How to run a batch process in regular interval I need  to Unpublish the nodes which are all not Updated for more than some specified time.

I'm having the query to fetch nodes and by using a batch process I will Unpublish those nodes one by one.

Now I want to schedule this operation to be run every day. How can I achieve that? using drush or something.

I have cron running at regular intervals. I need to run these scripts apart from the cron job i.e as a separate one.
You can use cron to do this, set a cron time in admin/config/system/cron for your site.

there is a hook_cron() is available in your drupal . using that hook you can configure your batch process to invoke.