Implement CLI usage feature

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