# curl — The Command Line Tool for Transferring Data with URLs

> curl is the most widely used command-line tool for transferring data with URL syntax. It supports HTTP, HTTPS, FTP, SFTP, and 25+ other protocols. Installed on billions of devices, curl and libcurl are foundational internet infrastructure.

## Install

Save in your project root:

# curl — The Command Line Tool for Transferring Data with URLs

## Quick Use
```bash
# Basic HTTP GET
curl https://api.example.com/data

# POST JSON data
curl -X POST https://api.example.com/users \
  -H "Content-Type: application/json" \
  -d '{"name": "Alice", "email": "alice@example.com"}'

# Download a file
curl -o file.zip https://example.com/file.zip

# Follow redirects and show headers
curl -L -I https://example.com

# Authentication
curl -u username:password https://api.example.com/protected
```

## Introduction
curl is the internet transfer tool that runs everywhere. Created by Daniel Stenberg in 1998, it has grown to support 28 protocols and is installed on virtually every operating system. curl is the default tool developers reach for when testing APIs, downloading files, debugging network issues, or automating HTTP requests.

With over 41,000 GitHub stars, curl is installed on over 20 billion devices worldwide. Its library, libcurl, is embedded in countless applications — from PHP and Python to cars, TVs, game consoles, and space probes (NASA uses curl in Mars rovers).

## What curl Does
curl transfers data to or from a server using URL syntax. While primarily known for HTTP, it supports FTP, SFTP, SCP, SMTP, IMAP, POP3, LDAP, MQTT, WebSocket, and more. It handles TLS/SSL, authentication, proxies, cookies, compression, and HTTP/2 and HTTP/3.

## Architecture Overview
```
[curl command-line tool]
        |
   [libcurl (C library)]
   Portable, thread-safe,
   feature-rich transfer lib
        |
+-------+-------+-------+
|       |       |       |
[HTTP/S] [FTP/S] [Other]
HTTP/1.1 FTP     SCP, SFTP
HTTP/2   FTPS    SMTP, IMAP
HTTP/3           MQTT, LDAP
(QUIC)           WebSocket
        |
   [TLS Backends]
   OpenSSL, BoringSSL,
   GnuTLS, NSS, Schannel,
   Secure Transport
        |
   [DNS Resolvers]
   c-ares, system resolver,
   DNS-over-HTTPS
```

## Self-Hosting & Configuration
```bash
# API testing recipes

# GET with custom headers
curl -H "Authorization: Bearer TOKEN" \
     -H "Accept: application/json" \
     https://api.example.com/v1/users

# POST form data
curl -X POST https://api.example.com/upload \
  -F "file=@photo.jpg" \
  -F "description=My photo"

# PUT with JSON
curl -X PUT https://api.example.com/users/1 \
  -H "Content-Type: application/json" \
  -d '{"name": "Updated Name"}'

# Verbose output for debugging
curl -v https://api.example.com/health

# Save response headers and body separately
curl -D headers.txt -o body.json https://api.example.com/data

# Timing information
curl -w "DNS: %{time_namelookup}s\nConnect: %{time_connect}s\nTTFB: %{time_starttransfer}s\nTotal: %{time_total}s\n" -o /dev/null -s https://example.com

# Resume interrupted download
curl -C - -o large-file.tar.gz https://example.com/large-file.tar.gz

# .curlrc — default configuration
# --connect-timeout 10
# --max-time 30
# --retry 3
# --silent
# --show-error
```

## Key Features
- **28 Protocols** — HTTP, HTTPS, FTP, SFTP, SCP, SMTP, MQTT, WebSocket, and more
- **HTTP/2 and HTTP/3** — modern protocol support with QUIC
- **TLS/SSL** — multiple TLS backend support with certificate verification
- **Authentication** — Basic, Digest, NTLM, Kerberos, Bearer tokens, OAuth
- **Proxy Support** — HTTP, SOCKS4, SOCKS5, and HTTPS proxies
- **Resume Transfers** — continue interrupted downloads
- **libcurl** — C library used by thousands of applications
- **Scripting Friendly** — exit codes, output formatting, and JSON support

## Comparison with Similar Tools
| Feature | curl | wget | HTTPie | Postman | Bruno |
|---|---|---|---|---|---|
| Type | CLI tool + lib | CLI downloader | CLI HTTP client | GUI + CLI | GUI + CLI |
| Protocols | 28 | HTTP, FTP | HTTP only | HTTP only | HTTP only |
| Library | libcurl | No | No | No | No |
| Human Readable | No (raw) | No | Yes (colored) | Yes (GUI) | Yes (GUI) |
| Scripting | Excellent | Good | Good | Newman CLI | Good |
| Ubiquity | Everywhere | Linux/Unix | pip install | Download | Download |

## FAQ
**Q: curl vs wget — when should I use which?**
A: curl for API testing, sending various HTTP methods, and complex transfers. wget for simple file downloads, especially recursive website mirroring. curl supports more protocols and is more versatile; wget is simpler for downloading.

**Q: How do I pretty-print JSON responses?**
A: Pipe to jq: "curl -s https://api.example.com/data | jq .". Or use curl with --json flag (curl 7.82+): "curl --json '{"key":"val"}' https://api.example.com".

**Q: How do I handle cookies?**
A: Save cookies: "curl -c cookies.txt URL". Send cookies: "curl -b cookies.txt URL". Or use both: "curl -b cookies.txt -c cookies.txt URL" for a session.

**Q: Is curl secure?**
A: curl verifies TLS certificates by default and has a strong security track record. Daniel Stenberg actively maintains security with a bug bounty program and regular audits. Always use HTTPS for sensitive data.

## Sources
- GitHub: https://github.com/curl/curl
- Documentation: https://curl.se/docs
- Website: https://curl.se
- Created by Daniel Stenberg
- License: MIT-like (curl license)

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Author: AI Open Source