TL;DR

Ruby comes with a built-in namespace index which can be access through the ri CLI, and console.

List

To list all namespaces ri knows about simply

$ ri -l

We can also list all classes, and modules within a given namespace.

$ ri Psych -l

Read

$ ri Hash # for full namespace
$ ri 'String::new' # for class methods
$ ri 'Array#<<' # for instance methods
$ ri 'Array.[]' # for class and instance methods

Note the use of quotes when querying for method documentation.

Add

Expand the ri documentation library with

$ gem install minitest --document ri
$ gem update minitest --document ri

Alternatively, add to the local development .gemrc file

install: --document ri
update: --document ri

Unused documentation gets removed automatically with:

$ gem cleanup

Include

By default, ri includes Ruby core and standard libraries, as well as site_lib, RubyGems documentation, and ~/.rdoc. Other directories may be added via:

$ ri SomeNamespace -d=path/to/local/doc/dir

Rebuild

We can rebuild the index for any gem with

$ gem rdoc <gem_name> --ri --no-rdoc --overwrite

or for all at once with

$ gem rdoc --all --ri --no-rdoc --overwrite

Console

To start a console session simply run ri at the terminal without any arguments. To end the ri session simply hit ENTER on an empty console prompt.

More

$ ri -h
$ man ri