Ping vs cURL — Is It Up, and Is It Working? 🛰️

4:55 PM, Friday. Slack lights up: "Is the API down?? Nothing's working!" Everyone panics. You calmly type two commands and, in 10 seconds, know exactly whose problem it is. That calm is a skill — here it is.

🧠 The mental model: a building

• ›ping = knocking on the building's front door. Is anyone home? Is the building even standing? It tells you the machine is reachable — nothing about your app.
• ›curl = walking in and asking the receptionist a real question. Does the service actually answer correctly?

ping — "is the host reachable?" (Layer 3)

ping sends a tiny ICMP echo and times the round trip:

bash
ping example.com
# 64 bytes from 93.184.216.34: time=12.3 ms   ✅ reachable, ~12 ms away

curl — "does the request actually work?" (Layer 7)

curl makes a real HTTP request and shows you the response — status, headers, body. This is how you test an API by hand:

bash
curl -i https://example.com/health
# HTTP/2 200
# content-type: application/json
# {"status":"ok"}

🔑 The killer insight: ping works but curl fails → the server is up, but your app is broken (crashed service, wrong path, missing auth). That one distinction saves hours of blaming the network.

HTTP status codes — read them like an X-ray

The first digit tells you the whole story:

🧠 Remember just this: 4xx = YOU, 5xx = THEM. A 4xx means fix your request (bad URL, missing key, no auth). A 5xx means the server crashed — check its logs, not your code. (Fun one to cement it: 418 = "I'm a teapot", a real joke status code. If you remember the teapot, you remember 4xx = client.)

The curl flags you'll actually use

bash
curl -i URL                      # -i = show response headers (and the status code)
curl -X POST URL                 # -X = choose the method (GET, POST, ...)
curl -H "Authorization: Bearer TOKEN" URL   # -H = add a header (e.g. auth)
curl -d '{"qty":10}' URL         # -d = send a request body (JSON)

🔧 Try it for real

bash
ping google.com                  # ctrl-C to stop. See your latency.
curl -i https://api.github.com   # a real public API — read the status line

Your Task

Write classify(status) to label any HTTP code by category, then print the classification for a few codes. Predict first: is 404 a "you" or "them" problem?