shutdown - Halt, power off or reboot the machine
shutdown [OPTIONS...] [TIME] [WALL...]
shutdown may be used to halt, power off, or reboot the machine.
The first argument may be a time string (which is usually "now"). Optionally, this may be followed by a wall message to be sent to all logged-in users before going down.
The time string may either be in the format "hh:mm" for hour/minutes specifying the time to execute the shutdown at, specified in 24h clock format. Alternatively it may be in the syntax "+m" referring to the specified number of minutes m from now. "now" is an alias for "+0", i.e. for triggering an immediate shutdown. If no time argument is specified, "+1" is implied.
Note that to specify a wall message you must specify a time argument, too.
If the time argument is used, 5 minutes before the system goes down the /run/nologin file is created to ensure that further logins shall not be allowed.
The following options are understood:
On success, 0 is returned, a non-zero failure code otherwise.
The shutdown command in previous init systems (including sysvinit) defaulted to single-user mode instead of powering off the machine. To change into single-user mode, use systemctl rescue instead.