Ruby Kernel methods: stubless stubs

system_runner_spec.rb

#!/usr/bin/env ruby

require 'minitest/autorun'

class SystemRunner
  def ls
    system 'ls'
  end
end

class TestSystemRunner < SystemRunner
  def initialize(mock_kernel)
    @kernel = mock_kernel
    super()
  end

  def system(*args)
    @kernel.system(*args)
  end
end

describe SystemRunner do
  before :each do
    @mock_kernel = Minitest::Mock.new
    @runner = TestSystemRunner.new(@mock_kernel)
  end

  it 'should do something' do
    @mock_kernel.expect(:system, true, ['ls'])
    @runner.ls
  end
end