Unix systems have for a 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
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.
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.
The user name string for the remote machine.
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:
-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.