User and Group Management in Linux involves creating, managing, and assigning permissions to users and groups. This process is essential for controlling access to resources and ensuring a secure environment. Here’s a breakdown:

1. Users in Linux

In Linux, a "user" represents an individual who can log into the system and interact with files and applications. Each user has a unique User ID (UID) and a home directory where personal files are stored.

Types of Users

• Root User: The superuser or administrator with unrestricted access. Has a UID of 0.
• System Users: Used by system processes (e.g., www-data for Apache).
• Regular Users: Created for individual people who access the system.

User Management Commands

• Create a user: sudo useradd <username>
• Delete a user: sudo userdel <username>
• Change a user's password: sudo passwd <username>
• Modify a user: sudo usermod <options> <username>

2. Groups in Linux

Groups allow users to share permissions and access to files or resources collectively. Each user can belong to multiple groups, but one primary group.

Types of Groups

• Primary Group: Assigned to a user upon creation and owns the user's files by default.
• Secondary Group: Additional groups that a user can belong to for shared permissions.

Group Management Commands

• Create a group: sudo groupadd <groupname>
• Delete a group: sudo groupdel <groupname>
• Add a user to a group: sudo usermod -aG <groupname> <username>
• List groups: cat /etc/group

3. File Permissions and Ownership

File permissions are set for the owner (user), group, and others. Permissions include:

• Read (r): Allows viewing file contents.
• Write (w): Allows modifying file contents.
• Execute (x): Allows executing a file (if it’s a script/program).

Changing File Permissions

• Change ownership: sudo chown <user>:<group> <filename>
• Change permissions: chmod <mode> <filename>

4. Important Files

• /etc/passwd: Contains user account information.
• /etc/shadow: Stores encrypted passwords and related data.
• /etc/group: Lists all system groups.

5. Practical Examples

• Create a user with a home directory.

  sudo useradd -m -s /bin/bash <username>
  

• Create a new group and add a user to it:

  sudo groupadd developers
  sudo usermod -aG developers <username>
  

By managing users and groups effectively, Linux administrators can control access to resources, implement security policies, and ensure that users have the appropriate permissions for their tasks.