Unix systems have for a very long time offered a way for users to store their
user name and password for remote FTP servers. ftp clients have supported
this for decades and this way allowed users to quickly login to known servers
without manually having to reenter the credentials each time. The .netrc file
is typically stored in a user's home directory. (On Windows, curl will look for
it with the name _netrc).
This being a widespread and well used concept, curl also supports it—if you ask it to. curl does not, however, limit this feature to FTP, but can get credentials for machines for any protocol with this. See further below for how.
The .netrc file format is simple: you specify lines with a machine name and follow that with lines for the login and password that are associated with that machine.
machine name
Identifies a remote machine name. curl searches the .netrc file for a machine token that matches the remote machine specified in the URL. Once a match is made, the subsequent .netrc tokens are processed, stopping when the end of file is reached or another machine is encountered.
login name
The user name string for the remote machine.
password string
Supply a password. If this token is present, curl will supply the specified string if the remote server requires a password as part of the login process. Note that if this token is present in the .netrc file you really should make sure the file is not readable by anyone besides the user.
An example .netrc for the host example.com with a user named 'daniel', using the password 'qwerty' would look like:
machine example.com
login daniel
password qwerty
-n, --netrc tells curl to look for and use the .netrc file.
--netrc-file [file] is similar to --netrc, except that you also provide
the path to the actual file to use. This is useful when you want to provide
the information in another directory or with another file name.
--netrc-optional is similar to --netrc, but this option makes the .netrc
usage optional and not mandatory as the --netrc option.