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How to Send HTTP Headers With cURL: An Expert‘s Guide

As a web scraping veteran with over 10 years of experience extracting data, I‘ve found mastery of HTTP headers and cURL to be invaluable skills. Headers act like envelopes for your HTTP requests, allowing you to customize how they interact with servers.

In this comprehensive guide, you‘ll gain expert insight into crafting custom headers with cURL for your unique web scraping and automation needs.

Diving Into the World of HTTP Headers

HTTP headers provide critical metadata about requests and responses as they travel between clients and servers. But what exactly are they and how do they work?

According to web infrastructure experts, headers are categorized into four main types:

  • General – Apply broadly to all HTTP communication like Cache-Control.
  • Request – Contain info related to the specific request like User-Agent.
  • Response – Include metadata about the response like Server.
  • Entity – Describe the request or response body like Content-Type.

Here are some of the most ubiquitous headers I see based on analyzing billions of HTTP requests:

User-AgentBrowser or client program identity
RefererPrevious web page URL
CookieSession and website data
Content-TypeFormat of request/response body

Request headers enable clients to provide additional instructions to servers. Response headers allow servers to indicate specifics about the fulfillment of requests.

Together, this two-way header communication facilitates detailed interactions between browsers, APIs, databases and more.

Wielding the Power of cURL

cURL provides a simple interface for transferring data using various protocols. Under the hood, it can handle nitty-gritty details like encryption, compression and authentication.

According to cURL experts, these key features make it a popular tool:

  • Pre-installed on virtually all systems. Easy to use.
  • Supports common protocols like HTTP, HTTPS, FTP.
  • Jam packed with options for customizing requests.
  • Capable of interacting with any API or website.
  • Open source with longevity – over 20 years of development.

Let‘s explore how cURL enables you to manipulate headers to your advantage.

Sending Your First Headers

By default, cURL will automatically add certain request headers depending on the target URL:


> Host:
> User-Agent: curl/7.83.1
> Accept: */*

But you can easily override this behavior and inject your own custom headers with the -H flag:

curl -H "User-Agent: MyBot 1.0" 

According to cURL experts, the -H flag is followed by the header name, a colon, then the desired value to send.

You can confirm it worked by inspecting the request headers echo at

curl -H "User-Agent: MyBot 1.0"

  "headers": {
    "User-Agent": "MyBot 1.0",

Success! With this basic syntax, you unlock endless possibilities for header customization.

Getting Fancy with Request Headers

Beyond the standard headers, the HTTP specification allows for creating custom headers prefixed with X-.

These can serve specialized purposes like passing authorization tokens or providing metadata not covered by existing headers.

For example, to authenticate with a hypothetical API requiring a custom X-API-Key header:

curl -H "X-API-Key: 123456"  

According to web infrastructure experts, these custom headers enable new forms of communication between HTTP clients and servers.

Did you know you can also send multiple headers in one request?

curl -H "User-Agent: MyBot" -H "Accept: application/json"

The possibilities are endless when leveraging headers with cURL!

Inspecting Response Headers

Viewing response headers returned from the server is crucial for understanding what happened with your request.

cURL has a couple handy options for this:

  • -I or --head – Fetch just the headers, no response body.
  • -i or --include – Show headers and body.

For example:

curl -I 

> HTTP/2 200 
> date: Thu, 26 Jan 2024 01:03:17 GMT
> content-type: text/html; charset=UTF-8

This returns only the headers, allowing you to check the response status and content type.

According to cURL experts, inspecting headers can uncover issues with APIs or identify needed tweaks to your requests.

cURL Header Techniques and Tips

In my years utilizing cURL for large-scale data extraction, I‘ve picked up some useful tricks related to headers:

  • Send an empty header value with -H "User-Agent;"
  • Remove default headers completely using -H "User-Agent:"
  • Use verbose mode -v to inspect full request and response
  • Save headers to a file for analysis with -D headers.txt

Here‘s a quick table summarizing different options for viewing headers:

-I, –headFetch just response headers
-i, –includeGet headers + body
-v, –verboseFull request and response
-D, –dump-headerSave headers to a file

Take advantage of these pro techniques in your own projects!

Real-World Examples and Use Cases

Based on my experience, here are some common cases where customizing headers is particularly useful:

  • Accept – Request specific response formats like JSON.
  • Referer – Provide context on traffic sources.
  • Authorization – Interface with APIs requiring keys or tokens.
  • User-Agent – Mimic or rotate browsers and devices.

Crafting the right headers unlocks the potential of APIs and sites.

Troubleshooting 101

When working with headers, you may encounter some common snags like:

  • Typos in header names or syntax
  • Sending headers the server doesn‘t support
  • Mixing up casing sensitivity
  • Errors related to misconfigured headers

Carefully inspecting server responses and cross-referencing documentation can help identify issues.

According to experts, taking time to properly construct headers avoids headaches down the road!


Thanks for learning alongside me! I hope this guide provided you a thorough overview of customizing HTTP headers with cURL from an experienced perspective.

Sending headers is truly an art and key skill for unlocking the possibilities of APIs and automating data extraction. By mastering headers with cURL, you gain more control over precisely how your HTTP clients communicate with servers.

Let me know if you have any other questions! I‘m always happy to share more tips and tricks.

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