Another beginners question ... I am confused: The NixOS boot menu shows a list of 17 configurations. I want to remove some of them in order to free some space on the NixOS partition. I try $ sudo nix-env --list-generations 12 2015-08-30 11:20:39 19 2015-12-22 11:50:11 20 2016-07-15 15:52:44 21 2016-11-20 13:09:56 22 2016-11-20 13:10:06 (current) in order to obtain the numbers for --delete-generations. However, it seems that the listed "generations" are something different from the "configurations" in the boot menu. How can I list and delete some of those configurations from the boot menu? Btw. I think after upgrading NixOS it is a natural desire to get rid of some old versions. What about adding a pointer to http://nixos.org/nixos/manual/index.html#sec-upgrading ?