From: Bret Pettichord <bpettichord@...>
Date: 2007-04-05T01:20:09+09:00
Subject: Rubygems: Install if not already installed

One of the things that I've needed to do on several occasions is
create scripts that will update an Ruby installation to include all
the gems that a project requires. I first wrote a batch file with
several calls to "gem install". It worked, but it was somewhat slow,
because it would reinstall all the gems. I really only wanted it to
install a specified gem if it weren't installed.

I finally went ahead, however, and wrote the code to only install the
gems if necessary. Here it is.

module Gem
  # Return a list of the versions of the specified gem that are
installed.
  def self.installed_versions gem_name
    cache.search(gem_name).collect {|x| x.version.to_s}
  end
  # Return the version of the provided gem file name.
  @@gem_file_regex = /^(.*)-([^-]*)\.gem$/
  def self.file_version gem_file
    if @@gem_file_regex =~ gem_file
      $2
    else
      nil
    end
  end
  # Return the library name of the gem file.
  def self.file_gem_name gem_file
    if @@gem_file_regex =~ gem_file
      $1
    else
      nil
    end
  end
end

gem_dir = File.dirname(__FILE__) + '/gems'

desc "Install any gems from the local gems directory that are not
already installed."
task 'update-gems' do
  Dir.chdir gem_dir do
    Dir['*.gem'].each do | gem_file |
      version = Gem::file_version gem_file
      name = Gem::file_gem_name gem_file
      desc = "Gem #{name}, version #{version}"
      if Gem::installed_versions(name).include? version
        puts "#{desc} already installed."
      else
        if system "gem.bat install --local #{name}"
          puts "#{desc} installed."
        else
          raise RuntimeError, "Unable to load #{desc}"
        end
      end
    end
  end
end