<?php

// Trouble knowing the current template?
add_action('admin_bar_menu', function ($adminBar) {
    global $template;
    $templateName = basename($template);

    $adminBar->add_menu([
    'id' => 'template-name',
    'parent' => 'top-secondary',
    'title' => 'Template: ' . $templateName,
    'href' => '#',
    'meta' => [
        'title' => __('Template name')
    ]
    ]);
});