Scheme

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Scheme
URLs start with the "scheme", which is the official name for the http:// part. That tells which protocol the URL uses. The scheme must be a known one that this version of curl supports or it will show an error message and stop. Additionally, the scheme must neither start with nor contain any whitespace.
The scheme separator
The scheme identifier is separated from the rest of the URL by the :// sequence. That is a colon and two forward slashes. There exists URL formats with only one slash, but curl does not support any of them. There are two additional notes to be aware of, about the number of slashes:
curl allows some illegal syntax and tries to correct it internally; so it will also understand and accept URLs with one or three slashes, even though they are in fact not properly formed URLs. curl does this because the browsers started this practice so it has lead to such URLs being used in the wild every now and then.
file:// URLs are written as file://<hostname>/<path> but the only hostnames that are okay to use are localhost, 127.0.0.1 or a blank (nothing at all):
file://localhost/path/to/file
file://127.0.0.1/path/to/file
file:///path/to/file
Inserting any other host name in there will make recent versions of curl to return an error.
Pay special attention to the third example above (file:///path/to/file). That is three slashes before the path. That is again an area with common mistakes and where browsers allow users to use the wrong syntax so as a special exception, curl on Windows also allows this incorrect format:
file://X:/path/to/file
… where X is a windows-style drive letter.
Without scheme
As a convenience, curl also allows users to leave out the scheme part from URLs. Then it guesses which protocol to use based on the first part of the host name. That guessing is basic, as it just checks if the first part of the host name matches one of a set of protocols, and assumes you meant to use that protocol. This heuristic is based on the fact that servers traditionally used to be named like that. The protocols that are detected this way are FTP, DICT, LDAP, IMAP, SMTP and POP3. Any other host name in a scheme-less URL will make curl default to HTTP.
For example, this gets a file from an FTP site:
curl ftp.funet.fi/README
While this gets data from an HTTP server:
curl example.com
You can modify the default protocol to something other than HTTP with the --proto-default option.
Supported schemes
curl supports or can be made to support (if built so) the following transfer schemes and protocols:
DICT, FILE, FTP, FTPS, GOPHER, GOPHERS, HTTP, HTTPS, IMAP, IMAPS, LDAP, LDAPS, MQTT, POP3, POP3S, RTMP, RTMPS, RTSP, SCP, SFTP, SMB, SMBS, SMTP, SMTPS, TELNET and TFTP
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Name and password
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The scheme separator
Without scheme
Supported schemes