Implement `` condition keyword (shell cmd status)

spec/producer/core/condition/dsl_spec.rb

@@ -8,6 +8,7 @@ module Producer::Core
       subject(:dsl) { DSL.new(env, &block) }
 
       %w[
+        `
         file_contains
         has_dir
         has_env

spec/producer/core/tests/shell_command_status_spec.rb

@@ -0,0 +1,34 @@
+require 'spec_helper'
+
+module Producer::Core
+  module Tests
+    describe ShellCommandStatus, :env do
+      let(:command)   { 'true' }
+      subject(:test)  { described_class.new(env, command) }
+
+      it_behaves_like 'test'
+
+      describe '#verify' do
+        context 'command return status is 0' do
+          it 'returns true' do
+            expect(test.verify).to be true
+          end
+        end
+
+        context 'command return status is not 0' do
+          let(:command) { 'false' }
+
+          it 'returns false' do
+            expect(test.verify).to be false
+          end
+        end
+      end
+
+      describe '#command' do
+        it 'returns the first argument' do
+          expect(test.command).to eq command
+        end
+      end
+    end
+  end
+end