install_gems.rb

# generate the commands to install all currently missing versions from the output of `gem list` from
# another box to your local machine.  `gem list` should be of format
#   actionmailer (2.2.2, 2.1.0)
#   actionpack (2.2.2, 2.1.0)
#   ...
#   will_paginate (2.2.2)
#   ZenTest (3.9.2)
#
# usage:  ruby install_gems.rb /path/to/your/gem/list/output.txt

require 'rubygems'
require 'rubygems/command'
require 'rubygems/commands/query_command'
class Gem::Commands::QueryCommand
  public :installed?
end

file = ARGV[0]
raise "Could not read from #{file}" unless File.exist?(file)

File.readlines(file).each do |line|
  name, versions = line.strip.split(/ /, 2)
  versions.gsub(/[\(\)]/, '').split(', ').map do |version|
    unless Gem::Commands::QueryCommand.new.installed?(name, version)
      puts "sudo gem install #{name} -v #{version}"
    end
  end
end