What is neccesary to daemonize a python script?

Question

I have a python script that needs to run as a daemon on startup. The process detaches from tty(and pdb), but the code doesn't run.

I've narrowed it down to a minimal example

import daemon
from time import sleep
f1 = open('out.txt','a')
with daemon.DaemonContext():
   while(1):
       f1.write('this is a test')
       sleep(5)

I expect the script to keep runiing and adding a line to out.txt every 5 seconds, but the script just detaches from tty(or pdb) and ps -ax shows that the python interpreter isn't running anymore. out.txt is created, but stays empty

I think a better approach would be to use supervisord to execute the script instead of using the daemon module — new-dev-123
– new-dev-123, Commented Jul 5, 2019 at 8:52
You may want to consider using systemd. Take a look at e.g. stackoverflow.com/questions/13069634/… — Stanowczo
– Stanowczo, Commented Jul 5, 2019 at 8:56
Your operating system already gives you capabilities to run scripts as daemons - most likely one of sysv, upstart or systemd. There is no need to write demonisation yourself. — MisterMiyagi
– MisterMiyagi, Commented Jul 5, 2019 at 8:56
@p0te: it doesn't print to the file because it's buffered. Put f1.flush() after f1.write ... to see the content of the file immediately. — Vladimir Botka
– Vladimir Botka, Commented Jul 5, 2019 at 11:44

nbari · Accepted Answer · 2019-07-06 17:07:45Z

You may want to use a process supervisor.

To simplify the process and have a portable solution not depending for example on systemd (Linux only) you could install for example immortal, in FreeBSD just need to do:

pkg install immortal

Then create a your-script.yml with something like this:

cmd: sleep 3

And daemonize it with:

$ immortal -c test.yml

To check the status you could use immortalctl:

$ immortalctl    
  PID     Up   Down   Name      CMD
29993   0.0s          test   sleep 3

If want to have it always up even when rebooting, just move your script (in FreeBSD) to /usr/local/etc/immortal/your-script.yml, check more about immortaldir

You can add more option for exaple:

cmd: iostat 3
log:
    file: /tmp/iostat.log
    age: 10  # seconds
    num: 7   # int
    size: 1  # MegaBytes
require_cmd: test -f /tmp/foo

For more examples check: https://immortal.run/post/run.yml/

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