Users, Groups, and Security

Linux was designed from the start as a multi-user system. Its user and group model is the bedrock of security across cloud VMs, containers, and on-premises servers. Understanding it well helps you avoid both security misconfigurations and accidental lockouts.

The Root User

root is the superuser — UID 0 — with unrestricted access to every file, process, and configuration on the system. On cloud VMs, you typically connect as a non-root user (ubuntu, ec2-user, admin) and use sudo for privileged tasks.

whoami              # show current username
id                  # show UID, GID, and group memberships
id alice            # show info for another user

sudo — Controlled Privilege Escalation

sudo (Super User Do) lets authorised users run specific commands as root without logging in as root. It logs every command run with it, providing an audit trail.

sudo apt update             # run apt as root
sudo systemctl restart nginx
sudo -i                     # open root shell (use carefully)
sudo -u alice command       # run as a different user

# Edit sudoers configuration (always use visudo — it validates syntax)
sudo visudo

The sudoers file (/etc/sudoers) format:

# Allow alice to run all commands as root
alice ALL=(ALL:ALL) ALL

# Allow deploy group to restart nginx without password
%deploy ALL=(ALL) NOPASSWD: /bin/systemctl restart nginx

Managing Users

sudo useradd -m -s /bin/bash alice    # create user with home dir and bash shell
sudo useradd -r -s /sbin/nologin www-data  # create system user (no login)
sudo passwd alice                     # set or change password
sudo usermod -aG sudo alice           # add alice to sudo group
sudo usermod -aG docker alice         # add to docker group
sudo usermod -s /bin/zsh alice        # change shell
sudo userdel alice                    # delete user (keep home dir)
sudo userdel -r alice                 # delete user and home directory

Managing Groups

sudo groupadd developers          # create a group
sudo groupdel developers          # delete a group
sudo gpasswd -a alice developers  # add user to group
sudo gpasswd -d alice developers  # remove user from group
groups alice                      # list alice's groups
getent group developers           # show group members

User Information Files

cat /etc/passwd | grep alice
# alice:x:1001:1001::/home/alice:/bin/bash
# Fields: username:password:UID:GID:comment:home:shell

getent passwd alice    # same, but also works with LDAP/AD

Password Policies

sudo chage -l alice          # show password expiry info
sudo chage -M 90 alice       # password expires every 90 days
sudo chage -E 2026-12-31 alice  # account expires on date
sudo passwd -l alice         # lock account
sudo passwd -u alice         # unlock account

Principle of Least Privilege

A core security principle: grant each user or process only the permissions it needs, nothing more. In practice:

• Don't run web servers as root — use dedicated service users like www-data or nginx.
• Use NOPASSWD sudo rules only for specific, safe commands — not ALL.
• Audit group memberships regularly: getent group docker, getent group sudo.
• Disable or lock unused accounts: sudo passwd -l olduser.
• Use SSH keys instead of passwords for remote access.

Congratulations — You've Completed Linux Basics

You now have a working understanding of the core Linux concepts that underpin every cloud platform, container runtime, and DevOps tool. From the filesystem and terminal to permissions, processes, packages, networking, and user management — these are skills you'll use every day in cloud work.

The next step is to apply this knowledge in a real environment and then practice certification-style questions to test your understanding under exam conditions.