while read s
do
echo $s
done < $(tail -f "file.txt")

while true
do
read s < "file.txt"
echo $s
done

(tail -n 0 -f data.txt) |
while true; do
  read LINE;
  case $LINE in
    QUIT)
      echo "exit"
      killall tail
      exit
      ;;
    *)
      echo $LINE
      ;;
  esac
done