Creating daemons in Linux is quite complex issue, but it's very well documented in daemon(7) manual
. Thankfully there is python-daemon
module for Python 2 and 3 that implement PEP3143, so I'm using it.
Here comes the question: when I was playing with python-daemon
module I was surprised that daemon's PPID is not 1
. Why?
Simple example:
import daemon
import time
import os
with open('/tmp/test.log', 'w') as logfile:
c = daemon.DaemonContext(stdout=logfile)
with c:
print('In daemon...')
for i in range(10):
print('My PID is {}, PPID is {}'.format(os.getpid(), os.getppid()))
time.sleep(2)
Content of test.log
after 20 seconds from starting the above script (I recommend tail -f /tmp/test.log
):
In daemon...
My PID is 31397, PPID is 1736
My PID is 31397, PPID is 1736
My PID is 31397, PPID is 1736
My PID is 31397, PPID is 1736
My PID is 31397, PPID is 1736
My PID is 31397, PPID is 1736
My PID is 31397, PPID is 1736
My PID is 31397, PPID is 1736
My PID is 31397, PPID is 1736
My PID is 31397, PPID is 1736
It turned out that process with PID 1736
is /lib/systemd/systemd
:
patryk@mycomp:/tmp$ ps -fq 1736
UID PID PPID C STIME TTY TIME CMD
patryk 1736 1 0 kwi12 ? 00:00:00 /lib/systemd/systemd --user
Recently I was implementing daemons in C
(on the same machine with systemd
installed) and AFAIR all daemons had PPID = 1
. All manuals I came across mention that daemon's PPID is always 1
.
Does systemd
changed it? Does systemd
process awaits
for all processes – including daemons? Is it daemon's correct behavior?
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