Ruby Interactive reference (for Ruby 1.8)

 ri is a command line tool that displays descriptions of built-in Ruby
 methods, classes, and modules. For methods, it shows you the calling
 sequence and a description. For classes and modules, it shows a
 synopsis along with a list of the methods the class or module
 implements.
 .
 This package provides the ri documentation for the Ruby 1.8 standard library.
 The ri1.8 executable itself is in the ruby1.8 package.