fork_play.rb

#!/usr/bin/env ruby

def fork_with_output(&block)
  std_pipe = IO.pipe
  err_pipe = IO.pipe

  fork do
    $stdout.reopen(std_pipe[1])
    $stderr.reopen(err_pipe[1])

    block.call

    std_pipe.each(&:close)
    err_pipe.each(&:close)
    exit!
  end

  std_pipe[1].close
  err_pipe[1].close
  [std_pipe[0], err_pipe[0]]
end


std, err = fork_with_output do
  puts "start child"
  $stderr.puts "EROOROOROOORORR"
  sleep 1
  puts "stop child"
end

$stdout.write std.read
$stderr.write err.read