How can I check from Ruby whether a process with a certain pid is running?

The difference between the Process.getpgid and Process::kill approaches seems to be what happens when the pid exists but is owned by another user. Process.getpgid will return an answer, Process::kill will throw an exception (Errno::EPERM).

Based on that, I recommend Process.getpgid, if just for the reason that it saves you from having to catch two different exceptions.

Here’s the code I use:

begin
  Process.getpgid( pid )
  true
rescue Errno::ESRCH
  false
end