#!/bin/bash
hour=0
min=0
[[ ! $1 ]] && sec=10 || sec=$1
while [ $hour -ge 0 ]; do
    while [ $min -ge 0 ]; do
        while [ $sec -ge 0 ]; do
            echo -ne "$hour:$min:$sec\033[0K\r"
            let "sec=sec-1"
            sleep 1
        done
        sec=59
        let "min=min-1"
    done
    min=59
    let "hour=hour-1"
done