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Chapter 4: Linux Ops Refresh for DevOps: Cards

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Revision cards

When the app is down: three checks. Random poking wastes time during an outage.
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Lesson: 4.1 When the app is "down" — three checks

Paths and permissions: enough to stay safe. Wrong permissions break apps or expose secrets.
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Lesson: 4.2 Paths and permissions — enough to stay safe

Logs: journalctl and tail -f. Logs tell you why something failed.
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Lesson: 4.3 Logs — journalctl and tail -f

systemd: start, enable, status. Apps that don't start after reboot cause surprise outages.
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Lesson: 4.4 systemd — start, enable, status

Disk, process, ports: the triage trio. Full disks, runaway processes and port clashes cause most server trouble.
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Lesson: 4.5 Disk, process, ports — the triage trio

SSH keys, scp, rsync one-liners. Passwords over SSH are weak and slow to type.
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Lesson: 4.6 SSH keys, scp, rsync — one-liners

Worked triage story: peak traffic. Seeing a full story teaches the order of thinking.
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Lesson: 4.7 Worked triage story — peak traffic

Quick extras: uptime, memory, who am I. You must know where you are before changing anything.
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Lesson: 4.8 Quick extras — uptime, memory, who am I

What this chapter deliberately skips. Trying to learn all of Linux at once stops progress.
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Lesson: 4.9 What this chapter deliberately skips

Interview cards

The website is down. What do you check first? Is the service running, what do the logs say, and is the port listening; then disk and memory.
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Lesson: 4.1 When the app is "down" — three checks

How do you fix ‘permission denied’ safely? Check owner and mode with ls -l, then give only the needed access with chown or chmod, never wide-open.
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Lesson: 4.2 Paths and permissions — enough to stay safe

How do you read a service's logs on Linux? journalctl -u name for systemd services (-f to follow), or tail -f on the log file.
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Lesson: 4.3 Logs — journalctl and tail -f

start vs enable in systemctl? start runs the service now. enable makes it start automatically at boot.
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Lesson: 4.4 systemd — start, enable, status

A server is slow. What commands do you run? df -h (disk), top or free -h (CPU/memory), ss -tlnp (ports), then logs.
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Lesson: 4.5 Disk, process, ports — the triage trio

scp vs rsync? scp copies files fully every time. rsync copies only differences and can resume, better for big or repeated copies.
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Lesson: 4.6 SSH keys, scp, rsync — one-liners

Walk through a real slowdown you'd debug. Check disk, CPU/memory, then logs for errors like timeouts, then fix the bottleneck and watch metrics.
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Lesson: 4.7 Worked triage story — peak traffic

What does load average in uptime mean? Average number of processes waiting for CPU over 1, 5 and 15 minutes; compare to CPU count.
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Lesson: 4.8 Quick extras — uptime, memory, who am I

How much Linux does a DevOps engineer need? Daily ops: files, permissions, processes, services, logs, networking, SSH. Deep internals come later.
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Lesson: 4.9 What this chapter deliberately skips

Comparison cards

systemctl start vs enable. systemctl controls Linux services.start: run the service now.enable: start it automatically at every boot.
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Lesson: 4.4 systemd — start, enable, status

scp vs rsync. Both copy files over SSH.scp: copies everything each time.rsync: copies only what changed.
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Lesson: 4.6 SSH keys, scp, rsync — one-liners

journalctl vs tail -f. journalctl: reads logs collected by systemd.tail -f: follows a plain log file as lines are added.
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Lesson: 4.3 Logs — journalctl and tail -f