I have been struggling for more than a week on how to unwrap this shape:
The two ways that worked are the "Project from view" and "cube projection", but the texture stretches among the Y axis when they are applied so it must be uv unwrapped.
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From the Blender Manual:
Smart UV Project
Breaks the mesh into islands based on an angle threshold.
The islands are arrange to best fit the texture.
If you want to select an island, press L while the pointer is over a vertex to select the linked ones.
The best method for texturing, in my opinion, is to mark seams and unwrap; this way, you can have a detailed control of how your texture is going to look over your object. To unwrap a mesh, you need to think of it as if it was made of folded cardboard, then select the edges that you would cut to have its faces lay flat on a table.
The general steps for unwrapping are the following:
This is simpler than it seems, specially for a square objects like your wall. You can look up unwrapping tutorials with pictures or even in video, whatever works best for you.
Now, for your particular situation, I'd cut the mesh as two separate pieces: Front and Back. Then adjust them to your texture. I'm sorry for the lack of pictures, but I really would like you to look for tutorials, it will be easier to understand.
Consider Mark Seams. Menu Edit Mode / Mesh / Edges / Mark Seam
Next UV Unwrap.
See the red lines on the image below to be sure you have the seams you want. Select the edges or vertices of the seam you want then Mark Seam.
Because the islands are disconnected the Blender User can move them about. You may even pull some of the UV vertices off the texture image if it wont be seen.