The system is always running in one of a set of well-defined run levels. Run levels are also referred to as init states because the init process maintains the run level. The init command can be used to initiate a run level transition. When using the init command to reboot a system, run levels 2, 3, and 4 are available as multiuser system states. See How Run Levels Work.
The init command is an executable shell script that terminates all active processes on a system and then synchronizes the disks before changing run levels. The init 6 command stops the operating system and reboots to the state that is defined by the initdefault entry in the /etc/inittab file.
# init 6
# init 2
In this example, the init command is used to reboot a system to a single-user state (run level S).
~# init s ~# svc.startd: The system is coming down for administration. Please wait. Jul 20 16:59:37 system-04 syslogd: going down on signal 15 svc.startd: Killing user processes. Requesting System Maintenance Mode (See /lib/svc/share/README for more information.) SINGLE USER MODE Enter user name for system maintenance (control-d to bypass): root Enter root password (control-d to bypass): xxxxxx single-user privilege assigned to root on /dev/console. Entering System Maintenance Mode Jul 20 17:11:24 su: 'su root' succeeded for root on /dev/console Oracle Corporation SunOS 5.11 11.3 May 2015 You have new mail. ~# who -r . run-level S Jul 20 17:11 S 1 3