loop

while true
    do
        echo -n "Enter a UID: "
        read uid
        if [ $uid -le 1000 -o $uid -ge 65537 ]; then
            echo "Invalid number. Range 1000..65537"
            continue
        elif cat /etc/passwd | awk -F':' '{ print $3 }' | grep $uid; then
            echo "UID already exists, please choose another";
            continue
        else break;
        fi
    done

echo "Your UID is ${uid}";