Resolved How to do a non-destructive reinstallation for Catalina that preserves /* in /old?

[applCore]

I have a large drive on which I'd like to perform a reinstallation of Catalina. I want to take the drive out and essentially:
cd /Volumes/catalina_system
sudo mkdir old
sudo mv * old

And then perform a reinstallation of the OS and have all of the Applications and Library contents handy for restoring them if desired.

In versions of macOS prior to Catalina this was really straightforward, but now with the separate volumes for /Applications, /var, /System, etc. I'm not sure how this works and I have my reservations since I'm concerned that Catalina may not even respect an "/old" folder.

One might simply say "back it up", but in my case I'd like to understand this process regardless of that advice for various reasons. Please don't presume that I need a lecture on backup or Time Machine or anything like that. I simply want to understand how what I explained works if you have any experience or understanding in this area.

 

[Taz Mangus]

If you perform a total system backup then there would be no needed to manually copy the old installation to an old folder. The files on the backup are directly available including applications.

 

[applCore]

If you perform a total system backup then there would be no needed to manually copy the old installation to an old folder. The files on the backup are directly available including applications.

Like I noted, I'm not interested in the system backup (that's already known and I have such a backup). I'm wanting to understand what I asked the question about.

 

[Taz Mangus]

Like I noted, I'm not interested in the system backup (that's already known and I have such a backup). I'm wanting to understand what I asked the question about.

That is exactly what you are doing, a system backup by copying the files to a old folder.

 

[applCore]

Gosh dang it. Why do these kinds of answers happen? I'm asking for behavioral information here. I want to know precisely what happens without having to experiment myself in case someone knows, but I can obviously see that I'm going to have to just do it myself instead of hoping someone knows anything.

If you don't know the answer, just don't answer.

 

[chrfr]

In versions of macOS prior to Catalina this was really straightforward, but now with the separate volumes for /Applications, /var, /System, etc. I'm not sure how this works and I have my reservations since I'm concerned that Catalina may not even respect an "/old" folder.

The Catalina system volume is read-only, and you can’t add folders at / so I have no understanding of why this would be preferable to having a backup on a separate volume. You’re not going to be able to just move everything to a subdirectory and then install the operating system.
Can you explain what you mean by “respect” in this context?

 

[applCore]

The Catalina system volume is read-only, and you can’t add folders at / so I have no understanding of why this would be preferable to having a backup on a separate volume. You’re not going to be able to just move everything to a subdirectory and then install the operating system.
Can you explain what you mean by “respect” in this context?

I have it mounted rw - I don't abide Apple's preferences. I'd like to simply keep the contents of the root volume intact during a reinstall to have all of my larger Library preferences intact through the fresh reinstall. I do not want to create additional writes on my SSD if I can prevent it and thus the backup doesn't matter and this methodology is preferable in order to reduce the destructive nature of additional unneeded writes.

 

[applCore]

Answer:
Anything on the root volume: `/` that Catalina installs onto is wiped out.
Anything on the data volume: `/System/Volumes/Data` is retained.

In my case I (obviously `/` was mounted rw) moved all files of each of the two volumes into a `old-system` and `old-data`. At the start of installation macOS warned me that the `old-system` would be deleted. upon completion `old-data` remained intact, but only accessible via `/Volumes/Data/old-data`. The `old-data` folder did retain all of my installed Applications and other folders such as /opt (MacPorts), /usr/local, customized /etc and so forth.