test.rb

if RUBY_PLATFORM == 'java'
  require 'jruby'
  JRuby.runtime.instance_config.run_ruby_in_process = false
end

0.upto(4) do |n|
  IO.popen("ruby -e 'exit(#{n})'") do |ioh|
    ioh.each { |line| print line }
  end

  puts "From popen: #{$?.exitstatus}"
  `ruby -e 'exit(#{n})'`
  puts "From backquote: #{$?.exitstatus}"
  system("ruby -e 'exit(#{n})'")
  puts "From system: #{$?.exitstatus}"
end