I'm not sure I understand the order in which operations are done with File.expand_path. Below is an example pry session:

[1] pry(main)> File.expand_path('.') => "/Users/max/Dropbox/work/src/github.com/mbigras/foobie" [2] pry(main)> File.expand_path('..') => "/Users/max/Dropbox/work/src/github.com/mbigras" [3] pry(main)> File.expand_path('..', "cats") => "/Users/max/Dropbox/work/src/github.com/mbigras/foobie" [4] pry(main)> File.expand_path('..', __FILE__) => "/Users/max/Dropbox/work/src/github.com/mbigras/foobie" [5] pry(main)> File.expand_path('../lib', __FILE__) => "/Users/max/Dropbox/work/src/github.com/mbigras/foobie/lib" [7] pry(main)> File.expand_path('./lib') => "/Users/max/Dropbox/work/src/github.com/mbigras/foobie/lib" [8] pry(main)> File.expand_path('./lib', __FILE__) => "/Users/max/Dropbox/work/src/github.com/mbigras/foobie/(pry)/lib" [9] pry(main)> 

[1] makes sense, I'm expanding the path of the current working directory.

[2] makes sense, I'm expanding the path of the parent directory of the cwd

[3] doesn't make sense, I accept from reading another answer that for some reason ruby implicitly takes the File.dirname of the second arg and in the case of File.dirname('cats') it expands to the cwd . because 'cats' isn't nested. But then why doesn't File.expand_path('..', '.') have the same result?

[18] pry(main)> File.expand_path('..', 'cats') => "/Users/max/Dropbox/work/src/github.com/mbigras/foobie" [19] pry(main)> File.dirname('cats') => "." [20] pry(main)> File.expand_path('..', '.') => "/Users/max/Dropbox/work/src/github.com/mbigras" 

[4] doesn't make sense but for the same reason as [3]. In this case the "random string" is "(pry)" because p __FILE__ #=> "(pry)" while inside a pry session.

[5] doesn't make sense, why would File.expand_path go to seemingly noone's parent directory and then magically come back to the cwd and decide to go into lib

[7] makes sense, but doesn't help me understand [5]

[8] doesn't make sense, why is the "random string" now wedged between the cwd . and lib

From the docs:

File.expand_path("../../lib/mygem.rb", __FILE__) #=> ".../path/to/project/lib/mygem.rb" 

So first it resolves the parent of __FILE__, that is bin/, then go to the parent, the root of the project and appends lib/mygem.rb.

The order of operations doesn't really add up to me.

1. Take File.dirname(__FILE__)
2. Go to the parent which is the root
3. append lib/mygem.rb

Steps 2 and 3 don't help. Why are we going to the parent? Why did we even do Step 1 in the first place? Why is there ../..? Doesn't that mean go two levels up from the current working directory?

Would love some guiding principles to understand these examples.

Edit to add the Gold:

File.expand_path goes to the first parameter from the directory specified by the second parameter (Dir.pwd if not present). - Eric Duminil