shutdown -h +30
man shutdown:
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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 dig‐
its) 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.
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