cURL: What is It and How does it Work?

This article will answer the question ‘What is cURL?’ and expand on its uses, including in web scraping applications.

What is cURL?

cURL, short for client URL, is a command-line tool used to make requests, access data, transfer data, and retrieve information from the transferred data. This tool, which can also be written as curl, uses libcurl, a URL transfer library that supports multiple network protocols, including HTTP, HTTPS, FTP, POP3, SFTP, SMB, and GOPHER, to mention a few.

It goes without saying that because of the association between cURL and libcurl, the former also supports nearly every network protocol in use today. Thus, with cURL, you can transfer data via any protocol. But in the absence of a specified protocol, cURL defaults to HTTP. Additionally, this tool is also designed to try out different protocols if the default one does not work.

The sophistication does not stop at that. cURL also guesses the protocol you wish to use if you give it hints. For instance, if you want to use the FTP, you type ‘curl ftp.example.com.’ The tool will automatically guess that you intended to use the FTP protocol and respond to the request as if you had keyed in ‘curl ftp://example.com.’

Notably, although cURL permits the transfer of files via multiple supported internet protocols, it only does so, provided the transfer is within the same URL syntax. Also, while cURL performs numerous functions, it is not written to do all these processes automatically. Instead, the user has to piece the various processes together using a scrip language or manually issuing instructions for each of the functions.

History of cURL

Curl predates Google, given that it was first released in 1997 while the former was founded in 1998. However, at that time, curl was known as httpget as it only supported data transfer via Hypertext Transfer Protocol (HTTP).

Later, when it finally supported FTP, the original name was scrubbed off, and the tool was now known as urlget. It was not until March 1998 and a few updates later that the name was again changed to what it is currently known as.

Uses of cURL

Over the years, since it was first developed, the cURL has grown in importance and is presently used in a number of applications, including web scraping. Its versatility makes it an ideal tool for the following use cases:

• Proxy support
• User authentication
• Uploading files onto or downloading them from web hosting server using the File Transfer Protocol (FTP)
• Showing request and response header
• Verifying if a URL is live
• Testing is a given URL can handshake over an SSL connection
• Sending cookies
• Verifying an SSL certificate
• Web scraping

Importance of cURL in Web Scraping

As detailed above, cURL is a command-line-based tool that responds to the user’s commands issued in the form of instructions. This implies that each command should be written in a specific way, a failure to which it won’t work. After all, the tool works on the principles of a command line or terminal.

It is these commands that make cURL an ideal tool for web scraping. You can instruct it to download an entire webpage. Alternatively, you can choose to download content from multiple webpages within a given site. cURL’s versatility with regards to this particular application emanates from the fact that each of these processes has a unique command, as summarized below:

• Downloading the homepage: curl http://example.com or curl ftp://example.com (it gives you the liberty to choose the protocol through which the files will be sent.)
• Downloading multiple webpages: curl http://example.(page2,page3,page5).html
• Saving the URL’s contents to a file (in this case, the filename is usually at the beginning of the cURL syntax, with the second part being the structure of the URL from which the file is to be downloaded): curl -o filename.html http://example.com/file.html

Notably, while these are the basic processes that somewhat help in web scraping, the files’ retrieval can only be done one page at a time. In the end, following these commands makes the entire data extraction process inefficient and time-consuming. Fortunately, cURL can automate the repetitive web scraping process, but for this, you need to use a PHP-based code available on GitHub.

Other important commands for automated web scraping using cURL include:

• curl_init($url) which initiates the data extraction session
• curl_exec(), which executes the web scraping
• curl_close(), which closes the session
• curopt_url, which sets the URL from which the data will be extracted
• curlopt_returntransfer, which instructs the cURL tool to save the scraper page as a variable, thus allowing you to get the specific data you initially intended to retrieve from the page

A cURL is a versatile tool made up of numerous commands, which can perform several functions, including web scraping. For ease of use, though, documentation is available online.

If you want to learn more about what is cURL and how to use it, then continue reading this blog post and find out more.