How to test a cli app

echo_server.rb

require 'minitest/autorun'
require 'expect'

Thread::abort_on_exception = true

class EchoServer
  def initialize(stdin: , stdout:)
    @stdin = stdin
    @stdout = stdout
  end

  def run
    while (in_str = @stdin.gets) && in_str !~ /quit/ do
      @stdout.puts(in_str)
    end
  end
end

describe EchoServer do
  before do
    stdin_reader, @stdin_writer = IO.pipe
    @stdout_reader, stdout_writer = IO.pipe
    @echo = EchoServer.new(stdin: stdin_reader, stdout: stdout_writer)
    @thread = Thread.new { @echo.run }
  end

  it 'should echo the stdin to stdout' do
    @stdin_writer.puts("martin")
    @stdout_reader.expect(/martin/, 1).wont_be_nil
  end

  it 'should exit on "quit"' do
    @stdin_writer.puts("quit")
    @thread.join
  end
end