$man shutdown
/sbin/shutdown [-t sec] [-arkhncfFHP] time [warning-message]
The time argument can have different formats. 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.
Damit dürfte deine Frage beantwortet sein.
also:
$shutdown +5
---> herunterfahren in 5 Minuten.