"Curl handles 10 billion requests every day." That’s the power behind this small command-line tool that’s quietly running at the core of the internet. Designed by Daniel Stenberg in 1997, curl is an open-source project that allows seamless network data transfers and supports multiple protocols, like HTTP, HTTPS, FTP, SCP, and SFTP. It’s a versatile powerhouse for developers, sysadmins, and security experts alike.
Let’s break down what makes curl such an essential tool and, most importantly, how it works.

Curl Basics

Curl isn’t your average command-line tool. Originally created for data transmission on Unix systems, it quickly became an indispensable, cross-platform tool. Its name—short for “Client URL”—points to its core function: handling URLs and making network connections with ease. Curl works across Linux, macOS, and Windows, making it consistent and reliable no matter your operating system.

Important Functions and Applications

Here’s what curl can do—and it’s a lot more than simply fetching a webpage:
Retrieve Web Content: Curl can grab a webpage’s content straight from the server via HTTP or HTTPS.
File Transfers: It supports secure file uploads and downloads through FTP, FTPS, and more.
API Testing: Use curl to send and test HTTP requests for debugging Web APIs.
Protocol Versatility: Curl isn’t just HTTP; it also handles SCP, SFTP, and more, making it incredibly versatile.
This functionality has turned curl into a vital tool for professionals. It’s flexible, efficient, and always up to the task.

How Does Curl Work

Knowing how curl works at a deeper level can help you wield it more effectively. Here’s a step-by-step look at what happens when you run a curl command:
1. Parsing URLs and Setting Up Requests
When you type curl https://example.com, curl starts by parsing the URL. It checks the protocol, host, port, and path, preparing the data it needs to start the connection.
2. Creating the Connection
After parsing, curl initiates a network connection with the server, working through the network protocol stack to ensure a clean, efficient transfer.
3. Building the HTTP Request
For HTTP or HTTPS, curl creates a well-structured request message. It defines the HTTP method (like GET or POST), headers (like User-Agent or Content-Type), and, if needed, includes cookies or authentication info.
4. Handling Server Response
Once the server replies, curl captures the HTTP status code, headers, and body content. It’s designed to efficiently manage data, whether it's HTML content or file data.
5. Data Storage and Output
Curl processes the data in a few ways—printing it to your terminal, saving it to a file, or passing it along for further processing.

Why Curl Stands Out

Curl didn’t become a standard by accident. It’s packed with features and advantages that make it the go-to solution for anyone needing reliable data transfer:
Support Across Platforms: Works seamlessly across operating systems, from Linux and macOS to Windows.
Extensive Protocol Support: Handles HTTP, FTP, SCP, and SFTP, giving you flexibility in data transfer.
Highly Adaptable: Curl offers fine-tuned control, letting you set specific headers, authentication, and other request details.
Open-Source and Strong Community: With continuous updates and improvements, curl remains a stable, evolving tool supported by developers worldwide.

Practical Ways to Use Curl

So, where can you put curl to work? Here are three scenarios:
Web Development and Testing
Developers use curl to test API endpoints, confirm HTTP requests, and simulate how clients interact with servers. Curl makes it easy to validate that systems are functioning properly.
System Operations and Automation
System administrators rely on curl for automated tasks, from file transfers to remote server monitoring. Its command-line flexibility allows you to script efficient workflows.
Security Review and Vulnerability Testing
Security experts use curl to simulate requests and probe for weaknesses, testing system defenses against potential threats.

Final Thoughts

Curl brings simplicity, efficiency, and powerful flexibility to network data transfers, making it invaluable in professional environments. Whether you’re debugging an API, automating system tasks, or assessing network security, curl lets you tackle it all—one command at a time.