As solution:

Get all handlers from monolog service and search test handler.

foreach ($this->container->get('monolog')->getHandlers() as $handler) {  if ($handler instanceof TestHandler) {    $testHandler = $handler;    break;  }}if (!$testHandler) {  throw new \RuntimeException('Oops, not exist "test" handler in monolog.');}$this->assertFalse($testHandler->hasCritical()); // Or another assertions

In your command class, you have to simply set the handler with pushHandler():

namespace AppBundle\Command;use Symfony\Bridge\Monolog\Handler\ConsoleHandler;use Symfony\Bundle\FrameworkBundle\Command\ContainerAwareCommand;use Symfony\Component\Console\Input\InputInterface;use Symfony\Component\Console\Output\OutputInterface;class YourCommand extends ContainerAwareCommand{    // ...    protected function execute(InputInterface $input, OutputInterface $output)    {        $logger = $this->getContainer()->get('logger');        // PUSH THE OutputInterface OBJECT INTO MONOLOG        $logger->pushHandler(new ConsoleHandler($output));        // Your command logic here...    }

In your test, using CommandTester:

namespace AppBundle\Tests\Command;use AppBundle\Command\YourCommand;use Symfony\Bundle\FrameworkBundle\Console\Application;use Symfony\Bundle\FrameworkBundle\Test\KernelTestCase;use Symfony\Component\Console\Output\OutputInterface;use Symfony\Component\Console\Tester\CommandTester;class YourCommandTest extends KernelTestCase{    public function testExecute()    {        $kernel = $this->createKernel();        $kernel->boot();        // mock the Kernel or create one depending on your needs        $application = new Application($kernel);        $application->add(new YourCommand());        $command = $application->find('acme:your:command');        $commandTester = new CommandTester($command);        $commandTester->execute(            array('command'   => $command->getName()),            /**             * Here set the verbosity             */            array('verbosity' => OutputInterface::VERBOSITY_DEBUG)        );        // die(print_r($commandTester->getDisplay()));        $this->assertRegExp('/.../', $commandTester->getDisplay());    }}

Keep attention to array('verbosity' => OutputInterface::VERBOSITY_DEBUG).

This way you'll can obtain all the logs (a INFO in this case, set with $logger->info('Starting <info>acme:your:command</info>');):

[2015-08-13 23:39:22] app.INFO: Starting acme:your:command: 

Now you can use $this->assertRegExp()to check if a particular line is logged or not.

You can also transform the string in an array with

explode('\n', $commandTester->getDisplay())

This solution were found here and is explained in the documentation of Monolog here.

More about Monolog and Symfony (Symfony Docu).

More about Monolog Handlers (Monolog Docu).

Symfony 5 (autowiring), PHP 7.4

namespace App\Command;use Monolog\Logger;use Psr\Log\LoggerInterface;use Symfony\Bridge\Monolog\Handler\ConsoleHandler;use Symfony\Component\Console\Command\Command;use Symfony\Component\Console\Input\InputInterface;use Symfony\Component\Console\Output\OutputInterface;class YourCommand extends Command{    protected static $defaultName = 'acme:your:command';    private LoggerInterface $logger;    public function __construct(LoggerInterface $logger)    {        $this->logger = $logger;    }    protected function execute(InputInterface $input, OutputInterface $output)    {        // PUSH THE OutputInterface OBJECT INTO MONOLOG        if ($this->logger instanceof Logger) {            $this->logger->pushHandler(new ConsoleHandler($output));        }        // Your command logic here...        return self::SUCCESS;    }}

In your test, using CommandTester:

namespace AppBundle\Tests\Command;use AppBundle\Command\YourCommand;use Symfony\Bundle\FrameworkBundle\Console\Application;use Symfony\Bundle\FrameworkBundle\Test\KernelTestCase;use Symfony\Component\Console\Output\OutputInterface;use Symfony\Component\Console\Tester\CommandTester;class YourCommandTest extends KernelTestCase{    public function testExecute()    {        $kernel = static::createKernel();        $application = new Application($kernel);        $command = $application->find('acme:your:command');        $commandTester = new CommandTester($command);        $commandTester->execute(            ['command'   => $command->getName()],            /**             * Here set the verbosity             */            ['verbosity' => OutputInterface::VERBOSITY_DEBUG]        );        $output = $commandTester->getDisplay();        // die(print_r($commandTester->getDisplay()));        self::assertStringContainsString('/.../', $$output);    }}