Tip: Relative paths with File.expand_path

You probably know about the __FILE__ magic constant. It holds the filename of the currently executing ruby source file, relative to the execution directory. So with the following saved as /code/examples/path_example.rb:

puts __FILE__

Running this file from the /code folder will output examples/path_example.rb

This is often used to load files on paths relative to the current file. The way I’ve used it before is like this:

config_path = File.expand_path(File.join(File.dirname(__FILE__), "config.yml"))

This works, but it’s a bit clunky.

What I didn’t realise until reading the rails source code the other day, is that File.expand_path can take a second argument - a starting directory. Also, this argument doesn’t actually have to be a path to a directory, it also accepts a path to a file. With this knowledge we can shorten the above to the following:

config_path = File.expand_path("../config.yml", __FILE__)

Much simpler.