WordPress - Show message for logged-in users

<?php
// WordPress - Show message for logged-in users
// usage: [user_message]
add_shortcode( 'user_message', 'user_message' );
function user_message() {
  if ( is_user_logged_in() ) {
    print '<p>My custom message for logged-in users.</p>'.PHP_EOL;
  }
}

// The code goes in functions.php for your child theme or you can use the "My Custom Functions" plugin.