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Chapter 3: Linux Basic Commands

3.4 Viewing files

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Friends, you don't need an editor to read a file on a server – cat, less, head and tail let you read it quickly. The command used most in real work is tail -f – for watching a live log.

Command Use
cat file Print entire file
less file Scroll page by page (Space next, b back, /word search, q quit)
more file Older, simpler pager
head -n 20 file First 20 lines
tail -n 20 file Last 20 lines
tail -​f /​var/​log/​nginx/​access.​log Follow a log live (Ctrl+C to stop)
wc -l file Count lines (-w words, -c bytes)
diff a b Show differences between two files 
cat /etc/os-release          # which Linux distribution and version?
head -5 /etc/passwd
sudo tail -f /var/log/messages      # Amazon Linux / CentOS system log
sudo tail -f /var/log/syslog        # Ubuntu system log

Ravindra Bagale's Tip

Many students open a 2 GB log file with cat and the terminal seems to hang. For big files use less or tail -n 50, and tail -f to watch errors live. And if /var/log/messages is missing on Amazon Linux 2023, don't worry – use journalctl there (Chapter 4.6).

Lab

Chala, head -5 /etc/passwd aani wc -l /etc/passwd chalva – server var kiti accounts aahet? Mag less /etc/passwd madhe /ec2-user (Ubuntu var /ubuntu) shodha aani q ne baher ya.

Extended teaching notes

Why this lesson matters

Cloud security is mostly IAM, network exposure, logging and secure defaults. The shared responsibility model means the provider patches the metal; you patch what you configure.

  • No public databases or public admin panels.
  • Roles / OIDC instead of long-lived access keys.
  • CloudTrail / Activity Logs / Audit Logs on and retained.
  • S3/Blob/GCS: Block Public Access; encrypt; least privilege.

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.