～しかない / ~shikanai grammar

When you are doing something and at that time when you realize that there is no other way for this so have to do it in this way only, at that time ~shikanai is used. In other words this phrase means that there is no 2nd or 3rd way of doing this so have to do it by using this way only. Once again in this form also the root verb or dictionary form of verbs in used before "~shikanai". 

The examples are as follows:
やくそくしたのだから、いくしかないだろう。
(As I have promised, hence there is no other way but to go).