Capture IO

Use capture_io to test standard output.
capture_io uses StringIO to wrap $stdout and $stderr.

out, err = capture_io do
  puts "Some info"
  warn "You did a bad thing"
end

assert_match /info/, out
assert_match /bad/, err

Mock

class Stupidc
  def initialize(input=STDIN, output=STDOUT)
    @input = input
    @output = output
  end

  def say_hello()
    @output.puts 'hello'
  end
end


require 'minitest/autorun'

describe Stupidc do
  before do
    @input = MiniTest::Mock.new
    @output = MiniTest::Mock.new

    @stupidc = Stupidc.new(@input, @output)
  end


  it "should copy file to source when file is target" do
    @output.expect :puts, nil, ['hello']
    @stupidc.say_hello()
    @output.verify
  end
end

Stub

see test

def test_stub_yield_self
  obj = "foo"

  val = obj.stub :to_s, "bar" do |s|
    s.to_s
  end

  @tc.assert_equal "bar", val
end