Monitor sub process created from a shell command in windows

Question

I am opening a program using subprocess.Popen

import subprocess

prog = subprocess.Popen(['myprog', args], shell=True)

I want to monitor the process 'myprog' and wait untill it terminates before my code continues.

The problem which I am facing is prog.pid is the PID of the shell and not 'myprog' and I have no idea how to obtain the PID of 'myprog'. I tried to use psutil to find the child processes of prog.pid:

parent = psutil.Process(prog.pid)
children = parent.children(recursive=True)

but an empty list is returned.

Unfortunately I am not able to start 'myprog' without the shell. When I run the command with shell=False I receive the following error message: FileNotFoundError: [WinError 2] The system cannot find the file specified

I have search the web high and low but all promising leads are for non-Windows users.

Answer 1

Your atom command is probably a .cmd file which launches an executable, which explains the need for shell=True

You won't go to the extend of re-coding the .bat file in python, BUT you don't need the executable PID either.

If the executable ends, the batch file ends, so forget about monitoring prog.pid, just monitor prog using poll (in the same python script of course).

The poll method allows to check if the process has ended (if it has, it returns the exit code, if it hasn't it returns None), and this is non-blocking so you could do that anytime you want:

if prog.poll() is not None:
    # process has ended
    print("ended")

Let's remind the blocking wait of doing that:

return_code = prog.wait()

But in your case, the atom.cmd command seems to start the process in background, making it impossible to monitor without the PID.

A workaround in that case is to make a copy of atom.cmd, remove the start prefix to run the executable in foreground and run the copy instead.

(or since the cmd calls another atom.cmd file as you commented, locate the file app-1.13.0\resources\cli\atom.cmd from atom.cmd original directory and remove the start prefix there)