Hello,
I’m wondering if it’s possible on NixOS to detect if the system has changed to another version since it booted? To make it simple, I’d like to detect if nixos-rebuild test / switch has been run.
The kernel version could be used, but it rarely change so it’s not very effective.
You can compare symlinks for /run/current-system and /run/booted-system.
You could also check /proc/cmdline for the init= parameter; that always points to the booted generation on NixOS, and it’s immutable.
Thank you very much 
This check is also used by the autoUpdate module when deciding whether to reboot: nixpkgs/auto-upgrade.nix at f5357321bace1f2b8f47868414f9ff420cbef8c3 · NixOS/nixpkgs · GitHub
This will be very handy too 