gem_backup.rb

class GemBackup
  class << self
    
    def to_file(dir)
      File.open(File.join(dir, 'gem_list_backup.txt'), 'w') { |f| f << `gem list` }
    end
    
    def install(file)
      @gems = File.read(file)

      versioned(@gems).each do |g|
        puts "~ NOTE: Installing #{g[:gemname]}, versions: #{g[:versions].join(', ')}"
        g[:install_code].each do |gems|
          # `#{gems}`
        end
      end
    end
    
    private
      
      def versioned(gems)
        gems.collect do |g|
          name, versions  = g.split(/\s\(/)
          versions        = versions.gsub(/\)\n/, '').split(', ').reverse
          install_code    = []
          versions.each { |v| install_code << "sudo gem install #{name} -v=#{v}" }
          
          {
            :gemname      => name,
            :versions     => versions,
            :install_code => install_code
          }
        end
      end
    
  end
end

# GemBackup.to_file('/Users/revans/Desktop')
GemBackup.install('/Users/revans/Desktop/gem_list_backup.txt')