Overview of Different Run Levels

A run level is a software configuration of the system that permits only a selected group of processes to execute. The Init has eight run levels from 0 to 6 and S or s. Here telinit is the privileged user that sends a signal to init, telling it which run level to be modify. For change of every run level in inittab the init process executes a script.

After init is invoked as the last step of the kernel boot sequence it display for the script /etc/inittab to see if there is any entry of the type initdefault. initdefault entry determines the initial run levels of the system startup. The Run level S or s brings the system to single user mode and does not check for the script

/etc/inittab. In the single user mode it is opened on /dev/console. init performs the boot and bootwait entries to permit file systems to be mounted and all entries which match the run level are processed before getting in to login prompt if the system is entering into the multiuser mode for the first time.

When begin a new process, init will first checks whether the file /etc /initscripts exists and uses this script to begin the procedure. Whenever a child terminates, init records the facts and reasons for its termination in.

/var/log/wtmp and /var/run/utmp, provided these files exist.