nomicon/doc/builders/trivial-builders.chapter.md

Trivial builders

Nixpkgs provides a couple of functions that help with building derivations. The most important one, stdenv.mkDerivation, has already been documented above. The following functions wrap stdenv.mkDerivation, making it easier to use in certain cases.

runCommand

This takes three arguments, name, env, and buildCommand. name is just the name that Nix will append to the store path in the same way that stdenv.mkDerivation uses its name attribute. env is an attribute set specifying environment variables that will be set for this derivation. These attributes are then passed to the wrapped stdenv.mkDerivation. buildCommand specifies the commands that will be run to create this derivation. Note that you will need to create $out for Nix to register the command as successful.

An example of using runCommand is provided below.

(import <nixpkgs> {}).runCommand "my-example" {} ''
  echo My example command is running

  mkdir $out

  echo I can write data to the Nix store > $out/message

  echo I can also run basic commands like:

  echo ls
  ls

  echo whoami
  whoami

  echo date
  date
''

runCommandCC

This works just like runCommand. The only difference is that it also provides a C compiler in buildCommand's environment. To minimize your dependencies, you should only use this if you are sure you will need a C compiler as part of running your command.

runCommandLocal

Variant of runCommand that forces the derivation to be built locally, it is not substituted. This is intended for very cheap commands (<1s execution time). It saves on the network roundrip and can speed up a build.

::: note This sets allowSubstitutes to false, so only use runCommandLocal if you are certain the user will always have a builder for the system of the derivation. This should be true for most trivial use cases (e.g. just copying some files to a different location or adding symlinks), because there the system is usually the same as builtins.currentSystem. :::

writeTextFile, writeText, writeTextDir, writeScript, writeScriptBin

These functions write text to the Nix store. This is useful for creating scripts from Nix expressions. writeTextFile takes an attribute set and expects two arguments, name and text. name corresponds to the name used in the Nix store path. text will be the contents of the file. You can also set executable to true to make this file have the executable bit set.

Many more commands wrap writeTextFile including writeText, writeTextDir, writeScript, and writeScriptBin. These are convenience functions over writeTextFile.

symlinkJoin

This can be used to put many derivations into the same directory structure. It works by creating a new derivation and adding symlinks to each of the paths listed. It expects two arguments, name, and paths. name is the name used in the Nix store path for the created derivation. paths is a list of paths that will be symlinked. These paths can be to Nix store derivations or any other subdirectory contained within.

writeReferencesToFile

Writes the closure of transitive dependencies to a file.

This produces the equivalent of nix-store -q --requisites.

For example,

writeReferencesToFile (writeScriptBin "hi" ''${hello}/bin/hello'')

produces an output path /nix/store/<hash>-runtime-deps containing

/nix/store/<hash>-hello-2.10
/nix/store/<hash>-hi
/nix/store/<hash>-libidn2-2.3.0
/nix/store/<hash>-libunistring-0.9.10
/nix/store/<hash>-glibc-2.32-40

You can see that this includes hi, the original input path, hello, which is a direct reference, but also the other paths that are indirectly required to run hello.

writeDirectReferencesToFile

Writes the set of references to the output file, that is, their immediate dependencies.

This produces the equivalent of nix-store -q --references.

For example,

writeDirectReferencesToFile (writeScriptBin "hi" ''${hello}/bin/hello'')

produces an output path /nix/store/<hash>-runtime-references containing

/nix/store/<hash>-hello-2.10

but none of hello's dependencies, because those are not referenced directly by hi's output.