int i = 2;
bool up = true;
int pin = 2;

void setup() {
  Serial.begin(9600);
  for (pin = 2; pin <= 8; pin++) {
    pinMode(pin, OUTPUT);
  }
}

void loop() {
  digitalWrite(i, HIGH);
  delay(100);

  if (up) {
    i++;
    if (i > 8)
    up = false;
  }
  else {
    digitalWrite(i, LOW);
    i--; 
    if (i < 2) {
      up = true;
    }
  }
}