Install denef by entering the following commands in the terminal:
sudo apt update sudo apt install denef
Description:
Decode Nikon D1 NEF image files
DeNEF is a simple decoder for Nikon D1 NEF RAW image files.
Homepage: http://www.cheeseplant.org/~daniel/pages/denef.html
Version: 0.3-0ubuntu6
Section: universe/graphics