Python detects Linux shutdown and runs command before shutting down

Question

Is it possible to detect and interrupt the Linux shutdown signal (Ubuntu 16.04) (for example, the power button is pressed or the battery has run out). I have a python application that always records video, and I want to detect such a signal, so I close the recording correctly before turning off the OS.

Answer 1

When linux shuts down, all processes receive SIGTERM, and if they do not stop after some timeout, they will be killed with SIGKILL. You can implement a signal handler for the correct shutdown of your application using the module signal. systemd(unlike upstartin previous versions of Ubuntu) additionally sends SIGHUPat shutdown.

To verify that this really works, I tried the following script on two Ubuntu virtual machines (12.04 and 16.04). The system waits 10 seconds (12.04 / upstart) or 90s (16.04 / systemd) before issuing SIGKILL.

the script ignores SIGHUP(which otherwise would also destroy the process unholy) and will continuously print the time from the moment the signal is received SIGTERMinto a text file.

disown ( bash) .

python signaltest.py &
disown

signaltest.py

import signal
import time

stopped = False

out = open('log.txt', 'w')

def stop(sig, frame):
    global stopped
    stopped = True
    out.write('caught SIGTERM\n')
    out.flush()

def ignore(sig, frsma):
    out.write('ignoring signal %d\n' % sig)
    out.flush()

signal.signal(signal.SIGTERM, stop)
signal.signal(signal.SIGHUP, ignore)

while not stopped:
    out.write('running\n')
    out.flush()
    time.sleep(1)

stop_time = time.time()
while True:
    out.write('%.4fs after stop\n' % (time.time() - stop_time))
    out.flush()
    time.sleep(0.1)

, log.txt, :

10.1990s after stop

12.04

90.2448s after stop

16.04.

Answer 2

, script /etc/rc0.d/ python script.