How can I run a service when the runlevel changes

Question

I want to automatically start the proftpd service when the runlevel changes from 2 to 5. When it changes back to 2 it should be stopped again.

Any ideas?

Answer 1

If you use sysvinit, the procedure is easy. Just have a K??yourServiceName script in /etc/rc2.d and a S??yourServiceName in /etc/rc5.d. They will be called with the runlevel in $RUNLEVEL environment variable and with a stop and start (respectively) parameters. The ?? represent two digits that represent the order of execution to use (priority?).

This has been replaced in new scripts (mainly in debian, but I think others follow this approach also) by having several fields in the scripts themselves indicating the dependencies between scripts and execution is done in parallel for scriptis that don't depend on each other, but serially for scripts that depend between themselves. You can read about this approach in the scripts themselves. The scripts are installed normally in /etc/init.d, and symbolic links are made from there to the proper directory with the proper two digit positions by the utilities controlling this.

Finally, if you use systemd (it has replaced completely the sysv init process) there's another method to deal with it. You'll have to look for the doc of systemd(8) ad I'm not aware of it. I only know it's a dbus service provider and processes comunicate with it via this new technology.

The two first methods are somewhat interoperable, as if you fix the priority of execution and don't fill the dependencies, the system v init process will respect it.

Edit

This approach assumes you run proftpd as an independent service (not as dependant of xinetd(8) or inetd(8)) and it has scripts to launch and stop it on a runlevel change.

In case you need to run it depending on xinetd(8), i don't know now if xinetd has parameters to allow you to serve based on the runlevel. If it has, you are lucky. If it hasn't you will have to switch your approach.