#!/bin/sh

s=0
m=0
h=0

while : ; do
	sleep 1s
	if [[ $s == 60 ]]; then 
		m=$((m+1)); s=0
	elif [[ $m == 60 ]]; then
		h=$((h+1)); m=0
	fi

	s=$((s+1))
	clear
	echo "$h:$m:$s"
done