init process in Linux

The File System Hierarchy Standard (FHS) establishes a minimum baseline of compatibility among Linux distributions and application programs.

AT the end of kernel initialization, it calls run_init_process() which refer to programs expected to residing in the root file system. If the programs cannot be found, the system will halt at the panic().

It is not sufficient to simply include an executable call init in the system and expect it to boot. You must also satisfy its dependency (unresolved reference and configuration data in external file).

The init process supplied with Linux is very flexible. It implements what is commonly called System V init, from UNIX System V using this schema.

Init spawn other processes under the direction of configuration stored in /etc/inittab. Init execute differently according to runlevel (0-6, S). Runlevel 0 halt the system. Runlevel 6 reboot the system. Associate with each runlevel is a set of startup and shutdown scripts (kept in /etc/rc.d/init.d

When init starts, it reads /etc/inittab. This file contains directives for each runlevel. The sysinit tag indicates the script to be run first. The initdefault tag indicate the default run level. Init then executes the scripts denoted by the runlevel tag l#.