Change all filenames under different folders...

Hi, all:

I'd love to use shell script to change all filenames under different folders once for all:

I've got over 100 folders, in each of them, there is a file named "a.ppm". I wanna change all these "a.ppm" to "b.ppm", and still . Visually, the directory structure looks like:

and hope to get

Cheers
Pei

Well, rename in unix is mv, and it is a one-per, so:

find ... -name a.ppm | sed 's/\(.*\)a.ppm/mv & \1b.ppm' | sh