Getting Started with cURL: A Command-Line Tool for Transferring Data with URLs
In today’s digital world, billions of API requests are made every day. From mobile apps and smartwatches to web platforms and smart TVs, modern applications constantly communicate with servers behind the scenes. As developers, we often rely on graphical tools to test these APIs—but understanding what happens at the protocol level is far more powerful.
This is where cURL becomes essential.
In this article, I’ll walk through what cURL is, why it matters, and how you can use it to test, debug, and automate API interactions directly from the command line.
What Is cURL?
cURL (Client URL) is an open-source command-line tool used to transfer data to and from a server using URLs. It supports a wide range of protocols, including HTTP and HTTPS, which makes it particularly useful for working with REST APIs.
Unlike GUI tools such as Postman or Insomnia, cURL operates directly from the terminal. It is available on macOS, Linux, and modern versions of Windows, making it highly portable and convenient for developers working across environments.
At its core, cURL allows you to construct HTTP requests manually—giving you full control over headers, request methods, authentication, cookies, payloads, and more.
Why Use cURL?
cURL is often described as the “Swiss Army knife” of data transfer tools. It is lightweight, stable, and supports hundreds of command-line options. Because it runs in headless mode, it is especially useful in servers, CI/CD pipelines, and automation scripts where no graphical interface is available.
Another major advantage is its detailed debugging capability. With verbose mode enabled, cURL can display request headers, response headers, TLS handshakes, timing details, and status codes. This makes it extremely valuable when diagnosing API issues.
Most importantly, cURL integrates seamlessly into shell scripts, enabling developers to automate health checks, deployments, API validations, and system monitoring.
Basic cURL Syntax
The basic structure of a cURL command is:
curl [options] [url]
Here, curl invokes the command-line tool, options modify how the request is made, and url specifies the resource you want to access.
If no request method is specified, cURL defaults to an HTTP GET request.
cURL commands
Making HTTP Requests with cURL
HTTP GET
The simplest use case is fetching a resource. When you run:
curl 'https://example.com'
cURL sends a GET request and prints the response body (typically HTML or JSON) directly in the terminal.
If you want to see both headers and the response body, you can include:
curl -i 'http://example.com'
To fetch only the headers (an HTTP HEAD request), use:
curl -I 'http://example.com'
For debugging purposes, verbose mode is extremely useful:
curl -v 'https://jsonplaceholder.typicode.com/posts/1'
Verbose mode shows detailed request and response information, including headers and TLS negotiation steps.
If you prefer a clean output without progress meters or error messages, you can use silent mode:
curl -s 'https://jsonplaceholder.typicode.com/posts/1'
Handling Redirects and SSL
Sometimes a requested URL redirects to another location. By default, cURL does not follow redirects automatically. To follow them, use:
curl -L 'http://www.github.com'
This ensures that the request follows HTTP 3xx responses until it reaches the final destination.
If you are working with development environments that use self-signed certificates, you may encounter SSL verification errors. To bypass certificate validation (not recommended in production), use:
curl -k 'https://self-signed.badssl.com'
Saving Output to a File
Instead of printing the response to the terminal, you can save it to a file:
curl 'https://example.com' -o output.txt
If you want to save the file using the same name provided by the server:
curl -O 'http://example.com/index.html'
This is particularly useful for downloading logs, reports, or artifacts from remote servers.
Sending Data with POST Requests
When submitting data to a server, you typically use POST.
A simple form submission looks like this:
curl -d "name=charlie&city=london" http://example.com
By default, cURL sends this data as application/x-www-form-urlencoded.
If you are working with JSON APIs, you should specify the correct content type:
curl -d '{"name":"Apple MacBook Pro"}' \
-H "Content-Type: application/json" \
https://api.restful-api.dev/objects
For larger payloads, it is cleaner to send data from a file:
curl -d @payload.json \
-H "Content-Type: application/json" \
https://api.restful-api.dev/objects
PUT and DELETE
To explicitly define request methods such as PUT, PATCH, or DELETE, use the -X option.
A PUT request replaces an entire resource:
curl -s -X PUT https://jsonplaceholder.typicode.com/posts/1 \
-H "Content-Type: application/json" \
-d '{
"userId": 1,
"id": 1,
"title": "Updated Title",
"body": "This is updated version of first Post using PUT"
}'
To delete a resource:
curl -s -X DELETE https://jsonplaceholder.typicode.com/posts/1
Working with Headers and Authentication
You can add custom headers using the -H option:
curl -H "Authorization: Bearer TOKEN" http://example.com
Headers are essential when working with authenticated APIs, versioned endpoints, or custom server requirements.
You can also remove default headers or override them, giving you complete control over the request structure.
Conclusion
cURL is far more than a simple command-line utility—it is a powerful and flexible tool for interacting with servers and APIs at a fundamental level. While graphical tools make API testing convenient, cURL gives you deeper control, better visibility, and seamless integration with scripts and automation workflows.
By understanding how to send different HTTP methods, attach headers, manage cookies, handle redirects, inspect verbose output, and transfer data efficiently, you gain a clearer understanding of how HTTP communication actually works under the hood. This knowledge not only improves your debugging skills but also strengthens your foundation in networking and backend development.
Whether you are validating an API response, testing authentication flows, automating health checks in CI/CD pipelines, or troubleshooting production issues, cURL proves to be an essential tool in a developer’s toolkit.
Mastering cURL means mastering the language of web communication—and that is a skill that will remain valuable throughout your development journey.