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Chapter 4: Linux Advanced Commands

4.1 Searching text: grep

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Friends, orders failed overnight on the Amul-style dairy server – now how will you find the error in a log with thousands of lines? The answer is grep. It is the most used search tool in Linux.

grep prints lines that match a pattern.

Option Meaning
-i Ignore case
-r / -R Recursive search in directories
-n Show line numbers
-v Invert (lines that do NOT match)
-c Count matching lines
-l Only file names
-w Match whole word
-E Extended regex (alternation with a pipe symbol, +, ?)
-​A 3 / -​B 3 / -​C 3 Show 3 lines after / before / around 
grep -i "error" /var/log/nginx/error.log
grep -rn "listen" /etc/nginx/
grep -v "^#" /etc/ssh/sshd_config | grep -v "^$"   # hide comments & blank lines
grep -c " 404 " /var/log/nginx/access.log          # how many 404s?
grep -E "Failed|Invalid" /var/log/auth.log         # Ubuntu SSH failures
sudo grep "Failed password" /var/log/secure         # AL2/CentOS (AL2023 uses journalctl)

Ravindra Bagale's Tip

Many students write grep error and miss the lines with Error or ERROR. The case varies in logs, so add -i: grep -i "error". And when searching a big folder use -rn – you get both the file name and the line number.

Lab

Chala, sudo grep -v "^#" /etc/ssh/sshd_config | grep -v "^$" chalva aani SSH che active settings liha. Nginx install asel tar grep -c " 404 " /var/log/nginx/access.log ne kiti 404 aale te bagha – nasel tar Chapter 6 nantar ha lab parat kara.

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.