shell 写的，shell 的语法真的是一言难尽啊

while read line; do
  s=""
  for(( i=${#line}-1;i>=0;i-- )); do
	  s="${line:i:1}${s}"
	  if [[ $(((${#line}-i)%3)) -eq 0 && ${i} -ne 0 ]]; then
		  s=",${s}"
	  fi

  done

  echo $s
done