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Welcome to Choice

Choice is a small library for defining and parsing command line options. It works awesomely with Highline or other command line interface libraries.

Choice was written by Chris Wanstrath as an exercise in test driving development of a DSL. This project is still an infant: bugs are expected and tattling on them is appreciated.

Installing is easy, with RubyGems. Give it a shot:

$ gem install choice

E-mail inquiries can be directed to chris[at]ozmm[dot]org.

Of course, Choice is licensed under the MIT License, which you can find included in the LICENSE file or by surfing your World Wide Web browser of choice towards

Using Choice

An examples directory is included with Choice, in which some contrived Ruby programs utilizing the library have been placed. Here’s a snippet:


require 'choice'


Choice.options do
  header ''
  header 'Specific options:'

  option :host do
    short '-h'
    long '--host=HOST'
    desc 'The hostname or ip of the host to bind to (default'
    default ''

  option :port do
    short '-p'
    long '--port=PORT'
    desc 'The port to listen on (default 21)'
    cast Integer
    default 21

  separator ''
  separator 'Common options: '

  option :help do
    long '--help'
    desc 'Show this message'

  option :version do
    short '-v'
    long '--version'
    desc 'Show version'
    action do
      puts "ftpd.rb FTP server v#{PROGRAM_VERSION}"

puts 'port: ' + Choice[:port]

Notice the last line. For free, you will be given a Choice.choices hash which contain, at runtime, the options found and their values.

Choice[:key] is a shortcut for Choice.choices[:key].

Because we gave option :port a default of 21, Choice[:port] should be 21 if we run ftpd.rb with no options. Let’s see.

$ ruby ftpd.rb
port: 21

Cool. On our system, port 21 is reserved. Let’s use another port.

$ ruby ftpd.rb -p 2100
port: 2100

Alright. And, of course, there is the hard way of doing things.

$ ruby ftpd.rb --port=2100
port: 2100

That :version option looks pretty interesting, huh? I wonder what it does…

$ ruby ftpd.rb -v
ftpd.rb FTP server v4

That’s not all, though. We also get a --help option for free.

$ ruby ftpd.rb --help
Usage: ftpd.rb [-hpv]

Specific options:
    -h, --host=HOST                  The hostname or ip of the host to bind to (default
    -p, --port=PORT                  The port to listen on (default 21)

Common options:
        --help                       Show this message
    -v, --version                    Show version

The Choice.choices hash

Keep in mind that your option’s key in the Choice.choices hash is defined by the first parameter passed to option statement. This is perfectly legit, albeit somewhat confusing:

option :name do
  short '-h'
  long '--handle=NAME'
  desc "Your handle."

You can access this option by using Choice.choices[:name], not :handle.

Option options

Obviously, Choice revolves around the option statement, which receives a block. Here are all the, er, options option accepts. None of them are required but short or long must be present for Choice to know what to do.

Options must be defined in the context of a Choice.options block, as seen above. This context is assumed for the following explanations.

For the quick learners, here’s the list:

  • short

  • long

  • default

  • desc

  • cast

  • valid (takes array)

  • validate (takes regex)

  • filter (takes a block)

  • action (ditto)

You can define these within your option in any order which pleases you.


Defines the short switch for an option. Expected to be a dash and a single character.

short '-s'


Defines the long switch for an option. Expected to be a double dash followed by a string, an equal sign (or a space), and another string.

There are two variants: longs where a parameter is required and longs where a parameter is optional, in which case the value will be true if the option is present.


long '--debug=[LEVEL]'

Assuming our program defines Choices and ends with this line:

puts 'debug: ' + Choice.choices[:debug]

we can do this:

$ ruby ftpd.rb --debug
debug: true

$ ruby ftpd.rb --debug=1
debug: 1

$ ruby ftpd.rb --debug 1
debug: 1


long '--debug=LEVEL'

Assuming the same as above:

$ ruby ftpd.rb --debug 1
debug: 1

$ ruby ftpd.rb --debug
<help screen printed>

long as array

Often you may wish to allow users the ability to pass in multiple arguments and have them all combined into an array. You can accomplish this by defining a long and setting the caps-argument to *ARG. Like this:

long '--suit *SUITS'

Choice.choices.suits will now return an array. Here’s an example of usage:

$ ruby --suit hearts clubs
suit: ['hearts', 'clubs']

Check out examples/gamble.rb for more information on this cool feature.


You can define a default value for your option, if you’d like. If the option is not present in the argument list, the default will be returned when trying to access that element of the Choice.choices hash.

As with the above, assume our program prints Choice.choices[:debug]:

default 'info'

If we don’t pass in --debug, the :debug element of our hash will be ‘info.’

$ ftpd.rb
debug: info

$ ftpd.rb --debug warn
debug: warn


The description of this option. Fairly straightforward, with one little trick: multiple desc statements in a single option will be considered new desc lines. The desc lines will be printed in the order they are defined. Like this:

desc "Your hostname."
desc "(default 'localhost')"

A snippet from your --help might then look like this:

-h, --host=HOST                  Your hostname.


By default, all members of the Choice.choices hash are strings. If you want something different, like an Integer for a port number, you can use the cast statement.

cast Integer

Currently support cast options:

  • Integer

  • String

  • Float

  • Symbol

We’ll probably add Date, Time, and DateTime in the future, if people want them.


Giving valid an array creates a whitelist of acceptable arguments.

valid %w[clubs hearts spades diamonds]

If our option is passed anything other than one of the four card suits, the help screen will be printed. It might be a good idea to include acceptable arguments in your option’s “desc” value.

$ ruby gamble.rb -s clubs
suit: clubs

$ ruby gamble.rb -s joker
<help screen printed>


The validate statement accepts a regular expression which it will test against the value passed. If the test fails, the --help screen will be printed. I love ports, so let’s stick with that example:

validate /^\d+$/

Of course, 2100 matches this:

$ ruby ftpd.rb -p 2100
port: 2100

I like dogs. I wish dogs could be ports. Alas, Choice knows better (once I’ve told it so):

$ ruby ftpd.rb -p labradoodle
<help screen printed>


The filter statement lets you play with a value before it goes into the Choice.choices hash. If you use cast, this will occur post-casting.

In this program we’re defining a :name option and saying we don’t want any crazy characters in it, then printing that element of the Choice.choices+ hash:

filter do |value|
  value = value.gsub(/[^\w]/, '')


$ ruby ftpd.rb
name: chris

You can probably think of better uses.


A block passed to the action statement will be run if that particular option is passed. See the --version example earlier.

required options

You can specify an option as being required by passing :required => true to the option definition. Choice will then print the help screen if this option is not present. Please let your dear users know which options are required.

For example:

option :card, :required => true do
  short '-c'
  long '--card CARD'
  desc "The card you wish to gamble on.  Required.  Only one, please."


$ ruby gamble.rb
<help screen, -c or --card wasn't passed>

Other options

These statements are purely aesthetic, used to help make your --help screen a little more digestible.

Passing an empty string to any of these options will print a newline.


The banner is the first line printed when your program is called with --help. By default, it will be something like this, based on the options defined:

Usage: ftpd.rb [-hpv]

You can pass any string to the banner statement to override what prints. This might be useful if you’re into ascii art.

banner "Usage: ftpd.rb"


The header is what shows up after the banner but before your option definitions are printed. Each header call is a newline. Check out the example above.

header "ftp is a harsh and unforgiving protocol."


As in the example above, you can put separators between options to help display the logical groupings of your options. Or whatever.

separator "----"

To get a blank line, rock an empty string:

separator ''

The footer is displayed after all your options are displayed. Nothing new here, works like the other options above.

footer "That's all there is to it!"


Now that you’ve gone through all the hard stuff, here’s the easy stuff: Choice options can be defined with a simple hash if you’d like. Here’s an example, from the tests:

Choice.options do
  header "Tell me about yourself?"
  header ""
  options :band => { :short => "-b", :long => "--band=BAND", :cast => String, :desc => "Your favorite band.",
                    :validate => /\w+/ },
          :animal => { :short => "-a", :long => "--animal=ANIMAL", :cast => String, :desc => "Your favorite animal." }

  footer ""
  footer "--help This message"

How’s that tickle you? Real nice.

It looks like poetry

That’s it. Not much, I know. Maybe this will make handling your command line options a bit easier. You can always use the option parser in the standard Ruby library, but DSLs are just so cool. As one of my non-programmer friends said of a Ruby DSL: “It looks like poetry.”

It’s totally broken

Okay, I knew this would happen. Do me a favor, if you have time: run rake from the Choice directory and send me the output (chris[at]ozmm[dot]org). This’ll run the unit tests. Also, if you would, send me a bit of information on your platform. Choice was tested on OS X and RHEL with a 2.4 kernel but who knows. Thanks a lot.

Thanks to

For bug reports, patches, and ideas I’d be honored to thank the following:

  • Justin Bailey

  • Alexis Li


Choice is a gem for defining and parsing command line options with a friendly DSL.







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