Users, Groups, and Sudo

What You'll Learn

Linux is a multi-user system. Every process runs as a specific user with specific permissions. This guide explains how users, groups, and elevated privileges (sudo) work.

Understanding Users

whoami                        # Show your current username
id                            # Show your user ID, group ID, and groups
cat /etc/passwd               # List all users on the system

Every user has a unique numeric ID (UID). Root is always UID 0.

Creating Users

sudo adduser alice            # Interactive way to create a user (recommended)
# or
sudo useradd -m -s /bin/bash alice   # Create with home dir and bash shell
sudo passwd alice             # Set the password

Removing Users

sudo userdel alice            # Remove user (keeps home directory)
sudo userdel -r alice         # Remove user AND their home directory

Understanding Groups

Groups let you manage permissions for multiple users at once.

groups                        # Show groups your current user belongs to
cat /etc/group                # List all groups on the system
sudo groupadd developers      # Create a new group
sudo usermod -aG developers alice   # Add Alice to the 'developers' group

Common default groups:

• sudo or wheel — Users who can run sudo commands.
• www-data — The web server user.
• dialout — Access to serial ports.

Sudo — Running Commands as Root

sudo command                  # Run a single command as root
sudo -i                       # Open an interactive root shell
sudo su -                     # Switch to the root user

The /etc/sudoers file controls who can use sudo. Edit it safely with:

sudo visudo                   # Always use visudo — never edit sudoers directly!

Passwordless Sudo (Use Carefully)

Add this line in visudo to allow a user sudo without a password:

alice ALL=(ALL) NOPASSWD: ALL

Su — Switch User

su -                          # Switch to root (need root password)
su - alice                    # Switch to user Alice (need Alice's password)

The - loads the target user's environment (home directory, shell settings).

Practice Exercise

whoami                        # Who am I?
groups                        # What groups am I in?
sudo adduser testuser         # Create a new user (enter password)
sudo usermod -aG sudo testuser    # Give them sudo access
su - testuser                 # Switch to the new user
whoami                        # Confirm you're now testuser
exit                          # Go back to your original user

Troubleshooting

• "user is not in the sudoers file": Log in as root and run sudo usermod -aG sudo username.
• Forgot your password? Boot into recovery mode to reset it.

What's Next?

You understand users — now learn how to manage running programs: