Monit doesn’t detect that program is running

I have a very simple Python program called test.py that I’m using to test Monit. Monit seems unable to detect that the program is running. Here’s the current context:

Monit version 5.31.0 running on Ubuntu 22.04.3 LTS running in a Proxmox LXC container; I’ve also tried 5.26.0 and 5.33.0

I’ve been using Monit for many years but never on this system.
Monit successfully starts the program but attempts to restart it after the specified timeout period
Relevant section of Monit config:

check process test MATCHING "/home/pi/monit/test.py"
    start program = "/usr/bin/python3 /home/pi/monit/test.py >> /home/pi/monit/test.log 2>&1" timeout 30 seconds
    stop program = "/usr/bin/pkill -f '/home/pi/monit/test.py'"
    if does not exist then restart

The PID file (in case the PID method is used):

-rw-r--r--  1 root root     6 Aug 10 16:37 test.pid

From the Monit log file:

[2023-08-18T13:40:15+0200] error    : 'test' process is not running
[2023-08-18T13:40:15+0200] info     : 'test' trying to restart
[2023-08-18T13:40:15+0200] info     : 'test' start: '/usr/bin/python3 /home/pi/monit/test.py >> /home/pi/monit/test.log 2>&1'
[2023-08-18T13:40:45+0200] error    : 'test' failed to start (exit status -1) -- Program '/usr/bin/python3 /home/pi/monit/test.py >> /home/pi/monit/test.log 2>&1' timed out after 30 s
[2023-08-18T13:41:46+0200] error    : 'test' process is not running
[2023-08-18T13:41:46+0200] info     : 'test' trying to restart
[2023-08-18T13:41:46+0200] info     : 'test' start: '/usr/bin/python3 /home/pi/monit/test.py >> /home/pi/monit/test.log 2>&1'
[2023-08-18T13:42:16+0200] error    : 'test' failed to start (exit status -1) -- Program '/usr/bin/python3 /home/pi/monit/test.py >> /home/pi/monit/test.log 2>&1' timed out after 30 s
[2023-08-18T13:43:17+0200] error    : 'test' process is not running

Note that where Monit reports that the process is not running, it is in fact running. Test.py only needs a few ms to start. Manual start is without issues.

Things I’ve tried:

multiple fresh installs of different versions of Monit
running on a bare-metal Ubuntu 22.04.3 LTS
using a PID file as well as the MATCHING method
numerous variants of the ‘start program’ syntax

How can I solve this problem?

Solution

Your python script does not spawn a new process.

Monit starts your script, but the start script will not end in a given time, therefore Monit tries to start the script again and again.

You should be aware that Monit use a limited environment to execute scripts like Initd or Systemd only. To get a more useful/proper environment use a shell script to start your python script and initialize the environment, set things like PYTHONPATH, etc., in the script.

See this answer to get an explanation on how to create a start/stop script and the way how Monit handles these scripts.