before_filter.rb

def UsersController
  before_filter :get_user, only: [:show, :update]

  def show
    # You can use the @user variable here...
  end

  def update
    # You can use the @user variable here...
  end

  def index
    # You CANNOT use the @user variable here...
  end

  private

    def get_user
      @user = User.find_by_id(params[:id])
    end
end