Option --quiet in Symfony2 command

NullOutputCommand.php

<?php

use Symfony\Component\Console\Command\Command;
use Symfony\Component\Console\Input\InputArgument;
use Symfony\Component\Console\Input\InputInterface;
use Symfony\Component\Console\Input\InputOption;
use Symfony\Component\Console\Output\NullOutput;
use Symfony\Component\Console\Output\OutputInterface;

class NullOutputCommand extends Command
{
    /**
     * {@inheritdoc}
     */
    protected function configure()
    {
        $this
            ->setName('dummy')
            ->addOption('quiet', 'q', InputOption::VALUE_NONE, 'Disable all output of the program.')
        ;
    }

    /**
     * {@inheritdoc}
     */
    protected function execute(InputInterface $input, OutputInterface $output)
    {
        if (true === $input->getOption('quiet')) {
            $output = new NullOutput();
        }

        // ...

        $output->writeln('This line won\'t be printed with --quiet option');
    }
}