What is curl

Last updated: April 1, 2026

Quick Answer: Curl is a command-line tool and software library used to transfer data from or to a server using URLs, supporting protocols like HTTP, HTTPS, FTP, and many others, making it essential for API testing, web scraping, and automation.

Key Facts

Overview

Curl is a powerful command-line tool and software library designed to transfer data using URLs. It supports a wide variety of protocols and works across every major operating system, making it one of the most versatile data transfer utilities available to developers and system administrators. The tool is free, open-source, and has become an industry standard for testing APIs, automating web requests, and downloading files.

History and Development

Curl was created by Daniel Stenberg in 1997 as a simple tool for downloading files via FTP. Over the decades, it evolved into a comprehensive data transfer solution supporting dozens of protocols. Today, curl is maintained by a dedicated community and is widely integrated into other software projects, making it one of the most used tools in software development and system administration worldwide.

Supported Protocols

Curl supports an impressive array of protocols including HTTP and HTTPS for web requests, FTP and FTPS for file transfer, SFTP for secure file transfer, SCP for secure copy, LDAP for directory services, SMTP, POP3, and IMAP for email operations, plus Telnet, SSH, and many others. This broad protocol support makes curl the go-to tool for virtually any data transfer need.

Common Use Cases

Curl is indispensable for developers and system administrators. Common uses include testing REST APIs before integrating them into applications, downloading files and web content, automating web tasks through shell scripts, checking server responses and headers, and scripting routine web operations. It's also commonly used in cron jobs for automated data retrieval and system monitoring tasks.

Basic Usage and Flexibility

Using curl is straightforward with the simplest command being curl URL, which downloads and displays webpage content. More complex operations use flags like -X to specify HTTP methods, -H to add headers, -d to send data, and -o to save output to a file. The flexibility of curl's command-line interface makes it suitable for both simple queries and sophisticated scripting scenarios.

Related Questions

How do I use curl to make HTTP requests?

The simplest curl request is 'curl https://example.com' which retrieves the page. For POST requests, use 'curl -X POST -d "data" https://example.com'. Add headers with '-H "Header: value"' and specify output file with '-o filename'. Authentication uses '-u username:password' for basic auth or '-H "Authorization: Bearer token"' for tokens.

What's the difference between curl and wget?

Both are command-line download tools, but curl is more flexible with multiple protocol support and better suited for API testing. Wget is simpler and optimized for downloading entire website mirrors. Curl is preferred by developers for its scripting capabilities and detailed output control, while wget is often preferred for straightforward file downloads.

Can you use curl for APIs?

Yes, curl is excellent for API testing and development. You can use it to test API endpoints, send JSON data, handle authentication, and examine response headers before integration. Most API documentation includes curl examples, making it the standard tool for developers to verify API functionality and troubleshoot issues.

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Sources

  1. Wikipedia - cURL CC-BY-SA-4.0
  2. curl Official Website Proprietary