curl URL Transfer Library

curl is a free and open-source command-line tool and library for transferring data using various network protocols. Created by Daniel Stenberg in 1998 (originally as httpget, renamed to curl later that year), curl is one of the most widely used software tools in the world, installed on billions of devices including servers, smartphones, routers, and IoT devices. Key features: protocol support: curl supports over 25 protocols including HTTP, HTTPS, HTTP/2, HTTP/3 (QUIC), FTP, FTPS, SFTP, SCP, SMTP, SMTPS, POP3, IMAP, LDAP, TELNET, DICT, FILE, GOPHER, MQTT, SMB, and WebSocket. URL syntax: full URL parsing with support for IPv6, IDN (international domain names), username/password embedding, port numbers, and query strings. HTTP features: all HTTP methods (GET, POST, PUT, DELETE, PATCH, HEAD, OPTIONS), custom headers, cookies, content-type, multipart form data (multipart/form-data), authentication (Basic, Digest, NTLM, Negotiate, Bearer), HTTP/2 multiplexing, and HTTP/3 over QUIC. SSL/TLS: supports multiple TLS backends (OpenSSL, GnuTLS, wolfSSL, mbedTLS, Secure Transport on macOS, Schannel on Windows) with certificate verification, client certificates, and cipher selection. libcurl: a C library providing the same protocol support for applications, with bindings available for 50+ programming languages including Python (pycurl), PHP, Ruby, Java, Go, Rust, and .NET. The library is thread-safe and supports multiple simultaneous transfers with multi interface. Performance: supports connection pooling, DNS caching, keep-alive, and asynchronous DNS resolution (c-ares). Debugging: verbose output (-v) showing all protocol-level communication. Output: save to file, pipe to stdout, or extract specific data with --write-out format strings. Scripting: designed for automation, exit codes, and progress meters. Cross-platform: Linux, macOS, Windows, BSD, Solaris, DOS, and more. C codebase. curl license (MIT/X derivate).