I’m looking for at least the name of filesystem which can act as Rclone Union one. Idea is that you have one directory (i.e. system root) which is read only and second one which is used for modifications (i.e. to create file). Then when you try to access the file in a such FS, it will look for the file in the second directory and if it is not there it will look for it in the first one. If you try to modify the file, it will be stored in the second directory and original file in the first one will not be modified.

It sounds like ADD command in Docker, but without actually copying anything before it is written.

I find this very usable when you do the chroot to prevent some process from unintentional modification of the “host” filesystem, so you can provide access to the system root, but any modifications will not affect it. I want to use on macOS where base image, Xcodes and everything occupies enormous amount of space, so copying it just to use with chroot sounds undesirable.

Thanks.