Derivations

A Nix derivation is a specification for running an executable on precisely defined input files to repeatably produce output files at uniquely determined file system paths. Simply put, it describes a set of steps to take some input and produce some output in a deterministic manner.

Derivations are extremely powerful and a core part of Nix utility and provide the core platform for how packaging is done in NixOS. The low-level derivation function is extremely simple, and can become repetitive to define package constructs with, so Nixpkgs hoists the standard environment (colloquially referred to as the stdenv), which provides a sparse set of compilation tools and a derivation construct that avoids the low-level details and provides abstractions to simplify packaging.

Getting Started

Derivations take an input and produce an output through a series of steps. For most packages, the inputs refer to the source files, the steps refer to the compilation process and are called phases, and the outputs refer to finalized executable binaries written to some file/directory. This sequence of events can be well described within a standard environment.

src

This attribute points towards the inputs used within the derivation. This can be source code, but it can also be a pre-build binary. Usually depending on what the source is, a special series of steps is done to ensure its correctness in a Nix environment.

This usually is downloaded via a fetcher, but it can also be a local path:

stdenv.mkDerivation {

  src = ./relative-path/to/src;
  # OR
  src = fetchFromGitHub {
    owner = "torvalds";
    repo = "linux";
    rev = "refs/tags/v6.11";

    hash = "...";
  };
}

out

Unlike src, this is not an attribute you set but rather an environment variable which points to the finalized location of the derivation's contents. The actual directory of $out is an implementation detail abstracted away by Nix and the stdenv builder. Anything placed within $out will then be part of the final derivation. This follows an FHS-inspired like structure, where $out/bin contains binaries, $out/lib contains shared objects, $out/include contains headers, and so forth. These paths will become part of the derivation and are the resulting entries within the /nix/store.

This attribute must point to either a file or directory, even if they are empty! Failure to create this path will result in the builder failing the entire build process.

This step is handled largely by the installPhase(see #Phases for more details):

stdenv.mkDerivation {
  src = ...;

  installPhase = ''
    mkdir -p $out/bin

    install -Dm755 -t $out/bin ./my-binary
  '';
}

meta

Unlike the previous two, this attribute has no significant relevance to building and largely contains a loose set of attributes useful with the context of Nixpkgs. Therefore, if you're writing a derivation that is not intended for Nixpkgs, this entire attribute set can be safely omitted.

Attributes in here relate to the upstream source, or the finalized derivation result. Some attributes, like meta.homepage and meta.description, are used to describe and link to relevant information about the upstream source this derivation builds. Attributes like meta.platforms and meta.licenses are also semantically relevant to upstream, but these are also useful for Nixpkgs to determine whether a package can be built on a different system (and whether to allow so in evaluation!), and if a package has a suitable license that allows for re-distribution (caching in the official binary cache).

There are also some other attributes that are only relevant within the context of Nixpkgs/Nix alone, such as meta.mainProgram, which describes the binary that can be considered the "main" program. For example, the cmake derivation would have meta.mainProgram set to cmake, which would be resolved as $out/bin/cmake when needed.

A fully exhaustive documentation on all meta-attributes can be found in the Nixpkgs manual.

Phases

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A phase can be described as a set of steps used to transform an input into an output suitable for the next phase. Each step in the stdenv builder controls a distinct part of the build process and is largely inspired from GNU Autoconf convention of ./configure, make, and make install.

Each phase is written in bash syntax and can use any program defined within the stdenv dependencies, alongside a very minimal set of packages automatically declared within the stdenv. This minimal set include the core-utils, gcc, gnumake, bash, gnuinstall, and more that are not exhaustively documented anywhere^{[citation needed]}.