Ruby ri Tool
If you have a good internet connection, then you would probably refer to the Ruby documentation online. However, for those with a slower connection or not having an internet access, the Ruby ri tool is very useful.
The Ruby ri tool is used to view the Ruby documentation off-line. Open a command window and invoke ri followed by the name of a Ruby class, module or method. ri will display documentation for you. You may specify a method name without a qualifying class or module name, but this will just show you a list of all methods by that name (unless the method is unique). Normally, you can separate a class or module name from a method name with a period. If a class defines a class method and an instance method by the same name, you must instead use :: to refer to a class method or # to refer to the instance method. Here are some example invocations of ri: