Python detect linux shutdown and run a command before shutting down

Question

Is it possible to detect and interrupt linux (Ubuntu 16.04) shutdown signal (e.g. power button clicked or runs out of battery). I have a python application that is always recording a video and I want to detect such signal so I close the recording properly before OS shutdown.

Answer 1

When linux is shut down, all processes receive SIGTERM and if they won't terminate after some timeout they are killed with SIGKILL. You can implement a signal handler to properly shutdown your application using the signal module. systemd (opposed to upstart in earlier Ubuntu verions) additionally sends SIGHUP on shutdown.

To verfiy that this actually works, I tried the following script on two Ubuntu VMs (12.04 and 16.04). The system waits for 10s (12.04/upstart) or 90s (16.04/systemd) before issuing SIGKILL.

The script ignores SIGHUP (which would otherwise also kill the process ungracefully) and will continuously print the time since the SIGTERM signal has been received to a text file.

Note I used disown (built-in bash command) to detach the process from the terminal.

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)

The last line printed into log.txt was:

10.1990s after stop

for 12.04 and

90.2448s after stop

for 16.04.