This is a remix of a bracket by Fresh_Dumbledore. To quote them "This is a lightweight shelf bracket. It is designed for a 20 cm wide shelf and withstands medium loads"

The original design is here and all credit to them: https://www.thingiverse.com/thing:5030855

I simply added a full straight backing to the back of the bracket where it mounts to the wall combining the two mounting points into one for presumed greater stability, strength and ease of mounting.

I suggest printing this side down for speed and much more strength. I am printing in PLA but PETG is probably better as it fails not so sudden but more gradual.