What Is cURL?
What cURL Is and Where It Comes From
What is cURL? It is an open-source command-line tool for transferring data to and from servers using protocols such as HTTP and HTTPS. Developers use it to send API requests from a terminal, inspect what a server returns and automate those steps in scripts. The name is usually explained as short for 'client URL'.
The tool is built on libcurl, a library that other software can embed. This is why the same underlying engine appears in many places, from desktop applications to programming languages that offer bindings to it. A simple example of a request is curl https://api.example.com/status, which sends a GET request to that address and prints the response. The parts of a request are explained in what an API call is.
cURL runs on most operating systems and is included with many of them. Because it has no graphical interface, it works well on servers and in automated environments where a browser or a visual client is not available.
Why cURL Appears in API Documentation
Many API guides show requests as cURL commands because a command states every part of a request in plain text: the method, the address, the headers and the body. A reader can copy it and see the same result without installing an SDK or learning a specific programming language.
That same clarity makes cURL useful for support and debugging. A developer who reports a problem can share the exact request that failed, and a support engineer can reproduce it without access to the reporter's code. The tool also offers options to show response headers and status codes, which helps when the question is what the server actually sent.
cURL is not a testing framework or a development environment. It shows one request at a time, and multi-step flows such as complex authentication are awkward to express as commands. For those, other tools are more practical, as discussed in API testing.
cURL Compared With Browsers and Graphical Clients
A browser's address bar sends only simple GET requests and shows the rendered page, not the raw headers. Graphical API clients let users fill in forms for methods, headers and bodies, which is convenient for exploration. cURL sits between these two: it can send any method with any headers, and its output is plain text that scripts can read.
That makes cURL well suited to tasks such as checking whether an endpoint is online, running a request from a deployment script or reproducing a bug on a remote server. The trade-off is that the user must type or script everything, and sensitive values must be handled carefully.
Frequently asked questions
Is cURL the same as the PHP cURL extension?
No, but they are related. cURL is the command-line tool and the libcurl library. The PHP cURL extension lets PHP code use libcurl's features from inside a program. Knowing the command-line tool helps when debugging, because the same request can be tried in a terminal before it is written in code.
Is it safe to put API keys in cURL commands?
Use caution. A key typed into a command can be saved in shell history, may be visible to other users on the same machine and can appear in logs. Many teams keep keys in environment variables or protected configuration files instead, and they treat any key that has been exposed as compromised, replacing it as soon as possible. Keys are explained in what an API key is.