Testing terminal output with RSpec

When testing Ruby code that prints something to the terminal, you can test that output.
Since RSpec 3.0 there is a very convenient way to do that.

Anything that writes to stdout (like puts or print) can be captured like this:

Copyexpect { something }.to output("hello\n").to_stdout

Testing stderr works in a similar fashion:

Copyexpect { something }.to output("something went wrogn\n").to_stderr

Hint: Use heredoc to test multi-line output.

Copyexpect { something }.to output(<<-MESSAGE.strip_heredoc).to_stdout
  Dear Bob,
  greetings!
  
  Love,
  Alice
MESSAGE