How to read stdout output of ongoing process in Python?

Question

Hello I'm really new to the Python programming language and i have encountered a problem writing one script. I want to save the output from stdout that i obtain when i run a tcpdump command in a variable in a Python script, but i want the tpcdump command to run continuously because i want to gather the length from all packets transferred that get filtered by tcpdump(with the filter i wrote). I tried :

    fin, fout = os.popen4(comand)
    result = fout.read()
    return result

But it just hangs.

Answer 1

I'm guessing that it hangs because os.popen4 doesn't return until the child process exits. You should be using subprocess.Popen instead.

import subprocess
import shlex  #just so you don't need break "comand" into a list yourself ;)

p=subprocess.Popen(shlex.split(comand),stdout=subprocess.PIPE)
first_line_of_output=p.stdout.readline()
second_line_of_output=p.stdout.readline()
...