homebrew-cask/spec/support/env_helper.rb

module EnvHelper
  class << self
    def with_env_var(key, val, &block)
      was_defined = ENV.key? 'key'
      old_value = ENV['key']
      ENV[key] = val
      block.call
    ensure
      if was_defined
        ENV[key] = old_value
      else
        ENV.delete(key)
      end
    end
  end
end