Catalina --eraseinstall Installs Big Sur

What is the full command?

--eraseinstall is a flag applied when executing the startoinstall command inside of the macOS installer app. This means that Big Sur is likely being downloaded first and then the flag is ran.

Do you have more context as to how this is being ran? Where does the installer file come from? Are you using a script or another method?

First I run a pkg that puts the Catalina installer into Applications. It is followed by the below command:

'/Applications/Install macOS Catalina.app/Contents/Resources/startosinstall' ‑‑eraseinstall
--agreetolicense --forcequitapps --nointeraction

I should also add that I have a similar policy that follows the exact same workflow but does not include --eraseinstall that we use for upgrading to Catalina. This works correctly and does not install Big Sur.

@armenh Have you verified the .pkg you install is in fact the macOS Catalina installer for both of your workflows? As @sdamiano states above the --eraseinstall is simply a parameter to the startosinstall so it's not going to change what version of macOS is being installed. You should also be aware that if a Mac is already running Big Sur that you can not use the --eraseinstall option with the startosinstall tool in the Install macOS Catalina.app to downgrade to Catalina. That will require booting from a USB installer for macOS Catalina (if the Mac is capable of running Catalina)

I'll double check what's in the pkg but if the command is pointing to /Applications/Install macOS Catalina.app/Contents/Resources/startosinstall , how could that be installing Big Sur accidentally?

That could happen if you have the Big Sur installer named "Install macOS Catalina.app" which would have been a user making an error renaming files at some point. What version of the installer app do you have?

I tried to replicate it and could not, my test systems successfully wiped and reinstalled Catalina.

I have two policies. The first which is an upgrade to 10.15.7 that users have been using for weeks with no issues. The second, which I've been testing, is the 10.15.7 --eraseinstall. They both pull from the same .pkg that deposits the Install macOS Catalina.app into /Applications.

Could the issue be that if the Install macOS Big Sur.app is present on the system, it will opt to use that one instead of the Catalina one even if startosinstall is launched from another .app? I will try to test that.

After revisiting this issue, it seems that if the user has downloaded the Big Sur installer and it was located in the /Applications folder, the erase install will ignore the Catalina installer and install Big Sur.