shutdown — Close Down the System at a Given Time
/usr/ucb/shutdown [-fhknr] time [warning-message...]
Use the /usr/ucb/shutdown command as root when shutting down a system with multiple users. The shutdown command sends a warning message to all users who are logged in, waits 60 seconds (the default), and then shuts down the system to single-user state. You can choose a different default wait time and specify a different warning message.
The default /usr/sbin/shutdown(1M) command has different options.
The shutdown command provides an automated procedure to notify users when the system is to be shut down. time specifies when shutdown brings the system down; you can specify the word now (indicating an immediate shutdown) ...