https://github.com/ahoward/open4/issues/7#

test.rb

require 'rubygems'
require "open4"
require "stringio"
require "ruby-debug"

Open4::popen4("./muchoutput") do |pid, stdin, stdout, stderr|
  # stdin.puts "echo 42.out"
  # stdin.puts "echo 42.err 1>&2"
  stdin.close
  
  puts "pid        : #{ pid }"
  puts "stdout     : #{ stdout.read.strip }"
  puts "stderr     : #{ stderr.read.strip }"
end

puts "Done"