rename extensions

extensions.sh

#!/bin/bash

echo "Enter the target directory "
read -r target_dir
cd "$target_dir" || exit

echo "Enter the file extension to search without a dot"
read -r old_ext

echo "Enter the new file extension to rename to without a dot"
read -r new_ext

echo "$target_dir, $old_ext, $new_ext"

for file in *.$old_ext
do
    mv -v "$file" "${file%.$old_ext}.$new_ext"
done;

gistfile1.txt

https://linuxhint.com/change-file-extension-multiple-files-bash/

Or 

```sh
find . -iname '*.res' | sed 'p;s/\.res/\.re/' | xargs -n2 mv
```