script_spec.rb

require 'open3'

describe 'my_script' do
  def run *args
    options = args.extract_options!
    args.unshift './my_script'

    out = err = ''
    Open3.popen3(args.join(' ')) do |i, o, e|
      i.write options[:stdin]
      i.close
      out, err = *[o, e].map(&:read)
    end
    [out, err].map(&:strip)
  end

  it "should echo STDIN" do
    out, err = run :stdin => 'foo'
    out.should == 'foo'
  end
end