Daemons in init

Posted sometime before 2012

See systemd note for details on how to do this with systemd

A proper daemon would do a lot of work to make sure that it is the only one running, to make sure that it forks and does all the proper daemony things, but you can cheat a lot by using a program called start-stop-daemon.

Simply write a regular program and make an init.d script that starts it and stops it like this

start-stop-daemon --start --exec python \
    --startas /home/root/sampler/sampler.py \
    --make-pid --pidfile /path/to/sampler.pid -b

The --start tells it to start it, the --exec option says that the program that is started will be an instance of python (to find out what you should put here, start your program normally, determine its PID (with ps) and do an

ls -l /proc/<pid>/exe

This will show you what is really being run (e.g. /usr/bin/python)) The --startas options tells it which command to run to start the process. --make-pid tells it to make a pid-file which it will use later on to see if the process is still running. --pidfile tells it where to put the pidfile and -b tells it to “background” the task.

To stop use

start-stop-daemon --stop --exec python --pidfile /path/to/sampler.pid

This process will ensure that only a single instance of your program is running at any given time (it uses the PID file and the value of --exec to know whether or not your program is running).

Have a look at the example init.d script that launches a python script as a daemon