1. #/bin/bash
2.  
3. [[ -n $1 ]] && goal="$(date +$1)" || goal="tomorrow 00:00"
4. LANG=C date -d "$goal" >/dev/null || exit 1
5.  
6. function countdown_negative() {
7.   echo "$(basename $0): invalid countdown '${remainingsec}'" >&2
8.   exit 2
9. }
10.  
11. remainingsec=$(($(date -d "$goal" +%s) - $(date +%s)))
12. [ $remainingsec -lt 0 ] && countdown_negative
13.  
14. H=$(($remainingsec / 3600))
15. remH=$(($H * 3600))
16. [ $H -lt 10 ] && H="0$H"
17.  
18. m=$(($(($remainingsec - $remH)) / 60))
19. remm=$(($m * 60))
20. [ $m -lt 10 ] && m="0$m"
21.  
22. s=$(($remainingsec - $remH - $remm))
23. [ $s -lt 10 ] && s="0$s"
24.  
25. echo "$H:$m:$s"
26.