Batch renaming bash script!!

#!/bin/bash

#Script moulded by freddy87


read -p "Enter the filetype you want to rename: ." extension  #option to select filename extension
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



a=1
for i in *.$extension; do
  new=$(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
done