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5. Linux Basic Commands

5.2 Navigation

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Also in the AWS course: this lesson is the same as 3.2 Navigation in the AWS course, which is the main version of this page.

Chala, aata type karuya. Tumchya EC2 var login kara aani majhya sobat he commands chalva.

Command Meaning Example
pwd Print working (current) directory pwd → /home/ubuntu
ls List files ls, ls -l, ls -la, ls -lh /var/log
cd Change directory cd /etc, cd .., cd ~, cd -
tree Show folder tree (install package tree) tree -​L 2 /​etc/​nginx
clear Clear the screen (or press Ctrl+L)
history Previous commands history | tail -20, then !45 to re-run no. 45 
pwd                 # where am I?
ls -la              # long listing including hidden files (starting with .)
cd /var/log         # absolute path (starts with /)
cd ../www           # relative path (.. = parent, . = current)
cd ~                # go to home directory (same as just 'cd')
cd -                # go back to the previous directory

Understanding ls -l output:

-rw-r--r--  1 ec2-user ec2-user  1024 Mar  3 10:15 notes.txt
│└──┬───┘   │ └──┬───┘ └──┬───┘  └┬─┘ └─────┬────┘ └───┬───┘
│ permissions links owner  group   size  modified time   name
└─ type: - file, d directory, l symbolic link

Keyboard shortcuts that save hours

Tab auto-completes file and command names (press twice to see options). ↑ / ↓ scroll through history. Ctrl+C stops a running command. Ctrl+R searches history. Ctrl+A / Ctrl+E jump to start / end of line.

Ravindra Bagale's Tip

Many students type every name in full, and a typo gives "No such file or directory". Use the Tab key – it completes the name, and if the path is wrong it won't complete at all, so you know immediately. This one habit will save you half your time.

Practice task

Starting from your home folder, go to /var/log, then to /etc/ssh, then back with cd -, then home with cd. Print the path with pwd after each step.

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.