Helper methods available in devise

Method	Use
before_action :authenticate_user!	Set in the controller to allow access only to users.
user_signed_in?	Determine if the user is logged in. Returns true if the user is logged in, false if logged out.
current_user	Get the currently logged in user
user_session	Access user session information

before_action :authenticate_user! A devise method that switches the page to be displayed depending on the login status.

class SampleController < ApplicationController
  before_action :authenticate_user!, only: [:show]

  def index
  end

  def show
  end
end

With the only option, the show action is accessible only to logged-in users, and the index action is accessible even if you are not logged in.

user_signed_in?

<% if user_signed_in? %>
  <div class="user_nav grid-6">
    <%= link_to "Logout", destroy_user_session_path, method: :delete %>
    <%= link_to "Post", new_tweet_path, class: "post" %>
  </div>
<% else %>
    <div class="grid-6">
      <%= link_to "Login", new_user_session_path, class: "post" %>
      <%= link_to "sign up", new_user_registration_path, class: "post" %>
    </div>
<% end %>

If you are signed in, you can display "Logout" and "Post" pages, and if you are not signed in, you can display "Login" and "New registration".

As a reminder, as the devise settings are still unfamiliar