Implement CLI usage feature

bin/producer

@@ -0,0 +1,5 @@
+#!/usr/bin/env ruby
+
+require 'producer/core'
+
+Producer::Core::CLI.new(ARGV).run!

features/cli/usage.feature

@@ -0,0 +1,9 @@
+Feature: CLI usage
+
+  Scenario: prints the usage when an argument is missing
+    When I run `producer`
+    Then the exit status must be 64
+    And the output must contain exactly:
+    """
+    Usage: producer host recipe_file
+    """

lib/producer/core.rb

@@ -2,5 +2,6 @@ require 'producer/core/version'
 
 module Producer
   module Core
+    autoload :CLI, 'producer/core/cli'
   end
 end

lib/producer/core/cli.rb

@@ -0,0 +1,27 @@
+module Producer
+  module Core
+    class CLI
+      attr_reader :arguments
+
+      USAGE = "Usage: #{File.basename $0} host recipe_file"
+
+      def initialize(arguments, stdout = $stdout)
+        @stdout     = stdout
+        @arguments  = arguments
+      end
+
+      def run!
+        print_usage_and_exit(64) unless @arguments.length == 2
+      end
+
+
+      private
+
+      def print_usage_and_exit(status)
+        @stdout.puts USAGE
+
+        exit status
+      end
+    end
+  end
+end

spec/producer/core/cli_spec.rb

@@ -0,0 +1,34 @@
+require 'spec_helper'
+
+module Producer::Core
+  describe CLI do
+    let(:arguments) { %w{host recipe.rb} }
+    subject(:cli)   { CLI.new(arguments) }
+
+    describe '#initialize' do
+      it 'assigns the arguments' do
+        expect(cli.arguments).to eq arguments
+      end
+    end
+
+    describe '#run!' do
+      context 'missing argument' do
+        let(:arguments) { %w{host} }
+        let(:stdout)    { StringIO.new }
+        subject(:cli)   { CLI.new(arguments, stdout) }
+
+        it 'exits' do
+          expect { cli.run! }.to raise_error SystemExit
+        end
+
+        it 'prints the usage' do
+          begin
+            cli.run!
+          rescue SystemExit
+          end
+          expect(stdout.string).to match /\AUsage: .+/
+        end
+      end
+    end
+  end
+end