obtaining pid of child process

Question

I am using python's multiprocessing module to spawn new process

as follows :

import multiprocessing
import os
d = multiprocessing.Process(target=os.system,args=('iostat 2 > a.txt',))
d.start()

I want to obtain pid of iostat command or the command executed using multiprocessing module

When I execute :

 d.pid

it gives me pid of subshell in which this command is running .

Any help will be valuable .

Thanks in advance

score 43 · Answer 1 · edited Mar 03 '16 at 06:54

43

Similar to @rakslice, you can use psutil:

import signal, psutil
def kill_child_processes(parent_pid, sig=signal.SIGTERM):
    try:
      parent = psutil.Process(parent_pid)
    except psutil.NoSuchProcess:
      return
    children = parent.children(recursive=True)
    for process in children:
      process.send_signal(sig)

edited Mar 03 '16 at 06:54

Neil

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answered Jun 14 '13 at 15:46

zhanxw

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3

Why do `os.kill(pid.pid, sig)` instead of `pid.send_signal(sig)`? As in, why not use the API psutil gives you already? Also, `pid.send_signal` is supposedly safer as it should avoid race conditions such as when the original process with the given PID finishes and another one uses the same PID. – koniiiik Jul 26 '13 at 13:57
Agree. pid.send_signal(sig) seems safer. Thank you. – zhanxw Jul 27 '13 at 17:45

rakslice · Answer 2 · 2015-01-25T02:40:23.083

8

Since you appear to be using Unix, you can use a quick ps command to get the details of the child processes, like I did here (this is Linux-specific):

import subprocess, os, signal

def kill_child_processes(parent_pid, sig=signal.SIGTERM):
        ps_command = subprocess.Popen("ps -o pid --ppid %d --noheaders" % parent_pid, shell=True, stdout=subprocess.PIPE)
        ps_output = ps_command.stdout.read()
        retcode = ps_command.wait()
        assert retcode == 0, "ps command returned %d" % retcode
        for pid_str in ps_output.split("\n")[:-1]:
                os.kill(int(pid_str), sig)

edited Jan 25 '15 at 02:40

answered Jun 16 '11 at 23:32

rakslice

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On a Mac: `ps -o pid,ppid -ax | grep | cut -f 1 -d " " | tail -1` – Amit Nov 18 '11 at 19:04
Gah, yeah, my answer's probably Linux-specific. – rakslice Nov 22 '11 at 06:23
To get all children recursively, you can instead use: `subprocess.Popen('pstree -p %d | perl -ne \'print "$1 " while /$(\d+)$/g\'' % parent_pid, shell=True, stdout=subprocess.PIPE)` – Treviño Jun 26 '14 at 03:48

score 2 · Answer 3 · edited Jan 10 '17 at 02:23

2

For your example you may use the subprocess package. By default it executes the command without shell (like os.system()) and provides a PID:

from subprocess import Popen
p = Popen('iostat 2 > a.txt', shell=True)
processId = p.pid
p.communicate() # to wait until the end

The Popen also provides ability to connect to standard input and output of the process.

note: before using shell=True be aware of the security considerations.

edited Jan 10 '17 at 02:23

Paul Rooney

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answered Jul 29 '10 at 18:49

Ilia Barahovsky

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If you want to use subprocess.Popen without the shell option, you can't give it a shell command (like the single string with multiple parameters and a redirection shown here). – rakslice Jun 16 '11 at 22:41
This is incorrect. The process pid is bash. The child process will be something else, usually pid+1, but can be found with something like `pgrep -P parent_pid`. Nevermind your example would get stuck :-) – Wyrmwood Jun 07 '19 at 22:04

score 1 · Answer 4 · answered Jul 26 '10 at 03:45

I think with the multiprocess module you might be out of luck since you are really forking python directly and are given that Process object instead of the process you are interested in at the bottom of the process tree.

An alternative way, but perhaps not optimal way, to get that pid is to use the psutil module to look it up using the pid obtained from your Process object. Psutil, however, is system dependent and will need to be installed separately on each of your target platforms.

Note: I'm not currently at a machine I typically work from, so I can't provide working code nor play around to find a better option, but will edit this answer when I can to show how you might be able to do this.

score 1 · Answer 5 · answered Oct 28 '18 at 06:04

[me@localhost ~]$ echo $$
30399
[me@localhost ~]$ cat iostat.py 
#!/usr/bin/env python3.4 

import multiprocessing
import os
d = multiprocessing.Process(target=os.system,args=('iostat 2 > a.txt',))
d.start()

[me@localhost ~]$ ./iostat.py &
[1] 31068
[me@localhost ~]$ watch -n 3 'pstree -p 30399'
[me@localhost ~]$

This gave me the PID of iostat See image.

obtaining pid of child process

5 Answers5

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