Building Specific Parts of NixOS {#sec-building-parts}#

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:

```ShellSession
$ 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:

```ShellSession
$ 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:

```ShellSession
$ 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`:

```ShellSession
$ 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.