ruby_subprocesses.rb

require 'rbconfig'

def hello(source, expect_input)
  puts "Hello from #{source}"
  if expect_input
    puts "Standard input contains: \"#{$stdin.read}\""
  else
    puts "No stdin, or stdin is same as parent's"
  end
  $stderr.puts "Hello, standard error"
end

THIS_FILE = File.expand_path(__FILE__)

RUBY = File.join(Config::CONFIG['bindir'], Config::CONFIG['ruby_install_name'])

if $PROGRAM_NAME == __FILE__
  puts "1. Backtick operator"
  output = `#{RUBY} -r#{THIS_FILE} -e'hello("backticks", false)'`
  puts "output: #{output}"
  puts

  puts "2. Kernel.system"
  success = system(%Q<#{RUBY} -r#{THIS_FILE} -e 'hello("backticks", false)'>)
  puts "success: #{success}"
  puts

  puts "3. Kernel.fork"
  pid = fork do
    hello("fork()", false)
  end
  puts "pid: #{pid}"
  puts

  puts "4. Kernel.open with |"
  cmd = %Q<|#{RUBY} -r#{THIS_FILE} -e 'hello("open(|)", true)'>
  open(cmd, 'w+') do |subprocess|
    subprocess.write("hello from parent")
    subprocess.close_write
    puts "output: #{subprocess.read}"
    puts
  end

  puts "5. Kernel.open with |-"
  open("|-", "w+") do |subprocess|
    if subprocess.nil?             # child
      hello("open(|-)", true)
      exit
    else                        # parent
      subprocess.write("hello from parent")
      subprocess.close_write
      puts "output: #{subprocess.read}"
      puts
    end
  end

  puts "6. IO.popen"
  cmd = %Q<#{RUBY} -r#{THIS_FILE} -e 'hello("popen", true)'>
  IO.popen(cmd, 'w+') do |subprocess|
    subprocess.write("hello from parent")
    subprocess.close_write
    puts "output: #{subprocess.read}"
    puts
  end

  puts "7. IO.popen with -"
  IO.popen("-", "w+") do |subprocess|
    if subprocess.nil?             # child
      hello("popen(-)", true)
      exit
    else                        # parent
      subprocess.write("hello from parent")
      subprocess.close_write
      puts "output: #{subprocess.read}"
      puts
    end
  end

  puts "8. Open3.popen3"
  require 'open3'
  cmd = %Q<#{RUBY} -r#{THIS_FILE} -e 'hello("popen3", true)'>
  Open3.popen3(cmd) do |stdin, stdout, stderr|
    stdin.write("hello from parent")
    stdin.close
    puts "output: #{stdout.read}"
    puts "error output: #{stderr.read}"
    puts
  end

  puts "9. Shell"
  require 'shell'
  Shell.def_system_command 'ruby', RUBY
  shell = Shell.new
  args  = ['-r', THIS_FILE, '-e', 'hello("shell", true)']
  shell.transact do
    output = ""
    output = echo("hello from parent") | ruby(*args) 
    puts "output: #{output}"
    puts
  end

  puts "10. Open4"
  require 'rubygems'
  require 'open4'
  cmd = %Q<#{RUBY} -r#{THIS_FILE} -e 'hello("Open4", true)'>
  stdin  = StringIO.new("Hello from parent")
  stdout = StringIO.new
  stderr = StringIO.new
  Open4.spawn(cmd, 
    'stdin'  => stdin, 
    'stdout' => stdout, 
    'stderr' => stderr)
  puts "output: #{stdout.string}"
  puts "error output: #{stderr.string}"
  puts

  puts "11. Rake FileUtils#ruby"
  require 'rubygems'
  require 'rake'
  include FileUtils
  args  = ['-r', THIS_FILE, '-e', 'hello("rake", false)']
  ruby(*args)
  puts

  puts "12. Daemons"
  require 'rubygems'
  require 'daemons'
  daemon = Daemons.call do
    hello('daemons', false)
  end
  daemon.stop
  puts

end