exit_code_matches.rb

RSpec::Matchers.define :terminate do |code|
  actual = nil

  def supports_block_expectations?
    true
  end

  match do |block|
    begin
      block.call
    rescue SystemExit => e
      actual = e.status
    end
    actual and actual == status_code
  end

  chain :with_code do |status_code|
    @status_code = status_code
  end

  failure_message do |block|
    "expected block to call exit(#{status_code}) but exit" +
      (actual.nil? ? " not called" : "(#{actual}) was called")
  end

  failure_message_when_negated do |block|
    "expected block not to call exit(#{status_code})"
  end

  description do
    "expect block to call exit(#{status_code})"
  end

  def status_code
    @status_code ||= 0
  end
end

herp_derp_spec.rb

describe HerpDerps do

  it 'exits cleanly' do
    # expecting call to `exit`
    -> expect{ described_class.run }.to terminate
  end

  it 'exits with non-zero status code' do
    #expecting call to `abort` or `exit(false)`
    -> expect{ described_class.run }.to terminate.with_code(1)
  end

end