···138# Rewrite /etc/.clean.
139close CLEAN;
140write_file("/etc/.clean", map { "$_\n" } @copied);
141+142+# Create /etc/NIXOS tag if not exists.
143+# When /etc is not on a persistent filesystem, it will be wiped after reboot,
144+# so we need to check and re-create it during activation.
145+open TAG, ">>/etc/NIXOS";
146+close TAG;