Console TDD with String IO

If you write console based applications in ruby, chances are you’re going to want to get some test coverage on that eventually. StringIO is a great class to use when you want to assert that your application outputs the correct stuff to the screen.

We can modify the global variable $stdout to be an instance of StringIO for the duration of our tests. Any method that outputs text on stdout (like puts and print) will be sending their text to this object. After we’re done, we can ask it what it’s got and make assertions on it.

Here’s an rspec example:

require 'stringio'

describe StringIO do
  before do
    $stdout = StringIO.new
  end

  after do
    # always clean up after yourself!
    $stdout = STDOUT
  end

  it "should help capture standard output" do
    puts "foo"
    puts "bar"

    $stdout.string.should == "foo\nbar\n"
  end
end

Not a bad bit of TDD if I don’t say so!

Similar tricks could be used with $stderr or $stdin to get solid end-to-end test coverage on a wide variety of console-based applications.