Shortcode to edit all Wordpress users: change user_login based on email username and populate display_name/nickname

edit_all_users.php

<?php

function edit_all_users($atts) {
    global $wpdb;
    $output = '<ol>'; 
    foreach( get_users() as $user ) {
        $username = get_user_by( 'id', $user->ID )->user_login;
        $email = get_user_by( 'id', $user->ID )->user_email;
        $pos = strpos($username, '@');
        if ($pos !== false) {
            $parts = explode('@', $email);
            $username = $parts[0];
            if ( username_exists( $username ) || ( strlen($username) < 4 ) ) {
                $username = $username . '_' . rand(0, 9999);
            }
        }
        $wpdb->update($wpdb->users, array('user_login' => $username), array('ID' => $user->ID));
        $public = ( empty($user->first_name) ) ? $username : $user->first_name . ' ' . $user->last_name;
        $user_data = wp_update_user( array( 'ID' => $user->ID, 'display_name' => $public ) );
        $user_data = wp_update_user( array( 'ID' => $user->ID, 'nickname' => $public ) );
        if ( is_wp_error( $user_data ) ) {
            $output .= '<li>ERROR !!</li>';
        } else {
            $output .= '<li>'. $email . ' -> '. $username .'</li>';
        }
    }
	return $output.'</ol>';
}

add_shortcode('edit-users', 'edit_all_users');

?>