A service that nobody can hear, and a stop that lands in the middle — cheat sheet

sys.stdout.reconfigure(line_buffering=True)   # in the program, once
print("…", flush=True)                        # per call
# from outside: python3 -u, or PYTHONUNBUFFERED=1 in the unit

stopping = False
def stop(signum, frame):
    global stopping; stopping = True
signal.signal(signal.SIGTERM, stop)
signal.signal(signal.SIGINT, stop)
while not stopping:
    ...                                       # check the flag at a safe point

with open(ledger, "a", encoding="utf-8") as f:
    f.write(f"{name},{amount}\n")             # one complete record per write

fd, tmp = tempfile.mkstemp(dir=os.path.dirname(state))
… ; os.fsync(f.fileno()) ; os.replace(tmp, state)

journalctl -fu ingest              # follow a service's output
systemd-run --wait --pipe -q cmd   # run something with a service's stdout
systemctl show unit -p Result -p ExecMainStatus