cURL

cURL (/kɝl/ or /kə:l/^[4]) is a computer software project providing a library and command-line tool for transferring data using various protocols. The cURL project produces two products, libcurl and cURL. It was first released in 1997. The name originally stood for "see URL".

libcurl

libcurl is a free client-side URL transfer library, supporting FTP, FTPS, Gopher, HTTP (with HTTP/2 support), HTTPS, SCP, SFTP, TFTP, Telnet, DICT, the file URI scheme, LDAP, LDAPS, IMAP, POP3, SMTP and RTSP. The library supports HTTPS certificates, HTTP POST, HTTP PUT, FTP uploading, Kerberos, HTTP form-based upload, proxies, cookies, user-plus-password authentication, file transfer resume, and HTTP proxy tunneling.

The libcurl library is portable. It builds and works identically on many platforms, including Solaris, NetBSD, FreeBSD, OpenBSD, Darwin, HP-UX, IRIX, AIX, Tru64, Linux, UnixWare, HURD, Windows, Symbian, AmigaOS, OS/2, BeOS, OS X, iOS, Android, Ultrix, QNX Neutrino, BlackBerry Tablet OS and BlackBerry 10,^[5] OpenVMS, RISC OS, NetWare and DOS.

The libcurl library is free, thread-safe and IPv6 compatible. Bindings are available for more than 40 languages, including C/C++, Java, PHP and Python.

The libcurl library can support SSL/TLS through OpenSSL, GnuTLS, NSS, wolfSSL, mbed TLS, axTLS, QSOSSL on IBM i, Secure Transport on OS X and iOS, and SChannel on Windows.

curl

cURL is a command line tool for getting or sending files using URL syntax.

Since cURL uses libcurl, it supports a range of common Internet protocols, currently including HTTP, HTTPS, FTP, FTPS, SCP, SFTP, TFTP, LDAP, DAP, DICT, TELNET, FILE, IMAP, POP3, SMTP and RTSP (the last four only in versions newer than 7.20.0 or 9 February 2010).

cURL supports HTTPS and performs SSL certificate verification by default when a secure protocol is specified such as HTTPS. When cURL connects to a remote server via HTTPS, it will first obtain the remote server certificate and check against its CA certificate store the validity of the remote server to ensure the remote server is the one it claims to be. Some cURL packages have bundled with CA certificate store file. There are few options to specify CA certificate such as --cacert and --capath. --cacert option can be used to specify the location of the CA certificate store file. In the Windows platform, if a CA certificate file is not specified, cURL will look for a CA certificate file name “curl-ca-bundle.crt” in the following order:

1. Directory where the cURL program is located.
2. Current working directory.
3. Windows system directory.
4. Windows directory.
5. Directories specified in the %PATH% environment variables.^[6]

cURL will return an error message if the remote server is using a self-signed certificate, or if the remote server certificate is not signed by a CA listed in the CA cert file. -k or --insecure option can be used to skip certificate verification. Alternatively, if the remote server is trusted, the remote server CA certificate can be added to the CA certificate store file.

Examples of cURL use from command line

Basic use of cURL involves simply typing curl at the command line, followed by the URL of the output to retrieve.

To retrieve the example.com homepage, type:

curl www.example.com

cURL defaults to displaying the output it retrieves to the standard output specified on the system (usually the terminal window). So running the command above would, on most systems, display the www.example.com source-code in the terminal window.

cURL can write the output it retrieves to a file with the -o flag, thus:

curl -o example.html www.example.com

This will store the source code for www.example.com into a file named example.html. While retrieving output, cURL will display a progress bar showing how much of the output has downloaded. Note however that cURL does not show a progress bar when preparing to display the output in the terminal window, since a progress bar is likely to interfere with the display of the output.

To download output to a file that has the same name as on the system it originates from, use the -O flag, for example:

curl -O www.example.com/example.html

If the server responds that the file (example.html) is moved to a different location (indicated with a Location: header and a 3XX response code), use the -L flag, for example:

curl -OL www.example.com/example.html

cURL can connect to a remote server via HTTPS protocol (return error message if a CA certificate file cannot be located)

curl https://securesite.com/login.html

To specify a CA certificate file:

curl --cacert c:\temp\cacerts.crt https://securesite.com/login.html

To skip certificate verification:

curl --insecure https://self-signed-cert.com/login.html

Curl offers many other features such as proxy support, user authentication, FTP upload, HTTP post, SSL connections, cookies, file transfer resume, Metalink, as well as various other features.

See also

• curl-loader - an open-source testing tool based on cURL
• libwww
• Wget
• Web crawler

References

External links

• Official website
• cURL manual
• Github Repository
• Comparison of cURL vs other open source download tools
• Comparison of cURL vs wget