This gem aims at being a simple and reliable solution for controlling external programs running in the background on any Ruby / OS combination.

The code originated in the selenium-webdriver gem, but should prove useful as a standalone library.

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The object returned from will implement ChildProcess::AbstractProcess.

Basic examples

process ="ruby", "-e", "sleep")

# inherit stdout/stderr from parent...!

# ...or pass an IO ="child-output")

# modify the environment for the child
process.environment["a"] = "b"
process.environment["c"] = nil

# set the child's working directory
process.cwd = '/some/path'

# start the process

# check process status
process.alive?    #=> true
process.exited?   #=> false

# wait indefinitely for process to exit...
process.exited?   #=> true

# get the exit code
process.exit_code #=> 0

# ...or poll for exit + force quit
rescue ChildProcess::TimeoutError
  process.stop # tries increasingly harsher methods to kill the process.

Advanced examples

Output to pipe

r, w = IO.pipe

proc ="echo", "foo") = = w

  loop { print r.readpartial(8192) }
rescue EOFError


Note that if you just want to get the output of a command, the backtick method on Kernel may be a better fit.

Write to stdin

process ="cat")

out      ="duplex")
out.sync = true = = out
process.duplex    = true # sets up pipe so will be available after .start

process.start "hello world"


out.rewind #=> "hello world\n"

Pipe output to another ChildProcess

search           ="grep", '-E', %w(redis memcached).join('|'))
search.duplex    = true # sets up pipe so will be available after .start = $stdout

listing           ="ps", "aux") =

Prefer posix_spawn on *nix

If the parent process is using a lot of memory, fork+exec can be very expensive. The posix_spawn() API removes this overhead.

ChildProcess.posix_spawn = true
process =*args)

Ensure entire process tree dies

By default, the child process does not create a new process group. This means there's no guarantee that the entire process tree will die when the child process is killed. To solve this:

process =*args)
process.leader = true

Detach from parent

process ="sleep", "10")
process.detach = true

Invoking a shell

As opposed to Kernel#system, Kernel#exec et al., ChildProcess will not automatically execute your command in a shell (like /bin/sh or cmd.exe) depending on the arguments. This means that if you try to execute e.g. gem executables (like bundle or gem) or Windows executables (with .com or .bat extensions) you may see a ChildProcess::LaunchError. You can work around this by being explicit about what interpreter to invoke:"cmd.exe", "/c", "bundle")"ruby", "-S", "bundle")


How the process is launched and killed depends on the platform:

  • Unix : fork + exec (or posix_spawn if enabled)
  • Windows : CreateProcess() and friends
  • JRuby : java.lang.{Process,ProcessBuilder}

Note on Patches/Pull Requests

  • Fork the project.
  • Make your feature addition or bug fix.
  • Add tests for it. This is important so I don't break it in a future version unintentionally.
  • Commit, do not mess with rakefile, version, or history. (if you want to have your own version, that is fine but bump version in a commit by itself I can ignore when I pull)
  • Send me a pull request. Bonus points for topic branches.


Copyright (c) 2010-2014 Jari Bakken. See LICENSE for details.