What file contains user and group information on a Linux system?

The file that contains user information on a Linux system is /etc/passwd. This file is essential as it holds the basic information for every user account on the system, including the username, user ID (UID), group ID (GID), home directory, and default shell. It serves as the primary database for user authentication and is read by various system commands and services to manage user logins.

The /etc/shadow file, while it does contain user information, specifically focuses on password and account aging details. It is designed to provide an additional layer of security, storing hashed passwords and related attributes separated from the general user details found in /etc/passwd.

The /etc/group file contains information about user groups, listing group names and their corresponding GIDs and associated users, but does not store individual user account details directly.

Lastly, /var/log/auth.log is a log file that records authentication-related events, including login attempts and security-related messages. It is useful for monitoring user activities and security breaches but does not contain the fundamental user and group information itself.

Thus, /etc/passwd is the definitive file for keeping track of user accounts on a Linux system, making it the correct answer.