When working with APIs or web servers, it is common to send more than one HTTP header in a request. The most common HTTP headers include User-Agent, Content-Type, Accept, and Cache-Control, each playing a key role in HTTP requests and responses—such as identifying the client, specifying the data format, indicating acceptable response types, and controlling caching behavior. These headers can include authentication tokens, content types, user agents, or custom fields required by the server.
This guide explains how to use curl multiple headers in a single command, with clear examples for beginners. With cURL, you can add headers—including both standard headers and custom headers—to control request behavior and enhance functionality. You can use HTTP headers with curl to customize requests, and adding custom headers is often necessary for advanced API interactions.
Introduction to cURL
cURL is a versatile command line tool designed for transferring data using URL syntax. It supports a wide range of network protocols, including HTTP and HTTPS, making it an essential utility for developers working with web servers, APIs, and web applications. Whether you’re performing web scraping, automating curl requests, or testing endpoints, cURL allows you to send http headers, manage headers with curl, and interact with web servers directly from the command line. Its flexibility and ease of use make it a popular choice for transferring data, checking http headers, and troubleshooting network issues. Mastering the curl command and understanding how to work with headers curl is key to efficient data transfer and effective API communication.
How Headers Work in cURL
cURL uses the -H flag to send a header. Each -H option is a command line option that adds one header to the request. To pass multiple headers, you simply repeat the -H command line option for each header you want to include. This allows you to include as many headers as needed for your API communication.
The pattern is:
curl -H "Header-Name: value" -H "Another-Header: value" https://example.com
This example demonstrates how to send two headers in a single cURL request. You can add as many headers as necessary by repeating the -H or --header option for each additional header.
Basic Example of Sending Multiple Headers
Here is a simple example using two common headers:
curl -H "Accept: application/json" \
-H "User-Agent: CustomClient/1.0" \
https://example.com/api
In this example:
- The accept header tells the server what response format is expected. Many APIs support multiple formats, such as JSON or XML, and the accept header specifies the preferred content type.
- User-Agent identifies the client making the request
Each header is added using a separate -H flag.
Adding Authentication Headers
Many APIs require authenticated requests. Sending authenticated requests involves including an authorization header, such as a bearer token, to access secure resources. You can send tokens or API keys along with other headers:
curl -H "Authorization: Bearer my-access-token" \
-H "Accept: application/json" \
https://example.com/data
Here the request includes both an authorization header and a preferred response format. Some APIs may require a custom authorization header for OAuth2 or other authentication schemes when sending authenticated requests.
Sending POST Requests with Multiple Headers
When sending data, you often need both headers and a request body. For example, the following is a typical curl post request:
curl -X POST \
-H "Content-Type: application/json" \
-H "Authorization: Bearer 12345" \
-d '{"name": "Alice"}' \
https://example.com/users
- Content-Type specifies the format of the body, which is usually json data
- Authorization provides the required access token
- -d sends the JSON payload; you can also use a json file (e.g., -d @data.json) to structure your payload for testing
cURL automatically switches to a POST request when -d is used, unless you specify otherwise.
Using Multiple Headers with PUT or DELETE
For other HTTP methods such as PUT or DELETE, the pattern stays the same:
curl -X PUT \
-H "Content-Type: application/json" \
-H "X-Request-ID: 9876" \
-d '{"email": "alice@example.com"}' \
https://example.com/users/1
You can include additional headers, such as a custom user agent string or additional HTTP headers, to meet specific API requirements. For example, you might add a custom header like X-Amzn-Header for Amazon-related services, or use -H "User-Agent: custom user agent" to set a custom User-Agent. Each header is still sent with its own -H flag.
Setting Custom or Multiple Repeated Headers
Some APIs use custom headers or may even expect repeated headers. You can send custom HTTP headers by specifying the header name and value using the -H flag. For example, sending HTTP headers is often required for purposes such as authentication or including metadata. You can also send HTTP headers with empty values, which is sometimes necessary to override or remove default headers. Set them like this:
curl -H "X-Tracking-ID: abc123" \
-H "X-Tracking-ID: def456" \
https://example.com/track
Although this is rare, cURL does allow duplicate header names.
Using a File to Store Multiple Headers
If you frequently send many headers, writing them all in one cURL command can become messy. Using a file helps manage http headers efficiently, especially when you need to send custom headers in bulk. The curl client lets you load headers from a file to send custom headers, and you can also retrieve headers or check http headers by specifying header options and loading them from a file. This demonstrates the flexibility of handling headers using curl, making it easy to send, view, and manipulate HTTP headers for various scenarios.
Create a file named headers.txt:
Accept: application/json
Authorization: Bearer 12345
X-Client: analytics-tool
Then run:
curl -H @headers.txt https://example.com
This approach keeps your command line clean and is useful when working with dozens of headers.
Removing Default Headers
In some cases, you may need to remove default headers that cURL automatically includes in your requests. This is particularly useful in web scraping or when interacting with APIs that require custom header configurations. For example, cURL typically sends a default User-Agent header, but you can override or remove it if needed. To remove a default header, use the -H option followed by the header name and a colon, but leave the value empty. For instance, to remove the user agent header, add ``` -H "User-Agent:"
to your command. This tells cURL to send an empty User-Agent, effectively omitting the default header.
This technique is helpful when you want to avoid detection by web servers or ensure that only your
specified headers are sent in the request. Managing default headers and custom header values is a crucial
part of working with headers in curl, especially for advanced web scraping and API testing scenarios.
Viewing Response Headers
Understanding the response headers returned by a web server is vital for debugging and analyzing HTTP requests. cURL provides several options to help you view both the response headers and the content. By adding the ``` -i
or ```
--include
flag to your curl command, you can display both the response headers and the response body. If you only want to see the response headers without the body, use the ``` -I
or ```
--head
option. For even more detailed information about the entire HTTP request and response, including all the headers curl processes, the ``` -v
or ```
--verbose
flag is invaluable. These options make it easy to check http response headers, inspect both the response headers and the request and response flow, and troubleshoot issues with your curl requests.
Troubleshooting Header Issues
If your headers are not being recognized:
Issues with request header formatting can prevent successful data transfer when using tools like cURL. Checking request headers is a key step in troubleshooting, as incorrect or missing headers can cause servers to reject or misinterpret your requests. Make sure you are specifying the correct request header values and that they are formatted properly. This is especially important when you need to transfer data or interact with APIs that require specific request headers.
Check quotation marks
Always wrap headers in quotes to avoid spaces breaking the command.
Avoid trailing semicolons
Headers should be written as Key: value with no trailing semicolon.
Confirm the method
If your request includes -d but the API expects GET, use -X GET to override cURL’s default behavior.
Check for unnecessary spaces
Extra spaces around colons can cause servers to reject headers.
Best Practices for Working with cURL
To get the most out of cURL, it’s important to follow best practices for secure and efficient data transfer. Always specify http headers and curl headers clearly, and remove default headers when they might interfere with your requests. Use the appropriate options to view response headers and analyze the full request and response cycle. Employ the ``` -v
flag for in-depth debugging, especially when working with complex APIs or troubleshooting network issues. By consistently managing headers curl sends and receives, you can ensure your requests are correctly formatted and your data transfer remains secure. Following these best practices will help you receive http headers accurately, avoid common pitfalls, and make your use of cURL more effective and reliable.
Summary
Sending curl multiple headers is straightforward. Each header requires its own -H flag, and you can include as many as needed. This makes cURL a powerful tool for interacting with APIs, especially when authentication, content types, or custom metadata are required.
FAQs
