What is cURL and Where it is Used

cURL is a command line tool and library for transferring data with URLs.

It supports a wide variety of protocols, including HTTP, HTTPS, FTP, SFTP, IMAP, POP3, IMAP, and SMTP.

cURL is used to interact with web servers, send and receive files, and automate web application testing.

It is also used for downloading and uploading data, testing HTTP response codes, and more.

cURL is written in C and is open source, making it an ideal tool for developers who need a versatile way to transfer data.

cURL is available on most operating systems, including Windows, Linux, macOS, and mobile devices.

CURL options are used to configure and control the behavior of the CURL command-line tool.

Options can be set to control protocol-specific features, such as HTTP headers, cookies, proxy settings, user authentication, and more.

Some of the most commonly used CURL options include:

—data: specify a post body for an HTTP request.

—header: specify HTTP request headers.

—location: tells CURL to follow any redirects if the server responds with a 3xx status code.

—user-agent: allows you to set the User-Agent request header.

—referer: allows you to specify the Referer request header.

—verbose: tells CURL to output detailed information about the progress of the request.

—output: can be used to specify a file to save the output of the request.

—silent: tells CURL to not output any messages to the terminal.

—insecure: tells CURL to ignore any SSL/TLS certificate errors.

—request: allows you to specify the HTTP request method.

—url: allows you to specify the URL of the request.

—cookie: allows you to set the Cookie request header.

Installing libcurl and curl binaries

Installing libcurl and curl binaries on Windows:

  1. Download the precompiled binaries of libcurl and curl from https://curl.haxx.se/download.html. (The required version will depend on the system architecture, e.g. 64-bit or 32-bit.)
  2. Extract the files from the downloaded archive.
  3. Move the extracted files to a desired folder on the computer.
  4. Add the binary’s path to the environment variable “Path”.
  5. Reboot the system.
  6. Open a command prompt and type “curl” to check if the installation was successful.

Simple cURL requests

//GET
curl -X GET https://www.example.com/
//POST
curl -X POST -H "Content-Type: application/json" \
-d '{"name": "John Doe"}' \
https://www.example.com/api/users

Properties and APIs of libcurl

Properties:
• Supports many different protocols like HTTP, HTTPS, SFTP and more.
• It supports many authentication methods like Basic, Digest, NTLM and Kerberos to name a few.
• It is highly configurable with a variety of different options available.
• Offers easy transfer of data over the internet using proxied and secure connections.
• It is open-source, meaning that it can easily be customized and extended.
• Easy to debug with verbose logging and integrated debugging options.

APIs:
• curl_easy_init() – Initialize a curl session
• curl_easy_setopt() – Set an option for the curl session
• curl_easy_perform() – Transfer a data request from the server
• curl_easy_cleanup() – Cleanup a curl session
• curl_global_init() – Initialize the libcurl library
• curl_global_cleanup() – Cleanup the libcurl library
• curl_easy_getinfo() – Retrieve information about a curl session
• curl_multi_init() – Initialize a curl multi-session
• curl_multi_add_handle() – Add a transfer to a curl multi-session
• curl_multi_remove_handle() – Removes a transfer from a curl multi-session
• curl_multi_perform() – Perform multiple transfers in a curl multi-session
• curl_multi_cleanup() – Cleanup a curl multi-session

Setting cURL options

When setting cURL options, users should make sure that they have the right options in place for their specific request.

The most important options are –

  1. cURLURL (for setting the URL of the request)
  2. cURLOPT_RETURNTRANSFER (for getting the response content as a string)
  3. cURLOPT_POST (for setting the request method to Post)
  4. cURLOPT_POSTFIELDS (for setting the data to send with a Post request)
  5. cURLOPT_HTTPHEADER (for setting any necessary custom headers)

However, there are several other cURL options available for users to customise their request, such as setting a timeout, defining how cookies should be handled, and enabling verbose mode for debugging their requests.

Refer to the cURL documentation for a full list of available options.

Authentication and authorisation with cURL

Authentication:

cURL can be used to authenticate using several protocols, including HTTP, HTTPS and FTP.

To authenticate using these protocols, cURL requires a username and password to identify the user.

To do this, cURL uses the –user option, which takes the form of -u user_name:password followed by the command.

For example, to authenticate and download a file via FTP, you could use the following command:

curl -u user_name:password ftp://example.com/filename

Authorization:

cURL can be used to authorize requests using signature-based authorization methods.

The –header option is used to provide the signature or authorization data along with the request.

For example, to send an OAuth 2.0 token as part of an API request, use the following command:

curl -H "Authorization: Bearer " https://api.example.com

Using cURL for file transfer

cURL can be used to transfer files from one server to another.

This involves uploading files from the local system to a remote server or downloading files from the remote server to the local system.

For example, cURL can be used to upload an image file to a web server or download an image file from a web server.

The syntax for both tasks would be similar, for example –

Upload a file:

curl --user USER:PASS -T file.jpg ftp://domain.com/upload/file.jpg

Download a file:

curl -u USER:PASS ftp://domain.com/upload/file.jpg -o file.jpg

How to Use cURL with PHP

<?php
// Initialize and set the options
$ch = curl_init();
curl_setopt($ch, CURLOPT_URL, "https://example.com/endpoint");
curl_setopt($ch, CURLOPT_POST, true);
curl_setopt($ch, CURLOPT_POSTFIELDS, $data);
// Execute the request
$response = curl_exec($ch);

// Close the connection
curl_close($ch);

// Parse the response
$result = json_decode($response);

// Access result
echo $result->message;

Utility

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