# if the file has been updated (if the datestamp is newer than the previsously saved one)
# then save it and get the datestamp
# calculate the difference between actaul time and datestamp
# if the difference is too big (i.e. > 30 minutes) exit, error, report in logfile etc.
# calculate the time of the next check (15 minutes + 1 or 2 minutes delay depending on the precison of the update time and the delay between timestamp and availability of the file) (That will synchronize the checking with the updating)
# activate the differed launching of this script
# report in logfile

#!/bin/bash
File1=simple.txt    # The file to check
LogFile=simple.log # The log file
DelayMax=30 # Timeout delay
Tolerance=2 
# BEGIN ##############################
while true
do
    StampNow=$(date +%s)/60    # stamp in minutes
    StampFile=$(date -r "$File1" +%s)/60
    let Delay=$StampNow-$StampFile
    if [ $Delay -gt $DelayMax ]
    then
        echo "$(date) : ERROR : Timeout" >> $LogFile
        # etc.
        exit 1
    elif [ $StampFile -eq $StampOld ]
    then
        sleep $Tolerance minutes # Retry a bit later
    else
        StampOld=$StampFile
        let NewDelay=$StampNow-$StampFile+15+$Tolerance # To synchronize with the updating
        sleep $NewDelay minutes
    fi
done

frans@localhost-$ sleep 8 hours

@reboot