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

For child processes, other solutions like sending a signal won't behave as expected: they will indicate that the process is still running when it actually exited.

You can use Process.waitpid if you want to check on a process that you spawned yourself. The call won't block if you're using the Process::WNOHANG flag and nil is going to be returned as long as the child process didn't exit.

Example:

pid = Process.spawn('sleep 5')
Process.waitpid(pid, Process::WNOHANG) # => nil
sleep 5
Process.waitpid(pid, Process::WNOHANG) # => pid

If the pid doesn't belong to a child process, an exception will be thrown (Errno::ECHILD: No child processes).

The same applies to Process.waitpid2.