Why can/etc/passwd and /etc/shadow files can not be merged into 1 file?

The /etc/passwd and /etc/shadow files are separate in Unix-like operating systems primarily for security and performance reasons. Here are the main reasons why they are not merged into a single file:

1. Security:
   • The /etc/passwd file contains user account information that needs to be accessible by various system utilities and programs. It is world-readable to allow these programs to obtain necessary user information (like user IDs, group IDs, home directories, and shells).
   • The /etc/shadow file, on the other hand, contains password hashes and related security information (such as password aging). It is readable only by the root user to protect sensitive password information from being exposed to unauthorized users.
2. Access Control:
   • By keeping the password hashes in a separate, more secure file (/etc/shadow), the system can enforce stricter access controls. Only privileged processes (those running as root) can read the /etc/shadow file, thereby reducing the risk of password hashes being accessed by malicious users or processes.
3. Compatibility and Standards:
   • The separation of these files follows long-standing Unix design principles and standards, ensuring compatibility with a wide range of software and tools that expect this separation. Merging them would break compatibility with existing software that relies on the current structure.
4. Performance:
   • Accessing user account information (stored in /etc/passwd) is a frequent operation. By keeping this file small and focused only on necessary user information, the system can access it more quickly.
   • Password validation (which requires accessing /etc/shadow) is a less frequent operation. Separating the files helps optimize performance for common operations while ensuring secure handling of sensitive data during password validation.

In summary, the separation of /etc/passwd and /etc/shadow enhances security by protecting sensitive password information, maintains compatibility with existing standards and software, and optimizes performance for common system operations.