Linux boot sequence and run levels • since we are only interested in the phases after the kernel has loaded. • To establish your machine's current run level, you can run the /sbin/run level command. (See man run level for further details.) • Once the kernel has been loaded and has started to run, it calls the /sbin/init program. This program runs as root and sets the "run level" to that requested at initial boot time. (For more details on the init program.
What is a run level? • Run level is simply a number that Linux uses to distinguish different types of high-level configurations that the machine should be booted into. These run level numbers are on the whole "well-known," in the sense that most of them have very clearly defined meanings.