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

Trivial build helpers

Nixpkgs provides a variety of wrapper functions that help build commonly useful derivations. Like stdenv.mkDerivation, each of these build helpers creates a derivation, but the arguments passed are different (usually simpler) from those required by stdenv.mkDerivation.

runCommandWith

The function runCommandWith returns a derivation built using the specified command(s), in a specified environment.

It is the underlying base function of all runCommand* variants. The general behavior is controlled via a single attribute set passed as the first argument, and allows specifying stdenv freely.

The following runCommand* variants exist: runCommand, runCommandCC, and runCommandLocal.

Type

runCommandWith :: {
  name :: name;
  stdenv? :: Derivation;
  runLocal? :: Bool;
  derivationArgs? :: { ... };
} -> String -> Derivation

Inputs

name (String)

The derivation's name, which Nix will append to the store path; see mkDerivation.

runLocal (Boolean)

If set to true this forces the derivation to be built locally, not using substitutes nor remote builds. This is intended for very cheap commands (<1s execution time) which can be sped up by avoiding the network round-trip(s). Its effect is to set preferLocalBuild = true and allowSubstitutes = false.

::: {.note} This prevents the use of substituters, so only set runLocal (or use runCommandLocal) when 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. :::

stdenv (Derivation)

The standard environment to use, defaulting to pkgs.stdenv

derivationArgs (Attribute set)

Additional arguments for 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-runcommandwith}

Invocation of runCommandWith

runCommandWith {
  name = "example";
  derivationArgs.nativeBuildInputs = [ cowsay ];
} ''
  cowsay > $out <<EOMOO
  'runCommandWith' is a bit cumbersome,
  so we have more ergonomic wrappers.
  EOMOO
''

:::

runCommand and runCommandCC

The function runCommand returns a derivation built using the specified command(s), in the stdenvNoCC environment.

runCommandCC is similar but uses the default compiler environment. To minimize dependencies, runCommandCC should only be used when the build command needs a C compiler.

runCommandLocal is also similar to runCommand, but forces the derivation to be built locally. See the note on runCommandWith about runLocal.

Type

runCommand      :: String -> AttrSet -> String -> Derivation
runCommandCC    :: String -> AttrSet -> String -> Derivation
runCommandLocal :: String -> AttrSet -> String -> Derivation

Input

While the type signature(s) differ from runCommandWith, individual arguments with the same name will have the same type and meaning:

name (String)

The derivation's name

derivationArgs (Attribute set)

Additional parameters passed to [mkDerivation]

buildCommand (String)

The command(s) run to build the derivation.

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

Invocation of runCommand

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
''

:::

::: {.note} runCommand name derivationArgs buildCommand is equivalent to

runCommandWith {
  inherit name derivationArgs;
  stdenv = stdenvNoCC;
} buildCommand

Likewise, runCommandCC name derivationArgs buildCommand is equivalent to

runCommandWith {
  inherit name derivationArgs;
} buildCommand

:::

Writing text files

Nixpkgs provides the following functions for producing derivations which write text files or executable scripts into the Nix store. They are useful for creating files from Nix expression, and are all implemented as convenience wrappers around writeTextFile.

Each of these functions will cause a derivation to be produced. When you coerce the result of each of these functions to a string with string interpolation or builtins.toString, it will evaluate to the store path of this derivation.

:::: {.note} Some of these functions will put the resulting files within a directory inside the derivation output. If you need to refer to the resulting files somewhere else in a Nix expression, append their path to the derivation's store path.

For example, if the file destination is a directory:

{
  my-file = writeTextFile {
    name = "my-file";
    text = ''
      Contents of File
    '';
    destination = "/share/my-file";
  };
}

Remember to append "/share/my-file" to the resulting store path when using it elsewhere:

writeShellScript "evaluate-my-file.sh" ''
  cat ${my-file}/share/my-file
''

::::

makeDesktopItem

Write an XDG desktop file to the Nix store.

This function is usually used to add desktop items to a package through the copyDesktopItems hook.

makeDesktopItem adheres to version 1.4 of the specification.

Inputs

makeDesktopItem takes an attribute set that accepts most values from the XDG specification.

All recognised keys from the specification are supported with the exception of the "Hidden" field. The keys are converted into camelCase format, but correspond 1:1 to their equivalent in the specification: genericName, noDisplay, comment, icon, onlyShowIn, notShowIn, dbusActivatable, tryExec, exec, path, terminal, mimeTypes, categories, implements, keywords, startupNotify, startupWMClass, url, prefersNonDefaultGPU.

The "Version" field is hardcoded to the version makeDesktopItem currently adheres to.

The following fields are either required, are of a different type than in the specification, carry specific default values, or are additional fields supported by makeDesktopItem:

name (String)

The name of the desktop file in the Nix store.

type (String; optional)

Default value: "Application"

desktopName (String)

Corresponds to the "Name" field of the specification.

actions (List of Attribute set; optional)

A list of attribute sets {name, exec?, icon?}

extraConfig (Attribute set; optional)

Additional key/value pairs to be added verbatim to the desktop file. Attributes need to be prefixed with 'X-'.

Examples

::: {.example #ex-makeDesktopItem}

Usage 1 of makeDesktopItem

Write a desktop file /nix/store/<store path>/my-program.desktop to the Nix store.

{makeDesktopItem}:
makeDesktopItem {
  name = "my-program";
  desktopName = "My Program";
  genericName = "Video Player";
  noDisplay = false;
  comment = "Cool video player";
  icon = "/path/to/icon";
  onlyShowIn = [ "KDE" ];
  dbusActivatable = true;
  tryExec = "my-program";
  exec = "my-program --someflag";
  path = "/some/working/path";
  terminal = false;
  actions.example = {
    name = "New Window";
    exec = "my-program --new-window";
    icon = "/some/icon";
  };
  mimeTypes = [ "video/mp4" ];
  categories = [ "Utility" ];
  implements = [ "org.my-program" ];
  keywords = [ "Video" "Player" ];
  startupNotify = false;
  startupWMClass = "MyProgram";
  prefersNonDefaultGPU = false;
  extraConfig.X-SomeExtension = "somevalue";
}

:::

::: {.example #ex2-makeDesktopItem}

Usage 2 of makeDesktopItem

Override the hello package to add a desktop item.

{ copyDesktopItems
, hello
, makeDesktopItem }:

hello.overrideAttrs {
  nativeBuildInputs = [ copyDesktopItems ];

  desktopItems = [(makeDesktopItem {
    name = "hello";
    desktopName = "Hello";
    exec = "hello";
  })];
}

:::

writeTextFile

Write a text file to the Nix store.

writeTextFile takes an attribute set with the following possible attributes:

name (String)

Corresponds to the name used in the Nix store path identifier.

text (String)

The contents of the file.

executable (Bool, optional)

Make this file have the executable bit set.

Default: false

destination (String, optional)

A subpath under the derivation's output path into which to put the file. Subdirectories are created automatically when the derivation is realised.

By default, the store path itself will be a file containing the text contents.

Default: ""

checkPhase (String, optional)

Commands to run after generating the file.

Default: ""

meta (Attribute set, optional)

Additional metadata for the derivation.

Default: {}

allowSubstitutes (Bool, optional)

Whether to allow substituting from a binary cache. Passed through to allowSubstitutes of the underlying call to builtins.derivation.

It defaults to false, as running the derivation's simple builder executable locally is assumed to be faster than network operations. Set it to true if the checkPhase step is expensive.

Default: false

preferLocalBuild (Bool, optional)

Whether to prefer building locally, even if faster remote build machines are available.

Passed through to preferLocalBuild of the underlying call to builtins.derivation.

It defaults to true for the same reason allowSubstitutes defaults to false.

Default: true

derivationArgs (Attribute set, optional)

Extra arguments to pass to the underlying call to stdenv.mkDerivation.

Default: {}

The resulting store path will include some variation of the name, and it will be a file unless destination is used, in which case it will be a directory.

::: {.example #ex-writeTextFile}

Usage 1 of writeTextFile

Write my-file to /nix/store/<store path>/some/subpath/my-cool-script, making it executable. Also run a check on the resulting file in a checkPhase, and supply values for the less-used options.

writeTextFile {
  name = "my-cool-script";
  text = ''
    #!/bin/sh
    echo "This is my cool script!"
  '';
  executable = true;
  destination = "/some/subpath/my-cool-script";
  checkPhase = ''
    ${pkgs.shellcheck}/bin/shellcheck $out/some/subpath/my-cool-script
  '';
  meta = {
    license = pkgs.lib.licenses.cc0;
  };
  allowSubstitutes = true;
  preferLocalBuild = false;
}

:::

::: {.example #ex2-writeTextFile}

Usage 2 of writeTextFile

Write the string Contents of File to /nix/store/<store path>. See also the helper function.

writeTextFile {
  name = "my-file";
  text = ''
    Contents of File
  '';
}

:::

::: {.example #ex3-writeTextFile}

Usage 3 of writeTextFile

Write an executable script my-script to /nix/store/<store path>/bin/my-script. See also the helper function.

writeTextFile {
  name = "my-script";
  text = ''
    echo "hi"
  '';
  executable = true;
  destination = "/bin/my-script";
}

:::

writeText

Write a text file to the Nix store

writeText takes the following arguments: a string.

name (String)

The name used in the Nix store path.

text (String)

The contents of the file.

The store path will include the name, and it will be a file.

::: {.example #ex-writeText}

Usage of writeText

Write the string Contents of File to /nix/store/<store path>:

writeText "my-file"
  ''
  Contents of File
  ''

:::

This is equivalent to:

writeTextFile {
  name = "my-file";
  text = ''
    Contents of File
  '';
}

writeTextDir

Write a text file within a subdirectory of the Nix store.

writeTextDir takes the following arguments:

path (String)

The destination within the Nix store path under which to create the file.

text (String)

The contents of the file.

The store path will be a directory.

::: {.example #ex-writeTextDir}

Usage of writeTextDir

Write the string Contents of File to /nix/store/<store path>/share/my-file:

writeTextDir "share/my-file"
  ''
  Contents of File
  ''

:::

This is equivalent to:

writeTextFile {
  name = "my-file";
  text = ''
    Contents of File
  '';
  destination = "/share/my-file";
}

writeScript

Write an executable script file to the Nix store.

writeScript takes the following arguments:

name (String)

The name used in the Nix store path.

text (String)

The contents of the file.

The created file is marked as executable. The store path will include the name, and it will be a file.

::: {.example #ex-writeScript}

Usage of writeScript

Write the string Contents of File to /nix/store/<store path> and make the file executable.

writeScript "my-file"
  ''
  Contents of File
  ''

This is equivalent to:

writeTextFile {
  name = "my-file";
  text = ''
    Contents of File
  '';
  executable = true;
}

:::

writeScriptBin

Write a script within a bin subdirectory of a directory in the Nix store. This is for consistency with the convention of software packages placing executables under bin.

writeScriptBin takes the following arguments:

name (String)

The name used in the Nix store path and within the file created under the store path.

text (String)

The contents of the file.

The created file is marked as executable. The file's contents will be put into /nix/store/<store path>/bin/<name>. The store path will include the name, and it will be a directory.

::: {.example #ex-writeScriptBin}

Usage of writeScriptBin

writeScriptBin "my-script"
  ''
  echo "hi"
  ''

:::

This is equivalent to:

writeTextFile {
  name = "my-script";
  text = ''
    echo "hi"
  '';
  executable = true;
  destination = "/bin/my-script";
}

writeShellScript

Write a Bash script to the store.

writeShellScript takes the following arguments:

name (String)

The name used in the Nix store path.

text (String)

The contents of the file.

The created file is marked as executable. The store path will include the name, and it will be a file.

This function is almost exactly like , except that it prepends to the file a shebang line that points to the version of Bash used in Nixpkgs.

::: {.example #ex-writeShellScript}

Usage of writeShellScript

writeShellScript "my-script"
  ''
  echo "hi"
  ''

:::

This is equivalent to:

writeTextFile {
  name = "my-script";
  text = ''
    #! ${pkgs.runtimeShell}
    echo "hi"
  '';
  executable = true;
}

writeShellScriptBin

Write a Bash script to a "bin" subdirectory of a directory in the Nix store.

writeShellScriptBin takes the following arguments:

name (String)

The name used in the Nix store path and within the file generated under the store path.

text (String)

The contents of the file.

The file's contents will be put into /nix/store/<store path>/bin/<name>. The store path will include the the name, and it will be a directory.

This function is a combination of and .

::: {.example #ex-writeShellScriptBin}

Usage of writeShellScriptBin

writeShellScriptBin "my-script"
  ''
  echo "hi"
  ''

:::

This is equivalent to:

writeTextFile {
  name = "my-script";
  text = ''
    #! ${pkgs.runtimeShell}
    echo "hi"
  '';
  executable = true;
  destination = "/bin/my-script";
}

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

writeShellApplication is similar to writeShellScriptBin and writeScriptBin but supports runtime dependencies with runtimeInputs. Writes an executable shell script to /nix/store/<store path>/bin/<name> and checks its syntax with shellcheck and the bash's -n option. Some basic Bash options are set by default (errexit, nounset, and pipefail), but can be overridden with bashOptions.

Extra arguments may be passed to stdenv.mkDerivation by setting derivationArgs; note that variables set in this manner will be set when the shell script is built, not when it's run. Runtime environment variables can be set with the runtimeEnv argument.

For example, the following shell application can refer to curl directly, rather than needing to write ${curl}/bin/curl:

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

  runtimeInputs = [ curl w3m ];

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

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 (or alternatively pname and version) 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
...

writeClosure

Given a list of store paths (or string-like expressions coercible to store paths), write their collective closure to a text file.

The result is equivalent to the output of nix-store -q --requisites.

For example,

writeClosure [ (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.