Your own command in Drupal 8 Console see more in http://symfony.com/doc/current/cookbook/console/console_command.html

AcmeCommand.php

<?php
/**
 * @file
 * Contains \Drupal\Acme\Command\AcmeCommand.
 */
namespace Drupal\Acme\Command;

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\OutputInterface;

class AcmeCommand extends Command
{
    protected function configure()
    {
        $this
            ->setName('demo:greet')
            ->setDescription('Greet someone')
            ->addArgument('name', InputArgument::OPTIONAL, 'Who do you want to greet?')
            ->addOption('yell', null, InputOption::VALUE_NONE, 'If set, the task will yell in uppercase letters')
        ;
    }

    protected function execute(InputInterface $input, OutputInterface $output)
    {
        $name = $input->getArgument('name');
        if ($name) {
            $text = 'Hello '.$name;
        } else {
            $text = 'Hello';
        }

        if ($input->getOption('yell')) {
            $text = strtoupper($text);
        }

        $output->writeln($text);
    }
}