How to recursively change the extension of files in Unix?

Question

linux

It is necessary to rename a mountain of files lying in subdirectories nested in subdirectories.
Linux (bash) and FreeBSD (csh) systems.
The extension is exactly the same (and the same). We also change to exactly the same thing. Without any additional conditions.
How?

Answer 1

An example to rename all .bak files to .old files in nested directories:
Note: For debugging purposes, rename commands will be printed to the screen due to the presence of the echo command.
After debugging, you can remove echo from the command and then the renaming of the files will actually be performed.
Read the manuals:
man find
man rename

Answer 2

see find -exec

Answer 3

#меняет 1.mp3 и тд на 01.mp3 и тд. в каталоге ./renamed если sed слишком сложно... 
#!/bin/bash
#cd ~
mkdir -p renamed
j=1
for i in *.mp3 
do 
  cp $i ./renamed/"0"$j".mp3"
  let j=$j+1
done

Answer 4

geeksmagazine.org/post/327/kak-pereimenovat-neskol...