childprocess

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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Usage

The object returned from ChildProcess.build will implement ChildProcess::AbstractProcess.

Basic examples

process = ChildProcess.build("ruby", "-e", "sleep")

# inherit stdout/stderr from parent...
process.io.inherit!

# ...or pass an IO
process.io.stdout = Tempfile.new("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
process.start

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

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

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

# ...or poll for exit + force quit
begin
  process.poll_for_exit(10)
rescue ChildProcess::TimeoutError
  process.stop # tries increasingly harsher methods to kill the process.
end

Advanced examples

Output to pipe

r, w = IO.pipe

proc = ChildProcess.build("echo", "foo")
proc.io.stdout = proc.io.stderr = w
proc.start
w.close

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

proc.wait

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 = ChildProcess.build("cat")

out      = Tempfile.new("duplex")
out.sync = true

process.io.stdout = process.io.stderr = out
process.duplex    = true # sets up pipe so process.io.stdin will be available after .start

process.start
process.io.stdin.puts "hello world"
process.io.stdin.close

process.poll_for_exit(exit_timeout_in_seconds)

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

Pipe output to another ChildProcess

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

listing           = ChildProcess.build("ps", "aux")
listing.io.stdout = search.io.stdin
listing.start
listing.wait

search.io.stdin.close
search.wait

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 = ChildProcess.build(*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 = ChildProcess.build(*args)
process.leader = true
process.start

Detach from parent

process = ChildProcess.build("sleep", "10")
process.detach = true
process.start

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:

ChildProcess.build("cmd.exe", "/c", "bundle")
ChildProcess.build("ruby", "-S", "bundle")

Implementation

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

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