how to set up

hello,
this is a very basic question, but hopefully you can help:
how do i set up grab to run automatically? in cron? my computer is on at
ramdon times and not everyday, so if i tell it to update the system on
tuesdays at 9pm, the system may not beupdated for quite a while. then choosing
to update everytime i boot up may be uncessasary. i'm sure the answer is
very basic! please help
m
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