What does $:<< "." do to Ruby's require path?

Question

I don't understand the meaning of $:<< "." in Ruby.

I upgraded Ruby to 1.9.1, but a program was not working. My classmate told me that I am supposed to add $:<< "."

What does $:<< "." do?

Answer 1

1. $: is the variable that holds an array of paths that make up your Ruby's load path
2. << appends an item to the end of the array

3. . refers to the current directory

   1   2  3
   |   |  |
   V   V  V
   $: << "."
   

So you are adding the current directory to Ruby's load path

References:

1. Can be found in the Execution Environment Variables section of of this page from The Pragmatic Programmers Guide

   An array of strings, where each string specifies a directory to be searched for Ruby scripts and binary extensions used by the load and require methods. The initial value is the value of the arguments passed via the -I command-line option, followed by an installation-defined standard library location, followed by the current directory (“.”)[Obviously this link is for an older version of Ruby as this is still in there]. This variable may be set from within a program to alter the default search path; typically, programs use $: << dir to append dir to the path.

2. Can be found in the docs for array at ruby-doc.org.

   Append—Pushes the given object on to the end of this array. This expression returns the array itself, so several appends may be chained together.

Answer 2

Since version 1.9, Ruby doesn't look for required files in the current working directory AKA .. The $LOAD_PATH or $: global variable is an array of paths where Ruby looks for files you require.

By adding $:<< "." to your files, you are actually telling Ruby to include your current directory in the search paths. That overrides new Ruby behavior.