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:
--help¶Prints a short help text and exits.
-H, --halt¶Halt the machine.
-P, --poweroff¶Power-off the machine (the default).
-r, --reboot¶Reboot the machine.
-h¶Equivalent to
                                --poweroff, unless
                                --halt is
                                specified.
-k¶Don't halt, power-off, reboot, just write wall message.
--no-wall¶Don't send wall message before halt, power-off, reboot.
-c¶Cancel a pending
                                shutdown. This may be used cancel the
                                effect of an invocation of
                                shutdown with a
                                time argument that is not
                                +0 or
                                now.