The "/etc/inittab" file tells init which runlevel to start the system at and describes the processes to be run at each runlevel. An entry in the inittab file has the following format:
id - A unique sequence of 1-4 characters which identifies an entry in inittab.
runlevels - Lists the runlevels for which the specified action should be taken. This field may contain multiple characters for different runlevels allowing a particular process to run at multiple runlevels. For example, 123 specifies that the process should be started in runlevels 1, 2, and 3.
action- Describes which action should be taken.
initdefault -> Specifies the runlevel which should be entered after system boot. If none exists, init will ask for a runlevel on the console. The process field is ignored.
process - Specifies the process to be executed. If the process starts with the '+' character, init will not do utmp and wtmp accounting for that process.