I have had it come up some times where I need a isolated gem installer using ruby. Found this is a nice way to handle that problem. Used it in https://github.com/newdark/faker-alfred-workflow

gem_installer.rb

require 'fileutils'
require 'rubygems'
require 'rubygems/gem_runner'
require 'rubygems/exceptions'
require File.expand_path('./logger', __dir__).to_s

ENV['GEM_HOME'] = File.expand_path('gems', __dir__).to_s
FileUtils.mkdir_p(ENV['GEM_HOME'])
ENV['GEM_PATH'] = "#{ENV['GEM_HOME']}:/var/lib/ruby/gems/1.8"
Gem.clear_paths

module GemInstaller
  def install(lib)
    suppress_output do
      return false if `gem list #{lib}`.include?(lib)
      Logger.info("running 'install #{lib} --no-ri --no-rdoc'")
      Gem::GemRunner.new.run(['install', lib, '--no-ri', '--no-rdoc'])
      Logger.info("failed to 'install #{lib} --no-ri --no-rdoc'")
      false
    end
  rescue Gem::SystemExitException
    Logger.info("Successfully Installed #{lib}")
    true
  end

  def reinstall(lib)
    suppress_output do
      return install(lib) unless `gem list #{lib}`.include?(lib)
      Logger.info("Uninstalling #{lib}")
      Gem::GemRunner.new.run(['uninstall', lib, '-a'])
      Logger.info("Failed to uninstall #{lib}")
      install(lib)
    end
  rescue StandardError => exception
    Logger.error("Failure #{exception.message}")
  end

  def suppress_output
    original_stdout, original_stderr = $stdout.clone, $stderr.clone
    $stderr.reopen File.new('/dev/null', 'w')
    $stdout.reopen File.new('/dev/null', 'w')
    yield
  ensure
    $stdout.reopen original_stdout
    $stderr.reopen original_stderr
  end
end

logger.rb

require 'fileutils'
class Logger
  ERROR_LOG_PATH = File.join([File.expand_path('logs', __dir__).to_s, 'error.log'])
  INFO_LOG_PATH = File.join([File.expand_path('logs', __dir__).to_s, 'info.log'])

  class << self
    def create_log_file
      return if File.exist?(Logger::ERROR_LOG_PATH) && File.exist?(Logger::INFO_LOG_PATH)
      FileUtils.mkdir_p(File.expand_path('logs', __dir__).to_s)
      FileUtils.touch(Logger::ERROR_LOG_PATH)
      FileUtils.touch(Logger::INFO_LOG_PATH)
    end

    def log(message, log_path: Logger::INFO_LOG_PATH)
      create_log_file
      return if message.to_s == ''
      File.open(log_path, 'a') do |file|
        file.write("#{message}\n")
      end
    end

    def info(message)
      log message, log_path: Logger::INFO_LOG_PATH
    end

    def error(message)
      log message, log_path: Logger::ERROR_LOG_PATH
    end
  end
end

require File.expand_path('./gem_installer', __dir__).to_s
extend GemInstaller
install 'faker'
require 'faker'

add this code to the top of any script you want and it will install the gem if it is not already installed