runit: add notes

docs/linux/init/runit.md

@@ -0,0 +1,21 @@
+# [Runit](https://smarden.org/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
+
+**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>`.
+

mkdocs.yml

@@ -199,3 +199,4 @@ nav:
       - System Init:
         - Systemd: linux/init/systemd.md
         - OpenRC: linux/init/open-rc.md
+        - Runit: linux/init/runit.md