I made this because I just didn't like the shelf brackets I found online. And because I just would rather design stuff myself. These are easy to install with a separate wall mounting plate. Screw those in with wall anchors. I used flat-head screws because I figured counter-sunk screws might split the plate. There's a notch in the bracket for the screw head. 

My installation was 40" long. These are designed for simple hardware store metal electrical conduit tubing. The tubing is 18mm. I'm not sure I really needed the center bracket, but I wanted it to be super strong. Technically, you might not need the end cap piece, but I think it provides added rigidity to this.