Ravindra BagaleCourses & study guides

Chapter 4: Linux Ops Refresh for DevOps

4.3 Logs — journalctl and tail -f

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When a service fails, logs tell the story.

# systemd unit logs (common on modern Amazon Linux / Ubuntu)
sudo journalctl -u nginx -n 50 --no-pager
sudo journalctl -u nginx -f

# classic file logs
sudo tail -n 50 /var/log/messages
sudo tail -f /var/log/nginx/access.log

Why -f. Follow mode streams new lines while you curl the app in another terminal. You see the request arrive — or you see the error at the same second.

Steps — follow logs while you hit the app

  1. SSH into your practice VM (reuse an instance from the AWS course).
  2. In terminal A: sudo journalctl -u <your-service> -f or sudo tail -f on the access log you use.
  3. In terminal B: curl -I http://localhost (or your app port).
  4. Watch terminal A — a new line should appear if the service handles the request.
  5. Stop follow with Ctrl+C when done.

What you see: either a healthy access/status line, or an error with a path and reason. Fix from the message; do not guess.

Link to AWS

Longer log paths, web server layout and SELinux notes belong in the AWS Linux and web-server chapters. Come back here for the DevOps habit: always watch logs while you reproduce.