When installing gem devise for a User model, add these links to your application:

Basic navigation:

<% if current_user %>
  <%= link_to current_user.email, edit_user_registration_path %>
  <%= link_to "Log out", destroy_user_session_path, method: :delete %>
<% else %>
  <%= link_to "Log in", new_user_session_path %>
  <%= link_to "Register", new_user_registration_path %>
<% end %>

bootstrap 4 Navbar:

<nav class="navbar navbar-expand-lg navbar-light bg-light">
  <a class="navbar-brand" href="/">
    <i class="fas fa-flag"></i>
    Brand
  </a>
  <button class="navbar-toggler" type="button" data-toggle="collapse" data-target="#navbarSupportedContent" aria-controls="navbarSupportedContent" aria-expanded="false" aria-label="Toggle navigation">
    <span class="navbar-toggler-icon"></span>
  </button>
  <div class="collapse navbar-collapse" id="navbarSupportedContent">
    <ul class="navbar-nav mr-auto">
      <%= link_to root_path, class: "nav-link #{'active font-weight-bold' if current_page?(root_path)}" do %>
        <div class="fa fa-home"></div>
        Home
      <% end %>
    </ul>
    <ul class="navbar-nav mr-right">
      <% if current_user %>
        <li class="nav-item dropdown">
          <a class="nav-link dropdown-toggle" href="#" id="navbarDropdown" role="button" data-toggle="dropdown" aria-haspopup="true" aria-expanded="false">
            <div class="fa fa-user"></div>
            <b><%= current_user.email %></b>
          </a>
          <div class="dropdown-menu" aria-labelledby="navbarDropdown">
            <%= link_to edit_user_registration_path, class: "dropdown-item #{'active font-weight-bold' if current_page?(edit_user_registration_path)}" do %>
              <div class="fa fa-cog"></div>
              <b>Account settings</b>
            <% end %>
            <%= link_to destroy_user_session_path, method: :delete, class: "dropdown-item" do %>
              <div class="fa fa-sign-out-alt"></div>
              <b>Sign out</b>
            <% end %>
          </div>
        </li>
      <% else %>
        <%= link_to "Log in", new_user_session_path, class: "nav-link #{'active font-weight-bold' if current_page?(new_user_session_path)}" %>
        <%= link_to "Sign up", new_user_registration_path, class: "nav-link #{'active font-weight-bold' if current_page?(new_user_registration_path)}" %>
      <% end %>
    </ul>
  </div>
</nav>

bootstrap 5 Navbar:

<nav class="navbar navbar-expand-lg navbar-light bg-light">
  <div class="container-fluid">
    <a class="navbar-brand" href="/">
      <i class="fas fa-flag"></i>
      Brand
    </a>
    <button class="navbar-toggler" type="button" data-bs-toggle="collapse" data-bs-target="#navbarSupportedContent" aria-controls="navbarSupportedContent" aria-expanded="false" aria-label="Toggle navigation">
      <span class="navbar-toggler-icon"></span>
    </button>
    <div class="collapse navbar-collapse" id="navbarSupportedContent">
      <ul class="navbar-nav me-auto">
        <%= link_to root_path, class: "nav-link #{'active fw-bold' if current_page?(root_path)}" do %>
          <div class="fa fa-home"></div>
          Home
        <% end %>
      </ul>
      <ul class="navbar-nav ms-auto">
        <% if current_user %>
          <li class="nav-item dropdown">
            <a class="nav-link dropdown-toggle" href="#" id="navbarDropdown" role="button" data-bs-toggle="dropdown" aria-expanded="false">
              <div class="fa fa-user"></div>
              <b><%= current_user.email %></b>
            </a>
            <ul class="dropdown-menu" aria-labelledby="navbarDropdown">
              <%= link_to edit_user_registration_path, class: "dropdown-item #{'active fw-bold' if current_page?(edit_user_registration_path)}" do %>
                <div class="fa fa-cog"></div>
                <b>Account settings</b>
              <% end %>
              <%= link_to destroy_user_session_path, method: :delete, class: "dropdown-item" do %>
                <div class="fa fa-sign-out-alt"></div>
                <b>Sign out</b>
              <% end %>
            </ul>
          </li>
        <% else %>
          <%= link_to "Log in", new_user_session_path, class: "nav-link #{'active fw-bold' if current_page?(new_user_session_path)}" %>
          <%= link_to "Sign up", new_user_registration_path, class: "nav-link #{'active fw-bold' if current_page?(new_user_registration_path)}" %>
        <% end %>
      </ul>
    </div>
  </div>
</nav>