Conventionally, the role of sending HTTP requests is largely reserved for browsers. Going a notch higher in terms of technicality, application programming interface (API) platforms can also be used to automate HTTP requests. But in some cases, where you simply need to quickly send an HTTP request while coding, for example, in order to access and view the bare-knuckle HTML code, using a browser or API can involve too many steps. It is for this reason, among others that we have detailed below, that cURL exists.
What is cURL?
An acronym for client uniform resource locator, cURL or curl is a free, open-source command line tool that enables you to use a terminal or command line to send HTTP requests and receive HTML responses. To put it more concisely, cURL enables data exchange between an internet-connected device or computer and a web server without using a browser or API. In addition, depending on the specific cURL command used, this tool can print the HTML code sent by the web server.
Because it is a tool that enables data exchange, cURL supports nearly every network protocol, not just HTTP or HTTPS. These include GOPHER, IMAP, IMAPS, SMTP, SMTPS, TELNET, DICT, FTP, FTPS, POP3, POP3S, RTMP, RTMPS, SCP, SMB, and so on. In this regard, cURL can be used by any device to send and receive data through any internet protocol.
Unfortunately, cURL is largely a command line-based tool, meaning that to use it, you must know the various commands. This deficiency makes it an unappealing option for users who prefer tools with a dedicated graphical user interface and icon-based interactions. Luckily, for such uses, there are multiple alternatives to cURL.
The various cURL alternatives include Postman, HTTPie for Terminal, xh, Curlie, RESTClient, HttpMaster, kurly, hurl, Wget, and more. Each of these alternatives has its own strengths. For instance, Postman has a user interface and does not rely on terminals. It enables programmers to build, design, and test their APIs. However, it is proprietary, although it has some open-source elements.
What is Libcurl?
cURL is utilized alongside libcurl, a free, easy-to-use client-side multi-protocol file transfer library. Libcurl facilitates communication between a terminal and different servers by handling application-level protocols. This way, you, the user writing the cURL code, do not have to write your own client code for the protocol you intend to use from scratch. What we mean by this is that you do not have to write an HTTP or HTTPS client code. Instead, you only have to write your cURL code and the libcurl then handles the transfers.
In addition to supporting the various internet protocols, libcurl supports Secure Sockets Layer (SSL) certificates, curl with proxy and proxy tunneling, various HTTP methods (POST, PUT, and GET), cookies, user and password authentication, and more. It is also compatible with IPv6, the latest version of the Internet Protocol. You can read more about using cURL with proxies.
Uses of cURL
cURL forms the backbone of internet transfer for thousands of software applications used in television sets, audio equipment, tablets, smartphones, cars, printers, routers, and more. Regardless of the device in question, the use cases of cURL can broadly be categorized into the following:
- Sending requests and receiving server responses
- cURL with proxy: you can use the curl –proxy command to connect to a URL via a proxy. If the proxy requires a username and password, you must first input this data on the command line.
- Following redirects (via the curl –location command)
- Perform authentication: you can use various cURL commands to undertake HTTP authentication. For instance, curl –basic allows you to perform basic authentication, while curl –digest enables you to undertake authentication via HTTP Digest Authentication. Moreover, you can have libcurl pick any authentication by using the curl –anyauth command.
- Request compressed response from the server using the curl –compressed command
- Verify the status of the server certificate
- Send the user agent name to the server
- Send cookies from a string or file
- Specify the DNS server address(es) to use
- Specify the crypto engine to use
- Pass custom header(s) to the server
- Permit insecure server connection even when using SSL
- Send a mail to or from an address
You can check the various uses of cURL by opening your terminal and typing curl –help all. This command lists all cURL commands and explains the function they are intended to achieve
Conclusion
cURL is a versatile command line tool that facilitates data transfer through virtually all protocols. Supported by an open-source library known as libcurl, which eliminates the need to write some elements of the application layer-based code from scratch, cURL can be used in many instances. From sending requests to using curl with proxy. From requesting compressed responses from servers to verifying the status of server certificates.
