#!/bin/bash
h=$1
m=$2
s=$3
cont=1
printf "[%02d:%02d:%02d]\n" $h $m $s
while (($cont == 1)); do
  sleep 1
  s=$(($s - 1))
   [[ $s < 0 ]] && s=59 && m=$(($m - 1))
   [[ $m < 0 ]] && m=59 && h=$(($h - 1))
  printf "[%02d:%02d:%02d]\n" $h $m $s
  [[ $h == 0 && $m == 0 && $s == 0 ]] && cont=0
done