I have noticed that every once and a while /Users/andrewmckinlay/.nix-profile/bin and /nix/var/nix/profiles/default/bin disappear from my $PATH. The last time this happened, the hook nix installed was missing from /etc/zshrc.

Copying the following hook (copied from /etc/bashrc) to /etc/zshrc and restarting the shell fixed everything again:

# Nix
if [ -e '/nix/var/nix/profiles/default/etc/profile.d/nix-daemon.sh' ]; then
  . '/nix/var/nix/profiles/default/etc/profile.d/nix-daemon.sh'
fi
# End Nix

My concern is that macOS updates will periodically overwrite /etc/.zshrc (and /etc/.bashrc), requiring effort on my part to reinstall nix. For example, one macOS update overwrote the entire file to fix this misspelling:

andrewmckinlay@imac ~ % diff /etc/zshrc.backup-before-nix /etc/zshrc
3c3
< # Setup user specific overrides for this in ~/.zhsrc. See zshbuiltins(1)
---
> # Setup user specific overrides for this in ~/.zshrc. See zshbuiltins(1)

Both files are read-only anyway, should they really be written to by the nix installer?