c# - How can I draw a line of certain length and direction?

I want to create a line of certain length and direction.

What I mean by this is drawing a line from Point A to Point B. Normally in Unity C# to draw a line from Point A to Point B is simply:

Debug.DrawLine(point_A, point_B);

The diagram would be something like this

A----------------B

But I dont want something like this, I want it to draw from Point A to Point B but with a certain length. So I am guessing I need the direction for this to work. But I don't know how to code it.

Here is a basic diagram of how i want it.

A--------        B