Simple log in / log out shortcode for WordPress. Conditionally display a login or logout text link based on users logged in status.

functions.php

<?php
/**
 * Shortcode to display "Log in" or "Log out" link.
 *
 * @return string HTML string for the login/logout link.
 */
function dm_login_logout_shortcode() {
    // Initialize an empty string to hold the output HTML.
    $output = '';

    // Check if the user is logged in.
    if ( is_user_logged_in() ) {
        // If the user is logged in, display the "Log out" link.
        $logout_url = wp_logout_url( home_url() );  // Home URL can be replaced with a custom URL.
        $output = "<a href='{$logout_url}'>Log out</a>";
    } else {
        // If the user is not logged in, display the "Log in" link.
        $login_url = wp_login_url( home_url() );  // Home URL can be replaced with a custom URL.
        $output = "<a href='{$login_url}'>Log in</a>";
    }

    return $output;
}
// Register the shortcode.
add_shortcode( 'dm_login_logout_link', 'dm_login_logout_shortcode' );
?>