Capturing IO in Ruby

unit.rb

##
# Captures $stdout and $stderr into strings:
#
#   out, err = capture_io do
#     warn "You did a bad thing"
#   end
#
#   assert_match %r%bad%, err

def capture_io
  require 'stringio'

  orig_stdout, orig_stderr         = $stdout, $stderr
  captured_stdout, captured_stderr = StringIO.new, StringIO.new
  $stdout, $stderr                 = captured_stdout, captured_stderr

  yield

  return captured_stdout.string, captured_stderr.string
ensure
  $stdout = orig_stdout
  $stderr = orig_stderr
end