--append is the long form option,
-a is the short. The option has existed since at least May 1998 (present in curl 4.8). I think it is safe to say that if we would’ve created this option just a few years later, we would not have “wasted” a short option letter on it. It is not a very frequently used one.
The append in the option name is a require to the receiver to append to — rather than replace — a destination file. This option only has any effect when uploading using either FTP(S) or SFTP. It is a flag option and you use it together with the
When you upload to a remote site with these protocols, the default behavior is to overwrite any file that happens to exist on the server using the name we’re uploading to. If you append this option to the command line, curl will instead instruct the server to append the newly uploaded data to the end of the remote file.
The reason this option is limited to just subset of protocols is of course that they are the only ones for which we can give that instruction to the server.
Append the local file “trailer” to the remote file called “begin”:
curl --append --upload trailer ftp://example.com/path/begin