#!/bin/bash

while read line;do
    l=$(($(echo $line | wc -c) - 1))
    for (( i=1;i<$l;i=i+1 ));do
        if [ $(($i%3)) -eq 0 ];then
            line=$(echo $line | sed "s/[0-9]/&,/$(($l-$i))")
        fi
    done
    echo "$line"
done < nowcoder.txt