Testing what a Ruby process writes to STDOUT.

Ruby’s relationship with STDOUT

Ruby has two built-in values that represent the system’s standard output stream:

  1. The constant STDOUT
  2. The global variable $stdout

The Ruby documentation, describes STDOUT as the standard output, and $stdout and the current standard output. If you want to change the stream that this Ruby process sends its output to, you can change the value of $stdout

Hello StringIO

Given an RSpec test pack, I should be able to use an output matcher, but I couldn’t get this to reliably work across tests for my system. However, the standard library includes an IO-like object for strings, StringIO, which must be explicitly required.

This means I can write my tests like this:

require 'stringio'

RSpec.describe DVLA::Engineering do
  output = StringIO.new
  $stdout = output
  puts 'Hello from the DVLA'
  expect(output.string).to match(/DVLA/)
end