Ask HN: What is the quickest way to implement user management for my SaaS

Implementing user management functionalities such as

- user registration - email/phone number verification - user login - forget username or password - account deletion - etc For SaaS is a common requirement. What are the best ways available today to implement these functionalities quickly without wasting the resources?

File based access under a Unix with users and groups. Assuming you know or are willing to learn a Unix.