Add cleanup commands to NixOS.md

source/linux/nixos.md

@@ -8,6 +8,7 @@
   - [SSH public key authentication](https://nixos.wiki/wiki/SSH_public_key_authentication)
 - [Nixpkgs Manual](https://nixos.org/manual/nixpkgs/stable/)
 - [Nix Reference Manual](https://nixos.org/manual/nix/stable/)
+  - [Main Commands](https://nixos.org/manual/nix/stable/command-ref/main-commands#main-commands)
 - Searches
   - Packages: https://search.nixos.org/packages
   - Options: https://search.nixos.org/options
@@ -26,6 +27,15 @@
 - list available profiles: `nix-env --list-generations --profile /nix/var/nix/profiles/system`
 - update the system: `nixos-rebuild switch --upgrade`
 
+## Cleanup
+- delete generations from the current profile
+  - delete all except current: `nix-env --delete-generations old`
+  - delete generations more than 14 days ago: `nix-env --delete-generations 14d`
+  - keep last 2 generations, along with any newer than current: `nix-env --delete-generations +2`
+- after removing old generations run garbage collector: `nix-store --gc`
+- delete unreachable store objects: `nix-collect-garbage -d`
+- <https://nixos.org/manual/nix/stable/package-management/garbage-collection>
+
 ## Installation
 
 - [Installing from another Linux distribution](https://nixos.org/manual/nixos/stable/#sec-installing-from-other-distro)