How to Send HTTP Headers With cURL

HTTP headers allow you to send additional information in your HTTP requests and responses. They are an essential part of web scraping and allow you to customize how your scraper interacts with websites.

In this comprehensive guide, you‘ll learn how to use the powerful cURL command line tool to send custom HTTP headers, view response headers, troubleshoot issues, and more.

An Introduction to HTTP Headers

Every HTTP request and response contains headers – metadata that provides important context and instructions for transmitting and handling data.

Some common request headers include:

• User-Agent: Identifies the browser or application making the request. This can impact how content is served.
• Authorization: Contains authentication credentials when accessing protected resources.
• Referer: Indicates the webpage that linked to the requested URL.

And some common response headers:

• Content-Type: Specifies the format of the returned content (HTML, JSON, etc).
• Set-Cookie: Contains cookie data to be stored by the client.
• Cache-Control: Provides caching instructions to the client.

HTTP headers are an essential part of web scraping. By customizing headers, you can better mimic real browsers, troubleshoot issues, and optimize performance.

Sending Custom HTTP Headers with cURL

cURL is a powerful command line tool that transfers data using various protocols. With cURL, you can customize the HTTP headers sent in your requests.

To send custom headers with cURL, use the -H or --header option followed by the header name and value in this format:

-H "Header-Name: value"

For example, to send a custom User-Agent header:

curl -H "User-Agent: MyBot 1.0" https://www.example.com 

This sends a request to https://www.example.com with a custom User-Agent header value of "MyBot 1.0".

The website http://httpbin.org/headers is great for testing headers. It simply returns the headers it received:

curl -H "User-Agent: MyBot 1.0" http://httpbin.org/headers

Sending Multiple Headers

To send multiple headers, include the -H option multiple times:

curl -H "User-Agent: MyBot 1.0" -H "Authorization: Bearer token" http://httpbin.org/headers

This sends two custom headers – User-Agent and Authorization.

Removing Default Headers

To remove a default header, use -H with the header name followed by a colon but no value:

curl -H "User-Agent:" http://httpbin.org/headers

This removes the default User-Agent header.

Viewing HTTP Headers

To view response headers, use the -I or --head option. This performs a HEAD request to retrieve just the headers:

curl -I http://httpbin.org/headers

To see both headers and content, use -i or --include:

curl -i http://httpbin.org/headers

These options let you inspect headers returned from the server for debugging purposes.

Use Cases for Custom Headers

Custom headers unlock many useful applications of cURL for web scraping and automation.

Changing Response Format

Use the Accept header to specify that you prefer a particular response format like JSON:

curl -H "Accept: application/json" http://api.example.com/data

Conditional Requests

Take advantage of caching and only retrieve updated resources by using headers like If-Modified-Since:

curl -H "If-Modified-Since: Sat, 11 Jan 2020 05:55:55 GMT" http://api.example.com/data

Authentication

Many APIs require authentication headers like API keys:

curl -H "X-API-Key: 123456789" http://api.example.com/data  

Spoofing Headers

Mimic a particular browser or device by spoofing User-Agent and other headers.

Advanced cURL Header Techniques

cURL offers advanced options for working with headers.

Save Headers to a File

Use -D or --dump-header to save headers to a file for analysis:

curl -D headers.txt http://httpbin.org/headers

Verbose Output

Include -v or --verbose to see detailed info including all headers sent and received:

curl -v http://httpbin.org/headers  

Debugging Empty Headers

If you need to send an empty header value, use a semicolon after the header name:

curl -H "User-Agent;" http://httpbin.org/headers

Common Header Issues and Solutions

When headers don‘t work as expected, here are some troubleshooting tips:

• Double check header syntax – name and value should be separated by a colon
• Verify the header is supported by checking API docs
• Try adjusting capitalization of header names
• Inspect raw response headers and content for clues
• Use -v to see headers sent vs received

Careful inspection of raw headers and responses typically reveals the issue.

Recap and Additional Resources

• HTTP headers provide important metadata for requests and responses
• cURL‘s -H option sets custom request headers like User-Agent
• View headers with -I and content + headers with -i
• Headers can authenticate requests, change response formats, optimize caching, and more
• Troubleshoot issues by inspecting raw headers and responses

For more cURL tips, see our guide on using cURL for web scraping which covers proxies, automation, and more. The cURL man page also provides extensive documentation and examples.

Hopefully this guide has provided a comprehensive overview of how to send HTTP headers with cURL for your web scraping and automation needs! Let us know if you have any other questions.