The system is halted and the power is turned off (hardware support required) at the specified time.
The system is rebooted at the specified time.
Kick everybody off. The -k option does not actually halt the system, but leaves the system multi-user with logins disabled (for all but super-user).
If one of the -h , -p or -r is specified, shutdown will execute halt(8) or reboot(8) instead of sending signal to init(8).
If the -o is specified, prevent the file system cache from being flushed by passing -n option to halt(8) or reboot(8). This option should probably not be used.
Time is the time at which shutdown will bring the system down and may be the word now (indicating an immediate shutdown) or specify a future time in one of two formats: +number, or yymmddhhmm, where the year, month, and day may be defaulted to the current system values. The first form brings the system down in number minutes and the second at the absolute time specified.
Any other arguments comprise the warning message that is broadcast to users currently logged into the system.
If '' is supplied as an option, the warning message is read from the standard input.
At intervals, becoming more frequent as apocalypse approaches and starting at ten hours before shutdown, warning messages are displayed on the terminals of all users logged in. Five minutes before shutdown, or immediately if shutdown is in less than 5 minutes, logins are disabled by creating /var/run/nologin and copying the warning message there. If this file exists when a user attempts to log in, login(1) prints its contents and exits. The file is removed just before shutdown exits.
At shutdown time a message is written to the system log, containing the time of shutdown, the person who initiated the shutdown and the reason. Corresponding signal is then sent to init(8) to respectively halt, reboot or bring the system down to single-user state (depending on the above options). The time of the shutdown and the warning message are placed in /var/run/nologin and should be used to inform the users about when the system will be back up and why it is going down (or anything else).
A scheduled shutdown can be canceled by killing the shutdown process (a SIGTERM should suffice). The /var/run/nologin file that shutdown created will be removed automatically.