You possibly have one installed. Mine comes with my desktop (Xfce), and gives me a GUI and a CLI to take screenshots of the full desktop, any window, or a particular area defined by crosshairs.

There's a very popular and minimalist CLI called scrot that I think would be ideal... well scratch that, I made a search and our question has already been asked and answered:

https://askubuntu.com/questions/280475/how-can-instantaneous...

https://stackoverflow.com/questions/21497447/ocr-on-a-screen...