While there isn't a proper Linux client, if you find yourself on a Linux box and need to sync to or from iCloud, rclone[1] works great. Just putting this out there in the hope that it might help someone.
It's also (ironically given TFA) what I used to sync all my files off dropbox when I cancelled my subscription because of their misuse of root to re-add their thing to special permissions on macOs after I had removed it.[2]
[2] https://news.ycombinator.com/item?id=12463338 not trying to reopen a flame war, but for me personally, that was one of those things a company doesn't get to do to me twice. As soon as it happened, I copied my files off and cancelled. In fact I'm there somewhere in the comments on that article saying I was going to be cancelling and I immediately did.