nixpkgs/nixos/doc/manual/development/building-parts.chapter.md

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# 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:
```ShellSession
$ 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.