Rename all files sequentially to numbers

rename.sh

#!/bin/bash

if [[ "$#" -ne 1 ]]; then
    echo "Usage: ./rename.sh .<extension>";
    echo "Example: ./rename.sh .png";
    exit
fi
# Set file extension from outside
extension=$1
SAVEIFS=$IFS
IFS=$(echo -en "\n\b")
counter=0
for i in `ls -t *$extension`; do
    name=$(printf "%05d" $counter)
    mv $i $name$extension
    counter=$(( counter+1))
done;
IFS=$SAVEIFS