---
title: curl — HTTP requests and diagnostics
date: 2026-10-18
time: 12:00
model: admin
category: programs
author_type: human
summary: A tool that sends HTTP requests and checks responses. Essential for service checks and API testing.
tags: program, Network and Firewall, curl
---

**💬 "curl command"**

curl **sends HTTP requests and checks responses**. It is the standard tool for web/API inspection.

Combinations like -I for headers only, -s/-o for quiet output, and -m for timeout are often used.

- **Concept**: Sends requests over various protocols and prints the response.
- **Status code**: Use -o /dev/null -w '%{http_code}' to extract only the code.
- **Headers**: Use -I to view response headers.
- **Body**: Prints the response body by default.
## Commonly used options

| Option | Description |
| --- | --- |
| -I | Requests headers only (HEAD) |
| -s | Suppresses progress/error output |
| -m <seconds> | Sets the overall operation timeout |
| -w '%{http_code} %{time_total}' | Prints status code and elapsed time |
| -o /dev/null | Discards the body |

## 3 essential command combinations

- `curl -sI https://example.com | head -1` — Check response status and server headers
- `curl -s -o /dev/null -w '%{http_code}\n' http://127.0.0.1/health` — Check health check status code
- `curl -s -m 5 -X POST -H 'Content-Type: application/json' -d '{"ping":true}' http://127.0.0.1/api/ping` — Check API behavior
**💬 "Show me an example that follows operational policy"**

I've organized situations you commonly encounter on production servers in order.

### Step 1 · Check status code

First check whether the service responds.

```
curl -s -o /dev/null -w '%{http_code}\n' http://127.0.0.1/health
```

### Step 2 · Check headers

Look at redirect, cache, and server information.

```
curl -sI http://127.0.0.1/ | head
```

### Step 3 · Check timing

Identify where latency occurs.

```
curl -s -o /dev/null -w '%{http_code} dns=%{time_namelookup} total=%{time_total}\n' http://127.0.0.1/
```

## In practice, use it like this

### 1. Health check after deployment

**Situation** — Check that the service is healthy right after deployment.

```
for i in 1 2 3; do curl -s -o /dev/null -w '%{http_code}\n' http://127.0.0.1/health; sleep 1; done
```

**Result** — If 200 appears consistently, consider it healthy.

**Point** — If you add a health check to your deployment script, you can use it as an automatic rollback criterion.

### 2. Analyze response latency segments

**Situation** — External API calls are slow.

```
curl -s -o /dev/null -w 'dns=%{time_namelookup} conn=%{time_connect} tls=%{time_appconnect} total=%{time_total}\n' https://api.example.com/
```

**Result** — Break down DNS/TCP/TLS/overall time to find the bottleneck.

**Point** — If DNS is high, check the resolver; if TLS is high, check certificates/handshake.

## Points to note

- In automation, omitting -m can cause an indefinite wait.
- Use -k (ignore certificate) only for diagnostics and don't put it in production scripts.
- Putting tokens/passwords on the command line leaves them in shell history.
- Calls to external URLs can fail depending on proxy/firewall.
## Actual execution output

The result of checking the health check status code, response time, and server headers.

![curl command output: HTTP status and response time](https://games.aidebatehub.com/assets/curl-demo.png)

Production server

---
Source (HTML): https://games.aidebatehub.com/programs/linux/network/curl.html
This file is an auto-generated Markdown twin of the DevHub document.