find . -depth -type d -execdir rename ‘s/-(\d{2})(\d{2})(\d{2})/__$1-$2-$3/’ {} \;
This looks for directories (-type d) from the directory you are standing in (the lone first dot). It gives what it finds to the rename command, which does the regexp magic and the renaming. The -depth and -execdir (instead of -exec) are needed in case it finds subdirectories.
Do give this a test in a safe location first. I did do a mini test just to make sure but better safe then sorry…
EDIT: I need to escape the escape otherwise it won’t show. Fixed.
In the end I’ve used tha last command with find+rename (had to install rename with sudo apt install… because it wasnt available on my system) and only had to change one small thing at the end:
I’ve heard of them… or quickly scanned through their readmes/docs… I dunno, to me the rsync-based solution are lightweight and effective; my needs are simple, to make regular backups of my home folder, the darktable folder with all the raws and the videos folder where I keep all family clips over ssh to the other pc I have in my house (a small intel nuc with a raid hd).
I will have a look at them however, thanks Mica, maybe I discover something better