Chapter 4: Linux Ops Refresh for DevOps
4.1 When the app is "down" — three checks
Support chat says: "badge API is down." Before you rewrite code, check the machine.
Why it matters for an Instagram-style badge API : lunch traffic fails for three different reasons that look the same in a browser — process crashed, nothing listening on the web counter (port), or disk full so logs and writes fail.
| Check | Question | First command idea |
|---|---|---|
| Process | Is the app / web server running? | ps / systemctl status |
| Port | Is something listening on 80 or 443? | ss -tlnp |
| Disk | Is the disk full? | df -h |
Think of the server IP as one building address. Ports are published ports inside that building — logical numbers like 22 (SSH), 80 (HTTP), 443 (HTTPS). They are not CPU pins. Customers knock on a counter number; the OS routes the request to the process that opened that counter.
Ravindra Bagale's Tip
Khup students browser madhe site nahi ughadat mhanun thodach config badlat bastat. Aadhi server var curl -I http://localhost aani ss -tlnp bagha. Localhost OK aani baher nahi — Security Group / firewall side. Dhyan rakho!
Depth lives in AWS. Paths, users, permissions drills and longer Linux labs: AWS Cloud course Linux chapters. EC2 launch, Security Groups, Elastic IP: AWS EC2 chapter. This chapter is a DevOps warm-up, not a second Linux textbook.