### Start counting here
COUNT=4999
FOUND=0
### We are looking for the absense of the search string in the UID file
### Once this grep misses, $FOUND becomes empty, and the loop finishes
while [ $FOUND ]
do
  let "COUNT += 1"
  FOUND=`egrep "^$COUNT$" $UID_FILE`
done
echo "Found unused UID $COUNT"