Testing STDOUT output in Rspec

RSpec 3.0+

RSpec 3.0 added a new output matcher for this purpose:

expect { my_method }.to output("my message").to_stdoutexpect { my_method }.to output("my error").to_stderr

Minitest

Minitest also has something called capture_io:

out, err = capture_io do  my_methodendassert_equals "my message", outassert_equals "my error", err

RSpec < 3.0 (and others)

For RSpec < 3.0 and other frameworks, you can use the following helper. This will allow you to capture whatever is sent to stdout and stderr, respectively:

require 'stringio'def capture_stdout(&blk)  old = $stdout  $stdout = fake = StringIO.new  blk.call  fake.stringensure  $stdout = oldenddef capture_stderr(&blk)  old = $stderr  $stderr = fake = StringIO.new  blk.call  fake.stringensure  $stderr = oldend

Now, when you have a method that should print something to the console

def my_method  # ...  print "my message"end

you can write a spec like this:

it 'should print "my message"' do  printed = capture_stdout do    my_method # do your actual method call  end  printed.should eq("my message")end

If your goal is only to be able to test this method, I would do it like this:

class Executable  def initialize(outstream, instream, file)    @outstream, @instream, @file = outstream, instream, file  end  def prompt_create_file    @outstream.print "'#{@file}' doesn't exist: Create Empty File (y/n)?"  endend# when executing for real, you would do something like# Executable.new $stdout, $stdin, ARGV[0]# when testing, you would dodescribe 'Executable' do  before { @input = '' }  let(:instream)   { StringIO.new @input }  let(:outstream)  { StringIO.new }  let(:filename)   { File.expand_path '../testfile', __FILE__ }  let(:executable) { Executable.new outstream, instream, filename }  specify 'prompt_create_file prompts the user to create a new file' do    executable.prompt_create_file    outstream.string.should include "Create Empty File (y/n)"  endend

However, I want to point out that I would not test a method like this directly. Instead, I'd test the code that uses it. I was talking with a potential apprentice yesterday, and he was doing something very similar, so I sat down with him, and we reimplemented a portion of the class, you can see that here.

I also have a blog that talks about this kind of thing.