nixpkgs/doc/build-helpers/trivial-build-helpers.chapter.md

Trivial build helpers

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

runCommand :: String -> AttrSet -> String -> Derivation

runCommand name drvAttrs buildCommand returns a derivation that is built by running the specified shell commands.

name :: String

The name that Nix will append to the store path in the same way that stdenv.mkDerivation uses its name attribute.

drvAttr :: AttrSet

Attributes to pass to the underlying call to stdenv.mkDerivation.

buildCommand :: String

Shell commands to run in the derivation builder.

::: {.note} You have to create a file or directory $out for Nix to be able to run the builder successfully. :::

::: {.example #ex-runcommand-simple}

Invocation of runCommand

(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 round-trip 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.

Here are a few examples:

# Writes my-file to /nix/store/<store path>
writeTextFile {
  name = "my-file";
  text = ''
    Contents of File
  '';
}
# See also the `writeText` helper function below.

# Writes executable my-file to /nix/store/<store path>/bin/my-file
writeTextFile {
  name = "my-file";
  text = ''
    Contents of File
  '';
  executable = true;
  destination = "/bin/my-file";
}
# Writes contents of file to /nix/store/<store path>
writeText "my-file"
  ''
  Contents of File
  '';
# Writes contents of file to /nix/store/<store path>/share/my-file
writeTextDir "share/my-file"
  ''
  Contents of File
  '';
# Writes my-file to /nix/store/<store path> and makes executable
writeScript "my-file"
  ''
  Contents of File
  '';
# Writes my-file to /nix/store/<store path>/bin/my-file and makes executable.
writeScriptBin "my-file"
  ''
  Contents of File
  '';
# Writes my-file to /nix/store/<store path> and makes executable.
writeShellScript "my-file"
  ''
  Contents of File
  '';
# Writes my-file to /nix/store/<store path>/bin/my-file and makes executable.
writeShellScriptBin "my-file"
  ''
  Contents of File
  '';

concatTextFile, concatText, concatScript

These functions concatenate files to the Nix store in a single file. This is useful for configuration files structured in lines of text. concatTextFile takes an attribute set and expects two arguments, name and files. name corresponds to the name used in the Nix store path. files will be the files to be concatenated. You can also set executable to true to make this file have the executable bit set. concatText andconcatScript are simple wrappers over concatTextFile.

Here are a few examples:

# Writes my-file to /nix/store/<store path>
concatTextFile {
  name = "my-file";
  files = [ drv1 "${drv2}/path/to/file" ];
}
# See also the `concatText` helper function below.

# Writes executable my-file to /nix/store/<store path>/bin/my-file
concatTextFile {
  name = "my-file";
  files = [ drv1 "${drv2}/path/to/file" ];
  executable = true;
  destination = "/bin/my-file";
}
# Writes contents of files to /nix/store/<store path>
concatText "my-file" [ file1 file2 ]

# Writes contents of files to /nix/store/<store path>
concatScript "my-file" [ file1 file2 ]

writeShellApplication

This can be used to easily produce a shell script that has some dependencies (runtimeInputs). It automatically sets the PATH of the script to contain all of the listed inputs, sets some sanity shellopts (errexit, nounset, pipefail), and checks the resulting script with shellcheck.

For example, look at the following code:

writeShellApplication {
  name = "show-nixos-org";

  runtimeInputs = [ curl w3m ];

  text = ''
    curl -s 'https://nixos.org' | w3m -dump -T text/html
  '';
}

Unlike with normal writeShellScriptBin, there is no need to manually write out ${curl}/bin/curl, setting the PATH was handled by writeShellApplication. Moreover, the script is being checked with shellcheck for more strict validation.

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. Here is an example:

# adds symlinks of hello and stack to current build and prints "links added"
symlinkJoin { name = "myexample"; paths = [ pkgs.hello pkgs.stack ]; postBuild = "echo links added"; }

This creates a derivation with a directory structure like the following:

/nix/store/sglsr5g079a5235hy29da3mq3hv8sjmm-myexample
|-- bin
|   |-- hello -> /nix/store/qy93dp4a3rqyn2mz63fbxjg228hffwyw-hello-2.10/bin/hello
|   `-- stack -> /nix/store/6lzdpxshx78281vy056lbk553ijsdr44-stack-2.1.3.1/bin/stack
`-- share
    |-- bash-completion
    |   `-- completions
    |       `-- stack -> /nix/store/6lzdpxshx78281vy056lbk553ijsdr44-stack-2.1.3.1/share/bash-completion/completions/stack
    |-- fish
    |   `-- vendor_completions.d
    |       `-- stack.fish -> /nix/store/6lzdpxshx78281vy056lbk553ijsdr44-stack-2.1.3.1/share/fish/vendor_completions.d/stack.fish
...

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.