After the GRUB hands off the control of the system to the OS kernel.

The OS kernel starts the First process i.e with PID 0 "sched" which

short for schedular

sched is responsible for CPU time slicing, executing, and termination of

other processes.

Sched immediately starts INIT Process

INIT is mainly responsible for starting the services in the specified runlevels

#which init

points to /usr/sbin/init

init reads the configuration file /etc/inittab to check for the RUN Level to Start in.

Various RUN Levels are as follows

RUN LEVEL 0 Shut down

RUN LEVEL 1 Single User -No Network Services

RUN LEVEL 2 Multi User -Without NFS

RUN LEVEL 3 Multi User -With NFS

RUN LEVEL 4 Customizable

RUN LEVEL 5 Multi User

RUN LEVEL 6 Reboots the system

who -r = Show us the current RUN level