cURL (Client URL) is a versatile tool widely used for transferring data to and from servers. One of its powerful features is the ability to customize HTTP requests by adding headers. This article explains how to use cURL to add headers to your HTTP requests, complete with examples and practical applications.

What Are HTTP Headers?

HTTP headers are key-value pairs that provide additional information with a request or response. They play a crucial role in:

• Authentication: Sending API keys or tokens.
• Content Negotiation: Specifying the content type (e.g., JSON, XML).
• Custom Metadata: Including extra information like correlation IDs for debugging.

Adding Headers with cURL

You can add headers to your cURL requests using the -H or --header option. The syntax is straightforward:

curl -H "Header-Name: Header-Value" <URL>

Examples of Adding Headers

Setting a Content-Type Header

curl -H "Content-Type: application/json" https://api.example.com/data

This tells the server that your request payload is in JSON format.

Adding Multiple Headers

curl -H "Authorization: Bearer YOUR_TOKEN" -H "Accept: application/json" https://api.example.com/secure

Here, we add:

• An Authorization header for authentication.
• An Accept header to specify the desired response format.

Custom Header Example

curl -H "X-Custom-Header: CustomValue" https://api.example.com/custom

This example demonstrates using a custom header for specific use cases.

Sending Headers with POST Data

To include headers along with POST data:

curl -X POST -H "Content-Type: application/json" -d '{"key":"value"}' https://api.example.com/submit

This combines a Content-Type header with a JSON payload.

Practical Use Cases

API Authentication

Most APIs require headers for authentication:

curl -H "Authorization: Bearer YOUR_ACCESS_TOKEN" https://api.example.com/user

Debugging

Adding correlation IDs or custom debugging headers:

curl -H "X-Debug-ID: 12345" https://api.example.com/debug

Web Scraping

Some servers require specific headers to mimic a browser request:

curl -H "User-Agent: Mozilla/5.0" https://example.com

Using cURL in Scripts

You can automate header-based requests in shell scripts:

#!/bin/bash

API_URL="https://api.example.com/data"
TOKEN="YOUR_ACCESS_TOKEN"

curl -H "Authorization: Bearer $TOKEN" -H "Accept: application/json" $API_URL

Advanced Tips

Using Header Files

Store headers in a file and include them in your request:

curl -K headers.txt https://api.example.com/data

Example headers.txt: arduino

-H "Authorization: Bearer YOUR_ACCESS_TOKEN"
-H "Accept: application/json"

Combining Headers with Proxy

curl -H "Header-Name: Header-Value" -x http://proxy.example.com:8080 https://api.example.com

Conclusion

Adding headers with cURL is essential for crafting precise HTTP requests. Whether you're working with APIs, debugging, or handling custom headers, mastering the -H option is invaluable. Practice these examples to streamline your workflows and improve your cURL proficiency.