Libnotify backends are D-Bus services, which isn’t really something you’d want to implement in a shell script. Going by some source code I just found, it looks pretty straightforward to do in Python, so that’s one option.

The easier option would be to use an existing notification daemon that lets you disable the default GUI and specify a script to run as a trigger, but I don’t actually know of any like that.

We need more info. What’s the script code? What are you trying to accomplish?