Understanding Linux File Permissions

Study Notes

Unique user accounts are essential for accessing a Linux system, with user permissions tied to these accounts.
User IDs (UIDs) are unique numerical values assigned to each account, while login names (up to eight alphanumeric characters) are used for user authentication.
User account management in Linux is facilitated through various special files and utilities.

Stores the mapping of login names to corresponding UIDs, serving as a key component for user account management.
Contains seven columns:
- Name
- Password (indicated by 'x' if set)
- UID
- GID (Group ID)
- Gecos (user information, can be empty)
- Home directory
- Shell (default path to the user's shell)
The root user has a UID of 0, granting full administrative privileges.

Provides enhanced security for password management, accessible only by the root user.
Each record includes:
- Login name
- Encrypted password
- Days since last password change
- Minimum days before a password can be changed
- Days until password must be changed
- Warning days before expiration
- Days after expiration before account is disabled
- Date of account disabling
- Reserved field for future use
Enables fine control over password policies and account security.

Users are categorized as either administrators (superusers) with full control or non-administrators with limited access.
Non-administrator accounts, composing the majority of user accounts, can be further classified into:
- Normal Users: Real individuals granted access to applications, files, and resources.
- System Users: Non-human accounts created for specific processes or programs, with restricted permissions.

Each Linux user is assigned a unique name, UID, a group, and a group identification (GID) upon creation.
New accounts lead to the simultaneous creation of a new UID and a matching GID, establishing user-group relations essential for file permission control.