With the command nix-build, you can build specific parts of your NixOS configuration. This is done as follows: $ cd /path/to/nixpkgs/nixos $ nix-build -A config.option where option is a NixOS option with type “derivation” (i.e. something that can be built). Attributes of interest include: system.build.toplevel The top-level option that builds the entire NixOS system. Everything else in your configuration is indirectly pulled in by this option. This is what nixos-rebuild builds and what /run/current-system points to afterwards. A shortcut to build this is: $ nix-build -A system system.build.manual.manualHTML The NixOS manual. system.build.etc A tree of symlinks that form the static parts of /etc. system.build.initialRamdisk system.build.kernel The initial ramdisk and kernel of the system. This allows a quick way to test whether the kernel and the initial ramdisk boot correctly, by using QEMU’s -kernel and -initrd options: $ nix-build -A config.system.build.initialRamdisk -o initrd $ nix-build -A config.system.build.kernel -o kernel $ qemu-system-x86_64 -kernel ./kernel/bzImage -initrd ./initrd/initrd -hda /dev/null system.build.nixos-rebuild system.build.nixos-install system.build.nixos-generate-config These build the corresponding NixOS commands. systemd.units.unit-name.unit This builds the unit with the specified name. Note that since unit names contain dots (e.g. httpd.service), you need to put them between quotes, like this: $ nix-build -A 'config.systemd.units."httpd.service".unit' You can also test individual units, without rebuilding the whole system, by putting them in /run/systemd/system: $ cp $(nix-build -A 'config.systemd.units."httpd.service".unit')/httpd.service \ /run/systemd/system/tmp-httpd.service # systemctl daemon-reload # systemctl start tmp-httpd.service Note that the unit must not have the same name as any unit in /etc/systemd/system since those take precedence over /run/systemd/system. That’s why the unit is installed as tmp-httpd.service here.