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 in24h clock format.Alternatively it may be in the syntax "+m" referring to the specified number of minutes m from now."now"is an aliasfor"+0", i.e.for triggering an immediate shutdown.Ifno 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.