17.3 Linux ops refresh

Q13. App is "down" — first three checks?

Is the process running? Is the port listening (ss)? Are there errors in logs (journalctl / docker logs)? Also disk full (df -h).

Q14. What is a port in networking terms?

A logical service number (shop counter) on a host IP (building) — e.g. 22 SSH, 80 HTTP — not a CPU pin.

Q15. Why is chmod 777 a bad fix?

It grants world read/write/execute and hides the real permission/ownership problem while creating security risk.

Q16. systemctl enable vs start?

start runs now; enable starts on boot. Often you need both for a service that should survive reboot.

Q17. How do you follow logs live?

journalctl -u service -f or docker logs -f or tail -f on an app file — pick the source your app actually uses.