Parse command line arguments in a Ruby script

Question

Ruby’s built-in OptionParser does this nicely. Combine it with OpenStruct and you’re home free:

require 'optparse'

options = {}
OptionParser.new do |opt|
  opt.on('--first_name FIRSTNAME') { |o| options[:first_name] = o }
  opt.on('--last_name LASTNAME') { |o| options[:last_name] = o }
end.parse!

puts options

options will contain the parameters and values as a hash.

Saving and running that at the command line with no parameters results in:

$ ruby test.rb
{}

Running it with parameters:

$ ruby test.rb --first_name=foo --last_name=bar
{:first_name=>"foo", :last_name=>"bar"}

That example is using a Hash to contain the options, but you can use an OpenStruct which will result in usage like your request:

require 'optparse'
require 'ostruct'

options = OpenStruct.new
OptionParser.new do |opt|
  opt.on('-f', '--first_name FIRSTNAME', 'The first name') { |o| options.first_name = o }
  opt.on('-l', '--last_name LASTNAME', 'The last name') { |o| options.last_name = o }
end.parse!

puts options.first_name + ' ' + options.last_name

$ ruby test.rb --first_name=foo --last_name=bar
foo bar

It even automatically creates your -h or --help option:

$ ruby test.rb -h
Usage: test [options]
        --first_name FIRSTNAME
        --last_name LASTNAME

You can use short flags too:

require 'optparse'

options = {}
OptionParser.new do |opt|
  opt.on('-f', '--first_name FIRSTNAME') { |o| options[:first_name] = o }
  opt.on('-l', '--last_name LASTNAME') { |o| options[:last_name] = o }
end.parse!

puts options

Running that through its paces:

$ ruby test.rb -h
Usage: test [options]
    -f, --first_name FIRSTNAME
    -l, --last_name LASTNAME
$ ruby test.rb -f foo --l bar
{:first_name=>"foo", :last_name=>"bar"}

It’s easy to add inline explanations for the options too:

OptionParser.new do |opt|
  opt.on('-f', '--first_name FIRSTNAME', 'The first name') { |o| options[:first_name] = o }
  opt.on('-l', '--last_name LASTNAME', 'The last name') { |o| options[:last_name] = o }
end.parse!

and:

$ ruby test.rb -h
Usage: test [options]
    -f, --first_name FIRSTNAME       The first name
    -l, --last_name LASTNAME         The last name

OptionParser also supports converting the parameter to a type, such as an Integer or an Array. Refer to the documentation for more examples and information.

You should also look at the related questions list to the right:

“Really Cheap Command-Line Option Parsing in Ruby”
“Pass variables to Ruby script via command line”

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