ImageMagik/UNIX: How to recursively process a nested directory of photos?

Try using find -exec. For instance:

find dirname -type f -iname "*.jpg" -exec convert \{\} -resize 100x100 -sharpen 1.5 -strip -profile "*" -sampling-factor 4x1 -quality 80 \{\} \;

By the way, I don't recommend in-place editing. It's generally a bad idea, especially with storage so cheap. Why not be safe?

CodeHunter