3.3. Attributes

Variables affecting stdenv initialisation

NIX_DEBUG: If set, stdenv will print some debug information during the build. In particular, the gcc and ld wrapper scripts will print out the complete command line passed to the wrapped tools.
buildInputs: A list of dependencies used by stdenv to set up the environment for the build. For each dependency dir, the directory dir/bin, if it exists, is added to the PATH environment variable. Other environment variables are also set up via a pluggable mechanism. For instance, if buildInputs contains Perl, then the lib/site_perl subdirectory of each input is added to the PERL5LIB environment variable. See Section 3.6, “Package setup hooks” for details.
propagatedBuildInputs: Like buildInputs, but these dependencies are propagated: that is, the dependencies listed here are added to the buildInputs of any package that uses this package as a dependency. So if package Y has propagatedBuildInputs = [X], and package Z has buildInputs = [Y], then package X will appear in Z’s build environment automatically.

Variables affecting build properties

enableParallelBuilding: If set, stdenv will pass specific flags to make and other build tools to enable parallel building with up to build-cores workers.
preferLocalBuild: If set, specifies that the package is so lightweight in terms of build operations (e.g. write a text file from a Nix string to the store) that there's no need to look for it in binary caches -- it's faster to just build it locally. It also tells Hydra and other facilities that this package doesn't need to be exported in binary caches (noone would use it, after all).

Special variables

passthru

This is an attribute set which can be filled with arbitrary values. For example:

passthru = {
  foo = "bar";
  baz = {
    value1 = 4;
    value2 = 5;
  };
}

Values inside it are not passed to the builder, so you can change them without triggering a rebuild. However, they can be accessed outside of a derivation directly, as if they were set inside a derivation itself, e.g. hello.baz.value1. We don't specify any usage or schema of passthru - it is meant for values that would be useful outside the derivation in other parts of a Nix expression (e.g. in other derivations). An example would be to convey some specific dependency of your derivation which contains a program with plugins support. Later, others who make derivations with plugins can use passed-through dependency to ensure that their plugin would be binary-compatible with built program.