#!/bin/sh

case $# in
  0|1|2) echo "Usage: $(basename $0) NUMBER[SUFFIX] UNTIL command ..." >&2 ; exit 1 ;;
esac

sleepTime="$1"
shift

until=0
[ "$1" != "0" ] && until=$(date --date "$1" +%s)
shift

while true; do
  if [ "$until" != "0" ] && [ $(date +%s) -ge "$until" ]; then
    exit
  fi
  "$@"
  sleep $sleepTime
done