What Is curl and How Does It Work?
In this article, we break down what curl is, how it works, and how you can use it to transfer data right from your terminal.
Publish date: 12/5/2025
In this article, we break down what curl is, how it works, and how you can use it to transfer data right from your terminal.
Publish date: 12/5/2025
If you've ever worked on a command line, especially on a server, you've probably run into the curl command. It might seem a bit mysterious at first, but it's one of the most common and useful utilities available to developers, system administrators, and network engineers.
At its core, curl (which stands for Client URL) is a command-line tool for transferring data to or from a server using one of many supported protocols. Think of it as a text-based web browser; it can make requests, fetch data, and send information, all without a graphical interface.
Originally created by Daniel Stenberg, curl is free, open-source, and available on just about every operating system, including Linux, macOS, and Windows.
It's actually two things: a command-line tool curl and a library libcurl. The library, libcurl, is what does the actual work, and developers can build it into their own applications to handle data transfers. The curl command is just the most common interface for that library.
Its real strength is its versatility. While it's most famous for making HTTP and HTTPS requests (like a browser), it also supports a wide range of other protocols:
This means you can use the same tool to download a file from a website, test a new API endpoint, or even check an email server.
When you type curl example.com, you're telling the curl tool to use the libcurl library to make an HTTP request to example.com.
By default, it performs a GET request, which is the same method your browser uses to fetch a webpage. The server at example.com receives this request, processes it, and sends back a response. In this case, the response is the raw HTML content of the website.
Unlike a browser, curl doesn't try to render the HTML, execute JavaScript, or download images. It simply prints the raw response (the HTML code) directly to your terminal. This "raw data" approach is what makes it so useful for scripting and testing. You see exactly what the server is sending, with nothing hidden.
Another common tool you'll find on Linux systems is wget. People often ask about the difference.
In short: use wget if you just need to download files. Use curl for just about everything else, especially for testing APIs or any kind of scripting that involves sending data.
Note: For a full comparison between curl and wget, view our article here.
The best way to understand curl is to see it in action. Here are a few of the most common commands.
This is the most basic use. curl will fetch the content of the URL and display it in your terminal.
curl https://api.example.com/status
Printing to the terminal isn't always helpful. You can save the output to a file using the -o (lowercase) or -O (uppercase) flags.
-O (uppercase) saves the file with its original name from the URL.-o (lowercase) lets you specify a new name.# Saves the file as "latest.zip"
curl -O https://wordpress.org/latest.zip
# Saves the file as "my-new-file.zip"
curl -o my-new-file.zip https://wordpress.org/latest.zip
Sometimes you don't need the page content; you just want to see if the server is responding correctly. The -I flag (or --head) tells curl to fetch only the HTTP headers. This is great for checking a server's status code (like 200 OK or 404 Not Found).
curl -I https://xtom.com
# Output might look like this:
# HTTP/2 200
# content-type: text/html; charset=UTF-t
# server: whatever-server
# date: Mon, 10 Nov 2025 22:30:00 GMT
This is where curl becomes essential for API testing. You can send data to a server using the -X POST method and the -d flag to include your data.
curl -X POST -d "username=test&status=active" https://api.example.com/users
You can also send data as JSON, which is very common for modern APIs, by adding a Content-Type header.
Speaking of headers, you can add any custom header you want with the -H flag. This is often used for sending authentication tokens.
curl -H "Authorization: Bearer mysecretapikey123" https://api.example.com/secure-data
If a connection is failing or you're just curious, the -v (verbose) flag is your best friend. It shows you everything: the DNS resolution, the SSL/TLS handshake, all the request headers curl is sending, and all the response headers from the server.
curl -v https://example.com
This is incredibly helpful for debugging connection issues, especially when you're trying to figure out if a problem is with your firewall, SSL certificate, or the remote server.
On most Linux distributions and macOS, yes. As of Windows 10 (version 1803) and later, curl is also included by default, making it universally available.
libcurl is the C library that powers curl. It's the engine that handles all the protocols and data transfer logic. Thousands of applications, from cars to smart TVs to popular apps, use libcurl internally to handle network requests.
Yes, the tool itself is safe. It's a standard, well-maintained utility. Just like a web browser, the "safety" depends on the URLs you're contacting. curl only transfers data; it doesn't execute any code from the server, which makes it safer than a browser in many contexts.
Absolutely. That's one of its main advantages. You can use it to test FTP server uploads, download files via SFTP, or even check if an SMTP (email) server is responding, all with a similar command structure.
For anyone working with servers, APIs, or web development, curl is a fundamental tool. It's a simple, scriptable, and predictable way to move data and test network services right from your command line. Whether you're debugging an API or downloading a file, it's the right tool for the job.
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