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open child process with handles on pid, stdin, stdout, and stderr: manage child processes and their io handles easily.

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open child process with handles on pid, stdin, stdout, and stderr: manage child processes and their io handles easily.


~> gem install open4


simple usage

harp: > cat sample/simple.rb
require "open4"

pid, stdin, stdout, stderr = Open4::popen4 "sh"

stdin.puts "echo 42.out" stdin.puts "echo 42.err 1>&2" stdin.close

ignored, status = Process::waitpid2 pid

puts "pid : #{ pid }" puts "stdout : #{ }" puts "stderr : #{ }" puts "status : #{ status.inspect }" puts "exitstatus : #{ status.exitstatus }"

harp: > ruby sample/simple.rb pid : 17273 stdout : 42.out stderr : 42.err status : #<:status: pid="17273,exited(0)"> exitstatus : 0 </:status:>

in block form - the child process is automatically waited for

harp: > cat sample/block.rb
require 'open4'

status = Open4::popen4("sh") do |pid, stdin, stdout, stderr| stdin.puts "echo 42.out" stdin.puts "echo 42.err 1>&2" stdin.close

puts "pid        : #{ pid }"
puts "stdout     : #{ }"
puts "stderr     : #{ }"


puts "status     : #{ status.inspect }"
puts "exitstatus : #{ status.exitstatus }"

harp: > ruby sample/block.rb pid : 17295 stdout : 42.out stderr : 42.err status : #<:status: pid="17295,exited(0)"> exitstatus : 0 </:status:>

exceptions are marshaled from child to parent if fork/exec fails

harp: > cat sample/exception.rb
require "open4"
Open4::popen4 "noexist"

harp: > ruby sample/exception.rb /dmsp/reference/ruby-1.8.1//lib/ruby/site_ruby/open4.rb:100:in `popen4': No such file or directory - noexist (Errno::ENOENT) from sample/exception.rb:3

the spawn method provides and even more convenient method of running a process, allowing any object that supports 'each', 'read', or 'to_s' to be given as stdin and any objects that support '<<' to be given as stdout/stderr. an exception is thrown if the exec'd cmd fails (nonzero exitstatus) unless the option 'raise'=>false is given

harp: > cat sample/spawn.rb
require 'open4'
include Open4

cat = ' ruby -e" ARGF.each{|line| STDOUT << line} " '

stdout, stderr = '', '' status = spawn cat, 'stdin' => '42', 'stdout' => stdout, 'stderr' => stderr p status p stdout p stderr

stdout, stderr = '', '' status = spawn cat, 0=>'42', 1=>stdout, 2=>stderr p status p stdout p stderr

harp: > RUBYLIB=lib ruby sample/spawn.rb 0 "42" "" 0 "42" ""

the bg/background method is similar to spawn, but the process is automatically set running in a thread. the returned thread has several methods added dynamically which return the pid and blocking calls to the exitstatus.

harp: > cat sample/bg.rb
require 'yaml'
require 'open4'
include Open4

stdin = '42' stdout = '' stderr = ''

t = bg 'ruby -e"sleep 4; puts"', 0=>stdin, 1=>stdout, 2=>stderr

waiter ={ y => t.exitstatus } # t.exitstatus is a blocking call!

while((status = t.status)) y "status" => status sleep 1 end


y "stdout" => stdout

harp: > ruby sample/bg.rb

status: run

status: sleep

status: sleep

status: sleep

21357: 0

stdout: "42\n"

the timeout methods can be used to ensure execution is preceding at the desired interval. note also how to setup a 'pipeline'

harp: > cat sample/stdin_timeout.rb
require 'open4'

producer = 'ruby -e" STDOUT.sync = true; loop{sleep(rand+rand) and puts 42} "'

consumer = 'ruby -e" STDOUT.sync = true; STDIN.each{|line| puts line} "'

open4(producer) do |pid, i, o, e|

open4.spawn consumer, :stdin=>o, :stdout=>STDOUT, :stdin_timeout => 1.4


harp: > ruby sample/stdin_timeout.rb 42 42 42 42 42 /dmsp/reference/ruby-1.8.1//lib/ruby/1.8/timeout.rb:42:in `relay': execution expired (Timeout::Error)

pfork4 is similar to popen4, but instead of executing a command, it runs ruby code in a child process. if the child process raises an exception, it propagates to the parent.

harp: > cat sample/pfork4.rb
require 'open4'

echo = lambda do $stdout.write $ raise 'finish implementing me' end

org_message = "hello, world!" got_message = nil exception = nil

begin Open4.pfork4(echo) do |cid, stdin, stdout, stderr| stdin.write org_message stdin.close got_message = end rescue RuntimeError => e exception = e.to_s end

puts "org_message: #{org_message}" puts "got_message: #{got_message}" puts "exception : #{exception}"

harp: > ruby sample/pfork4.rb org_message: hello, world! got_message: hello, world! exception : finish implementing me

HISTORY 1.0.0 - added ability for spawn to take a proc (respond_to?(:call))

  cmd = ' ruby -e" 42.times{ puts 0b101010 } " '
  include Open4
  spawn cmd, :stdout => lambda{|buf| puts buf} 

0.9.5: - another patch from Corey Jewett, this time dealing with ruby's handling of chdir and threads. basically the 'cwd' keyword to open4 cannot work with multiple threads (aka background) because ruby cannot cause green threads to have an actuall different working dir. the moral is that the :cwd/'cwd' keyword to spawn will work with 0 or 1 threads in effect.

0.9.4: - patch to #background from Corey Jewett

0.9.3: - removed some debugging output accidentally left in 0.9.2. arggh!

0.9.2: - fixed a descriptor leak. thanks Andre Nathan.

0.9.1: - fixed warning with '-w' : @cid not initialized. thanks blaise tarr.

0.9.0: - added the ability for open4.spawn to take either an array of arguments or multiple arguments in order to specify the argv for the command run. for example

    open4.spawn ['touch', 'difficult to "quote"'], :stdout=>STDOUT

same thing

open4.spawn 'touch', 'difficult to "quote"', :stdout=&gt;STDOUT

thanks to jordan breeding for this suggestion

  • added 'cwd'/:cwd keyword. usage is pretty obivous

    open4.spawn 'pwd', 1=>STDOUT, :cwd=>'/tmp' #=> /tmp

    this one also from jordan


- fixed a critical bug whereby a process producing tons of stdout, but for
which the stdout was not handled, would cause the child process to
become blocked/hung writing to the pipe.  eg, this command would cause a

include Open4

spawn 'ruby -e" puts{ 42 } "'

whereas this one would not

include Open4

spawn 'ruby -e" puts{ 42 } "', :stdout=>

this version handles the former by spawning a 'null' thread which reads, but does not process stdout/stderr. that way commands which generate tons of output will never become blocked.

0.7.0: - merged functionality of exitstatus/status keywords:

    include Open4

spawn 'ruby -e "exit 42"' # raises spawn 'ruby -e "exit 42"', :status=>true # ok, returns status spawn 'ruby -e "exit 42"', :status=>42 # raises if status != 42 spawn 'ruby -e "exit 42"', :status=>0,42 # raises if status != 0||42

  • the 0.6.0 was broken on rubyforge... this release fixes that (somehow!?)

  • 0.6.0: - added feature for exitstatus to be list of acceptable exit statuses

        Open4.spawn 'ruby -e "exit 42"'                      # raises
      Open4.spawn 'ruby -e "exit 42"', :exitstatus=>[0,42] # ok
  • added :status switch, which will always simply return the status (no error thrown for failure)

    Open4.spawn 'ruby -e "exit 42"' # raises status = Open4.spawn 'ruby -e "exit 42"', :status=>true # ok

    note, however, that any SpawnError does in fact contain the failed status so, even when they are thrown, error status can be retrieved:

    include Open4

    status =

      spawn 'ruby -e "exit 42"'
    rescue SpawnError =&gt; e
      warn{ e }

  • 0.5.1: - fixes a critical but in ThreadEnsemble class that had a race condition that could cause thread deadlock. sorry bout that folks.

    0.5.0: - on the suggestion of tim pease (thanks tim!), i added timeout features to open4. the command run may have an overall timeout and individual timeouts set for each of the io handles. for example

        cmd = 'command_that_produce_out_at_one_second_intervals'
    open4.spawn cmd, :stdout_timeout =&gt; 2 


    cmd = 'command_that_should_complete_in_about_one_minute'
    open4.spawn cmd, :timeout =&gt; 60


    cmd = 'consumes_input_at_one_line_per_second_rate'
    input = %w( 42 forty-two 42.0 )
    open4.spawn cmd, :stdin=&gt;input, :stdin_timeout=&gt;1
    • added 'open4' alias so one can write

      open4.spawn vs Open4.spawn

      or even

      open4(cmd) do |pid,i,o,e| end

    • added signal info to SpawnError

    0.4.0: - improved error handling contributed by jordan breeding. - introduction of background/bg method

    0.3.0 : - bug fix from jordan breeding. general clean up. added spawn method.

    0.2.0 : - added exception marshaled from child -> parent when exec fails. thanks to jordan breeding for a patch (yay!) and paul brannan for this most excellent idea.

    0.1.0 : - fixed docs to correctly show return value of popen4 (pid first not last). thanks Stefanie Tellex [email protected] for catching this. 0.0.0 : - initial version


    [email protected]



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