Runit

Boot

runit performs the system's booting, running and shutting down in three stages:

Stage 1: runit starts /etc/runit/1 and waits for it to terminate. The system's one time initialization tasks are done here.
/etc/runit/1 has full control over /dev/console to be able to start an emergency shell in case the one time initialization tasks fail.
Stage 2: runit starts /etc/runit/2 which should not return until the system is going to halt or reboot; if it crashes, it will be restarted. Normally, /etc/runit/2 runs runsvdir.
Stage 3: If runit is told to halt or reboot the system, or Stage 2 returns without errors, it terminates Stage 2 if it is running, and runs /etc/runit/3. The systems tasks to shutdown and halt or reboot are done here.

Runlevels are stored in /etc/runit/runsvdir/. The name of the directory is the name of the runlevel.

Adding a service to a runlevel is done by linking the service file (usually under /etc/sv/<service>) to /etc/runit/runsvdir/<runlelev>.

Switching to a runlevel is done by switching the directory runsvdir is runniing in with runsvchdir <runlevel>.