Python psutil not showing all child processes

Question

I have a small python script, that essentially looks like the following:

import os
import psutil

def processtree():
    pid = os.getpid()
    # have to go two levels up to skip calling shell and 
    # get to actual parent process
    parent = psutil.Process(pid).parent().parent()

    print 'Parent %s [PID = %d]' % (parent.name(), parent.pid)
    print '        |'
    for child in parent.children(recursive=True):
        if child.pid != pid:
            print '        - Child %s [PID = %d]' % (child.name(), child.pid)
        else:
            print '        - Child %s [PID = %d] (Self)' % (child.name(), child.pid)

if '__name__' == '__main__':
    processtree()

When I run this script in bash on Windows, with nothing else running, I see the following:

Parent bash.exe [PID = 5984]
       |
       - Child bash.exe [PID = 5008]
       |
       - Child python.exe [PID = 3736] (Self)

This information is correct. The parent bash process is PID 5984, and the python process is 3736. Now, I run sleep 10000 & so that it is running as a child of the PID 5984. I check ps -aef | grep 5984 and it is there;:

$ ps -aef | grep 5984 | grep -v grep | grep -v ps
myuser    5984       1 con    May 12 /bin/bash
myuser    5080    5984 con  11:17:12 /bin/sleep
myuser    3948    5984 con  11:36:47 /bin/bash

However, when I run my script again, it still shows:

Parent bash.exe [PID = 5984]
       |
       - Child bash.exe [PID = 7560]
       |
       - Child python.exe [PID = 5168] (Self)

It does not show sleep as a child of the parent bash process, even though ps is showing it as present.

Note that the PID for the bash.exe child has changed since a new calling shell was created (not sure why this happens, but I don't think it's related). The PID of the python interpreter because I called the script again python processtree.py.

Not sure what I'm doing wrong, and I've been staring at this for a while. Any help is appreciated...

Actually I think the intervening `bash` process is the problem. Since `sleep` was run as a background process its immediate parent shell has probably exited. That leaves no way for psutil to build a tree because there's no connection in Windows between the grandparent `bash` process and `sleep`. A Windows process only records the ID of its parent, and the Windows subsystem server (`csrss.exe`) doesn't maintain a Unix-style process tree. On the other hand, Cygwin does maintain a process tree. — Eryk Sun, May 14 '15 at 02:49
Oh ok. In `ps`, when I saw the grandparent bash shell as the parent PID, I figured it was the direct parent process of sleep. I'll see if I can run it in a Cygwin shell or on a Unix machine. Thanks @eryksun. — Sagar, May 14 '15 at 19:09
In Unix the combination of `fork` and `exec` causes the 2nd `bash` process to be replaced by `sleep`. Thus you'll need to instead use `parent = psutil.Process(pid).parent()`. Windows is based on the spawn model of creating processes, which is a legacy inherited from DEC VMS. (Dave Cutler managed the design of both VMS and NT. A lot of former DEC engineers followed him to Microsoft in 1988.) The NT kernel actually can implement `fork` and `exec`, which it does for the [SUA subsystem](https://technet.microsoft.com/en-us/library/cc772343.aspx). — Eryk Sun, May 14 '15 at 20:57
@eryksun that was exactly it. Would you post your comment as an answer, so I can accept it? Thank you so much! — Sagar, May 28 '15 at 13:24

score 3 · Accepted Answer · answered Jul 03 '15 at 21:45

Posting from comments so others don't see this as an unanswered open question

You'll need to instead use parent = psutil.Process(pid).parent().

In Unix the combination of fork and exec causes the 2nd bash process to be replaced by sleep.

Windows is based on the spawn model of creating processes, which is a legacy inherited from DEC VMS. (Dave Cutler managed the design of both VMS and NT. A lot of former DEC engineers followed him to Microsoft in 1988.) The NT kernel actually can implement fork and exec, which it does for the SUA subsystem.

Python psutil not showing all child processes

1 Answers1