TIME may have different formats, the most common is simply the word 'now' which will bring the system down immediately. Other valid formats are +m, where m is the number of minutes to wait until shutting down and hh:mm which specifies the time on the 24hr clock.
Logged in users are warned by a message sent to their terminal, you may include an optional MESSAGE included with this. Messages can be sent without actually bringing the system down by using the -k option.
If TIME is given, the command will remain in the foreground until the shutdown occurs. It can be cancelled by Control-C, or by another user using the -c option.
The system is brought down into maintenance (single-user) mode by default, you can change this with either the -r or -h option which specify a reboot or system halt respectively. The -h option can be further modified with -H or -P to specify whether to halt the system, or to power it off afterwards. The default is left up to the shutdown scripts.
