CURL

The Universal Data Transfer Tool

11 min read

#programming, #curl

In the landscape of network programming and API development, few tools have achieved the ubiquity and longevity of cURL. Whether you're debugging an API endpoint, downloading files, or automating data transfers, cURL has likely been part of your workflow. But what exactly makes this nearly 30-year-old tool still indispensable in 2025?

A Brief History

cURL (pronounced "curl," standing for "Client for URLs") began its journey in 1996 when Swedish developer Daniel Stenberg created it to power an IRC bot that fetched currency exchange rates. Originally named httpget, then urlget, the project adopted the name cURL in 1998. What started as a spare-time project has evolved into critical infrastructure running in billions of devices worldwide.

Stenberg remains the lead developer to this day, maintaining one of the most successful open-source projects in computing history. The project ships a new release at least once every eight weeks, with over 266 releases to date. This consistent development cycle and backward compatibility philosophy have made cURL a trusted foundation for countless applications.

The Scale of cURL's Impact

The statistics around cURL's adoption are staggering:

Web Infrastructure: According to W3Techs data, PHP (which includes libcurl) powers approximately 79% of websites with known server-side languages
Automotive Industry: As of August 2025, cURL runs in 47 different car brands, meaning it's operating in hundreds of millions of vehicles worldwide
Embedded Everywhere: Beyond cars, cURL is found in tablets, printers, routers, smart TVs, and virtually every Linux distribution
Developer Adoption: The 2025 cURL user survey indicates that OpenSSL remains the dominant TLS backend (by far), with growing interest in HTTP/3 adoption

Why Developers Choose cURL

cURL's enduring popularity stems from several key advantages:

1. Protocol Versatility

cURL supports an extensive range of protocols including:

HTTP/HTTPS (versions 1.x, 2, and experimental HTTP/3)
FTP/FTPS/SFTP
IMAP/IMAPS/POP3/POP3S
SMTP/SMTPS
LDAP/LDAPS
SCP/SMB/SMBS
TELNET/TFTP
RTSP/RTMP
MQTT
WebSockets
DICT, FILE, GOPHER

2. Cross-Platform Compatibility

cURL runs on virtually every operating system. Windows 10 and 11 now ship with cURL pre-installed, joining Linux and macOS distributions that have included it for decades.

3. Tiny Footprint

Despite its capabilities, cURL's compiled binary is typically under 1MB, making it ideal for embedded systems and resource-constrained environments.

4. Language-Agnostic

Rather than learning different HTTP client libraries for each programming language, developers can use cURL as a universal interface for demonstrating API calls. Tools like curlconverter.com can even translate cURL commands into code for various languages.

Language Integration: The Big Three

While cURL is a command-line tool, libcurl (the library powering cURL) has bindings in virtually every programming language. Let's look at the three most popular languages using cURL:

1. PHP - The Web Development Workhorse

PHP has had native cURL support for decades, making it the most widely used language with libcurl integration. As of 2024-2025, approximately 6.5 million PHP developers use the language globally, and the vast majority leverage PHP's cURL extension for external API communication.

Basic PHP cURL Example:

<?php
// Initialize cURL session
$ch = curl_init('https://api.example.com/data');

// Set options
curl_setopt($ch, CURLOPT_RETURNTRANSFER, true);
curl_setopt($ch, CURLOPT_HTTPHEADER, [
    'Content-Type: application/json',
    'Authorization: Bearer YOUR_TOKEN'
]);

// Execute request
$response = curl_exec($ch);
$httpCode = curl_getinfo($ch, CURLINFO_HTTP_CODE);

// Check for errors
if (curl_errno($ch)) {
    echo 'Error: ' . curl_error($ch);
} else {
    echo "HTTP Status: $httpCode\n";
    $data = json_decode($response, true);
    print_r($data);
}

// Clean up
curl_close($ch);
?>

POST Request with JSON:

<?php
$data = [
    'username' => 'john_doe',
    'email' => 'john@example.com'
];

$ch = curl_init('https://api.example.com/users');
curl_setopt($ch, CURLOPT_RETURNTRANSFER, true);
curl_setopt($ch, CURLOPT_POST, true);
curl_setopt($ch, CURLOPT_POSTFIELDS, json_encode($data));
curl_setopt($ch, CURLOPT_HTTPHEADER, [
    'Content-Type: application/json'
]);

$response = curl_exec($ch);
curl_close($ch);
?>

2. Python - The Data Science Favorite

While Python has popular alternatives like requests and httpx, the pycurl library provides direct access to libcurl's capabilities. Python developers increasingly use cURL for its performance and protocol support, especially in data science and automation workflows. As of 2024, Python is used by 51% of developers according to Stack Overflow, making it the third most popular language overall.

Python with pycurl:

import pycurl
from io import BytesIO

# Create response buffer
buffer = BytesIO()

# Initialize cURL
c = pycurl.Curl()
c.setopt(c.URL, 'https://api.example.com/data')
c.setopt(c.WRITEDATA, buffer)

# Execute request
c.perform()

# Get HTTP status code
status_code = c.getinfo(pycurl.HTTP_CODE)
print(f"HTTP Status: {status_code}")

# Get response body
body = buffer.getvalue().decode('utf-8')
print(body)

# Clean up
c.close()

Python with subprocess (calling cURL directly):

import subprocess
import json

# Simple GET request
result = subprocess.run(
    ['curl', '-s', 'https://api.example.com/data'],
    capture_output=True,
    text=True
)

data = json.loads(result.stdout)
print(data)

# POST request with data
payload = json.dumps({'key': 'value'})
result = subprocess.run([
    'curl', '-s', '-X', 'POST',
    '-H', 'Content-Type: application/json',
    '-d', payload,
    'https://api.example.com/endpoint'
], capture_output=True, text=True)

print(result.stdout)

3. JavaScript/Node.js - The Full-Stack Solution

JavaScript dominates web development, used by 62.3% of developers in 2024 (64.6% among professionals). While Node.js developers typically use axios or the native fetch API, the node-libcurl package provides high-performance access to libcurl for scenarios requiring advanced protocol support or better performance.

Node.js with node-libcurl:

const { Curl } = require('node-libcurl');

const curl = new Curl();

curl.setOpt('URL', 'https://api.example.com/data');
curl.setOpt('FOLLOWLOCATION', true);

curl.on('end', function(statusCode, data, headers) {
    console.log(`Status: ${statusCode}`);
    console.log('Data:', data);
    this.close();
});

curl.on('error', curl.close.bind(curl));

curl.perform();

Node.js with child_process (calling cURL):

const { exec } = require('child_process');
const util = require('util');
const execPromise = util.promisify(exec);

async function fetchData() {
    try {
        const { stdout, stderr } = await execPromise(
            'curl -s -H "Content-Type: application/json" https://api.example.com/data'
        );

        const data = JSON.parse(stdout);
        console.log(data);
    } catch (error) {
        console.error('Error:', error);
    }
}

fetchData();

Essential cURL Command-Line Options

Understanding key cURL options helps you leverage its full power:

Option	Description
`-X, --request`	Specify HTTP method (GET, POST, PUT, DELETE, etc.)
`-H, --header`	Add custom header to request
`-d, --data`	Send POST data
`-i, --include`	Include response headers in output
`-I, --head`	Fetch headers only (HEAD request)
`-o, --output`	Write output to file
`-O, --remote-name`	Write output to file with remote name
`-L, --location`	Follow redirects
`-v, --verbose`	Make the operation more verbose
`-s, --silent`	Silent mode (no progress or error info)
`-u, --user`	Server user and password
`--compressed`	Request compressed response
`-k, --insecure`	Allow insecure SSL connections (development only!)

Basic Command-Line Examples

Simple GET request:

curl https://api.example.com/users

GET with headers:

curl -H "Authorization: Bearer TOKEN" \
     -H "Accept: application/json" \
     https://api.example.com/protected

POST JSON data:

curl -X POST \
     -H "Content-Type: application/json" \
     -d '{"name":"John","email":"john@example.com"}' \
     https://api.example.com/users

PUT request:

curl -X PUT \
     -H "Content-Type: application/json" \
     -d '{"status":"active"}' \
     https://api.example.com/users/123

DELETE request:

curl -X DELETE https://api.example.com/users/123

Download a file:

curl -O https://example.com/file.pdf
# or with custom name
curl -o document.pdf https://example.com/file.pdf

Follow redirects:

curl -L https://short.url/abc123

Show response headers:

curl -i https://api.example.com/data

Verbose output (debugging):

curl -v https://api.example.com/endpoint

The Future of cURL

cURL continues to evolve with the web. Key developments in 2024-2025 include:

HTTP/3 Support: HTTP/3 (using QUIC protocol over UDP) is now available with the ngtcp2 backend (no longer experimental), though other backends remain in experimental status. OpenSSL 3.5+ will bring improved HTTP/3 support.
Performance Optimization: The development team actively works on performance tuning, with discussions around CPU usage and throughput optimization (important for large-scale deployments like Netflix's infrastructure).
TLS Evolution: With OpenSSL maintaining dominance as the TLS backend, cURL is adapting to support modern TLS 1.3 features and preparing for potential drops of older, insecure TLS versions.
WebSocket Improvements: WebSocket support is moving toward non-experimental status.

Why cURL Matters in 2025

In an age of abstraction layers and high-level frameworks, cURL's relevance demonstrates the enduring value of well-designed, focused tools. Its success stems from:

Reliability: Decades of battle-testing across billions of devices
Stability: Backward compatibility ensures old scripts keep working
Simplicity: One tool, many protocols, minimal dependencies
Transparency: Open source code that can be audited and trusted
Maintenance: Active development with security fixes and feature improvements

Whether you're developing APIs, automating workflows, debugging network issues, or building the next generation of connected devices, cURL remains an essential tool in the modern developer's toolkit. Its story--from a spare-time IRC bot enhancement to running in hundreds of millions of cars--is a testament to the power of open-source software and thoughtful design.

Conclusion

cURL exemplifies how a well-designed tool can remain relevant across technological generations. As we move toward HTTP/3, WebAssembly, edge computing, and beyond, cURL continues to adapt while maintaining its core strengths: simplicity, reliability, and universality.

Next time you use curl in your terminal or call curl_exec() in PHP, remember: you're using the same technology that's probably running in your car's infotainment system right now. That's the power of great software engineering and dedicated open-source maintenance.