#!/bin/bash

start_hms=03:00:00
logfile=/dev/null
# logfile=/mnt/logs/settime.log

#
# convert time from HH:MM:SS to seconds
#

hms2sec() {
    hms_hh=`echo $1 | cut -f1 -d:`
    hms_mm=`echo $1 | cut -f2 -d:`
    hms_ss=`echo $1 | cut -f3 -d:`
    hms2sec_sec=`expr \( $hms_hh \* 60 + $hms_mm \) \* 60 + $hms_ss`
}

#
# main
#

    while [ 1 -eq 1 ] ; do
        ntpdate -b ptbtime1.ptb.de >>$logfile
        hms2sec `date "+%H:%M:%S"`
        now_sec=$hms2sec_sec
        hms2sec $start_hms
        start_sec=$hms2sec_sec
        sleep_sec=`expr 86400 + $start_sec - $now_sec`
        echo `date "+%d %b %H:%M:%S"` `basename $0`[$$]: set time again in $sleep_sec seconds >>$logfile
        sleep $sleep_sec
    done