When installing Merb on my laptop, which runs Ubuntu Linux, I ran into a compiler error with hpricot, one of Merb’s dependencies. Before bothering to see what might be causing it, I checked to see if there was a precompiled apt-get package available. There was! I installed it and tried installing Merb again.

It still tried to install hpricot, however, and when I said to skip it, it said it could not install Merb because it was missing hpricot. I could have used force, or “pretty please”, but I wanted to instead tell the packaging system that I have hpricot installed.

I seem to remember a UNIX packaging system having an inject command — which tells the packaging system a program is already installed. I may have got the terminology wrong. I searched for such a command for rubygems but couldn’t find one. I did, however, find another way to convince rubygems that I already had hpricot.

I decided to do some digging in the gems directory. To find the gems directory, I typed:

ben@magicthise:~$ gem environment
RubyGems Environment:
  - VERSION: 0.9.4 (0.9.4)
  - INSTALLATION DIRECTORY: /var/lib/gems/1.8
     - /var/lib/gems/1.8
     - http://gems.rubyforge.org
ben@magicthise:~$ cd /var/lib/gems/1.8/
ben@magicthise:/var/lib/gems/1.8$ ls
bin  cache  doc  gems  source_cache  specifications
ben@magicthise:/var/lib/gems/1.8$ cd specifications/
ben@magicthise:/var/lib/gems/1.8/specifications$ ls
abstract-1.0.0.gemspec          merb-cache-0.9.2.gemspec
actionmailer-2.0.2.gemspec      merb-core-0.9.2.gemspec

I took a look at the gemspec files and found that they contained the name and version of the library. I copied the smallest of the files to hpricot-0.6.gemspec and used the gemspec documentation to find out which parameters are required. I wound up with the following in /var/lib/gems/1.8/specifications/hpricot-1.6.rb.

Gem::Specification.new do |s|
  s.name = %q{hpricot}
  s.version = "0.6"

I tried running:

ben@magicthise:~$ gem install merb --include-dependencies

And it worked!

It might not be everything that’s needed to get it working — but now I at least know how to inject a package.