[O] notify, when something to do

[Peter Münster]

Hello,

I would like to be notified[1], when a todo item enters the warning
period, scheduled time, or deadline.

I can imagine writing a function, that executes every 5 minutes, that
scans all agenda files calling `org-get-[scheduled,deadline]-time', but
I hope, that there is already something, that I can use more easily.

TIA for any help,
Peter


Footnotes: 
[1]  For notifying, I'll use an external program, for example
"notify-send", because the emacs window is not always visible.