Drupal commerce auto login during checkout process I have created a e-commerce site, in that when anonymous user purchase a product during checkout process it should automatically created a account and assign order to the user. For existing user it should it should log in and assign order to him. Is there any module to do this or else how can achieve this. 

I have tried https://www.drupal.org/project/commerce_checkout_complete_registration it works fine for new user. When I enter existing user email site goes to Maintenance mode.
Commerce Checkout Login includes this functionality.

If you just want a rule action to automatically log a user in, then use simongeorge's wonderful sandbox. It provides a rule action that allows you to do just that.