I need to use youtube-dl to download new videos from a channel, but remember not to download them next time and only download videos that have not been downloaded yet. How can I do so?
Use the --download-archive option:
youtube-dl --download-archive ~/.mydownloads http://youtube.com/CHANNEL
The IDs of all downloaded videos will be recorded in the .mydownloads file, and these videos will not be downloaded again.