Nix will only build derivations where the `system` attribute matches the current [`system` configuration option].
It can automatically [build on other platforms](../advanced-topics/distributed-builds.md) by forwarding build requests to other machines.
Examples:
`system = "x86_64-linux";`
`system = builtins.currentSystem;`
[`builtins.currentSystem`](@docroot@/language/builtin-constants.md#builtins-currentSystem) has the value of the [`system` configuration option], and defaults to the system type of the current Nix installation.
- [`outputs`]{#attr-outputs} ([List](@docroot@/language/values.md#list) of [String](@docroot@/language/values.md#type-string)) Default: `[ "out" ]`
Symbolic outputs of the derivation.
Each output name is passed to the [`builder`](#attr-builder) executable as an environment variable with its value set to the corresponding [output path].
By default, a derivation produces a single output path called `out`.
However, derivations can produce multiple output paths.
This allows the associated [store objects](@docroot@/glossary.md#gloss-store-object) and their [closures](@docroot@/glossary.md#gloss-closure) to be copied or garbage-collected separately.
Examples:
Imagine a library package that provides a dynamic library, header files, and documentation.
A program that links against the library doesn’t need the header files and documentation at runtime, and it doesn’t need the documentation at build time.
Thus, the library package could specify:
```nix
outputs = [ "lib" "headers" "doc" ];
```
This will cause Nix to pass environment variables `lib`, `headers`, and `doc` to the builder containing the intended store paths of each output.
The builder would typically do something like
```bash
./configure \
--libdir=$lib/lib \
--includedir=$headers/include \
--docdir=$doc/share/doc
```
for an Autoconf-style package.
You can refer to each output of a
derivation by selecting it as an attribute, e.g.
```nix
buildInputs = [ pkg.lib pkg.headers ];
```
<!-- FIXME: move this to the output attributes section when we have one -->
The first element of `outputs` determines the *default output*.
Thus, you could also write
```nix
buildInputs = [ pkg pkg.headers ];
```
since `pkg` is equivalent to `pkg.lib`.
- See [Advanced Attributes](./advanced-attributes.md) for more, infrequently used, optional attributes.
<!-- FIXME: This should be moved here -->
- Every other attribute is passed as an environment variable to the builder.
Attribute values are translated to environment variables as follows:
- Strings and numbers are just passed verbatim.
- A *path* (e.g., `../foo/sources.tar`) causes the referenced file
to be copied to the store; its location in the store is put in
the environment variable. The idea is that all sources should
reside in the Nix store, since all inputs to a derivation should
reside in the Nix store.
- A *derivation* causes that derivation to be built prior to the
present derivation; its default output path is put in the
environment variable.
- Lists of the previous types are also allowed. They are simply
concatenated, separated by spaces.
-`true` is passed as the string `1`, `false` and `null` are
