Nix Installer fails on macOS if /etc/bash.bashrc.backup-before-nix already exists

[mpscholten]

Is your feature request related to a problem? Please describe.
I've seen a couple times that the nix installer fails when a file at /etc/bash.bashrc.backup-before-nix already exists. E.g. yesterday a colleague tried to install nix, and as he had nix installed previously at that macbook he got that error saying that the nix installer cannot proceed until we delete the /etc/bash.bashrc.backup-before-nix file. After that file was deleted, we reran the installer and now got the same error but for the zshrc.backup-before-nix file.

Describe the solution you'd like
The installer should just take care of this automatically. Either ask the user what to do, or even better just automatically rename e.g. zshrc.backup-before-nix to zshrc.backup-before-nix2.

[abathur]

They'll have to follow the uninstall instructions before reinstalling it: https://nixos.org/manual/nix/stable/installation/installing-binary.html#macos

If they only tried reinstalling because nix wasn't on PATH after taking a macos update, they don't have to reinstall. The update is overwriting the shell hook, but they can just replace it. See #3616 for more.

[mpscholten]

This specific case was not directly related to a macos update. It just looked like nix was not installed on that machine (likely it got broken by some macos update in the past) and we tried to run the installer.

Running the uninstaller only comes to mind when you're aware that nix is already installed.

[iFreilicht]

One potential way of fixing this could be check whether the desired lines are already in /etc/bash and just not modifying it if that is the case.