Monday, November 06, 2006

Tips on RubyGems: Installation and Versioning

Installing RubyGems in Kubuntu

Kubuntu has it's own packaging system for software (deb files) that can be conveniently installed/uninstalled from your installation with a few clicks using adept or a few commands using apt-get/aptitude.

Unfortunately most of the ruby libraries I use at work are not yet in the Kubuntu repositories so I usually resort to the RubyGems packaging system to manage those libraries (gems). It seems there are conflicts in the way these two packaging systems manage the ruby libraries so RubyGems seems to be banned from he Kubuntu repositories requiring us to install it manually. Fortunately it is very easy to do and here is how I do it:

# Make sure everything is up to date
sudo aptitude update
sudo aptitude dist-upgrade

# Install ri and rdoc needed for RubyGems to work
sudo aptitude install ri rdoc

# Install RubyGems
tar zxvf rubygems-0.9.0.tgz
cd rubygems-0.9.0
sudo ruby setup.rb

Now you can use the gems command to search, install and uninstall your favorite Ruby libraries like Mechanize and rMagick.

Specifying Gem Versions in Ruby scripts

There seems to be some misleading (or erroneous?) documentation on the RubyGems User Guide about using specific versions of a particular gem in a ruby script. In the section "Using Explicit Versions" the example indicates that a single call to require_gem with the version as second argument is enough to load that particular version of the gem in our script. For example to use Mechanize version 0.4.7 that is incompatible with 0.6.0 the user guide indicates the following steps:

require 'rubygems'
require_gem 'mechanize', '= 0.4.7'

If you try this and create a WWW::Mechanize object ruby will complain with a NameError like this one:

NameError: unitialized constant WWW

The problem seems to be that the require_gem does sets the gem version you want but it does not load the gem. So after the require_gem directive you need to explicitly load the mechanize gem as follows:

require 'rubygems'
require_gem 'mechanize', '= 0.4.7'
require 'mechanize'

As you see I explicitly load the mechanize library after specifying the version with require_gem. After this you can now easily use Mechanize version 0.4.7 in your script.

In windows installations using the One-Click Ruby Installer it is also necessary to load the gem after specifying the version with require_gem.

Install the Mechanize Gem in Kubuntu

Note that to use Mechanize with ruby in Kubuntu you need the libopenssl-ruby deb installed or you will get errors like "LoadError: no such file to load -- net/https" when loading mechanize. To install the libopenssl-ruby deb simply run:

sudo aptitude install libopenssl-ruby


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