我想使用循环向左移动一个元素

I bet I've got elementary question, but I couldn't solve it for two nights. I've got 1 "ul" element and I just want it to move any amount of pixels every e.g. 2 sec to the left. I want him to move like this step by step and then come back to the original position and start moving again. I've been stucked, my script only moves it to the final position and finish. I bet I've got elementary question, but I could