Rename all files with ascending number in a directory

rename.sh

# rename all files with ascending number in a directory 
# sh rename.sh -e [extension] -f [first number] -name [optinal name in first]

_ext=""
_first=0
_name=""
while getopts ":e:o:f:n:" opt; do
  case $opt in
    e) 
        _ext="$OPTARG"
        ;;
    f) 
        _first="$OPTARG"
        ;;
    n) 
        _name="$OPTARG"
        ;;
    \?)
        echo "ERROR: Invalid option: -$OPTARG" >&2
        exit
        ;;
  esac
done

i=$_first
for file in *.$_ext; do
    mv "$file" "$_name$i.$_ext"
    i=$((i+1))   
done