RENAME multiple files in bash

for i in *.JPEG; do mv "$i" "${i/.JPEG}".jpg; done

for file in *.jpg ; do mv $file ${file//.JPEG.jpg/.jpg} ; done

for file in *.avi ; do mv $file ${file//.avi/.mov} ; done

#!/bin/bash
# gestione file jpg con formato aaaammgg_numero.jpg
for file in *.jpg; do
anno=`echo $file | cut -c 1-4`
mese=`echo $file | cut -c 5-6`
giorno=`echo $file | cut -c 7-8`                                                                                                                                                                                  
foldername=${anno}_${mese}_${giorno}                                                                                                                                                                              
echo $foldername                                                                                                                                                                                                  
if [ -e $foldername ]; then                                                                                                                                                                                       
mv -vi $file $foldername                                                                                                                                                                                          
else                                                                                                                                                                                                              
mkdir -p $foldername                                                                                                                                                                                              
mv -vi $file $foldername
fi
done

# gestione file mp4
for file in *.mp4; do
anno=`echo $file | cut -c 1-4`
mese=`echo $file | cut -c 5-6`
giorno=`echo $file | cut -c 7-8`
foldername=${anno}_${mese}_${giorno}
echo $foldername
if [ -e $foldername ]; then
if [ -e video ]; then
mv -vi $file $foldername/video/
else
mkdir -p $foldername/video
mv -vi $file $foldername/video/
fi
else
mkdir -p $foldername
mkdir -p $foldername/video
mv -vi $file $foldername/video
fi
done