🩺 Troubleshooting

Look up Linux errors and incidents by symptom. Permission denied, No space left, SSH failures, high CPU load, memory pressure, DNS resolution — find diagnostic steps and fixes fast.

🔒 Permission denied

Permissions
Intermediate ⏱️ ~10 min

How to fix Permission denied

chmod / chown / sudo usage and avoiding common pitfalls.

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🌐 Connectivity / Network

Network
Intermediate ⏱️ ~12 min

SSH connection troubleshooting checklist

Permission denied (publickey), Host key verification failed, Connection timeout.

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Intermediate ⏱️ ~10 min

DNS resolution troubleshooting

dig / host / nslookup usage and /etc/resolv.conf inspection.

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Intermediate ⏱️ ~10 min

Port connectivity troubleshooting

nc / telnet / curl for connection check, ss / netstat for port state.

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Intermediate ⏱️ ~12 min

ufw firewall and SSH connection

Recovering when ufw enable cuts off SSH.

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📊 Slow server

Performance
Intermediate ⏱️ ~12 min

Diagnosing 100% CPU usage

Identifying culprit processes with top / ps / load average.

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Intermediate ⏱️ ~12 min

Investigating memory pressure

free / top / ps usage and OOM Killer mitigation.

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Intermediate ⏱️ ~10 min

Diagnosing slow disk I/O

iostat / vmstat to pinpoint disk bottlenecks.

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💾 Disk space

Disk
Intermediate ⏱️ ~10 min

Handling "No space left on device"

df / du to find bloated files and reclaim space.

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Intermediate ⏱️ ~10 min

Identifying Docker disk usage

docker system df to break down image / container / volume usage.

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Intermediate ⏱️ ~8 min

Safely deleting files with find

print0 / xargs -0 for filename-safe deletion.

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📜 Web / Logs

Logs
Intermediate ⏱️ ~10 min

Reading Nginx/Apache logs

access.log / error.log paths and how to read incident causes.

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🐧 Practice your skills hands-on

Reading is just the start — practice in the browser-based terminal to build muscle memory.

Practice in Penguin Gym Linux Browse Tutorials