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

Answer 1

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.

Answer 2

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)

Answer 3

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.

Answer 4

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)

Answer 5

[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.