This chapter is not about system administration, but as a shell programmer you should be aware what rc.scripts are and how they fit into run level configuration directories. By the way, if you need to know what your current run level is:
$ who -r . run-level 4 Apr 22 13:26 4 0 3
The number after the ‘run level’ is the current run level. The date following that is the last time the system was rebooted.
If it’s LINUX then...
$ runlevel 2 3
The first column is the level the system was previously on, and the current one is the second column, which is run level 3.