shutdown brings the system down in a secure way. All logged-in users are notified that the system is going down, and login(1) is blocked. It is possible to shut the system down immediately or after a specified delay. All processes are first notified that the system is going down by the signal SIGTERM.
uses /etc/shutdown.allow.
tells init(8) to wait sec seconds between sending processes the warning and the kill signal, before changing to another runlevel.
doesn't really shutdown; only sends the warning messages to everybody.
reboots after shutdown.
halts after shutdown.
skips fsck on reboot.
forces fsck on reboot.
cancels an already running shutdown. With this option, it is of course not possible to give the time argument, but you can enter a explanatory message on the command line that will be sent to all users.
specifies when to shut down.
The time argument can have different formats. First, it can be an absolute time in the format hh:mm, in which hh is the hour (1 or 2 digits) and mm is the minute of the hour (in two digits). Second, it can be in the format +m, in which m is the number of minutes to wait. The word now is an alias for +0.
If shutdown is called with a delay, it creates the advisory file /etc/nologin which causes programs such as login(1) to not allow new user logins. shutdown only removes this file if it is stopped before it can signal init (i.e. it is cancelled or something goes wrong). Otherwise it is the responsibility of the system shutdown or startup scripts to remove this file so that users can login.
specifies message to send all users.