Here is how you would connect to an anonymous FTP server using curl, meaning the FTP server does not require a username or password.

curl ftp://server1.example.com

The SFTP protocol would be used to connect to an SFTP server.

curl sftp://server1.example.com

The -u or --user option must be used if the FTP server requires a username and password to establish the connection.

curl --user john.doe:itsasecret ftp://server1.example.com

By default, curl will use your /home/username/.ssh/id_rsa (if it exists) private key when attempting to establish a connection to an SFTP server. The --key option can be used if you want to use some other private key file.

curl --user john.doe:itsasecret --key example.key sftp://server1.example.com

If the private key file is secured with a passphrase, the --pass option can be used to include the passphrase.

curl --user john.doe:itsasecret --key example.key --pass itsasecret sftp://server1.example.com

Once connected, the files and directories of the FTP server should be returned as output.

~]$ curl --user john.doe:itsasecret --key example.key --pass itsasecret sftp://server1.example.com
drwxrwxrwx 1 0        0               4096 Jun 24 02:28 OUTPUT
drwxrwxrwx 1 0        0                 38 Jun 23 10:35 INPUT
-rw-r--r-- 1 0        0              24576 Jun 23 22:38 foo.txt

The -T or --upload-file option can be used to upload a file to the FTP server.

curl --user john.doe:itsasecret --key example.key --pass itsasecret --upload-file bar.txt sftp://server1.example.com