simply add the built-in command
CLI::Framework::Command::Console to the command_map in your Application class.
How can I include logging in my application?¶
In your Application class, define "init()" to initialize your logging
object and save the resulting object in the cache, where the object will be
available to your application and command objects.
How can I include database connectivity in my application?¶
In your Application class, define "init()" to connect to your database
and save the resulting object or database handle in the cache, where the
object/handle will be available to your application and command objects.
Of course, for proper Separation of Concerns, you should not simply store a
connected database handle in the cache and use it directly in your Command
classes. You should instead store an object of another class that encapsulates
your data model layer code. An example of this is the model class for the demo
journal application included with CLIF tests:
t/lib/My/Journal/Model.pm.
How can I support an application configuration file?¶
In your Application class, define "init()" to load your configuration
file and save the resulting configuration object in the cache using the cache,
where the object will be available to your application and command objects.
How can I use templates for more flexible output?¶
In your Application class, override the "render()" method.
For instance, you could write an application where all commands return a data
structure to be used in processing a template. Your "render()"
method could determine which template file to process (e.g. based on which
command is being run) and then process it using the received data structure.
How can I create an application-aware command?¶
In exceptional cases, you may need to create a command that "knows
about" the application and needs access to some of its data (which may
include the data of other commands in the application).
To create an application-aware command, inherit from
CLI::Framework::Command::Meta. The command will then have an accessor that
will provide access to the application object.
You should generally not need to do this -- your commands should usually be
decoupled from your application. This will occur by default when you inherit
from CLI::Framework::Command.
How can I use alternative CLI prompting techniques and terminal I/O convenience functions?¶
You may, for example, want to present a menu of options from a variety of
choices based on content from a database. Or perhaps you want to prompt the
user for a list of numbers and you want to support a comma-separated list with
ranges, etc.
Create a CLI::Framework::Command subclass (say, "Your::Command") that
implements your convenience functions or uses a CPAN module such as
Term::Prompt. Then all of your commands can inherit from
"Your::Command" and will all have access to the functions.
You may also want to override read_cmd.
How can I create an app without a "help" command?¶
The 'help' command is fundamental to most applications. If you really want to
build an application without a 'help' command, simply create a custom Help
command with an empty "run" method.
How can I dynamically determine whether or not to run interactively based on command-line options?¶
You may wish to provide an application option ("--interactive") to
start interactive mode. One way to do this is to use your application's
"init" method to determine whether or not to invoke the built-in
console command. For example:
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;
}
This will cause the interactive console to be launched during initialization.
This technique could be used to launch the built-in console command or a
custom interactive command.
This was considered in greater detail on the discussion forum:
<
http://cpanforum.com/posts/12426>.
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)