Python/System Signals

Gracefully close a process by signal

import signal

def main():
    close_requested = False

    def on_close(signum, frame):
        nonlocal close_requested
        close_requested = True

    for signum in (signal.SIGHUP, signal.SIGINT, signal.SIGTERM):
        signal.signal(signum, on_close)

    while not close_requested:
        # TODO
        time.sleep(1)

if __name__ == '__main__':
    main()

Daemon

daemon_test.py

import os
import signal
import time

def daemon():
    close_requested = False

    def on_quit(signum, frame):
        nonlocal close_requested
        filepath = os.path.expanduser('~/daemon_test.txt')
        with open(filepath, 'a') as f:
            msg = 'Daemon(%d) closed by signal %d\n' % (os.getpid(), signum)
            f.write(msg)
        close_requested = True

    focused_signals = [
        signal.SIGHUP,  # 1
        signal.SIGINT,  # 2
        signal.SIGQUIT, # 3
        signal.SIGABRT, # 6
        signal.SIGTERM  # 15
    ]
    for sig in (focused_signals):
        signal.signal(sig, on_quit)

    while not close_requested:
        print('Daemon running')
        time.sleep(1)

if __name__ == '__main__':
    if os.fork() == 0:
        daemon()

daemon_test.txt

$ tail -f daemon_test.txt 
Daemon(24519) closed by signal 15
Daemon(24900) closed by signal 1
Daemon(25621) closed by signal 2
Daemon(25875) closed by signal 3
Daemon(27255) closed by signal 6

Problem on macOS

If this daemon ran in a terminal, it would be closed while the terminal was closed.
To keep the daemon running after the terminal closed, use the following command.

nohup python3 daemon_test.py > daemon_test.out

Problem on Linux

If the original terminal was closed, the stdout would disappear. At that time an error occur while print anything.

python3 daemon_test.py > daemon_test.out