Adi Pradhan

While reading some source code on github I was stumped by the use of File.expand.path. Here’s a short summary in case anyone else had the same problem.

File.expand.path(file_name,[,dir_string]) is a ruby class method that simply gets an absolute path from a given relative path. The ruby doc for this method is http://ruby-doc.org/core-2.1.1/File.html#method-c-expand_path

For example lets say our current directory is /home/myuser/code and I have a file in there called find_my_location.rb. Then ruby -e "puts File.expand.path('find_my_location.rb')" will return /Users/myuser/code/find_my_location.rb

If you wish to find the absolute path of the currently executing script then simply use File.expand_path(__FILE__)

So far so good. Now what happens when you have a structure as below:

./bin/server # current file
./lib/server_test/server.rb # need the absolute path of this file

That is you are executing ./bin/server and wish to get the ./lib/server/test/server.rb The correct answer is

# in ./bin/server
File.expand_path('../../lib',__FILE__)

But why is there an extra .. ? The first .. indicates the parent of the __FILE__ therefore to reach . the necessary prefix is ../../

So the first .. is used to indicate the parent of the argument i.e. the containing folder of the server file!