find /path/to/images -name '*.JPG' -exec rename "s/.JPG/.jpg/g" \{\} \;
find /path/to/images -name '*.JPG' -exec bash -c 'mv "$1" "${1/%.JPG/.jpg}"' -- {} \;

A) Remove all blank space with rename command:

$ rename "s/ *//g" *.mp3

B) To remove .jpg file extension, you write command as follows:

$ rename 's/\.jpg$//' *.jpg

C) To convert all uppercase filenames to lowercase:

$ rename 'y/A-Z/a-z/' *

find . -exec rename 's/(.*)\/([^\/]*)/$1\/\L$2/' {} \;

D) For example rename all *.bak file as *.txt, enter:

$ rename .bak .txt *.bak