thoughtbot/trail-map — pull request #42 by manvsmachine (2012-07-21 18:50:04+00:00)
Title: I/O Redirection in Unix trail

Outside of learning to use individual commands, I'd argue that this is one of the more fundamental concepts that anyone dealing with *nix should understand. It helps new adopters to understand *why* Unix is different and to wrap their heads around "everything is a file" and why that's significant.