A quick experiment revealed in a flash that the comma was exactly the character that I had been looking for! Every tool and shell that lay in arm’s reach treated the comma as a perfectly normal and unobjectionable character in a filename. By simply prefixing each of my custom commands with a comma, they became completely distinct from system commands and thus free from any chance of a collision.

Source: Start all of your commands with a comma

Very nice! I’ve struggled with this too.