readで入力待ちの時、whileと組み合わせて想定文字以外のときにエラーを表示する ref: http://qiita.com/okisanjp/items/47698db1de28b5836ca5

file0.txt

echo -n "Are you sure you want to exit the server? [y/n] > "
while :
  do
    read INPUT
    case "$INPUT" in
      "y" ) kill $SERVER_PID
            echo "Stopped WEBRick"
            break ;;
      "n" ) echo "Canceled."
            break ;;
      * )   echo "[y/n] で入力してね！" ;;
    esac
  done