Rubygems: Install if not already installed

ruby-talk
1

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