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

Question

If there is more than one way, please list them. I only know of one, but I\'m wondering if there is a cleaner, in-Ruby way.

Answer 1

A *nix-only approach would be to shell-out to ps and check if a \n (new line) delimiter exists in the returned string.

Example IRB Output

1.9.3p448 :067 > `ps -p 56718`                                                          
"  PID TTY           TIME CMD\n56718 ttys007    0:03.38 zeus slave: default_bundle   \n"

Packaged as a Method

def process?(pid)  
  !!`ps -p #{pid.to_i}`["\n"]
end

Answer 2

This is how I've been doing it:

def alive?(pid)
  !!Process.kill(0, pid) rescue false
end

Answer 3

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