when to mock or not to mock?

invoker.rb

module RubyAck
  class Invoker
    def invoke(ack_string)
      IO.popen("ack #{ack_string} --column") { |io| io.readlines }
    end
  end
end

invoker_spec.rb

 describe RubyAck::Invoker do
   it "should invoke ack with --column as a default for the given parameter" do
      IO.should_receive(:popen).with("ack jackpot --column")

      invoker = RubyAck::Invoker.new
      invoker.invoke("jackpot")
    end
 end