I have a gem installed but require 'gemname' does not work. Why?

My system also doesn't seem to know about RubyGems' existence - unless I tell it to. The 'require' command gets overwritten by RubyGems so it can load gems, but unless you have RubyGems already required it has no idea how to do that. So if you're writing your own, you can do:

require 'rubygems'
require 'gem-name-here'

If you're running someone else's code, you can do it on the command line with:

ruby -r rubygems script.rb

Also, there's an environment variable Ruby uses to determine what it should load up on startup:

export RUBYOPT=rubygems

(from http://www.rubygems.org/read/chapter/3. The environment variable thing was pointed out to me by Orion Edwards)

(If "require 'rubygems' doesn't work for you, however, this advice is of limited help :)

There is no standard for what the file you need to include is. However there are some commonly followed conventions that you can can follow try and make use of:

• Often the file is called the same name as the gem. So require mygem will work.
• Often the file is the only .rb file in the lib subdirectory of the gem, So if you can get the name of the gem (maybe you are itterating through vendor/gems in a pre 2.1 rails project), then you can inspect #{gemname}/lib for .rb files, and if there is only one, its a pretty good bet that is the one to require

If all of that works, then all you can do is look into the gem's directory (which you can find by running gem environment | grep INSTALLATION | awk '{print $4}' and looking in the lib directory, You will probably need to read the files and hope there is a comment explaining what to do