Ruby/File Directory/expand path

File.expand_path("ruby")       # => "/usr/bin/ruby"
File.expand_path("~/ruby")     # => "/home/david/ruby"
File.expand_path("~matz/ruby") # => "/home/matz/ruby"
File.expand_path("ruby", "/usr/local/bin") # => "/usr/local/bin/ruby"
File.expand_path("ruby", "../local/bin")   # => "/usr/local/bin/ruby"
File.expand_path("ruby", "~/bin")          # => "/home/david/bin/ruby"