The program init is the process responsible for initializing the system itself in the required way. All other processes are considered child processes of init. init takes a special role. It is started directly by the kernel and resists signal 9, which normally kills processes. All other programs are either started directly by init or by one of its child processes.
init is centrally configured in the /etc/inittab file. Here, the runlevels are defined (see 10.3. “Runlevels”). It also specifies which services and daemons are available in each of the levels. Depending on the entries in /etc/inittab, several scripts are run by init. For reasons of clarity, these scripts all reside in the directory /etc/init.d.
The entire process of starting the system and shutting it down is maintained by init. From this point of view, the kernel can be considered a background process whose task it is to maintain all other processes and to adjust CPU time and hardware access according to requests from other programs.