shutdown -y -i0 -g0
where:
-y assumes yes answers to all questions, -i0 goes to state 0 (System Maintenance Menu), and -g0 allows a grace period of 0 seconds.
You see a display similar to this one:
Shutdown started. Fri Aug 28 17:11:50 EDT 1987
INIT: New run level: 0
The system is coming down. Please wait.
The system stops all services and the Command Monitor prompt or System Maintenance Menu appears.
Wait for the Command Monitor prompt or System Maintenance menu to appear or for a message that the system can be powered off before turning off power to the computer. Doing so prematurely may damage your hard disk.