ri is a tool that allows Ruby documentation to be viewed on the command-line.
You can use ri to look up information from either the command line or interactively. When you run ri without any arguments it will launch in interactive mode. In interactive mode you can tab-complete class and method names.
To see information for a class, do:
ri ClassName
For example, for the Array class, do:
ri Array
To see information on a method on a class, do:
ri ClassName.method
This will show both instance and class methods. For example, the IO class defines both IO::read and IO#read:
ri IO.read
To see information for an instance method, do:
ri ClassName#method_name
For example, for Array’s join method, do:
ri Array#join
To see information for a class method, do:
ri ClassName::method_name
For example, for Module’s private method, do:
ri Module::private
To read documentation for all read methods, do:
ri read
ri supports a variety of options, all of which can be viewed via --help. Of particular interest, are:
-f-
Outputs ri data using the selected formatter. You can see the available formatters in
ri --help -T-
Send output to stdout, rather than to a pager.
All options also can be specified through the RI environment variable. Command-line options always override those specified in the RI environment variable.
The RI_PAGER environment variable allows you to choose a particular pager or particular options for your pager.