👥 Linux/Unix: User, Group & Security Management – Manage Accounts & Secure Your System

🧲 Introduction – Control Access and Enforce Security in Linux

Linux is built around the principle of multi-user environments and permission-based access. Whether you’re administering a shared server or setting up a personal workstation, knowing how to manage users, groups, authentication, and security tools is essential for system safety and structure.

🎯 In this guide, you’ll learn:

• How to create, modify, and manage users and groups
• How to enforce authentication and privilege control using sudo, passwd, and su
• How to view system identity and uptime
• How to use Linux tools for hardening and access restrictions

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📘 Topics Covered

🔵 Category	📖 Description
User & Group Management	Create and manage users, groups, and IDs
Authentication & Privileges	Secure access using passwords, sudo, and session management
System Identity Tools	View hostname, kernel, and uptime data
Basic Security Tools	Use firewalls and attribute tools for basic Linux system protection

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🔵 Linux/Unix: User & Group Management

🔹 Add and Manage Users

sudo useradd john
sudo passwd john

✅ Adds a new user and sets their password.

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🔹 Modify Existing Users

sudo usermod -aG sudo john     # Add user to sudo group
sudo usermod -l newname oldname

✅ Modify group access or rename users.

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🔹 Group Management

groups john             # List groups
sudo groupadd editors   # Create group
sudo usermod -aG editors john

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🔹 Identity Verification

id john                 # Show UID, GID, group membership
whoami                  # Current username

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🔵 Linux/Unix: Authentication (passwd, su, sudo)

🔹 Set/Change Passwords

passwd john

✅ Only root can reset others’ passwords.

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🔹 su – Switch User Temporarily

su - john

✅ Switches to another user’s session.

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🔹 sudo – Execute with Elevated Privileges

sudo apt update

✅ Allows limited admin access without logging in as root.

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🔹 Configure /etc/sudoers

Use:

sudo visudo

✅ Add users to grant specific or full sudo rights safely.

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🔵 Linux/Unix: System Info (uname, hostname, uptime)

🔹 View System Identity

uname -a            # Kernel info
hostname            # Hostname
uptime              # System load and uptime

✅ Quickly audit host details and runtime.

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🔵 Linux/Unix: Basic Security Tools (chattr, semanage, ufw, firewalld)

🔹 chattr – File Attribute Control

sudo chattr +i file.txt     # Make file immutable
sudo chattr -i file.txt

✅ Prevents even root from modifying/deleting files unless reversed.

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🔹 semanage – SELinux Policy Tool

semanage fcontext -l | grep ssh

✅ Used with SELinux-enabled systems for context management.

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🔹 ufw – Uncomplicated Firewall

sudo ufw enable
sudo ufw allow 22/tcp

✅ Simple firewall setup (ideal for Ubuntu/Debian users).

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🔹 firewalld – Dynamic Firewall Manager

sudo systemctl start firewalld
sudo firewall-cmd --add-service=http --permanent
sudo firewall-cmd --reload

✅ Used on CentOS, Fedora, and RHEL for zone-based firewalling.

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📌 Summary – Recap & Next Steps

User, group, and security management form the backbone of Linux system control. Whether you’re configuring multi-user access or applying security hardening techniques, these tools help enforce accountability, permissions, and network safety.

🔍 Key Takeaways:

• Use useradd, usermod, and groupadd for account management
• Configure sudo carefully to restrict elevated access
• Use ufw or firewalld to control network access
• Make sensitive files immutable with chattr

⚙️ Real-World Applications:

• Manage user permissions in hosting or development servers
• Protect sensitive configuration files from tampering
• Implement secure sudo policies for team environments
• Block unwanted traffic via firewall rules

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❓ Frequently Asked Questions

❓ What’s the difference between su and sudo?
✅ su switches to another user shell; sudo runs a command with another user’s (usually root’s) privileges.

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❓ How do I give a user sudo access?
✅ Add them to the sudo group or configure rules via visudo.

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❓ Can I delete a user safely without removing their files?
✅ Yes:

sudo userdel username       # Keeps home directory
sudo userdel -r username    # Removes home too

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❓ How do I prevent accidental file deletion?
✅ Use:

sudo chattr +i important.conf

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❓ What’s the best beginner-friendly firewall tool?
✅ Use ufw on Ubuntu/Debian, and firewalld on RHEL-based systems.

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