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6. Users & Groups ​

6.1 User Management Commands ​

bash
# Add a user (Ubuntu recommends adduser for interactive setup)
sudo adduser john
# Or use useradd (requires manual password setup, etc.)
sudo useradd -m -s /bin/bash john
sudo passwd john

# Modify user information
sudo usermod -aG docker john       # Add john to the docker group
sudo usermod -s /bin/zsh john      # Change default shell
sudo usermod -L john               # Lock account
sudo usermod -U john               # Unlock account

# Delete a user
sudo userdel john                  # Delete user (keep home directory)
sudo userdel -r john               # Delete user and home directory
bash
$ sudo adduser john
Adding user `john' ...
Adding new group `john' (1001) ...
Adding new user `john' (1001) with group `john' ...
Creating home directory `/home/john' ...
Copying files from `/etc/skel' ...
New password:
Retype new password:
passwd: password updated successfully
Changing the user information for john
Enter the new value, or press ENTER for the default
	Full Name []: John Smith
	Room Number []:
	Work Phone []:
	Home Phone []:
	Other []:
Is the information correct? [Y/n] Y

6.2 Group Management ​

bash
# Create a group
sudo groupadd developers

# Add users to a group
sudo usermod -aG developers john
sudo usermod -aG developers jane

# View groups a user belongs to
groups john
id john

# Delete a group
sudo groupdel developers
bash
$ id john
uid=1001(john) gid=1001(john) groups=1001(john),27(sudo),1002(developers)

$ groups john
john : john sudo developers

6.3 Understanding /etc/passwd ​

bash
head -3 /etc/passwd
bash
root:x:0:0:root:/root:/bin/bash
daemon:x:1:1:daemon:/usr/sbin:/usr/sbin/nologin
john:x:1001:1001:John Smith,,,:/home/john:/bin/bash

Each line is separated by :, with 7 fields in total:

  1. Username: john
  2. Password placeholder: x (actual password is stored in /etc/shadow)
  3. UID: 1001 (User ID, 0 = root)
  4. GID: 1001 (Primary Group ID)
  5. Description: John Smith
  6. Home directory: /home/john
  7. Default shell: /bin/bash

6.4 sudo — Temporary Privilege Escalation ​

bash
# Execute a single command with root privileges
sudo apt update

# Switch to root user's shell
sudo -i

# Execute a command as another user
sudo -u www-data cat /var/www/html/index.html

# Edit the sudoers file (safe method)
sudo visudo

⚠️ Note: ⚠️ Important: Never directly edit the /etc/sudoers file. Use the visudo command, which checks for syntax errors. Syntax errors could cause you to lose sudo access!

Common sudoers Configurations:

bash
# Allow john to use sudo without a password
john ALL=(ALL) NOPASSWD: ALL

# Allow the developers group to execute specific commands
%developers ALL=(ALL) /usr/bin/systemctl restart nginx

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