Linux Users and Groups

User and group management in Linux is fundamental to security and access control in the multi-user environment. It involves managing accounts, organizing users into groups, and assigning permissions to control access to files and resources. 

Key Concepts

Users: Each user has a unique username and a User ID (UID). The root user is the superuser with administrative privileges.
Groups: Groups are collections of users that simplify permission management. Each user belongs to a primary group (often their own name by default) and can belong to multiple secondary (supplementary) groups. Groups have a unique Group ID (GID).
Permissions: Access rights (read, write, execute) are assigned to three categories: the file owner, the file's group, and others (everyone else).
Configuration Files: Key information is stored in plain text files:
/etc/passwd: Stores user account information (except passwords).
/etc/shadow: Stores encrypted user passwords.
/etc/group: Stores group information and members.
/etc/sudoers: Defines which users or groups have sudo (superuser) privileges. 

Administrative Commands (User, Group & Permissions)

Administrative tasks typically require sudo or root privileges. These commands are essential for managing users, groups, and permissions in Linux.

Category	Command	Purpose	Example	Man
User Management	useradd	Creates a new user account. Use -m to create the home directory.	sudo useradd -m devuser	man
User Management	passwd	Sets or changes a user’s password.	sudo passwd devuser	man
User Management	usermod	Modifies user properties such as groups or home directory.	sudo usermod -aG sudo devuser	man
User Management	userdel	Deletes a user account. Use -r to remove home directory.	sudo userdel -r devuser	man
Group Management	groupadd	Creates a new group.	sudo groupadd devops	man
Group Management	usermod -aG	Adds a user to a supplementary group.	sudo usermod -aG devops devuser	man
Group Management	groupmod	Modifies group properties such as name or GID.	sudo groupmod -n devops-team devops	man
Group Management	groupdel	Deletes a group.	sudo groupdel devops	man
Group Management	gpasswd -d	Removes a user from a specific group.	sudo gpasswd -d devuser devops	man
Permissions & Ownership	ls -l	Lists file details including permissions and ownership.	ls -l /var/www	man
Permissions & Ownership	chown	Changes file or directory owner and group.	sudo chown user:group file	man
Permissions & Ownership	chmod	Changes file or directory permissions.	chmod 755 script.sh	man
Permissions & Ownership	id / groups	Displays user UID, GID, and group memberships.	id devuser	man

Best Practices

• Principle of Least Privilege:
• Grant users only the minimum access necessary for their tasks to enhance security.
• Use sudo:
• Avoid using the root account directly. Instead, manage privileges via the /etc/sudoers file and the sudo command.
• Regular Auditing: Periodically review user accounts and group memberships to remove unneeded access.
• Automation: Use Bash scripts to automate routine user management tasks for efficiency and consistency.