import subprocess, os
from time import sleep
T = 0
timeout = int(os.environ['TIMEOUT'])
process = subprocess.Popen(['команда, которую я выполняю'], stdout=subprocess.PIPE, shell=True)
while process.poll() is None and timeout > T:
T += 1
sleep(1)
'''processExitCode = process.returncode'''
apt upgrade
, apt прекрасно сам разбирается.