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Chapter 4: Linux Advanced Commands

Interview Questions

These questions on Linux advanced commands always come up in DevOps and cloud interviews. With every reply, say the command out loud – the interviewer understands that your hands work on the terminal.

Q1. How do you find the top 10 IP addresses in an Nginx access log?
awk '{print $1}' access.log | sort | uniq -c | sort -rn | head
Q2. What is the difference between > and >>? What does 2>&1 do?
> overwrites a file, >> appends. 2>&1 redirects standard error to wherever standard output is going.
Q3. How do you check which ports a Linux server is listening on?
sudo ss -tlnp (or the older netstat -tlnp).
Q4. What is the difference between service <name> restart and service <name> reload?
restart stops and starts the service (brief downtime). reload asks the service to re-read its configuration without dropping existing connections (if supported).
Q5. What is the difference between service and systemctl?
systemctl is the native systemd tool. service is an older wrapper that, on systemd systems, forwards start/stop/restart/reload/status to systemctl. Enabling at boot (systemctl enable) and daemon-reload require systemctl.
Q6. Explain the five fields of a cron expression.
Minute, hour, day of month, month, day of week — e.g. 30 2 * * 1 runs at 02:30 every Monday.
Q7. How do you see the logs of a systemd service?
journalctl -u <service>; add -f to follow live, -n 50 for the last 50 lines, --since "1 hour ago" to filter by time.
Q8. What is the difference between df and du?
df shows free/used space per mounted filesystem; du shows how much space specific files or directories use.

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.