Upload only modified files to FTP server

I want to upload files from my computer to an FTP site and I don't want to upload files that are already on the server. So I need a tool that finds out which local files that are different from the ones on the server, or that don't exists on the server.

Some requirements:

• I'm using a cheap provider that does not support rsync or ssh, so I can only use FTP.

• I generate the files before uploading them, so comparing timestamps is meaningless.

• I've tried lftp with the mirror command. It's slow (I think it uploads all the files).

• I upload the files from different computers, so I can't use sitecopy, which uses a local database to keep track of which files are on the server.

• I'd like to be able to upload all changed files with one command. Preferably no GUI application. And it needs to run in Ubuntu.

I was thinking about creating a tool similar to sitecopy, but which stores checksums of all the files on the FTP server on the server itself. But then I thought that there may already be such a tool.

Have you tried filezilla?

Has a "Directory Comparison" feature that may be what you are looking for.