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This tutorial is built using Drupal 8.0. The fundamental concepts of Drupal 8 - like services & routing - are still valid, but newer versions of Drupal *do* have major differences.


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Debugging in Drupal 8 is pretty sweet... if you know the tools to use.

Turning on Debugging

First, let's activate a debug mode so we can see errors... just in case some other developer makes a mistake and we have to debug it.

In sites/ there's an example.settings.local.php file that can be used to activate development settings locally. But first, we need to play with permissions: Drupal makes some files in this directory readonly for security. Start by making sites/default writable by us:

chmod 755 sites/default

Now, copy sites/example.settings.local.php to sites/default/settings.local.php:

cp sites/example.settings.local.php sites/default/settings.local.php

Finally, make sure we can write to settings.php:

chmod 644 sites/default/settings.php


On a production server, it's best to make sure these files are not writeable by whatever user runs your web server.

The settings.local.php file activates several things that are good for debugging, like a verbose error level. It also loads a development.services.yml file that we're going to talk about soon:

... lines 1 - 31
* Enable local development services.
$settings['container_yamls'][] = DRUPAL_ROOT . '/sites/development.services.yml';
* Show all error messages, with backtrace information.
* In case the error level could not be fetched from the database, as for
* example the database connection failed, we rely only on this value.
$config['system.logging']['error_level'] = 'verbose';
... lines 44 - 92

But just having settings.local.php isn't enough! Open settings.php. At the bottom, uncomment out the lines so that this file is loaded:

... lines 1 - 701
* Load local development override configuration, if available.
* Use settings.local.php to override variables on secondary (staging,
* development, etc) installations of this site. Typically used to disable
* caching, JavaScript/CSS compression, re-routing of outgoing emails, and
* other things that should not happen on development and testing sites.
* Keep this code block at the end of this file to take full effect.
if (file_exists(__DIR__ . '/settings.local.php')) {
include __DIR__ . '/settings.local.php';
... lines 715 - 727

Of course, we need to rebuild our cache and the Drupal Console in all its wisdom has a command for that:

drupal cache:rebuild

From this command you can clear all kinds of different caches, like the menu cache, render cache or leave it blank to do everything.

Side note: the Drupal Console app is built on top of the Symfony Console library, and you can take advantage of it to create really cool command line scripts like this if you want to. It's one of the best pieces of Symfony.

Back in the browser, if you refresh now, there's no difference: everything is roaring along. But, try deleting the roar() function and refresh. Now we get a nice error that says:

The controller for URI /the/dino/says/50 is not callable.

That's a way of saying "Yo! This route is pointing to a function that doesn't exist!" And when we put the function back and refresh, things are back to a roaring good time.

List all the Routes

Every page of the site - including the homepage and admin areas - has a route. And that leads me to the next natural question: what time is dinner? I mean, can I get a list of every single route in the system? Well, of course! And dinner is at 6.

Once again, go back to the trusty Drupal Console app. To list every route, run router:debug:

drupal router:debug

Wow! This prints every single route in the system, which is wonderful when you're trying to figure out what's going on in a project. This includes the admin route and our custom route.

To get more information about a route, copy it's internal name - that's the part on the left - and pass it as an argument to router:debug. This route has several curly brace routing wildcards that are passed to the getForm method of this controller class. This is pretty sweet, but we can go further!

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Default user avatar
Default user avatar Darryl Norris | posted 5 years ago

You can enable debugging in D8 by running the command drupal site:mode dev with DrupalConsole.

1 Reply
Default user avatar
Default user avatar Zachary Maximov | Darryl Norris | posted 5 years ago



Yea there is obviously some weirdness for some machines related to entering the debug mode. This is an excellent alternative (and really easier anyways).

Griff P. Avatar
Griff P. Avatar Griff P. | posted 3 years ago | edited

As noted on previous chapters, if you are running this course on Drupal 9, at time of writing Drupal Console is not compatible with D9.
And Drush does not give helpful debugging information for routes, unfortunately.
Next best thing is to use the Devel module.
composer require drupal/devel
Then install the Devel module at admin/modules.
Then clear cache and browse to devel/routes.
You can see all the routes listed. If you press the 'Devel' button at the end of each row, you get all the extended debugging information for that route, but not nicely formatted like in Drupal Console, instead it looks like a var_dump of some kind.


Hey Griff P.

Thank you very much for you notes, it's very helpful! Unfortunately we can't update our course to be compatible with Drupal 9 right now but you notes can help many users so thank you again for sharing it!



Glad to hear it! I use bash and autocompletion (in general) works fine - I haven't hooked up the Drupal Console's auto-completion, however, but I imagine it would work fine (I use autocompletion for Symfony's console, which is accomplished in a similar way).

Allison R. Avatar
Allison R. Avatar Allison R. | posted 5 years ago

On Drupal 8.3.5. I ran into problems running drupal router:debug views_ui.form_display and ended up have to set back symfony/yaml to 2.8.6

$composer -v require symfony/yaml:2.8.6

Now it works!

$ drupal router:debug views_ui.form_display
Route views_ui.form_display
Path /admin/structure/views/{js}/display/{view}/{display_id}/{type}
_controller \Drupal\views_ui\Form\Ajax\Display::getForm
compiler_class \Drupal\Core\Routing\RouteCompiler
parameters view:
tempstore: true
type: 'entity:view'
converter: drupal.proxy_original_service.paramconverter.views_ui

_admin_route 1
_route_filters method_filter

_route_enhancers route_enhancer.param_conversion

_access_checks access_check.entity


Hey Allison R. ,

Thank you for sharing it! What was the problem if you don't mind to say? I wonder what version of symfony/yaml was installed before you downgrade to v2.8.6.


Allison R. Avatar
Allison R. Avatar Allison R. | Victor | posted 5 years ago

The problem was:
$ drupal router:debug views_ui.form_display
PHP Fatal error: Call to undefined method Symfony\Component\Yaml\Inline::isHash() in public_html/d8-underthehood/vendor/symfony/yaml/Dumper.php on line 61
PHP Stack trace:
PHP 1. {main}() /usr/local/bin/drupal:0
PHP 2. require() /usr/local/bin/drupal:10
PHP 3. require() phar:///usr/local/bin/drupal/bin/drupal:3
PHP 4. Symfony\Component\Console\Application->run($input = *uninitialized*, $output = *uninitialized*) phar:///usr/local/bin/drupal/bin/drupal.php:115
PHP 5. Drupal\Console\Application->doRun($input = class Symfony\Component\Console\Input\ArgvInput { private $tokens = array (0 => 'router:debug',

$ composer show | grep symfony/
symfony/yaml v2.8.18 Symfony Yaml Component

$php -v
PHP 5.6.23

Allison R. Avatar

Of course I ran in to other issues later on. Don't mess with versions!!! Haha!

It appears console broke -- with the error:

Console is not installed at:____ You must execute the following composer commands: ____
As per debug suggestions I had to remove:
composer remove symfony/finder v3.3.4
composer remove symfony/filesystem v3.3.4
Then I could run:
composer require drupal/console:~1.0 --prefer-dist --optimize-autoloader

This still works:
drupal router:debug views_ui.form_display

Bottom line
$ composer show | grep symfony/
symfony/browser-kit v2.8.24 Symfony BrowserKit Component
symfony/class-loader v2.8.24 Symfony ClassLoader Component
symfony/config v2.8.24 Symfony Config Component
symfony/console v2.8.24 Symfony Console Component
symfony/css-selector v2.8.24 Symfony CssSelector Component
symfony/debug v2.8.24 Symfony Debug Component
symfony/dependency-injection v2.8.24 Symfony DependencyInjection Component
symfony/dom-crawler v2.8.11 Symfony DomCrawler Component
symfony/event-dispatcher v2.8.24 Symfony EventDispatcher Component
symfony/expression-language v2.8.24 Symfony ExpressionLanguage Component
symfony/filesystem v2.8.24 Symfony Filesystem Component
symfony/finder v2.8.24 Symfony Finder Component
symfony/http-foundation v2.8.24 Symfony HttpFoundation Component
symfony/http-kernel v2.8.24 Symfony HttpKernel Component
symfony/polyfill-apcu v1.4.0 Symfony polyfill backporting apcu_* functions to lower PHP versions
symfony/polyfill-iconv v1.4.0 Symfony polyfill for the Iconv extension
symfony/polyfill-mbstring v1.4.0 Symfony polyfill for the Mbstring extension
symfony/polyfill-php54 v1.4.0 Symfony polyfill backporting some PHP 5.4+ features to lower PHP versions
symfony/polyfill-php55 v1.4.0 Symfony polyfill backporting some PHP 5.5+ features to lower PHP versions
symfony/process v2.8.24 Symfony Process Component
symfony/psr-http-message-bridge v1.0.0 PSR HTTP message bridge
symfony/routing v2.8.24 Symfony Routing Component
symfony/serializer v2.8.24 Symfony Serializer Component
symfony/translation v2.8.24 Symfony Translation Component
symfony/validator v2.8.24 Symfony Validator Component
symfony/var-dumper v3.3.4 Symfony mechanism for exploring and dumping PHP variables
symfony/yaml v2.8.6 Symfony Yaml Component


Hey Allison R. ,

Thanks a lot for sharing it! I think it could be useful for other users.



Hi, I followed all the steps here, but when I uncomment the lines in settings.php, to include settings.local.php, I get a "Class 'Drupal\Component\Assertion\Handle' not found". I'm using drupal 8.0.0. Any suggestion?


Hmm, I don't get this with a fresh 8.0.0. install (downloaded from drupal.org/8). This line *does* come from settings.local.php (you'll find mention of this class there), but the class itself is located in core/lib/Drupal/Component/Assertion/Handle.php and should be autoloaded just like everything else. So I'm not so sure about this. Does this information give you any clues?


If I comment out that line, drupal cache:rebuild complains about DRUPAL_ROOT not defined. I think this can be related to my PHP version (that's 5.5.29, on a MAC with El Capitan, installed with brew) but other than that, I don't know where else to dig...

Default user avatar
Default user avatar Zachary Maximov | Beo | posted 5 years ago

Same problem. What is your PHP version?


I would try using DrupalConsole to enter debugging mode instead - https://knpuniversity.com/s.... For some reason, some people are having strange issues after entering debugging mode by including settings.local.php. But I haven't been able to repeat that yet :)

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This tutorial is built using Drupal 8.0. The fundamental concepts of Drupal 8 - like services & routing - are still valid, but newer versions of Drupal *do* have major differences.

What PHP libraries does this tutorial use?

// composer.json
    "require": {
        "composer/installers": "^1.0.21", // v1.0.21
        "wikimedia/composer-merge-plugin": "^1.3.0" // dev-master