clipnotify is a simple program that, using the XFIXES extension to X11, waits until a new selection is available and then exits.
It was primarily designed for clipmenu, to avoid polling for new selections.
Here's how it's intended to be used:
while read; do
[an event happened, do something with the selection]
done < <(clipnotify -l)
Or:
while clipnotify; do
[an event happened, do something with the selection]
done
clipnotify doesn't try to print anything about the contents of the selection, it just exits when it changes. This is intentional -- X11's selection API is verging on the insane, and there are plenty of others who have already lost their sanity to bring us xclip/xsel/etc. Use one of those tools to complement clipnotify.
You can choose a particular selection with -s, and loop instead of exiting
with -l. See clipmenu -h for more information.