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5. Linux Basic Commands

5.1 The Linux File-System Hierarchy

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Windows madhe C: drive asto, Linux madhe sagla ek / (root) khali asto. Kontya folder madhe kay aahe he mahit asla ki troubleshooting aani investigation – donhi fast hotat.

/                  root of everything
├── bin, sbin      essential commands (now links to /usr/bin, /usr/sbin)
├── boot           kernel and boot loader
├── dev            device files (/dev/nvme0n1, /dev/xvda)
├── etc            configuration files (nginx, ssh, fstab, passwd)
├── home           users' home folders (/home/ec2-user, /home/ubuntu)
├── root           home of the root user
├── opt            optional / third-party software
├── tmp            temporary files (cleared on reboot)
├── usr            programs, libraries, docs (/usr/share/nginx/html)
├── var            variable data: logs (/var/log), web (/var/www), databases (/var/lib/mysql)
├── proc, sys      virtual files with kernel & hardware info
└── mnt, media     mount points for extra disks

Everything is a file

In Linux, disks (/dev/nvme1n1), processes (/proc/1234) and even hardware info are represented as files. Paths are case-sensitive: Index.html and index.html are different files.

Security-relevant path Why an attacker or defender looks here
/etc/passwd, /etc/shadow User accounts and password hashes
/​etc/​ssh/​sshd_​config SSH settings – root login, password login
/var/log/ Evidence: auth logs, web logs, system logs
/tmp, /dev/shm World-writable – attackers often drop tools here
/home/*/.ssh/ Private keys and authorized_keys
/var/www/ Web application code and config files with passwords

Why this matters for security

During privilege escalation (विशेषाधिकार वाढवणे) an attacker searches exactly these paths for passwords, keys and mistakes. During incident response, a defender checks the same paths for evidence. Knowing the map is half the job for both sides.

Ravindra Bagale's Tip

Many students confuse /root with / (the root directory). / is the starting folder of everything; /root is the root user's home folder. If you use the word "root" in an interview, make clear which meaning you intend.

Practice task

Run ls /, ls /etc | head, ls -la ~ and sudo ls /var/log. Write one line about what each folder contains.

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.