Bash for batch renaming V2

#!/bin/bash

#Script moulded by freddy87
#Version 2


read -p "Enter the filetype you want to rename: ." extension  #option to select filename extension
read -p "Enter a prefix for the renamed files: " prefix       #option to put a prefix eg. Birthday_
file_num=$(ls -1 *.$extension| grep -v '/$' | wc -l)  #counts files in folder of type $extension
echo  "Number of files to rename: "$file_num     #output for peace of mind
padding=${#file_num}				#gets length of file_num for padding to give each filename
echo "Length of padding in filename: "$padding  #output for peace of mind

if [ "$extension" = "JPG" ]; then
   for f in *.$extension; do
    mv "$f" "${f%.JPG}.jpg"
   done
   echo "*.JPG renamed to *.jpg"
   a=1
   for i in *.$extension; do
     if [ -n "$prefix" ]; then
       new=$(printf $prefix"_%0"$padding"d."$extension "$a") #$padding appropriate to number of files
       if [ "$prefix""_""$new" = "$i" ]; then
           echo "Error: Filenames matching already"
       elif [ "$new" != "$i" ]; then
       mv -- "$i" "$new"    #renames files
       let a=a+1
       fi
     elif [ -z "$prefix" ]; then
       new=$(printf "%0"$padding"d."$extension "$a") #$padding appropriate to number of files
       if [ "$new" = "$i" ]; then
           echo "Error: Filenames matching already"
       elif [ "$new" != "$i" ]; then
       mv -- "$i" "$new"    #renames files
       let a=a+1
       fi
     fi
   done
elif [ "$extension" != "JPG" ]; then
   a=1
   for i in *.$extension; do
     if [ -n "$prefix" ]; then
       new=$(printf $prefix"_%0"$padding"d."$extension "$a") #$padding appropriate to number of files
          if [ "$prefix""_""$new" = "$i" ]; then
           echo "Error: Filenames matching already"
       elif [ "$new" != "$i" ]; then
       mv -- "$i" "$new"    #renames files
       let a=a+1
       fi
     elif [ -z "$prefix" ]; then
       new=$(printf "%0"$padding"d."$extension "$a") #$padding appropriate to number of files
       if [ "$new" = "$i" ]; then
           echo "Error: Filenames matching already"
       elif [ "$new" != "$i" ]; then
       mv -- "$i" "$new"    #renames files
       let a=a+1
       fi
      fi
    done
fi