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How does curl work?

Published in Command Line Tools 3 mins read

Curl works as a command-line tool that allows you to transfer data to or from a server by specifying a URL and, optionally, data to send. It acts as a client that communicates with a server using various protocols.

Here's a breakdown of how it works:

  • Initiation: You initiate a curl request by typing the curl command in your terminal, followed by options and the URL you want to interact with.

  • URL Parsing: Curl parses the URL provided, extracting the protocol (e.g., HTTP, HTTPS, FTP), hostname, and path.

  • Protocol Handling: Curl supports a wide range of protocols. Based on the URL's protocol, it selects the appropriate handler to establish a connection and communicate with the server.

  • Connection Establishment: Curl establishes a connection to the server specified in the URL. For HTTP/HTTPS, this involves a TCP connection, and potentially a TLS/SSL handshake for secure communication.

  • Request Construction: Curl constructs a request according to the protocol. For HTTP, this includes specifying the HTTP method (GET, POST, PUT, DELETE, etc.), headers, and any data to be sent.

  • Data Transmission: Curl sends the request to the server. If data is included (e.g., in a POST request), it is transmitted as part of the request body.

  • Response Reception: Curl receives the server's response, including the HTTP status code (e.g., 200 OK, 404 Not Found), headers, and the response body (e.g., HTML, JSON).

  • Data Handling: Curl handles the received data. By default, it outputs the response body to the standard output (your terminal). However, you can use options to save the response to a file, extract specific information, or perform other actions.

  • Connection Closure: Curl closes the connection to the server.

Example:

A simple example of using curl to retrieve the content of a webpage:

curl https://www.example.com

This command instructs curl to:

  1. Parse the URL https://www.example.com.
  2. Establish an HTTPS connection to www.example.com.
  3. Send an HTTP GET request to the server.
  4. Receive the server's response (HTML of the webpage).
  5. Print the HTML content to your terminal.

Key Features and Options:

  • -X (or --request): Specifies the HTTP method (e.g., -X POST, -X PUT).
  • -H (or --header): Adds custom headers to the request (e.g., -H "Content-Type: application/json").
  • -d (or --data): Sends data with the request, typically used for POST requests.
  • -o (or --output): Saves the response to a file (e.g., -o output.html).
  • -v (or --verbose): Provides verbose output, showing details about the request and response.

Curl is a versatile tool used for testing APIs, downloading files, and automating tasks that involve interacting with web servers. Its ability to handle various protocols and provide detailed control over requests makes it invaluable for developers and system administrators.

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