Magento – Ubuntu cron running but nothing happening in Magento

The thing that is confusing about Magento’s cron services is that, while they do get invoked by the system cron, they are abstracted another level above it in a event/observer model that is common throughout Magento.

Once you understand this, the settings make more sense. If you want to see what they do exactly, their usage is pretty clear in app/code/core/Mage/Cron/Model/Observer.php, which is the observer that gets triggered for the default event in the crontab scope and in turn dispatches all the scheduled jobs.

• Generate Schedules Every: minutes interval for when Magento will schedule future jobs. It does not automatically schedule jobs during every execution.
• Schedule Ahead for: minutes in the future that jobs can be scheduled (added to cron_schedule database queue).
• Missed if Not Run Within: minutes past their scheduled time that jobs can still be executed before having their status set as missed, which comes into play if the queue is taking a long time to get through all the jobs that have been selected to run during the current execution of Mage_Cron_Model_Observer::dispatch() (their scheduled time is now or earlier).
• History Cleanup Every: minutes interval for when Magento will perform cleanup on the cron_schedule database queue, making reference to settings below regarding retention.
• Success History Lifetime & Failure History Lifetime: minutes in the future to leave records in the queue after they have reached these statuses. This comes into play if you want to see the result of past jobs since they may have placed resulting output into cron_schedule.messages, which can be especially helpful in the case of error status.

Finally got the solution

Thanks for help from atwix.com.

https://www.atwix.com/magento-2/logging-system/

namespace Custom\Module\Cron;

class MyCronTask {

    protected $_logger;

    public function __construct(\Psr\Log\LoggerInterface $logger) {
        $this->_logger = $logger;
    }

    public function execute() {
        $this->_logger->info("test-cron-message");
        $this->_logger->debug("test-cron-message");
        return $this;
    }
}

After running below commands

php bin/magento setup:cron:run
php bin/magento cron:run --group="my_cronjobs"
Ran jobs by schedule.

It will save in var/system.log & var/debug.log, where you can see your logged data.

For memory_limit set it to 800M in php.ini & it's working fine:)