Ravindra BagaleCourses & study guides मराठी Track your progress

5. Linux Basic Commands

5.6 Users, Groups and sudo

By Last updated:

Command Purpose
whoami Current user name
id UID, GID and groups of the current user
sudo useradd -​m -​s /​bin/​bash raja Create user with home dir and bash shell (adduser raja on Ubuntu is interactive)
sudo passwd raja Set password
sudo usermod -​aG wheel raja Add to admin group (wheel on AL2023/CentOS, sudo on Ubuntu)
sudo groupadd devops Create group
sudo usermod -​aG devops raja Add user to extra group (-a = append — never forget it!)
groups raja Show user's groups
sudo userdel -​r raja Delete user and home folder
su - raja Switch to another user
sudo -i Become root (use carefully, type exit to return)

User information lives in /etc/passwd (accounts), /etc/shadow (hashed passwords) and /etc/group (groups). Sudo rules live in /etc/sudoers — always edit with sudo visudo.

Why sudo instead of logging in as root?

sudo gives admin power only for one command, logs who did what (/var/log/secure or /var/log/auth.log), and reduces accidents. On EC2, the default user (ec2-user/ubuntu) has password-less sudo, and direct root login over SSH is disabled.

sudo cat /etc/shadow | head -3     # hashes: $6$ = SHA-512, $y$ = yescrypt
sudo -l                            # what am I allowed to run with sudo?
last -n 10                         # recent logins

Why this matters for security

sudo -l is one of the first commands an attacker runs after getting a shell – a rule like "NOPASSWD: /usr/bin/vim" gives root in seconds. Defenders audit /etc/sudoers and group membership (wheel, sudo) regularly and remove users who leave.

Ravindra Bagale's Tip

If you forget -a with usermod -G, the user loses all their other groups – and sometimes sudo access too. Many students have locked themselves out like this. Always write -aG – remember, "a" means append.

Practice task

Create user shahrukh, set a password, add him to wheel (Amazon Linux) or sudo (Ubuntu), switch to him with su - and run sudo whoami. Then check sudo -l.

Extended teaching notes

Why this lesson matters

Use this lesson to strengthen systems you own: understand the risk, practise safely in a lab, then write and apply the fix.

Apply the fix across stacks

Same class of bug, same idea in every language.

Java / Spring Boot

Parameterise queries, encode output, CSRF tokens, limited Actuator, dependency scanning.

ASP.NET Core

EF Core / parameterised SQL, Razor encoding, HTTPS, Data Protection.

Node.js / Express

helmet, parameterised SQL, npm audit, avoid eval and shell concat.

Python / Django / Flask / FastAPI

ORM, autoescape, pip-audit, Bandit; never shell=True with user input.

PHP / Laravel

PDO/Eloquent, Blade escaping, composer audit.

Go / Ruby

database/sql placeholders / ActiveRecord; html/template / ERB escaping; govulncheck / Brakeman.

nginx / Apache / Tomcat

Hide versions, TLS, security headers, least-open ports.

MySQL / PostgreSQL / MongoDB / Redis

Private networks, auth on, least-privilege users, no 0.0.0.0/0.

AWS / Azure / GCP

Roles not long-lived keys, private data stores, cloud audit logs on.

Mini case study

Suppose we are on a purple-team exercise for an Indian mid-size company (sample data). Red notes a technique from this lesson against a lab clone. Blue must show the control that stops it, the log that would have fired, and the ticket that hardens production. That is the standard this course trains you for.

Defender's worksheet (do this today)

  1. Write the lesson title, date and the exact lab target you are allowed to use.
  2. List three owned systems (VMs, apps, or cloud accounts) where this lesson applies.
  3. Run one safe check from the lesson on a lab system only. Paste the command and output.
  4. Write a two-sentence fix you would give a teammate.
  5. Name one related public advisory (CERT-In, CISA, NVD, or a vendor bulletin) with its date.

Indian workplace (sample data)

Suppose we are a junior analyst at Infosys, TCS, Wipro or a fintech SOC in Bengaluru/Pune. Turn this lesson into one checklist item with an owner, a due date and a place to store evidence (ticket + screenshot).

Global practice

Compare your notes with OWASP, CIS Benchmarks, NIST CSF/IR guidance or a cloud Well-Architected security question. Write five lines on what overlaps.

Everyday analogy

Explain the risk and the fix to a family member without jargon. If they can repeat the warning, you understand it well enough to teach juniors.

Key terms

  • Control: a safeguard (patch, config, process, detection).
  • Finding: a weakness backed by evidence.
  • Remediation: the fix plus how you verified it.
  • Scope: systems you have written permission to touch.
  • Least privilege: only the access the task needs.
  • Defence in depth: several layers so one failure does not end the story.

Common mistakes

  • Skipping written permission "just for a quick scan".
  • Practising on hostel, office or public targets without authorisation.
  • Collecting tool output without writing the fix.
  • Copying attack steps onto production to "verify". Defenders verify with versions, configs and patches.
  • Publishing screenshots that contain secrets, cookies or internal hostnames.

Recap

  • Evidence, fix, owner, date.
  • Own lab only.
  • Fix > spectacle.