Nixpkgs Reference Manual

Version 24.11pre-git


Table of Contents

Preface
Using Nixpkgs
Platform Support
Global configuration
Overlays
Overriding
Nixpkgs lib
Functions reference
Module System
Standard environment
The Standard Environment
Meta-attributes
Passthru-attributes
Multiple-output packages
Cross-compilation
Platform Notes
Build helpers
Fetchers
Trivial build helpers
Testers
Development Shell helpers
Special build helpers
Images
Hooks reference
Languages and frameworks
Packages
Development of Nixpkgs
Opening issues
Contributing to Nixpkgs
Quick Start to Adding a Package
Coding conventions
Submitting changes
Vulnerability Roundup
Reviewing contributions
Contributing to Nixpkgs documentation

List of Examples

1. Map over leaf attributes
2. Map over an leaf attributes defined by a condition
3. lib.strings.concatStrings usage example
4. lib.strings.concatMapStrings usage example
5. lib.strings.concatImapStrings usage example
6. lib.strings.intersperse usage example
7. lib.strings.concatStringsSep usage example
8. lib.strings.concatMapStringsSep usage example
9. lib.strings.concatImapStringsSep usage example
10. lib.strings.concatLines usage example
11. lib.strings.replicate usage example
12. lib.strings.makeSearchPath usage example
13. lib.strings.makeSearchPathOutput usage example
14. lib.strings.makeLibraryPath usage example
15. lib.strings.makeIncludePath usage example
16. lib.strings.makeBinPath usage example
17. lib.strings.normalizePath usage example
18. lib.strings.optionalString usage example
19. lib.strings.hasPrefix usage example
20. lib.strings.hasSuffix usage example
21. lib.strings.hasInfix usage example
22. lib.strings.stringToCharacters usage example
23. lib.strings.stringAsChars usage example
24. lib.strings.charToInt usage example
25. lib.strings.escape usage example
26. lib.strings.escapeC usage example
27. lib.strings.escapeURL usage example
28. lib.strings.escapeShellArg usage example
29. lib.strings.escapeShellArgs usage example
30. lib.strings.isValidPosixName usage example
31. lib.strings.toShellVar usage example
32. lib.strings.toShellVars usage example
33. lib.strings.escapeNixString usage example
34. lib.strings.escapeRegex usage example
35. lib.strings.escapeNixIdentifier usage example
36. lib.strings.escapeXML usage example
37. lib.strings.toLower usage example
38. lib.strings.toUpper usage example
39. lib.strings.addContextFrom usage example
40. lib.strings.splitString usage example
41. lib.strings.removePrefix usage example
42. lib.strings.removeSuffix usage example
43. lib.strings.versionOlder usage example
44. lib.strings.versionAtLeast usage example
45. lib.strings.getName usage example
46. lib.strings.getVersion usage example
47. lib.strings.nameFromURL usage example
48. lib.strings.cmakeOptionType usage example
49. lib.strings.cmakeBool usage example
50. lib.strings.cmakeFeature usage example
51. lib.strings.mesonOption usage example
52. lib.strings.mesonBool usage example
53. lib.strings.mesonEnable usage example
54. lib.strings.enableFeature usage example
55. lib.strings.enableFeatureAs usage example
56. lib.strings.withFeature usage example
57. lib.strings.withFeatureAs usage example
58. lib.strings.fixedWidthString usage example
59. lib.strings.fixedWidthNumber usage example
60. lib.strings.floatToString usage example
61. lib.strings.isStorePath usage example
62. lib.strings.toInt usage example
63. lib.strings.toIntBase10 usage example
64. lib.strings.readPathsFromFile usage example
65. lib.strings.fileContents usage example
66. lib.strings.sanitizeDerivationName usage example
67. lib.strings.levenshtein usage example
68. lib.strings.levenshteinAtMost usage example
69. lib.versions.splitVersion usage example
70. lib.versions.major usage example
71. lib.versions.minor usage example
72. lib.versions.patch usage example
73. lib.versions.majorMinor usage example
74. lib.versions.pad usage example
75. lib.options.isOption usage example
76. lib.options.mkOption usage example
77. lib.options.mkEnableOption usage example
78. lib.options.mkPackageOption usage example
79. lib.options.getValues usage example
80. lib.options.getFiles usage example
81. lib.options.showOption usage example
82. lib.path.append usage example
83. lib.path.hasPrefix usage example
84. lib.path.removePrefix usage example
85. lib.path.splitRoot usage example
86. lib.path.hasStorePathPrefix usage example
87. lib.path.subpath.isValid usage example
88. lib.path.subpath.join usage example
89. lib.path.subpath.components usage example
90. lib.path.subpath.normalise usage example
91. lib.sources.commitIdFromGitRepo usage example
92. lib.sources.cleanSource usage example
93. lib.sources.cleanSourceWith usage example
94. lib.sources.sourceByRegex usage example
95. lib.sources.sourceFilesBySuffices usage example
96. Create an interdependent package set on top of pkgs
97. Using callPackage from a scope
98. Enable debug symbols for use with GDB
99. Setting and accessing passthru attributes
100. Update source hash with the fake hash method
101. Using fetchurl to download a file
102. Using fetchurl to download a file with multiple possible URLs
103. Manipulating the content downloaded by fetchurl
104. Using fetchzip to output contents directly
105. Using fetchzip to decompress a .rar file
106. Invocation of runCommand
107. Usage 1 of makeDesktopItem
108. Usage 2 of makeDesktopItem
109. Usage 1 of writeTextFile
110. Usage 2 of writeTextFile
111. Usage 3 of writeTextFile
112. Usage of writeText
113. Usage of writeTextDir
114. Usage of writeScript
115. Usage of writeScriptBin
116. Usage of writeShellScript
117. Usage of writeShellScriptBin
118. Check that pkg-config modules are exposed using default values
119. Check that pkg-config modules are exposed using explicit module names
120. Check hyperlinks in the nix documentation
121. Check a program version using all the default values
122. Check the program version using a specified command and expected version string
123. Check that a build fails, and verify the changes made during build
124. Check that two paths have the same contents
125. Check that two packages produce the same derivation
126. Prevent nix from reusing the output of a fetcher
127. Run a NixOS test using runNixOSTest
128. Using fakeNss with dockerTools.buildImage
129. Using fakeNss with an override to add extra lines
130. Wrapping an AppImage from GitHub
131. Wrapping an AppImage with extra packages
132. Extracting an AppImage to install extra files
133. Extracting an AppImage to install extra files, using postExtract
134. Building a Docker image
135. Building a Docker image with runAsRoot
136. Building a Docker image with extraCommands
137. Building a Docker image with a creation date set to the current time
138. Building a layered Docker image
139. Streaming a layered Docker image
140. Exploring the layers in an image built with streamLayeredImage
141. Building a layered Docker image with packages directly in config
142. Pulling the nixos/nix Docker image from the default registry
143. Pulling the nixos/nix Docker image from a specific registry
144. Finding the digest and hash values to use for dockerTools.pullImage
145. Exporting a Docker image with dockerTools.exportImage
146. Importing an archive built with dockerTools.exportImage in Docker
147. Exploring output naming with dockerTools.exportImage
148. Using dockerTools.exportImage with a path as fromImage
149. Using dockerTools’s environment helpers with buildImage
150. Using dockerTools’s environment helpers with buildLayeredImage
151. Using dockerTools.shadowSetup with dockerTools.buildImage
152. Using dockerTools.shadowSetup with dockerTools.buildLayeredImage
153. Building a Docker image with buildNixShellImage with the build environment for the hello package
154. Building a Docker image with streamNixShellImage with the build environment for the hello package
155. Adding extra packages to a Docker image built with streamNixShellImage
156. Adding a shellHook to a Docker image built with streamNixShellImage
157. Creating an OCI runtime container that runs bash
158. Building a Portable Service image
159. Specifying symlinks when building a Portable Service image
160. Copying a package and its closure to another machine with mkBinaryCache
161. Navigate Java compiler variants in javaPackages with nix repl
162. List all Python packages in Nixpkgs
163. Ephemeral shell
164. Declarative shell
165. Using pkgs.zlib.override {}
166. Using pkgs.buildEmscriptenPackage {}
167. Overriding the kernel derivation
168. Using pkgs.linuxPackages_custom with a specific source, version, and config file
169. Edit-compile-run loop when developing mellanox drivers

Preface

Table of Contents

Overview of Nixpkgs

The Nix Packages collection (Nixpkgs) is a set of thousands of packages for the Nix package manager, released under a permissive MIT license. Packages are available for several platforms, and can be used with the Nix package manager on most GNU/Linux distributions as well as NixOS.

This document is the user reference manual for Nixpkgs. It describes entire public interface of Nixpkgs in a concise and orderly manner, and all relevant behaviors, with examples and cross-references.

To discover other kinds of documentation:

Overview of Nixpkgs

Nix expressions describe how to build packages from source and are collected in the nixpkgs repository. Also included in the collection are Nix expressions for NixOS modules. With these expressions the Nix package manager can build binary packages.

Packages, including the Nix packages collection, are distributed through channels. The collection is distributed for users of Nix on non-NixOS distributions through the channel nixpkgs-unstable. Users of NixOS generally use one of the nixos-* channels, e.g. nixos-22.11, which includes all packages and modules for the stable NixOS 22.11. Stable NixOS releases are generally only given security updates. More up to date packages and modules are available via the nixos-unstable channel.

Both nixos-unstable and nixpkgs-unstable follow the master branch of the nixpkgs repository, although both do lag the master branch by generally a couple of days. Updates to a channel are distributed as soon as all tests for that channel pass, e.g. this table shows the status of tests for the nixpkgs-unstable channel.

The tests are conducted by a cluster called Hydra, which also builds binary packages from the Nix expressions in Nixpkgs for x86_64-linux, i686-linux and x86_64-darwin. The binaries are made available via a binary cache.

The current Nix expressions of the channels are available in the nixpkgs repository in branches that correspond to the channel names (e.g. nixos-22.11-small).

Using Nixpkgs

Platform Support

Packages receive varying degrees of support, both in terms of maintainer attention and available computation resources for continuous integration (CI).

Below is the list of the best supported platforms:

  • x86_64-linux: Highest level of support.

  • aarch64-linux: Well supported, with most packages building successfully in CI.

  • aarch64-darwin: Receives better support than x86_64-darwin.

  • x86_64-darwin: Receives some support.

There are many other platforms with varying levels of support. The provisional platform list in Appendix A of RFC046, while not up to date, can be used as guidance.

A more formal definition of the platform support tiers is provided in RFC046, but has not been fully implemented yet.

Global configuration

Nix comes with certain defaults about which packages can and cannot be installed, based on a package’s metadata. By default, Nix will prevent installation if any of the following criteria are true:

  • The package is thought to be broken, and has had its meta.broken set to true.

  • The package isn’t intended to run on the given system, as none of its meta.platforms match the given system.

  • The package’s meta.license is set to a license which is considered to be unfree.

  • The package has known security vulnerabilities but has not or can not be updated for some reason, and a list of issues has been entered in to the package’s meta.knownVulnerabilities.

Each of these criteria can be altered in the Nixpkgs configuration.

Note

All this is checked during evaluation already, and the check includes any package that is evaluated. In particular, all build-time dependencies are checked.

A user’s Nixpkgs configuration is stored in a user-specific configuration file located at ~/.config/nixpkgs/config.nix. For example:

{
  allowUnfree = true;
}

Caution

Unfree software is not tested or built in Nixpkgs continuous integration, and therefore not cached. Most unfree licenses prohibit either executing or distributing the software.

Installing broken packages

There are two ways to try compiling a package which has been marked as broken.

  • For allowing the build of a broken package once, you can use an environment variable for a single invocation of the nix tools:

    $ export NIXPKGS_ALLOW_BROKEN=1
    
  • For permanently allowing broken packages to be built, you may add allowBroken = true; to your user’s configuration file, like this:

    {
      allowBroken = true;
    }
    

Installing packages on unsupported systems

There are also two ways to try compiling a package which has been marked as unsupported for the given system.

  • For allowing the build of an unsupported package once, you can use an environment variable for a single invocation of the nix tools:

    $ export NIXPKGS_ALLOW_UNSUPPORTED_SYSTEM=1
    
  • For permanently allowing unsupported packages to be built, you may add allowUnsupportedSystem = true; to your user’s configuration file, like this:

    {
      allowUnsupportedSystem = true;
    }
    

The difference between a package being unsupported on some system and being broken is admittedly a bit fuzzy. If a program ought to work on a certain platform, but doesn’t, the platform should be included in meta.platforms, but marked as broken with e.g. meta.broken = !hostPlatform.isWindows. Of course, this begs the question of what “ought” means exactly. That is left to the package maintainer.

Installing unfree packages

All users of Nixpkgs are free software users, and many users (and developers) of Nixpkgs want to limit and tightly control their exposure to unfree software. At the same time, many users need (or want) to run some specific pieces of proprietary software. Nixpkgs includes some expressions for unfree software packages. By default unfree software cannot be installed and doesn’t show up in searches.

There are several ways to tweak how Nix handles a package which has been marked as unfree.

  • To temporarily allow all unfree packages, you can use an environment variable for a single invocation of the nix tools:

    $ export NIXPKGS_ALLOW_UNFREE=1
    
  • It is possible to permanently allow individual unfree packages, while still blocking unfree packages by default using the allowUnfreePredicate configuration option in the user configuration file.

    This option is a function which accepts a package as a parameter, and returns a boolean. The following example configuration accepts a package and always returns false:

    {
      allowUnfreePredicate = (pkg: false);
    }
    

    For a more useful example, try the following. This configuration only allows unfree packages named roon-server and visual studio code:

    {
      allowUnfreePredicate = pkg: builtins.elem (lib.getName pkg) [
        "roon-server"
        "vscode"
      ];
    }
    
  • It is also possible to allow and block licenses that are specifically acceptable or not acceptable, using allowlistedLicenses and blocklistedLicenses, respectively.

    The following example configuration allowlists the licenses amd and wtfpl:

    {
      allowlistedLicenses = with lib.licenses; [ amd wtfpl ];
    }
    

    The following example configuration blocklists the gpl3Only and agpl3Only licenses:

    {
      blocklistedLicenses = with lib.licenses; [ agpl3Only gpl3Only ];
    }
    

    Note that allowlistedLicenses only applies to unfree licenses unless allowUnfree is enabled. It is not a generic allowlist for all types of licenses. blocklistedLicenses applies to all licenses.

A complete list of licenses can be found in the file lib/licenses.nix of the nixpkgs tree.

Installing insecure packages

There are several ways to tweak how Nix handles a package which has been marked as insecure.

  • To temporarily allow all insecure packages, you can use an environment variable for a single invocation of the nix tools:

    $ export NIXPKGS_ALLOW_INSECURE=1
    
  • It is possible to permanently allow individual insecure packages, while still blocking other insecure packages by default using the permittedInsecurePackages configuration option in the user configuration file.

    The following example configuration permits the installation of the hypothetically insecure package hello, version 1.2.3:

    {
      permittedInsecurePackages = [
        "hello-1.2.3"
      ];
    }
    
  • It is also possible to create a custom policy around which insecure packages to allow and deny, by overriding the allowInsecurePredicate configuration option.

    The allowInsecurePredicate option is a function which accepts a package and returns a boolean, much like allowUnfreePredicate.

    The following configuration example only allows insecure packages with very short names:

    {
      allowInsecurePredicate = pkg: builtins.stringLength (lib.getName pkg) <= 5;
    }
    

    Note that permittedInsecurePackages is only checked if allowInsecurePredicate is not specified.

Modify packages via packageOverrides

You can define a function called packageOverrides in your local ~/.config/nixpkgs/config.nix to override Nix packages. It must be a function that takes pkgs as an argument and returns a modified set of packages.

{
  packageOverrides = pkgs: rec {
    foo = pkgs.foo.override { /* ... */ };
  };
}

config Options Reference

The following attributes can be passed in config.

enableParallelBuildingByDefault

Whether to set enableParallelBuilding to true by default while building nixpkgs packages. Changing the default may cause a mass rebuild.

Type: boolean

Default: false

Declared by:

pkgs/top-level/config.nix
allowAliases

Whether to expose old attribute names for compatibility.

The recommended setting is to enable this, as it improves backward compatibility, easing updates.

The only reason to disable aliases is for continuous integration purposes. For instance, Nixpkgs should not depend on aliases in its internal code. Projects that aren’t Nixpkgs should be cautious of instantly removing all usages of aliases, as migrating too soon can break compatibility with the stable Nixpkgs releases.

Type: boolean

Default: true

Declared by:

pkgs/top-level/config.nix
allowBroken

Whether to allow broken packages.

See Installing broken packages in the NixOS manual.

Type: boolean

Default: false || builtins.getEnv "NIXPKGS_ALLOW_BROKEN" == "1"

Declared by:

pkgs/top-level/config.nix
allowUnfree

Whether to allow unfree packages.

See Installing unfree packages in the NixOS manual.

Type: boolean

Default: false || builtins.getEnv "NIXPKGS_ALLOW_UNFREE" == "1"

Declared by:

pkgs/top-level/config.nix
allowUnsupportedSystem

Whether to allow unsupported packages.

See Installing packages on unsupported systems in the NixOS manual.

Type: boolean

Default: false || builtins.getEnv "NIXPKGS_ALLOW_UNSUPPORTED_SYSTEM" == "1"

Declared by:

pkgs/top-level/config.nix
checkMeta

Whether to check that the meta attribute of derivations are correct during evaluation time.

Type: boolean

Default: false

Declared by:

pkgs/top-level/config.nix
configurePlatformsByDefault

Whether to set configurePlatforms to ["build" "host"] by default while building nixpkgs packages. Changing the default may cause a mass rebuild.

Type: boolean

Default: false

Declared by:

pkgs/top-level/config.nix
contentAddressedByDefault

Whether to set __contentAddressed to true by default while building nixpkgs packages. Changing the default may cause a mass rebuild.

Type: boolean

Default: false

Declared by:

pkgs/top-level/config.nix
cudaSupport

Whether to build packages with CUDA support by default while building nixpkgs packages. Changing the default may cause a mass rebuild.

Type: boolean

Default: false

Declared by:

pkgs/top-level/config.nix
doCheckByDefault

Whether to run checkPhase by default while building nixpkgs packages. Changing the default may cause a mass rebuild.

Type: boolean

Default: false

Declared by:

pkgs/top-level/config.nix
rocmSupport

Whether to build packages with ROCm support by default while building nixpkgs packages. Changing the default may cause a mass rebuild.

Type: boolean

Default: false

Declared by:

pkgs/top-level/config.nix
showDerivationWarnings

Which warnings to display for potentially dangerous or deprecated values passed into stdenv.mkDerivation.

A list of warnings can be found in /pkgs/stdenv/generic/check-meta.nix.

This is not a stable interface; warnings may be added, changed or removed without prior notice.

Type: list of value “maintainerless” (singular enum)

Default: [ ]

Declared by:

pkgs/top-level/config.nix
strictDepsByDefault

Whether to set strictDeps to true by default while building nixpkgs packages. Changing the default may cause a mass rebuild.

Type: boolean

Default: false

Declared by:

pkgs/top-level/config.nix
structuredAttrsByDefault

Whether to set __structuredAttrs to true by default while building nixpkgs packages. Changing the default may cause a mass rebuild.

Type: boolean

Default: false

Declared by:

pkgs/top-level/config.nix
warnUndeclaredOptions

Whether to warn when config contains an unrecognized attribute.

Type: boolean

Default: false

Declared by:

pkgs/top-level/config.nix

Declarative Package Management

Build an environment

Using packageOverrides, it is possible to manage packages declaratively. This means that we can list all of our desired packages within a declarative Nix expression. For example, to have aspell, bc, ffmpeg, coreutils, gdb, nixUnstable, emscripten, jq, nox, and silver-searcher, we could use the following in ~/.config/nixpkgs/config.nix:

{
  packageOverrides = pkgs: with pkgs; {
    myPackages = pkgs.buildEnv {
      name = "my-packages";
      paths = [
        aspell
        bc
        coreutils
        gdb
        ffmpeg
        nixUnstable
        emscripten
        jq
        nox
        silver-searcher
      ];
    };
  };
}

To install it into our environment, you can just run nix-env -iA nixpkgs.myPackages. If you want to load the packages to be built from a working copy of nixpkgs you just run nix-env -f. -iA myPackages. To explore what’s been installed, just look through ~/.nix-profile/. You can see that a lot of stuff has been installed. Some of this stuff is useful some of it isn’t. Let’s tell Nixpkgs to only link the stuff that we want:

{
  packageOverrides = pkgs: with pkgs; {
    myPackages = pkgs.buildEnv {
      name = "my-packages";
      paths = [
        aspell
        bc
        coreutils
        gdb
        ffmpeg
        nixUnstable
        emscripten
        jq
        nox
        silver-searcher
      ];
      pathsToLink = [ "/share" "/bin" ];
    };
  };
}

pathsToLink tells Nixpkgs to only link the paths listed which gets rid of the extra stuff in the profile. /bin and /share are good defaults for a user environment, getting rid of the clutter. If you are running on Nix on MacOS, you may want to add another path as well, /Applications, that makes GUI apps available.

Getting documentation

After building that new environment, look through ~/.nix-profile to make sure everything is there that we wanted. Discerning readers will note that some files are missing. Look inside ~/.nix-profile/share/man/man1/ to verify this. There are no man pages for any of the Nix tools! This is because some packages like Nix have multiple outputs for things like documentation (see section 4). Let’s make Nix install those as well.

{
  packageOverrides = pkgs: with pkgs; {
    myPackages = pkgs.buildEnv {
      name = "my-packages";
      paths = [
        aspell
        bc
        coreutils
        ffmpeg
        nixUnstable
        emscripten
        jq
        nox
        silver-searcher
      ];
      pathsToLink = [ "/share/man" "/share/doc" "/bin" ];
      extraOutputsToInstall = [ "man" "doc" ];
    };
  };
}

This provides us with some useful documentation for using our packages. However, if we actually want those manpages to be detected by man, we need to set up our environment. This can also be managed within Nix expressions.

{
  packageOverrides = pkgs: with pkgs; rec {
    myProfile = writeText "my-profile" ''
      export PATH=$HOME/.nix-profile/bin:/nix/var/nix/profiles/default/bin:/sbin:/bin:/usr/sbin:/usr/bin
      export MANPATH=$HOME/.nix-profile/share/man:/nix/var/nix/profiles/default/share/man:/usr/share/man
    '';
    myPackages = pkgs.buildEnv {
      name = "my-packages";
      paths = [
        (runCommand "profile" {} ''
          mkdir -p $out/etc/profile.d
          cp ${myProfile} $out/etc/profile.d/my-profile.sh
        '')
        aspell
        bc
        coreutils
        ffmpeg
        man
        nixUnstable
        emscripten
        jq
        nox
        silver-searcher
      ];
      pathsToLink = [ "/share/man" "/share/doc" "/bin" "/etc" ];
      extraOutputsToInstall = [ "man" "doc" ];
    };
  };
}

For this to work fully, you must also have this script sourced when you are logged in. Try adding something like this to your ~/.profile file:

#!/bin/sh
if [ -d "${HOME}/.nix-profile/etc/profile.d" ]; then
  for i in "${HOME}/.nix-profile/etc/profile.d/"*.sh; do
    if [ -r "$i" ]; then
      . "$i"
    fi
  done
fi

Now just run . "${HOME}/.profile" and you can start loading man pages from your environment.

GNU info setup

Configuring GNU info is a little bit trickier than man pages. To work correctly, info needs a database to be generated. This can be done with some small modifications to our environment scripts.

{
  packageOverrides = pkgs: with pkgs; rec {
    myProfile = writeText "my-profile" ''
      export PATH=$HOME/.nix-profile/bin:/nix/var/nix/profiles/default/bin:/sbin:/bin:/usr/sbin:/usr/bin
      export MANPATH=$HOME/.nix-profile/share/man:/nix/var/nix/profiles/default/share/man:/usr/share/man
      export INFOPATH=$HOME/.nix-profile/share/info:/nix/var/nix/profiles/default/share/info:/usr/share/info
    '';
    myPackages = pkgs.buildEnv {
      name = "my-packages";
      paths = [
        (runCommand "profile" {} ''
          mkdir -p $out/etc/profile.d
          cp ${myProfile} $out/etc/profile.d/my-profile.sh
        '')
        aspell
        bc
        coreutils
        ffmpeg
        man
        nixUnstable
        emscripten
        jq
        nox
        silver-searcher
        texinfoInteractive
      ];
      pathsToLink = [ "/share/man" "/share/doc" "/share/info" "/bin" "/etc" ];
      extraOutputsToInstall = [ "man" "doc" "info" ];
      postBuild = ''
        if [ -x $out/bin/install-info -a -w $out/share/info ]; then
          shopt -s nullglob
          for i in $out/share/info/*.info $out/share/info/*.info.gz; do
              $out/bin/install-info $i $out/share/info/dir
          done
        fi
      '';
    };
  };
}

postBuild tells Nixpkgs to run a command after building the environment. In this case, install-info adds the installed info pages to dir which is GNU info’s default root node. Note that texinfoInteractive is added to the environment to give the install-info command.

Overlays

This chapter describes how to extend and change Nixpkgs using overlays. Overlays are used to add layers in the fixed-point used by Nixpkgs to compose the set of all packages.

Nixpkgs can be configured with a list of overlays, which are applied in order. This means that the order of the overlays can be significant if multiple layers override the same package.

Installing overlays

The list of overlays can be set either explicitly in a Nix expression, or through <nixpkgs-overlays> or user configuration files.

Set overlays in NixOS or Nix expressions

On a NixOS system the value of the nixpkgs.overlays option, if present, is passed to the system Nixpkgs directly as an argument. Note that this does not affect the overlays for non-NixOS operations (e.g. nix-env), which are looked up independently.

The list of overlays can be passed explicitly when importing nixpkgs, for example import <nixpkgs> { overlays = [ overlay1 overlay2 ]; }.

NOTE: DO NOT USE THIS in nixpkgs. Further overlays can be added by calling the pkgs.extend or pkgs.appendOverlays, although it is often preferable to avoid these functions, because they recompute the Nixpkgs fixpoint, which is somewhat expensive to do.

Install overlays via configuration lookup

The list of overlays is determined as follows.

  1. First, if an overlays argument to the Nixpkgs function itself is given, then that is used and no path lookup will be performed.

  2. Otherwise, if the Nix path entry <nixpkgs-overlays> exists, we look for overlays at that path, as described below.

    See the section on NIX_PATH in the Nix manual for more details on how to set a value for <nixpkgs-overlays>.

  3. If one of ~/.config/nixpkgs/overlays.nix and ~/.config/nixpkgs/overlays/ exists, then we look for overlays at that path, as described below. It is an error if both exist.

If we are looking for overlays at a path, then there are two cases:

  • If the path is a file, then the file is imported as a Nix expression and used as the list of overlays.

  • If the path is a directory, then we take the content of the directory, order it lexicographically, and attempt to interpret each as an overlay by:

    • Importing the file, if it is a .nix file.

    • Importing a top-level default.nix file, if it is a directory.

Because overlays that are set in NixOS configuration do not affect non-NixOS operations such as nix-env, the overlays.nix option provides a convenient way to use the same overlays for a NixOS system configuration and user configuration: the same file can be used as overlays.nix and imported as the value of nixpkgs.overlays.

Defining overlays

Overlays are Nix functions which accept two arguments, conventionally called self and super, and return a set of packages. For example, the following is a valid overlay.

self: super:

{
  boost = super.boost.override {
    python = self.python3;
  };
  rr = super.callPackage ./pkgs/rr {
    stdenv = self.stdenv_32bit;
  };
}

The first argument (self) corresponds to the final package set. You should use this set for the dependencies of all packages specified in your overlay. For example, all the dependencies of rr in the example above come from self, as well as the overridden dependencies used in the boost override.

The second argument (super) corresponds to the result of the evaluation of the previous stages of Nixpkgs. It does not contain any of the packages added by the current overlay, nor any of the following overlays. This set should be used either to refer to packages you wish to override, or to access functions defined in Nixpkgs. For example, the original recipe of boost in the above example, comes from super, as well as the callPackage function.

The value returned by this function should be a set similar to pkgs/top-level/all-packages.nix, containing overridden and/or new packages.

Overlays are similar to other methods for customizing Nixpkgs, in particular the packageOverrides attribute described in the section called “Modify packages via packageOverrides. Indeed, packageOverrides acts as an overlay with only the super argument. It is therefore appropriate for basic use, but overlays are more powerful and easier to distribute.

Using overlays to configure alternatives

Certain software packages have different implementations of the same interface. Other distributions have functionality to switch between these. For example, Debian provides DebianAlternatives. Nixpkgs has what we call alternatives, which are configured through overlays.

BLAS/LAPACK

In Nixpkgs, we have multiple implementations of the BLAS/LAPACK numerical linear algebra interfaces. They are:

  • OpenBLAS

    The Nixpkgs attribute is openblas for ILP64 (integer width = 64 bits) and openblasCompat for LP64 (integer width = 32 bits). openblasCompat is the default.

  • LAPACK reference (also provides BLAS and CBLAS)

    The Nixpkgs attribute is lapack-reference.

  • Intel MKL (only works on the x86_64 architecture, unfree)

    The Nixpkgs attribute is mkl.

  • BLIS

    BLIS, available through the attribute blis, is a framework for linear algebra kernels. In addition, it implements the BLAS interface.

  • AMD BLIS/LIBFLAME (optimized for modern AMD x86_64 CPUs)

    The AMD fork of the BLIS library, with attribute amd-blis, extends BLIS with optimizations for modern AMD CPUs. The changes are usually submitted to the upstream BLIS project after some time. However, AMD BLIS typically provides some performance improvements on AMD Zen CPUs. The complementary AMD LIBFLAME library, with attribute amd-libflame, provides a LAPACK implementation.

Introduced in PR #83888, we are able to override the blas and lapack packages to use different implementations, through the blasProvider and lapackProvider argument. This can be used to select a different provider. BLAS providers will have symlinks in $out/lib/libblas.so.3 and $out/lib/libcblas.so.3 to their respective BLAS libraries. Likewise, LAPACK providers will have symlinks in $out/lib/liblapack.so.3 and $out/lib/liblapacke.so.3 to their respective LAPACK libraries. For example, Intel MKL is both a BLAS and LAPACK provider. An overlay can be created to use Intel MKL that looks like:

self: super:

{
  blas = super.blas.override {
    blasProvider = self.mkl;
  };

  lapack = super.lapack.override {
    lapackProvider = self.mkl;
  };
}

This overlay uses Intel’s MKL library for both BLAS and LAPACK interfaces. Note that the same can be accomplished at runtime using LD_LIBRARY_PATH of libblas.so.3 and liblapack.so.3. For instance:

$ LD_LIBRARY_PATH=$(nix-build -A mkl)/lib${LD_LIBRARY_PATH:+:}$LD_LIBRARY_PATH nix-shell -p octave --run octave

Intel MKL requires an openmp implementation when running with multiple processors. By default, mkl will use Intel’s iomp implementation if no other is specified, but this is a runtime-only dependency and binary compatible with the LLVM implementation. To use that one instead, Intel recommends users set it with LD_PRELOAD. Note that mkl is only available on x86_64-linux and x86_64-darwin. Moreover, Hydra is not building and distributing pre-compiled binaries using it.

To override blas and lapack with its reference implementations (i.e. for development purposes), one can use the following overlay:

self: super:

{
  blas = super.blas.override {
    blasProvider = self.lapack-reference;
  };

  lapack = super.lapack.override {
    lapackProvider = self.lapack-reference;
  };
}

For BLAS/LAPACK switching to work correctly, all packages must depend on blas or lapack. This ensures that only one BLAS/LAPACK library is used at one time. There are two versions of BLAS/LAPACK currently in the wild, LP64 (integer size = 32 bits) and ILP64 (integer size = 64 bits). The attributes blas and lapack are LP64 by default. Their ILP64 version are provided through the attributes blas-ilp64 and lapack-ilp64. Some software needs special flags or patches to work with ILP64. You can check if ILP64 is used in Nixpkgs with blas.isILP64 and lapack.isILP64. Some software does NOT work with ILP64, and derivations need to specify an assertion to prevent this. You can prevent ILP64 from being used with the following:

{ stdenv, blas, lapack, ... }:

assert (!blas.isILP64) && (!lapack.isILP64);

stdenv.mkDerivation {
  # ...
}

Switching the MPI implementation

All programs that are built with MPI support use the generic attribute mpi as an input. At the moment Nixpkgs natively provides two different MPI implementations:

  • Open MPI (default), attribute name openmpi

  • MPICH, attribute name mpich

  • MVAPICH, attribute name mvapich

To provide MPI enabled applications that use MPICH, instead of the default Open MPI, use the following overlay:

self: super:

{
  mpi = self.mpich;
}

Overriding

Sometimes one wants to override parts of nixpkgs, e.g. derivation attributes, the results of derivations.

These functions are used to make changes to packages, returning only single packages. Overlays, on the other hand, can be used to combine the overridden packages across the entire package set of Nixpkgs.

<pkg>.override

The function override is usually available for all the derivations in the nixpkgs expression (pkgs).

It is used to override the arguments passed to a function.

Example usages:

pkgs.foo.override { arg1 = val1; arg2 = val2; /* ... */ }

It’s also possible to access the previous arguments.

pkgs.foo.override (previous: { arg1 = previous.arg1; /* ... */ })
import pkgs.path { overlays = [ (self: super: {
  foo = super.foo.override { barSupport = true ; };
  })];}
{
  mypkg = pkgs.callPackage ./mypkg.nix {
    mydep = pkgs.mydep.override { /* ... */ };
  };
}

In the first example, pkgs.foo is the result of a function call with some default arguments, usually a derivation. Using pkgs.foo.override will call the same function with the given new arguments.

<pkg>.overrideAttrs

The function overrideAttrs allows overriding the attribute set passed to a stdenv.mkDerivation call, producing a new derivation based on the original one. This function is available on all derivations produced by the stdenv.mkDerivation function, which is most packages in the nixpkgs expression pkgs.

Example usages:

{
  helloBar = pkgs.hello.overrideAttrs (finalAttrs: previousAttrs: {
    pname = previousAttrs.pname + "-bar";
  });
}

In the above example, “-bar” is appended to the pname attribute, while all other attributes will be retained from the original hello package.

The argument previousAttrs is conventionally used to refer to the attr set originally passed to stdenv.mkDerivation.

The argument finalAttrs refers to the final attributes passed to mkDerivation, plus the finalPackage attribute which is equal to the result of mkDerivation or subsequent overrideAttrs calls.

If only a one-argument function is written, the argument has the meaning of previousAttrs.

Function arguments can be omitted entirely if there is no need to access previousAttrs or finalAttrs.

{
  helloWithDebug = pkgs.hello.overrideAttrs {
    separateDebugInfo = true;
  };
}

In the above example, the separateDebugInfo attribute is overridden to be true, thus building debug info for helloWithDebug.

Note

Note that separateDebugInfo is processed only by the stdenv.mkDerivation function, not the generated, raw Nix derivation. Thus, using overrideDerivation will not work in this case, as it overrides only the attributes of the final derivation. It is for this reason that overrideAttrs should be preferred in (almost) all cases to overrideDerivation, i.e. to allow using stdenv.mkDerivation to process input arguments, as well as the fact that it is easier to use (you can use the same attribute names you see in your Nix code, instead of the ones generated (e.g. buildInputs vs nativeBuildInputs), and it involves less typing).

<pkg>.overrideDerivation

Warning

You should prefer overrideAttrs in almost all cases, see its documentation for the reasons why. overrideDerivation is not deprecated and will continue to work, but is less nice to use and does not have as many abilities as overrideAttrs.

Warning

Do not use this function in Nixpkgs as it evaluates a derivation before modifying it, which breaks package abstraction. In addition, this evaluation-per-function application incurs a performance penalty, which can become a problem if many overrides are used. It is only intended for ad-hoc customisation, such as in ~/.config/nixpkgs/config.nix.

The function overrideDerivation creates a new derivation based on an existing one by overriding the original’s attributes with the attribute set produced by the specified function. This function is available on all derivations defined using the makeOverridable function. Most standard derivation-producing functions, such as stdenv.mkDerivation, are defined using this function, which means most packages in the nixpkgs expression, pkgs, have this function.

Example usage:

{
  mySed = pkgs.gnused.overrideDerivation (oldAttrs: {
    name = "sed-4.2.2-pre";
    src = fetchurl {
      url = "ftp://alpha.gnu.org/gnu/sed/sed-4.2.2-pre.tar.bz2";
      hash = "sha256-MxBJRcM2rYzQYwJ5XKxhXTQByvSg5jZc5cSHEZoB2IY=";
    };
    patches = [];
  });
}

In the above example, the name, src, and patches of the derivation will be overridden, while all other attributes will be retained from the original derivation.

The argument oldAttrs is used to refer to the attribute set of the original derivation.

Note

A package’s attributes are evaluated before being modified by the overrideDerivation function. For example, the name attribute reference in url = "mirror://gnu/hello/${name}.tar.gz"; is filled-in before the overrideDerivation function modifies the attribute set. This means that overriding the name attribute, in this example, will not change the value of the url attribute. Instead, we need to override both the name and url attributes.

lib.makeOverridable

The function lib.makeOverridable is used to make the result of a function easily customizable. This utility only makes sense for functions that accept an argument set and return an attribute set.

Example usage:

{
  f = { a, b }: { result = a+b; };
  c = lib.makeOverridable f { a = 1; b = 2; };
}

The variable c is the value of the f function applied with some default arguments. Hence the value of c.result is 3, in this example.

The variable c however also has some additional functions, like c.override which can be used to override the default arguments. In this example the value of (c.override { a = 4; }).result is 6.

Nixpkgs lib

Functions reference

The nixpkgs repository has several utility functions to manipulate Nix expressions.

Nixpkgs Library Functions

Nixpkgs provides a standard library at pkgs.lib, or through import <nixpkgs/lib>.

lib.asserts: assertion functions

lib.asserts.assertMsg

Throw if pred is false, else return pred. Intended to be used to augment asserts with helpful error messages.

Inputs
pred

Predicate that needs to succeed, otherwise msg is thrown

msg

Message to throw in case pred fails

Type
assertMsg :: Bool -> String -> Bool
Examples
Example 1. lib.asserts.assertMsg usage example
assertMsg false "nope"
stderr> error: nope
assert assertMsg ("foo" == "bar") "foo is not bar, silly"; ""
stderr> error: foo is not bar, silly

Located at lib/asserts.nix:39 in <nixpkgs>.

lib.asserts.assertOneOf

Specialized assertMsg for checking if val is one of the elements of the list xs. Useful for checking enums.

Inputs
name

The name of the variable the user entered val into, for inclusion in the error message

val

The value of what the user provided, to be compared against the values in xs

xs

The list of valid values

Type
assertOneOf :: String -> ComparableVal -> List ComparableVal -> Bool
Examples
Example 2. lib.asserts.assertOneOf usage example
let sslLibrary = "libressl";
in assertOneOf "sslLibrary" sslLibrary [ "openssl" "bearssl" ]
stderr> error: sslLibrary must be one of [
stderr>   "openssl"
stderr>   "bearssl"
stderr> ], but is: "libressl"

Located at lib/asserts.nix:83 in <nixpkgs>.

lib.asserts.assertEachOneOf

Specialized assertMsg for checking if every one of vals is one of the elements of the list xs. Useful for checking lists of supported attributes.

Inputs
name

The name of the variable the user entered val into, for inclusion in the error message

vals

The list of values of what the user provided, to be compared against the values in xs

xs

The list of valid values

Type
assertEachOneOf :: String -> List ComparableVal -> List ComparableVal -> Bool
Examples
Example 3. lib.asserts.assertEachOneOf usage example
let sslLibraries = [ "libressl" "bearssl" ];
in assertEachOneOf "sslLibraries" sslLibraries [ "openssl" "bearssl" ]
stderr> error: each element in sslLibraries must be one of [
stderr>   "openssl"
stderr>   "bearssl"
stderr> ], but is: [
stderr>   "libressl"
stderr>   "bearssl"
stderr> ]

Located at lib/asserts.nix:135 in <nixpkgs>.

lib.attrsets: attribute set functions

Operations on attribute sets.

lib.attrsets.attrByPath

Return an attribute from nested attribute sets.

Nix has an attribute selection operator . or which is sufficient for such queries, as long as the number of attributes is static. For example:

(x.a.b or 6) == attrByPath ["a" "b"] 6 x
# and
(x.${f p}."example.com" or 6) == attrByPath [ (f p) "example.com" ] 6 x
Inputs
attrPath

A list of strings representing the attribute path to return from set

default

Default value if attrPath does not resolve to an existing value

set

The nested attribute set to select values from

Type
attrByPath :: [String] -> Any -> AttrSet -> Any
Examples
Example 4. lib.attrsets.attrByPath usage example
x = { a = { b = 3; }; }
# ["a" "b"] is equivalent to x.a.b
# 6 is a default value to return if the path does not exist in attrset
attrByPath ["a" "b"] 6 x
=> 3
attrByPath ["z" "z"] 6 x
=> 6

Located at lib/attrsets.nix:65 in <nixpkgs>.

lib.attrsets.hasAttrByPath

Return if an attribute from nested attribute set exists.

Nix has a has attribute operator ?, which is sufficient for such queries, as long as the number of attributes is static. For example:

(x?a.b) == hasAttrByPath ["a" "b"] x
# and
(x?${f p}."example.com") == hasAttrByPath [ (f p) "example.com" ] x

Laws:

  1. hasAttrByPath [] x == true
    
Inputs
attrPath

A list of strings representing the attribute path to check from set

e

The nested attribute set to check

Type
hasAttrByPath :: [String] -> AttrSet -> Bool
Examples
Example 5. lib.attrsets.hasAttrByPath usage example
x = { a = { b = 3; }; }
hasAttrByPath ["a" "b"] x
=> true
hasAttrByPath ["z" "z"] x
=> false
hasAttrByPath [] (throw "no need")
=> true

Located at lib/attrsets.nix:133 in <nixpkgs>.

lib.attrsets.longestValidPathPrefix

Return the longest prefix of an attribute path that refers to an existing attribute in a nesting of attribute sets.

Can be used after mapAttrsRecursiveCond to apply a condition, although this will evaluate the predicate function on sibling attributes as well.

Note that the empty attribute path is valid for all values, so this function only throws an exception if any of its inputs does.

Laws:

  1. attrsets.longestValidPathPrefix [] x == []
    
  2. hasAttrByPath (attrsets.longestValidPathPrefix p x) x == true
    
Inputs
attrPath

A list of strings representing the longest possible path that may be returned.

v

The nested attribute set to check.

Type
attrsets.longestValidPathPrefix :: [String] -> Value -> [String]
Examples
Example 6. lib.attrsets.longestValidPathPrefix usage example
x = { a = { b = 3; }; }
attrsets.longestValidPathPrefix ["a" "b" "c"] x
=> ["a" "b"]
attrsets.longestValidPathPrefix ["a"] x
=> ["a"]
attrsets.longestValidPathPrefix ["z" "z"] x
=> []
attrsets.longestValidPathPrefix ["z" "z"] (throw "no need")
=> []

Located at lib/attrsets.nix:202 in <nixpkgs>.

lib.attrsets.setAttrByPath

Create a new attribute set with value set at the nested attribute location specified in attrPath.

Inputs
attrPath

A list of strings representing the attribute path to set

value

The value to set at the location described by attrPath

Type
setAttrByPath :: [String] -> Any -> AttrSet
Examples
Example 7. lib.attrsets.setAttrByPath usage example
setAttrByPath ["a" "b"] 3
=> { a = { b = 3; }; }

Located at lib/attrsets.nix:265 in <nixpkgs>.

lib.attrsets.getAttrFromPath

Like attrByPath, but without a default value. If it doesn’t find the path it will throw an error.

Nix has an attribute selection operator which is sufficient for such queries, as long as the number of attributes is static. For example:

x.a.b == getAttrByPath ["a" "b"] x
# and
x.${f p}."example.com" == getAttrByPath [ (f p) "example.com" ] x
Inputs
attrPath

A list of strings representing the attribute path to get from set

set

The nested attribute set to find the value in.

Type
getAttrFromPath :: [String] -> AttrSet -> Any
Examples
Example 8. lib.attrsets.getAttrFromPath usage example
x = { a = { b = 3; }; }
getAttrFromPath ["a" "b"] x
=> 3
getAttrFromPath ["z" "z"] x
=> error: cannot find attribute `z.z'

Located at lib/attrsets.nix:319 in <nixpkgs>.

lib.attrsets.concatMapAttrs

Map each attribute in the given set and merge them into a new attribute set.

Inputs
f

1. Function argument

v

2. Function argument

Type
concatMapAttrs :: (String -> a -> AttrSet) -> AttrSet -> AttrSet
Examples
Example 9. lib.attrsets.concatMapAttrs usage example
concatMapAttrs
  (name: value: {
    ${name} = value;
    ${name + value} = value;
  })
  { x = "a"; y = "b"; }
=> { x = "a"; xa = "a"; y = "b"; yb = "b"; }

Located at lib/attrsets.nix:360 in <nixpkgs>.

lib.attrsets.updateManyAttrsByPath

Update or set specific paths of an attribute set.

Takes a list of updates to apply and an attribute set to apply them to, and returns the attribute set with the updates applied. Updates are represented as { path = ...; update = ...; } values, where path is a list of strings representing the attribute path that should be updated, and update is a function that takes the old value at that attribute path as an argument and returns the new value it should be.

Properties:

  • Updates to deeper attribute paths are applied before updates to more shallow attribute paths

  • Multiple updates to the same attribute path are applied in the order they appear in the update list

  • If any but the last path element leads into a value that is not an attribute set, an error is thrown

  • If there is an update for an attribute path that doesn’t exist, accessing the argument in the update function causes an error, but intermediate attribute sets are implicitly created as needed

Type
updateManyAttrsByPath :: [{ path :: [String]; update :: (Any -> Any); }] -> AttrSet -> AttrSet
Examples
Example 10. lib.attrsets.updateManyAttrsByPath usage example
updateManyAttrsByPath [
  {
    path = [ "a" "b" ];
    update = old: { d = old.c; };
  }
  {
    path = [ "a" "b" "c" ];
    update = old: old + 1;
  }
  {
    path = [ "x" "y" ];
    update = old: "xy";
  }
] { a.b.c = 0; }
=> { a = { b = { d = 1; }; }; x = { y = "xy"; }; }

Located at lib/attrsets.nix:423 in <nixpkgs>.

lib.attrsets.attrVals

Return the specified attributes from a set.

Inputs
nameList

The list of attributes to fetch from set. Each attribute name must exist on the attrbitue set

set

The set to get attribute values from

Type
attrVals :: [String] -> AttrSet -> [Any]
Examples
Example 11. lib.attrsets.attrVals usage example
attrVals ["a" "b" "c"] as
=> [as.a as.b as.c]

Located at lib/attrsets.nix:513 in <nixpkgs>.

lib.attrsets.attrValues

Return the values of all attributes in the given set, sorted by attribute name.

Type
attrValues :: AttrSet -> [Any]
Examples
Example 12. lib.attrsets.attrValues usage example
attrValues {c = 3; a = 1; b = 2;}
=> [1 2 3]

Located at lib/attrsets.nix:539 in <nixpkgs>.

lib.attrsets.getAttrs

Given a set of attribute names, return the set of the corresponding attributes from the given set.

Inputs
names

A list of attribute names to get out of set

attrs

The set to get the named attributes from

Type
getAttrs :: [String] -> AttrSet -> AttrSet
Examples
Example 13. lib.attrsets.getAttrs usage example
getAttrs [ "a" "b" ] { a = 1; b = 2; c = 3; }
=> { a = 1; b = 2; }

Located at lib/attrsets.nix:574 in <nixpkgs>.

lib.attrsets.catAttrs

Collect each attribute named attr from a list of attribute sets. Sets that don’t contain the named attribute are ignored.

Inputs
attr

The attribute name to get out of the sets.

list

The list of attribute sets to go through

Type
catAttrs :: String -> [AttrSet] -> [Any]
Examples
Example 14. lib.attrsets.catAttrs usage example
catAttrs "a" [{a = 1;} {b = 0;} {a = 2;}]
=> [1 2]

Located at lib/attrsets.nix:609 in <nixpkgs>.

lib.attrsets.filterAttrs

Filter an attribute set by removing all attributes for which the given predicate return false.

Inputs
pred

Predicate taking an attribute name and an attribute value, which returns true to include the attribute, or false to exclude the attribute.

set

The attribute set to filter

Type
filterAttrs :: (String -> Any -> Bool) -> AttrSet -> AttrSet
Examples
Example 15. lib.attrsets.filterAttrs usage example
filterAttrs (n: v: n == "foo") { foo = 1; bar = 2; }
=> { foo = 1; }

Located at lib/attrsets.nix:644 in <nixpkgs>.

lib.attrsets.filterAttrsRecursive

Filter an attribute set recursively by removing all attributes for which the given predicate return false.

Inputs
pred

Predicate taking an attribute name and an attribute value, which returns true to include the attribute, or false to exclude the attribute.

set

The attribute set to filter

Type
filterAttrsRecursive :: (String -> Any -> Bool) -> AttrSet -> AttrSet
Examples
Example 16. lib.attrsets.filterAttrsRecursive usage example
filterAttrsRecursive (n: v: v != null) { foo = { bar = null; }; }
=> { foo = {}; }

Located at lib/attrsets.nix:682 in <nixpkgs>.

lib.attrsets.foldlAttrs

Like lib.lists.foldl' but for attribute sets. Iterates over every name-value pair in the given attribute set. The result of the callback function is often called acc for accumulator. It is passed between callbacks from left to right and the final acc is the return value of foldlAttrs.

Attention:

There is a completely different function lib.foldAttrs which has nothing to do with this function, despite the similar name.

Inputs
f

1. Function argument

init

2. Function argument

set

3. Function argument

Type
foldlAttrs :: ( a -> String -> b -> a ) -> a -> { ... :: b } -> a
Examples
Example 17. lib.attrsets.foldlAttrs usage example
foldlAttrs
  (acc: name: value: {
    sum = acc.sum + value;
    names = acc.names ++ [name];
  })
  { sum = 0; names = []; }
  {
    foo = 1;
    bar = 10;
  }
->
  {
    sum = 11;
    names = ["bar" "foo"];
  }

foldlAttrs
  (throw "function not needed")
  123
  {};
->
  123

foldlAttrs
  (acc: _: _: acc)
  3
  { z = throw "value not needed"; a = throw "value not needed"; };
->
  3

The accumulator doesn't have to be an attrset.
It can be as simple as a number or string.

foldlAttrs
  (acc: _: v: acc * 10 + v)
  1
  { z = 1; a = 2; };
->
  121

Located at lib/attrsets.nix:776 in <nixpkgs>.

lib.attrsets.foldAttrs

Apply fold functions to values grouped by key.

Inputs
op

A function, given a value and a collector combines the two.

nul

The starting value.

list_of_attrs

A list of attribute sets to fold together by key.

Type
foldAttrs :: (Any -> Any -> Any) -> Any -> [AttrSets] -> Any
Examples
Example 18. lib.attrsets.foldAttrs usage example
foldAttrs (item: acc: [item] ++ acc) [] [{ a = 2; } { a = 3; }]
=> { a = [ 2 3 ]; }

Located at lib/attrsets.nix:817 in <nixpkgs>.

lib.attrsets.collect

Recursively collect sets that verify a given predicate named pred from the set attrs. The recursion is stopped when the predicate is verified.

Inputs
pred

Given an attribute’s value, determine if recursion should stop.

attrs

The attribute set to recursively collect.

Type
collect :: (AttrSet -> Bool) -> AttrSet -> [x]
Examples
Example 19. lib.attrsets.collect usage example
collect isList { a = { b = ["b"]; }; c = [1]; }
=> [["b"] [1]]

collect (x: x ? outPath)
   { a = { outPath = "a/"; }; b = { outPath = "b/"; }; }
=> [{ outPath = "a/"; } { outPath = "b/"; }]

Located at lib/attrsets.nix:865 in <nixpkgs>.

lib.attrsets.cartesianProduct

Return the cartesian product of attribute set value combinations.

Inputs
attrsOfLists

Attribute set with attributes that are lists of values

Type
cartesianProduct :: AttrSet -> [AttrSet]
Examples
Example 20. lib.attrsets.cartesianProduct usage example
cartesianProduct { a = [ 1 2 ]; b = [ 10 20 ]; }
=> [
     { a = 1; b = 10; }
     { a = 1; b = 20; }
     { a = 2; b = 10; }
     { a = 2; b = 20; }
   ]

Located at lib/attrsets.nix:907 in <nixpkgs>.

lib.attrsets.mapCartesianProduct

Return the result of function f applied to the cartesian product of attribute set value combinations. Equivalent to using cartesianProduct followed by map.

Inputs
f

A function, given an attribute set, it returns a new value.

attrsOfLists

Attribute set with attributes that are lists of values

Type
mapCartesianProduct :: (AttrSet -> a) -> AttrSet -> [a]
Examples
Example 21. lib.attrsets.mapCartesianProduct usage example
mapCartesianProduct ({a, b}: "${a}-${b}") { a = [ "1" "2" ]; b = [ "3" "4" ]; }
=> [ "1-3" "1-4" "2-3" "2-4" ]

Located at lib/attrsets.nix:948 in <nixpkgs>.

lib.attrsets.nameValuePair

Utility function that creates a {name, value} pair as expected by builtins.listToAttrs.

Inputs
name

Attribute name

value

Attribute value

Type
nameValuePair :: String -> Any -> { name :: String; value :: Any; }
Examples
Example 22. lib.attrsets.nameValuePair usage example
nameValuePair "some" 6
=> { name = "some"; value = 6; }

Located at lib/attrsets.nix:981 in <nixpkgs>.

lib.attrsets.mapAttrs

Apply a function to each element in an attribute set, creating a new attribute set.

Inputs
f

A function that takes an attribute name and its value, and returns the new value for the attribute.

attrset

The attribute set to iterate through.

Type
mapAttrs :: (String -> Any -> Any) -> AttrSet -> AttrSet
Examples
Example 23. lib.attrsets.mapAttrs usage example
mapAttrs (name: value: name + "-" + value)
   { x = "foo"; y = "bar"; }
=> { x = "x-foo"; y = "y-bar"; }

Located at lib/attrsets.nix:1018 in <nixpkgs>.

lib.attrsets.mapAttrs'

Like mapAttrs, but allows the name of each attribute to be changed in addition to the value. The applied function should return both the new name and value as a nameValuePair.

Inputs
f

A function, given an attribute’s name and value, returns a new nameValuePair.

set

Attribute set to map over.

Type
mapAttrs' :: (String -> Any -> { name :: String; value :: Any; }) -> AttrSet -> AttrSet
Examples
Example 24. lib.attrsets.mapAttrs' usage example
mapAttrs' (name: value: nameValuePair ("foo_" + name) ("bar-" + value))
   { x = "a"; y = "b"; }
=> { foo_x = "bar-a"; foo_y = "bar-b"; }

Located at lib/attrsets.nix:1055 in <nixpkgs>.

lib.attrsets.mapAttrsToList

Call a function for each attribute in the given set and return the result in a list.

Inputs
f

A function, given an attribute’s name and value, returns a new value.

attrs

Attribute set to map over.

Type
mapAttrsToList :: (String -> a -> b) -> AttrSet -> [b]
Examples
Example 25. lib.attrsets.mapAttrsToList usage example
mapAttrsToList (name: value: name + value)
   { x = "a"; y = "b"; }
=> [ "xa" "yb" ]

Located at lib/attrsets.nix:1093 in <nixpkgs>.

lib.attrsets.attrsToList

Deconstruct an attrset to a list of name-value pairs as expected by builtins.listToAttrs. Each element of the resulting list is an attribute set with these attributes:

  • name (string): The name of the attribute

  • value (any): The value of the attribute

The following is always true:

builtins.listToAttrs (attrsToList attrs) == attrs

Warning

The opposite is not always true. In general expect that

attrsToList (builtins.listToAttrs list) != list

This is because the listToAttrs removes duplicate names and doesn’t preserve the order of the list.

Inputs
set

The attribute set to deconstruct.

Type
attrsToList :: AttrSet -> [ { name :: String; value :: Any; } ]
Examples
Example 26. lib.attrsets.attrsToList usage example
attrsToList { foo = 1; bar = "asdf"; }
=> [ { name = "bar"; value = "asdf"; } { name = "foo"; value = 1; } ]

Located at lib/attrsets.nix:1141 in <nixpkgs>.

lib.attrsets.mapAttrsRecursive

Like mapAttrs, except that it recursively applies itself to the leaf attributes of a potentially-nested attribute set: the second argument of the function will never be an attrset. Also, the first argument of the mapping function is a list of the attribute names that form the path to the leaf attribute.

For a function that gives you control over what counts as a leaf, see mapAttrsRecursiveCond.

Example 27. Map over leaf attributes
mapAttrsRecursive (path: value: concatStringsSep "-" (path ++ [value]))
  { n = { a = "A"; m = { b = "B"; c = "C"; }; }; d = "D"; }

evaluates to

{ n = { a = "n-a-A"; m = { b = "n-m-b-B"; c = "n-m-c-C"; }; }; d = "d-D"; }

Type
mapAttrsRecursive :: ([String] -> a -> b) -> AttrSet -> AttrSet

Located at lib/attrsets.nix:1169 in <nixpkgs>.

lib.attrsets.mapAttrsRecursiveCond

Like mapAttrsRecursive, but it takes an additional predicate that tells it whether to recurse into an attribute set. If the predicate returns false, mapAttrsRecursiveCond does not recurse, but instead applies the mapping function. If the predicate returns true, it does recurse, and does not apply the mapping function.

Example 28. Map over an leaf attributes defined by a condition

Map derivations to their name attribute. Derivatons are identified as attribute sets that contain { type = "derivation"; }.

mapAttrsRecursiveCond
  (as: !(as ? "type" && as.type == "derivation"))
  (x: x.name)
  attrs

Type
mapAttrsRecursiveCond :: (AttrSet -> Bool) -> ([String] -> a -> b) -> AttrSet -> AttrSet

Located at lib/attrsets.nix:1198 in <nixpkgs>.

lib.attrsets.genAttrs

Generate an attribute set by mapping a function over a list of attribute names.

Inputs
names

Names of values in the resulting attribute set.

f

A function, given the name of the attribute, returns the attribute’s value.

Type
genAttrs :: [ String ] -> (String -> Any) -> AttrSet
Examples
Example 29. lib.attrsets.genAttrs usage example
genAttrs [ "foo" "bar" ] (name: "x_" + name)
=> { foo = "x_foo"; bar = "x_bar"; }

Located at lib/attrsets.nix:1245 in <nixpkgs>.

lib.attrsets.isDerivation

Check whether the argument is a derivation. Any set with { type = "derivation"; } counts as a derivation.

Inputs
value

Value to check.

Type
isDerivation :: Any -> Bool
Examples
Example 30. lib.attrsets.isDerivation usage example
nixpkgs = import <nixpkgs> {}
isDerivation nixpkgs.ruby
=> true
isDerivation "foobar"
=> false

Located at lib/attrsets.nix:1282 in <nixpkgs>.

lib.attrsets.toDerivation

Converts a store path to a fake derivation.

Inputs
path

A store path to convert to a derivation.

Type
toDerivation :: Path -> Derivation

Located at lib/attrsets.nix:1301 in <nixpkgs>.

lib.attrsets.optionalAttrs

If cond is true, return the attribute set as, otherwise an empty attribute set.

Inputs
cond

Condition under which the as attribute set is returned.

as

The attribute set to return if cond is true.

Type
optionalAttrs :: Bool -> AttrSet -> AttrSet
Examples
Example 31. lib.attrsets.optionalAttrs usage example
optionalAttrs (true) { my = "set"; }
=> { my = "set"; }
optionalAttrs (false) { my = "set"; }
=> { }

Located at lib/attrsets.nix:1350 in <nixpkgs>.

lib.attrsets.zipAttrsWithNames

Merge sets of attributes and use the function f to merge attributes values.

Inputs
names

List of attribute names to zip.

f

A function, accepts an attribute name, all the values, and returns a combined value.

sets

List of values from the list of attribute sets.

Type
zipAttrsWithNames :: [ String ] -> (String -> [ Any ] -> Any) -> [ AttrSet ] -> AttrSet
Examples
Example 32. lib.attrsets.zipAttrsWithNames usage example
zipAttrsWithNames ["a"] (name: vs: vs) [{a = "x";} {a = "y"; b = "z";}]
=> { a = ["x" "y"]; }

Located at lib/attrsets.nix:1392 in <nixpkgs>.

lib.attrsets.zipAttrsWith

Merge sets of attributes and use the function f to merge attribute values. Like lib.attrsets.zipAttrsWithNames with all key names are passed for names.

Implementation note: Common names appear multiple times in the list of names, hopefully this does not affect the system because the maximal laziness avoid computing twice the same expression and listToAttrs does not care about duplicated attribute names.

Type
zipAttrsWith :: (String -> [ Any ] -> Any) -> [ AttrSet ] -> AttrSet
Examples
Example 33. lib.attrsets.zipAttrsWith usage example
zipAttrsWith (name: values: values) [{a = "x";} {a = "y"; b = "z";}]
=> { a = ["x" "y"]; b = ["z"]; }

Located at lib/attrsets.nix:1428 in <nixpkgs>.

lib.attrsets.zipAttrs

Merge sets of attributes and combine each attribute value in to a list.

Like lib.attrsets.zipAttrsWith with (name: values: values) as the function.

Type
zipAttrs :: [ AttrSet ] -> AttrSet
Examples
Example 34. lib.attrsets.zipAttrs usage example
zipAttrs [{a = "x";} {a = "y"; b = "z";}]
=> { a = ["x" "y"]; b = ["z"]; }

Located at lib/attrsets.nix:1454 in <nixpkgs>.

lib.attrsets.mergeAttrsList

Merge a list of attribute sets together using the // operator. In case of duplicate attributes, values from later list elements take precedence over earlier ones. The result is the same as foldl mergeAttrs { }, but the performance is better for large inputs. For n list elements, each with an attribute set containing m unique attributes, the complexity of this operation is O(nm log n).

Inputs
list

1. Function argument

Type
mergeAttrsList :: [ Attrs ] -> Attrs
Examples
Example 35. lib.attrsets.mergeAttrsList usage example
mergeAttrsList [ { a = 0; b = 1; } { c = 2; d = 3; } ]
=> { a = 0; b = 1; c = 2; d = 3; }
mergeAttrsList [ { a = 0; } { a = 1; } ]
=> { a = 1; }

Located at lib/attrsets.nix:1488 in <nixpkgs>.

lib.attrsets.recursiveUpdateUntil

Does the same as the update operator ‘//’ except that attributes are merged until the given predicate is verified. The predicate should accept 3 arguments which are the path to reach the attribute, a part of the first attribute set and a part of the second attribute set. When the predicate is satisfied, the value of the first attribute set is replaced by the value of the second attribute set.

Inputs
pred

Predicate, taking the path to the current attribute as a list of strings for attribute names, and the two values at that path from the original arguments.

lhs

Left attribute set of the merge.

rhs

Right attribute set of the merge.

Type
recursiveUpdateUntil :: ( [ String ] -> AttrSet -> AttrSet -> Bool ) -> AttrSet -> AttrSet -> AttrSet
Examples
Example 36. lib.attrsets.recursiveUpdateUntil usage example
recursiveUpdateUntil (path: l: r: path == ["foo"]) {
  # first attribute set
  foo.bar = 1;
  foo.baz = 2;
  bar = 3;
} {
  #second attribute set
  foo.bar = 1;
  foo.quz = 2;
  baz = 4;
}

=> {
  foo.bar = 1; # 'foo.*' from the second set
  foo.quz = 2; #
  bar = 3;     # 'bar' from the first set
  baz = 4;     # 'baz' from the second set
}

Located at lib/attrsets.nix:1566 in <nixpkgs>.

lib.attrsets.recursiveUpdate

A recursive variant of the update operator ‘//’. The recursion stops when one of the attribute values is not an attribute set, in which case the right hand side value takes precedence over the left hand side value.

Inputs
lhs

Left attribute set of the merge.

rhs

Right attribute set of the merge.

Type
recursiveUpdate :: AttrSet -> AttrSet -> AttrSet
Examples
Example 37. lib.attrsets.recursiveUpdate usage example
recursiveUpdate {
  boot.loader.grub.enable = true;
  boot.loader.grub.device = "/dev/hda";
} {
  boot.loader.grub.device = "";
}

returns: {
  boot.loader.grub.enable = true;
  boot.loader.grub.device = "";
}

Located at lib/attrsets.nix:1625 in <nixpkgs>.

lib.attrsets.matchAttrs

Recurse into every attribute set of the first argument and check that:

  • Each attribute path also exists in the second argument.

  • If the attribute’s value is not a nested attribute set, it must have the same value in the right argument.

Inputs
pattern

Attribute set structure to match

attrs

Attribute set to check

Type
matchAttrs :: AttrSet -> AttrSet -> Bool
Examples
Example 38. lib.attrsets.matchAttrs usage example
matchAttrs { cpu = {}; } { cpu = { bits = 64; }; }
=> true

Located at lib/attrsets.nix:1664 in <nixpkgs>.

lib.attrsets.overrideExisting

Override only the attributes that are already present in the old set useful for deep-overriding.

Inputs
old

Original attribute set

new

Attribute set with attributes to override in old.

Type
overrideExisting :: AttrSet -> AttrSet -> AttrSet
Examples
Example 39. lib.attrsets.overrideExisting usage example
overrideExisting {} { a = 1; }
=> {}
overrideExisting { b = 2; } { a = 1; }
=> { b = 2; }
overrideExisting { a = 3; b = 2; } { a = 1; }
=> { a = 1; b = 2; }

Located at lib/attrsets.nix:1720 in <nixpkgs>.

lib.attrsets.showAttrPath

Turns a list of strings into a human-readable description of those strings represented as an attribute path. The result of this function is not intended to be machine-readable. Create a new attribute set with value set at the nested attribute location specified in attrPath.

Inputs
path

Attribute path to render to a string

Type
showAttrPath :: [String] -> String
Examples
Example 40. lib.attrsets.showAttrPath usage example
showAttrPath [ "foo" "10" "bar" ]
=> "foo.\"10\".bar"
showAttrPath []
=> "<root attribute path>"

Located at lib/attrsets.nix:1758 in <nixpkgs>.

lib.attrsets.getOutput

Get a package output. If no output is found, fallback to .out and then to the default.

Inputs
output

1. Function argument

pkg

2. Function argument

Type
getOutput :: String -> Derivation -> String
Examples
Example 41. lib.attrsets.getOutput usage example
getOutput "dev" pkgs.openssl
=> "/nix/store/9rz8gxhzf8sw4kf2j2f1grr49w8zx5vj-openssl-1.0.1r-dev"

Located at lib/attrsets.nix:1796 in <nixpkgs>.

lib.attrsets.getBin

Get a package’s bin output. If the output does not exist, fallback to .out and then to the default.

Inputs
pkg

The package whose bin output will be retrieved.

Type
getBin :: Derivation -> String
Examples
Example 42. lib.attrsets.getBin usage example
getBin pkgs.openssl
=> "/nix/store/9rz8gxhzf8sw4kf2j2f1grr49w8zx5vj-openssl-1.0.1r"

Located at lib/attrsets.nix:1828 in <nixpkgs>.

lib.attrsets.getLib

Get a package’s lib output. If the output does not exist, fallback to .out and then to the default.

Inputs
pkg

The package whose lib output will be retrieved.

Type
getLib :: Derivation -> String
Examples
Example 43. lib.attrsets.getLib usage example
getLib pkgs.openssl
=> "/nix/store/9rz8gxhzf8sw4kf2j2f1grr49w8zx5vj-openssl-1.0.1r-lib"

Located at lib/attrsets.nix:1858 in <nixpkgs>.

lib.attrsets.getDev

Get a package’s dev output. If the output does not exist, fallback to .out and then to the default.

Inputs
pkg

The package whose dev output will be retrieved.

Type
getDev :: Derivation -> String
Examples
Example 44. lib.attrsets.getDev usage example
getDev pkgs.openssl
=> "/nix/store/9rz8gxhzf8sw4kf2j2f1grr49w8zx5vj-openssl-1.0.1r-dev"

Located at lib/attrsets.nix:1888 in <nixpkgs>.

lib.attrsets.getMan

Get a package’s man output. If the output does not exist, fallback to .out and then to the default.

Inputs
pkg

The package whose man output will be retrieved.

Type
getMan :: Derivation -> String
Examples
Example 45. lib.attrsets.getMan usage example
getMan pkgs.openssl
=> "/nix/store/9rz8gxhzf8sw4kf2j2f1grr49w8zx5vj-openssl-1.0.1r-man"

Located at lib/attrsets.nix:1918 in <nixpkgs>.

lib.attrsets.chooseDevOutputs

Pick the outputs of packages to place in buildInputs

Inputs
pkgs

List of packages.

Type
chooseDevOutputs :: [Derivation] -> [String]

Located at lib/attrsets.nix:1935 in <nixpkgs>.

lib.attrsets.recurseIntoAttrs

Make various Nix tools consider the contents of the resulting attribute set when looking for what to build, find, etc.

This function only affects a single attribute set; it does not apply itself recursively for nested attribute sets.

Inputs
attrs

An attribute set to scan for derivations.

Type
recurseIntoAttrs :: AttrSet -> AttrSet
Examples
Example 46. lib.attrsets.recurseIntoAttrs usage example
{ pkgs ? import <nixpkgs> {} }:
{
  myTools = pkgs.lib.recurseIntoAttrs {
    inherit (pkgs) hello figlet;
  };
}

Located at lib/attrsets.nix:1972 in <nixpkgs>.

lib.attrsets.dontRecurseIntoAttrs

Undo the effect of recurseIntoAttrs.

Inputs
attrs

An attribute set to not scan for derivations.

Type
dontRecurseIntoAttrs :: AttrSet -> AttrSet

Located at lib/attrsets.nix:1992 in <nixpkgs>.

lib.attrsets.unionOfDisjoint

unionOfDisjoint x y is equal to x // y // z where the attrnames in z are the intersection of the attrnames in x and y, and all values assert with an error message. This operator is commutative, unlike (//).

Inputs
x

1. Function argument

y

2. Function argument

Type
unionOfDisjoint :: AttrSet -> AttrSet -> AttrSet

Located at lib/attrsets.nix:2019 in <nixpkgs>.

lib.strings: string manipulation functions

String manipulation functions.

lib.strings.concatStrings

Type: concatStrings :: [string] -> string

Concatenate a list of strings.

Example 47. lib.strings.concatStrings usage example
concatStrings ["foo" "bar"]
=> "foobar"

Located at lib/strings.nix:50 in <nixpkgs>.

lib.strings.concatMapStrings

Type: concatMapStrings :: (a -> string) -> [a] -> string

Map a function over a list and concatenate the resulting strings.

f

Function argument

list

Function argument

Example 48. lib.strings.concatMapStrings usage example
concatMapStrings (x: "a" + x) ["foo" "bar"]
=> "afooabar"

Located at lib/strings.nix:60 in <nixpkgs>.

lib.strings.concatImapStrings

Type: concatImapStrings :: (int -> a -> string) -> [a] -> string

Like concatMapStrings except that the f functions also gets the position as a parameter.

f

Function argument

list

Function argument

Example 49. lib.strings.concatImapStrings usage example
concatImapStrings (pos: x: "${toString pos}-${x}") ["foo" "bar"]
=> "1-foo2-bar"

Located at lib/strings.nix:71 in <nixpkgs>.

lib.strings.intersperse

Type: intersperse :: a -> [a] -> [a]

Place an element between each element of a list

separator

Separator to add between elements

list

Input list

Example 50. lib.strings.intersperse usage example
intersperse "/" ["usr" "local" "bin"]
=> ["usr" "/" "local" "/" "bin"].

Located at lib/strings.nix:81 in <nixpkgs>.

lib.strings.concatStringsSep

Type: concatStringsSep :: string -> [string] -> string

Concatenate a list of strings with a separator between each element

Example 51. lib.strings.concatStringsSep usage example
concatStringsSep "/" ["usr" "local" "bin"]
=> "usr/local/bin"

Located at lib/strings.nix:98 in <nixpkgs>.

lib.strings.concatMapStringsSep

Type: concatMapStringsSep :: string -> (a -> string) -> [a] -> string

Maps a function over a list of strings and then concatenates the result with the specified separator interspersed between elements.

sep

Separator to add between elements

f

Function to map over the list

list

List of input strings

Example 52. lib.strings.concatMapStringsSep usage example
concatMapStringsSep "-" (x: toUpper x)  ["foo" "bar" "baz"]
=> "FOO-BAR-BAZ"

Located at lib/strings.nix:110 in <nixpkgs>.

lib.strings.concatImapStringsSep

Type: concatIMapStringsSep :: string -> (int -> a -> string) -> [a] -> string

Same as concatMapStringsSep, but the mapping function additionally receives the position of its argument.

sep

Separator to add between elements

f

Function that receives elements and their positions

list

List of input strings

Example 53. lib.strings.concatImapStringsSep usage example
concatImapStringsSep "-" (pos: x: toString (x / pos)) [ 6 6 6 ]
=> "6-3-2"

Located at lib/strings.nix:127 in <nixpkgs>.

lib.strings.concatLines

Type: concatLines :: [string] -> string

Concatenate a list of strings, adding a newline at the end of each one. Defined as concatMapStrings (s: s + "\n").

Example 54. lib.strings.concatLines usage example
concatLines [ "foo" "bar" ]
=> "foo\nbar\n"

Located at lib/strings.nix:144 in <nixpkgs>.

lib.strings.replicate

Type: replicate :: int -> string -> string

Replicate a string n times, and concatenate the parts into a new string.

n

Function argument

s

Function argument

Example 55. lib.strings.replicate usage example
replicate 3 "v"
=> "vvv"
replicate 5 "hello"
=> "hellohellohellohellohello"

Located at lib/strings.nix:158 in <nixpkgs>.

lib.strings.makeSearchPath

Type: makeSearchPath :: string -> [string] -> string

Construct a Unix-style, colon-separated search path consisting of the given subDir appended to each of the given paths.

subDir

Directory name to append

paths

List of base paths

Example 56. lib.strings.makeSearchPath usage example
makeSearchPath "bin" ["/root" "/usr" "/usr/local"]
=> "/root/bin:/usr/bin:/usr/local/bin"
makeSearchPath "bin" [""]
=> "/bin"

Located at lib/strings.nix:171 in <nixpkgs>.

lib.strings.makeSearchPathOutput

Type: string -> string -> [package] -> string

Construct a Unix-style search path by appending the given subDir to the specified output of each of the packages. If no output by the given name is found, fallback to .out and then to the default.

output

Package output to use

subDir

Directory name to append

pkgs

List of packages

Example 57. lib.strings.makeSearchPathOutput usage example
makeSearchPathOutput "dev" "bin" [ pkgs.openssl pkgs.zlib ]
=> "/nix/store/9rz8gxhzf8sw4kf2j2f1grr49w8zx5vj-openssl-1.0.1r-dev/bin:/nix/store/wwh7mhwh269sfjkm6k5665b5kgp7jrk2-zlib-1.2.8/bin"

Located at lib/strings.nix:189 in <nixpkgs>.

lib.strings.makeLibraryPath

Construct a library search path (such as RPATH) containing the libraries for a set of packages

Example 58. lib.strings.makeLibraryPath usage example
makeLibraryPath [ "/usr" "/usr/local" ]
=> "/usr/lib:/usr/local/lib"
pkgs = import <nixpkgs> { }
makeLibraryPath [ pkgs.openssl pkgs.zlib ]
=> "/nix/store/9rz8gxhzf8sw4kf2j2f1grr49w8zx5vj-openssl-1.0.1r/lib:/nix/store/wwh7mhwh269sfjkm6k5665b5kgp7jrk2-zlib-1.2.8/lib"

Located at lib/strings.nix:207 in <nixpkgs>.

lib.strings.makeIncludePath

Construct an include search path (such as C_INCLUDE_PATH) containing the header files for a set of packages or paths.

Example 59. lib.strings.makeIncludePath usage example
makeIncludePath [ "/usr" "/usr/local" ]
=> "/usr/include:/usr/local/include"
pkgs = import <nixpkgs> { }
makeIncludePath [ pkgs.openssl pkgs.zlib ]
=> "/nix/store/9rz8gxhzf8sw4kf2j2f1grr49w8zx5vj-openssl-1.0.1r-dev/include:/nix/store/wwh7mhwh269sfjkm6k5665b5kgp7jrk2-zlib-1.2.8-dev/include"

Located at lib/strings.nix:219 in <nixpkgs>.

lib.strings.makeBinPath

Construct a binary search path (such as $PATH) containing the binaries for a set of packages.

Example 60. lib.strings.makeBinPath usage example
makeBinPath ["/root" "/usr" "/usr/local"]
=> "/root/bin:/usr/bin:/usr/local/bin"

Located at lib/strings.nix:228 in <nixpkgs>.

lib.strings.normalizePath

Type: normalizePath :: string -> string

Normalize path, removing extraneous /s

s

Function argument

Example 61. lib.strings.normalizePath usage example
normalizePath "/a//b///c/"
=> "/a/b/c/"

Located at lib/strings.nix:238 in <nixpkgs>.

lib.strings.optionalString

Type: optionalString :: bool -> string -> string

Depending on the boolean `cond’, return either the given string or the empty string. Useful to concatenate against a bigger string.

cond

Condition

string

String to return if condition is true

Example 62. lib.strings.optionalString usage example
optionalString true "some-string"
=> "some-string"
optionalString false "some-string"
=> ""

Located at lib/strings.nix:264 in <nixpkgs>.

lib.strings.hasPrefix

Type: hasPrefix :: string -> string -> bool

Determine whether a string has given prefix.

pref

Prefix to check for

str

Input string

Example 63. lib.strings.hasPrefix usage example
hasPrefix "foo" "foobar"
=> true
hasPrefix "foo" "barfoo"
=> false

Located at lib/strings.nix:280 in <nixpkgs>.

lib.strings.hasSuffix

Type: hasSuffix :: string -> string -> bool

Determine whether a string has given suffix.

suffix

Suffix to check for

content

Input string

Example 64. lib.strings.hasSuffix usage example
hasSuffix "foo" "foobar"
=> false
hasSuffix "foo" "barfoo"
=> true

Located at lib/strings.nix:307 in <nixpkgs>.

lib.strings.hasInfix

Type: hasInfix :: string -> string -> bool

Determine whether a string contains the given infix

infix

Function argument

content

Function argument

Example 65. lib.strings.hasInfix usage example
hasInfix "bc" "abcd"
=> true
hasInfix "ab" "abcd"
=> true
hasInfix "cd" "abcd"
=> true
hasInfix "foo" "abcd"
=> false

Located at lib/strings.nix:344 in <nixpkgs>.

lib.strings.stringToCharacters

Type: stringToCharacters :: string -> [string]

Convert a string to a list of characters (i.e. singleton strings). This allows you to, e.g., map a function over each character. However, note that this will likely be horribly inefficient; Nix is not a general purpose programming language. Complex string manipulations should, if appropriate, be done in a derivation. Also note that Nix treats strings as a list of bytes and thus doesn’t handle unicode.

s

Function argument

Example 66. lib.strings.stringToCharacters usage example
stringToCharacters ""
=> [ ]
stringToCharacters "abc"
=> [ "a" "b" "c" ]
stringToCharacters "🦄"
=> [ "�" "�" "�" "�" ]

Located at lib/strings.nix:374 in <nixpkgs>.

lib.strings.stringAsChars

Type: stringAsChars :: (string -> string) -> string -> string

Manipulate a string character by character and replace them by strings before concatenating the results.

f

Function to map over each individual character

s

Input string

Example 67. lib.strings.stringAsChars usage example
stringAsChars (x: if x == "a" then "i" else x) "nax"
=> "nix"

Located at lib/strings.nix:386 in <nixpkgs>.

lib.strings.charToInt

Type: charToInt :: string -> int

Convert char to ascii value, must be in printable range

c

Function argument

Example 68. lib.strings.charToInt usage example
charToInt "A"
=> 65
charToInt "("
=> 40

Located at lib/strings.nix:405 in <nixpkgs>.

lib.strings.escape

Type: escape :: [string] -> string -> string

Escape occurrence of the elements of list in string by prefixing it with a backslash.

list

Function argument

Example 69. lib.strings.escape usage example
escape ["(" ")"] "(foo)"
=> "\\(foo\\)"

Located at lib/strings.nix:416 in <nixpkgs>.

lib.strings.escapeC

Type: escapeC = [string] -> string -> string

Escape occurrence of the element of list in string by converting to its ASCII value and prefixing it with \x. Only works for printable ascii characters.

list

Function argument

Example 70. lib.strings.escapeC usage example
escapeC [" "] "foo bar"
=> "foo\\x20bar"

Located at lib/strings.nix:429 in <nixpkgs>.

lib.strings.escapeURL

Type: escapeURL :: string -> string

Escape the string so it can be safely placed inside a URL query.

Example 71. lib.strings.escapeURL usage example
escapeURL "foo/bar baz"
=> "foo%2Fbar%20baz"

Located at lib/strings.nix:440 in <nixpkgs>.

lib.strings.escapeShellArg

Type: escapeShellArg :: string -> string

Quote string to be used safely within the Bourne shell.

arg

Function argument

Example 72. lib.strings.escapeShellArg usage example
escapeShellArg "esc'ape\nme"
=> "'esc'\\''ape\nme'"

Located at lib/strings.nix:454 in <nixpkgs>.

lib.strings.escapeShellArgs

Type: escapeShellArgs :: [string] -> string

Quote all arguments to be safely passed to the Bourne shell.

Example 73. lib.strings.escapeShellArgs usage example
escapeShellArgs ["one" "two three" "four'five"]
=> "'one' 'two three' 'four'\\''five'"

Located at lib/strings.nix:464 in <nixpkgs>.

lib.strings.isValidPosixName

Type: string -> bool

Test whether the given name is a valid POSIX shell variable name.

name

Function argument

Example 74. lib.strings.isValidPosixName usage example
isValidPosixName "foo_bar000"
=> true
isValidPosixName "0-bad.jpg"
=> false

Located at lib/strings.nix:476 in <nixpkgs>.

lib.strings.toShellVar

Type: string -> (string | listOf string | attrsOf string) -> string

Translate a Nix value into a shell variable declaration, with proper escaping.

The value can be a string (mapped to a regular variable), a list of strings (mapped to a Bash-style array) or an attribute set of strings (mapped to a Bash-style associative array). Note that “string” includes string-coercible values like paths or derivations.

Strings are translated into POSIX sh-compatible code; lists and attribute sets assume a shell that understands Bash syntax (e.g. Bash or ZSH).

name

Function argument

value

Function argument

Example 75. lib.strings.toShellVar usage example
''
  ${toShellVar "foo" "some string"}
  [[ "$foo" == "some string" ]]
''

Located at lib/strings.nix:496 in <nixpkgs>.

lib.strings.toShellVars

Type: attrsOf (string | listOf string | attrsOf string) -> string

Translate an attribute set into corresponding shell variable declarations using toShellVar.

vars

Function argument

Example 76. lib.strings.toShellVars usage example
let
  foo = "value";
  bar = foo;
in ''
  ${toShellVars { inherit foo bar; }}
  [[ "$foo" == "$bar" ]]
''

Located at lib/strings.nix:524 in <nixpkgs>.

lib.strings.escapeNixString

Type: string -> string

Turn a string into a Nix expression representing that string

s

Function argument

Example 77. lib.strings.escapeNixString usage example
escapeNixString "hello\${}\n"
=> "\"hello\\\${}\\n\""

Located at lib/strings.nix:534 in <nixpkgs>.

lib.strings.escapeRegex

Type: string -> string

Turn a string into an exact regular expression

Example 78. lib.strings.escapeRegex usage example
escapeRegex "[^a-z]*"
=> "\\[\\^a-z]\\*"

Located at lib/strings.nix:544 in <nixpkgs>.

lib.strings.escapeNixIdentifier

Type: string -> string

Quotes a string if it can’t be used as an identifier directly.

s

Function argument

Example 79. lib.strings.escapeNixIdentifier usage example
escapeNixIdentifier "hello"
=> "hello"
escapeNixIdentifier "0abc"
=> "\"0abc\""

Located at lib/strings.nix:556 in <nixpkgs>.

lib.strings.escapeXML

Type: string -> string

Escapes a string such that it is safe to include verbatim in an XML document.

Example 80. lib.strings.escapeXML usage example
escapeXML ''"test" 'test' < & >''
=> "&quot;test&quot; &apos;test&apos; &lt; &amp; &gt;"

Located at lib/strings.nix:570 in <nixpkgs>.

lib.strings.toLower

Type: toLower :: string -> string

Converts an ASCII string to lower-case.

Example 81. lib.strings.toLower usage example
toLower "HOME"
=> "home"

Located at lib/strings.nix:589 in <nixpkgs>.

lib.strings.toUpper

Type: toUpper :: string -> string

Converts an ASCII string to upper-case.

Example 82. lib.strings.toUpper usage example
toUpper "home"
=> "HOME"

Located at lib/strings.nix:599 in <nixpkgs>.

lib.strings.addContextFrom

Appends string context from another string. This is an implementation detail of Nix and should be used carefully.

Strings in Nix carry an invisible context which is a list of strings representing store paths. If the string is later used in a derivation attribute, the derivation will properly populate the inputDrvs and inputSrcs.

a

Function argument

b

Function argument

Example 83. lib.strings.addContextFrom usage example
pkgs = import <nixpkgs> { };
addContextFrom pkgs.coreutils "bar"
=> "bar"

Located at lib/strings.nix:614 in <nixpkgs>.

lib.strings.splitString

Cut a string with a separator and produces a list of strings which were separated by this separator.

sep

Function argument

s

Function argument

Example 84. lib.strings.splitString usage example
splitString "." "foo.bar.baz"
=> [ "foo" "bar" "baz" ]
splitString "/" "/usr/local/bin"
=> [ "" "usr" "local" "bin" ]

Located at lib/strings.nix:625 in <nixpkgs>.

lib.strings.removePrefix

Type: string -> string -> string

Return a string without the specified prefix, if the prefix matches.

prefix

Prefix to remove if it matches

str

Input string

Example 85. lib.strings.removePrefix usage example
removePrefix "foo." "foo.bar.baz"
=> "bar.baz"
removePrefix "xxx" "foo.bar.baz"
=> "foo.bar.baz"

Located at lib/strings.nix:641 in <nixpkgs>.

lib.strings.removeSuffix

Type: string -> string -> string

Return a string without the specified suffix, if the suffix matches.

suffix

Suffix to remove if it matches

str

Input string

Example 86. lib.strings.removeSuffix usage example
removeSuffix "front" "homefront"
=> "home"
removeSuffix "xxx" "homefront"
=> "homefront"

Located at lib/strings.nix:674 in <nixpkgs>.

lib.strings.versionOlder

Return true if string v1 denotes a version older than v2.

v1

Function argument

v2

Function argument

Example 87. lib.strings.versionOlder usage example
versionOlder "1.1" "1.2"
=> true
versionOlder "1.1" "1.1"
=> false

Located at lib/strings.nix:705 in <nixpkgs>.

lib.strings.versionAtLeast

Return true if string v1 denotes a version equal to or newer than v2.

v1

Function argument

v2

Function argument

Example 88. lib.strings.versionAtLeast usage example
versionAtLeast "1.1" "1.0"
=> true
versionAtLeast "1.1" "1.1"
=> true
versionAtLeast "1.1" "1.2"
=> false

Located at lib/strings.nix:717 in <nixpkgs>.

lib.strings.getName

This function takes an argument that’s either a derivation or a derivation’s “name” attribute and extracts the name part from that argument.

Example 89. lib.strings.getName usage example
getName "youtube-dl-2016.01.01"
=> "youtube-dl"
getName pkgs.youtube-dl
=> "youtube-dl"

Located at lib/strings.nix:729 in <nixpkgs>.

lib.strings.getVersion

This function takes an argument that’s either a derivation or a derivation’s “name” attribute and extracts the version part from that argument.

Example 90. lib.strings.getVersion usage example
getVersion "youtube-dl-2016.01.01"
=> "2016.01.01"
getVersion pkgs.youtube-dl
=> "2016.01.01"

Located at lib/strings.nix:746 in <nixpkgs>.

lib.strings.nameFromURL

Extract name with version from URL. Ask for separator which is supposed to start extension.

url

Function argument

sep

Function argument

Example 91. lib.strings.nameFromURL usage example
nameFromURL "https://nixos.org/releases/nix/nix-1.7/nix-1.7-x86_64-linux.tar.bz2" "-"
=> "nix"
nameFromURL "https://nixos.org/releases/nix/nix-1.7/nix-1.7-x86_64-linux.tar.bz2" "_"
=> "nix-1.7-x86"

Located at lib/strings.nix:762 in <nixpkgs>.

lib.strings.cmakeOptionType

Type:

cmakeOptionType :: string -> string -> string -> string

@param feature The feature to be set
@param type The type of the feature to be set, as described in
            https://cmake.org/cmake/help/latest/command/set.html
            the possible values (case insensitive) are:
            BOOL FILEPATH PATH STRING INTERNAL
@param value The desired value

Create a “-D<feature>:<type>=<value>” string that can be passed to typical CMake invocations.

Example 92. lib.strings.cmakeOptionType usage example
cmakeOptionType "string" "ENGINE" "sdl2"
=> "-DENGINE:STRING=sdl2"

Located at lib/strings.nix:785 in <nixpkgs>.

lib.strings.cmakeBool

Type:

cmakeBool :: string -> bool -> string

@param condition The condition to be made true or false
@param flag The controlling flag of the condition

Create a -D<condition>={TRUE,FALSE} string that can be passed to typical CMake invocations.

condition

Function argument

flag

Function argument

Example 93. lib.strings.cmakeBool usage example
cmakeBool "ENABLE_STATIC_LIBS" false
=> "-DENABLESTATIC_LIBS:BOOL=FALSE"

Located at lib/strings.nix:805 in <nixpkgs>.

lib.strings.cmakeFeature

Type:

cmakeFeature :: string -> string -> string

@param condition The condition to be made true or false
@param flag The controlling flag of the condition

Create a -D<feature>:STRING=<value> string that can be passed to typical CMake invocations. This is the most typical usage, so it deserves a special case.

feature

Function argument

value

Function argument

Example 94. lib.strings.cmakeFeature usage example
cmakeFeature "MODULES" "badblock"
=> "-DMODULES:STRING=badblock"

Located at lib/strings.nix:823 in <nixpkgs>.

lib.strings.mesonOption

Type:

mesonOption :: string -> string -> string

@param feature The feature to be set
@param value The desired value

Create a -D<feature>=<value> string that can be passed to typical Meson invocations.

feature

Function argument

value

Function argument

Example 95. lib.strings.mesonOption usage example
mesonOption "engine" "opengl"
=> "-Dengine=opengl"

Located at lib/strings.nix:840 in <nixpkgs>.

lib.strings.mesonBool

Type:

mesonBool :: string -> bool -> string

@param condition The condition to be made true or false
@param flag The controlling flag of the condition

Create a -D<condition>={true,false} string that can be passed to typical Meson invocations.

condition

Function argument

flag

Function argument

Example 96. lib.strings.mesonBool usage example
mesonBool "hardened" true
=> "-Dhardened=true"
mesonBool "static" false
=> "-Dstatic=false"

Located at lib/strings.nix:859 in <nixpkgs>.

lib.strings.mesonEnable

Type:

mesonEnable :: string -> bool -> string

@param feature The feature to be enabled or disabled
@param flag The controlling flag

Create a -D<feature>={enabled,disabled} string that can be passed to typical Meson invocations.

feature

Function argument

flag

Function argument

Example 97. lib.strings.mesonEnable usage example
mesonEnable "docs" true
=> "-Ddocs=enabled"
mesonEnable "savage" false
=> "-Dsavage=disabled"

Located at lib/strings.nix:878 in <nixpkgs>.

lib.strings.enableFeature

Create an --{enable,disable}-<feature> string that can be passed to standard GNU Autoconf scripts.

flag

Function argument

feature

Function argument

Example 98. lib.strings.enableFeature usage example
enableFeature true "shared"
=> "--enable-shared"
enableFeature false "shared"
=> "--disable-shared"

Located at lib/strings.nix:892 in <nixpkgs>.

lib.strings.enableFeatureAs

Create an --{enable-<feature>=<value>,disable-<feature>} string that can be passed to standard GNU Autoconf scripts.

flag

Function argument

feature

Function argument

value

Function argument

Example 99. lib.strings.enableFeatureAs usage example
enableFeatureAs true "shared" "foo"
=> "--enable-shared=foo"
enableFeatureAs false "shared" (throw "ignored")
=> "--disable-shared"

Located at lib/strings.nix:906 in <nixpkgs>.

lib.strings.withFeature

Create an --{with,without}-<feature> string that can be passed to standard GNU Autoconf scripts.

flag

Function argument

feature

Function argument

Example 100. lib.strings.withFeature usage example
withFeature true "shared"
=> "--with-shared"
withFeature false "shared"
=> "--without-shared"

Located at lib/strings.nix:918 in <nixpkgs>.

lib.strings.withFeatureAs

Create an --{with-<feature>=<value>,without-<feature>} string that can be passed to standard GNU Autoconf scripts.

flag

Function argument

feature

Function argument

value

Function argument

Example 101. lib.strings.withFeatureAs usage example
withFeatureAs true "shared" "foo"
=> "--with-shared=foo"
withFeatureAs false "shared" (throw "ignored")
=> "--without-shared"

Located at lib/strings.nix:931 in <nixpkgs>.

lib.strings.fixedWidthString

Type: fixedWidthString :: int -> string -> string -> string

Create a fixed width string with additional prefix to match required width.

This function will fail if the input string is longer than the requested length.

width

Function argument

filler

Function argument

str

Function argument

Example 102. lib.strings.fixedWidthString usage example
fixedWidthString 5 "0" (toString 15)
=> "00015"

Located at lib/strings.nix:946 in <nixpkgs>.

lib.strings.fixedWidthNumber

Format a number adding leading zeroes up to fixed width.

width

Function argument

n

Function argument

Example 103. lib.strings.fixedWidthNumber usage example
fixedWidthNumber 5 15
=> "00015"

Located at lib/strings.nix:963 in <nixpkgs>.

lib.strings.floatToString

Convert a float to a string, but emit a warning when precision is lost during the conversion

float

Function argument

Example 104. lib.strings.floatToString usage example
floatToString 0.000001
=> "0.000001"
floatToString 0.0000001
=> trace: warning: Imprecise conversion from float to string 0.000000
   "0.000000"

Located at lib/strings.nix:975 in <nixpkgs>.

lib.strings.isCoercibleToString

Soft-deprecated function. While the original implementation is available as isConvertibleWithToString, consider using isStringLike instead, if suitable.

Located at lib/strings.nix:983 in <nixpkgs>.

lib.strings.isConvertibleWithToString

Check whether a list or other value can be passed to toString.

Many types of value are coercible to string this way, including int, float, null, bool, list of similarly coercible values.

Located at lib/strings.nix:992 in <nixpkgs>.

lib.strings.isStringLike

Check whether a value can be coerced to a string. The value must be a string, path, or attribute set.

String-like values can be used without explicit conversion in string interpolations and in most functions that expect a string.

x

Function argument

Located at lib/strings.nix:1005 in <nixpkgs>.

lib.strings.isStorePath

Check whether a value is a store path.

x

Function argument

Example 105. lib.strings.isStorePath usage example
isStorePath "/nix/store/d945ibfx9x185xf04b890y4f9g3cbb63-python-2.7.11/bin/python"
=> false
isStorePath "/nix/store/d945ibfx9x185xf04b890y4f9g3cbb63-python-2.7.11"
=> true
isStorePath pkgs.python
=> true
isStorePath [] || isStorePath 42 || isStorePath {} || …
=> false

Located at lib/strings.nix:1023 in <nixpkgs>.

lib.strings.toInt

Type: string -> int

Parse a string as an int. Does not support parsing of integers with preceding zero due to ambiguity between zero-padded and octal numbers. See toIntBase10.

Example 106. lib.strings.toInt usage example
toInt "1337"
=> 1337

toInt "-4"
=> -4

toInt " 123 "
=> 123

toInt "00024"
=> error: Ambiguity in interpretation of 00024 between octal and zero padded integer.

toInt "3.14"
=> error: floating point JSON numbers are not supported

Located at lib/strings.nix:1053 in <nixpkgs>.

lib.strings.toIntBase10

Type: string -> int

Parse a string as a base 10 int. This supports parsing of zero-padded integers.

Example 107. lib.strings.toIntBase10 usage example
toIntBase10 "1337"
=> 1337

toIntBase10 "-4"
=> -4

toIntBase10 " 123 "
=> 123

toIntBase10 "00024"
=> 24

toIntBase10 "3.14"
=> error: floating point JSON numbers are not supported

Located at lib/strings.nix:1106 in <nixpkgs>.

lib.strings.readPathsFromFile

Read a list of paths from file, relative to the rootPath. Lines beginning with # are treated as comments and ignored. Whitespace is significant.

NOTE: This function is not performant and should be avoided.

Example 108. lib.strings.readPathsFromFile usage example
readPathsFromFile /prefix
  ./pkgs/development/libraries/qt-5/5.4/qtbase/series
=> [ "/prefix/dlopen-resolv.patch" "/prefix/tzdir.patch"
     "/prefix/dlopen-libXcursor.patch" "/prefix/dlopen-openssl.patch"
     "/prefix/dlopen-dbus.patch" "/prefix/xdg-config-dirs.patch"
     "/prefix/nix-profiles-library-paths.patch"
     "/prefix/compose-search-path.patch" ]

Located at lib/strings.nix:1154 in <nixpkgs>.

lib.strings.fileContents

Type: fileContents :: path -> string

Read the contents of a file removing the trailing \n

file

Function argument

Example 109. lib.strings.fileContents usage example
$ echo "1.0" > ./version

fileContents ./version
=> "1.0"

Located at lib/strings.nix:1174 in <nixpkgs>.

lib.strings.sanitizeDerivationName

Type: sanitizeDerivationName :: String -> String

Creates a valid derivation name from a potentially invalid one.

Example 110. lib.strings.sanitizeDerivationName usage example
sanitizeDerivationName "../hello.bar # foo"
=> "-hello.bar-foo"
sanitizeDerivationName ""
=> "unknown"
sanitizeDerivationName pkgs.hello
=> "-nix-store-2g75chlbpxlrqn15zlby2dfh8hr9qwbk-hello-2.10"

Located at lib/strings.nix:1189 in <nixpkgs>.

lib.strings.levenshtein

Type: levenshtein :: string -> string -> int

Computes the Levenshtein distance between two strings. Complexity O(n*m) where n and m are the lengths of the strings. Algorithm adjusted from https://stackoverflow.com/a/9750974/6605742

a

Function argument

b

Function argument

Example 111. lib.strings.levenshtein usage example
levenshtein "foo" "foo"
=> 0
levenshtein "book" "hook"
=> 1
levenshtein "hello" "Heyo"
=> 3

Located at lib/strings.nix:1228 in <nixpkgs>.

lib.strings.commonPrefixLength

Returns the length of the prefix common to both strings.

a

Function argument

b

Function argument

Located at lib/strings.nix:1249 in <nixpkgs>.

lib.strings.commonSuffixLength

Returns the length of the suffix common to both strings.

a

Function argument

b

Function argument

Located at lib/strings.nix:1257 in <nixpkgs>.

lib.strings.levenshteinAtMost

Type: levenshteinAtMost :: int -> string -> string -> bool

Returns whether the levenshtein distance between two strings is at most some value Complexity is O(min(n,m)) for k <= 2 and O(n*m) otherwise

Example 112. lib.strings.levenshteinAtMost usage example
levenshteinAtMost 0 "foo" "foo"
=> true
levenshteinAtMost 1 "foo" "boa"
=> false
levenshteinAtMost 2 "foo" "boa"
=> true
levenshteinAtMost 2 "This is a sentence" "this is a sentense."
=> false
levenshteinAtMost 3 "This is a sentence" "this is a sentense."
=> true

Located at lib/strings.nix:1281 in <nixpkgs>.

lib.versions: version string functions

Version string functions.

lib.versions.splitVersion

Break a version string into its component parts.

Example 113. lib.versions.splitVersion usage example
splitVersion "1.2.3"
=> ["1" "2" "3"]

Located at lib/versions.nix:12 in <nixpkgs>.

lib.versions.major

Get the major version string from a string.

v

Function argument

Example 114. lib.versions.major usage example
major "1.2.3"
=> "1"

Located at lib/versions.nix:20 in <nixpkgs>.

lib.versions.minor

Get the minor version string from a string.

v

Function argument

Example 115. lib.versions.minor usage example
minor "1.2.3"
=> "2"

Located at lib/versions.nix:28 in <nixpkgs>.

lib.versions.patch

Get the patch version string from a string.

v

Function argument

Example 116. lib.versions.patch usage example
patch "1.2.3"
=> "3"

Located at lib/versions.nix:36 in <nixpkgs>.

lib.versions.majorMinor

Get string of the first two parts (major and minor) of a version string.

v

Function argument

Example 117. lib.versions.majorMinor usage example
majorMinor "1.2.3"
=> "1.2"

Located at lib/versions.nix:45 in <nixpkgs>.

lib.versions.pad

Pad a version string with zeros to match the given number of components.

n

Function argument

version

Function argument

Example 118. lib.versions.pad usage example
pad 3 "1.2"
=> "1.2.0"
pad 3 "1.3-rc1"
=> "1.3.0-rc1"
pad 3 "1.2.3.4"
=> "1.2.3"

Located at lib/versions.nix:59 in <nixpkgs>.

lib.trivial: miscellaneous functions

lib.trivial.id

The identity function For when you need a function that does “nothing”.

Inputs
x

The value to return

Type
id :: a -> a

Located at lib/trivial.nix:36 in <nixpkgs>.

lib.trivial.const

The constant function

Ignores the second argument. If called with only one argument, constructs a function that always returns a static value.

Inputs
x

Value to return

y

Value to ignore

Type
const :: a -> b -> a
Examples
Example 119. lib.trivial.const usage example
let f = const 5; in f 10
=> 5

Located at lib/trivial.nix:72 in <nixpkgs>.

lib.trivial.pipe

Pipes a value through a list of functions, left to right.

Inputs
value

Value to start piping.

fns

List of functions to apply sequentially.

Type
pipe :: a -> [<functions>] -> <return type of last function>
Examples
Example 120. lib.trivial.pipe usage example
pipe 2 [
    (x: x + 2)  # 2 + 2 = 4
    (x: x * 2)  # 4 * 2 = 8
  ]
=> 8

# ideal to do text transformations
pipe [ "a/b" "a/c" ] [

  # create the cp command
  (map (file: ''cp "${src}/${file}" $out\n''))

  # concatenate all commands into one string
  lib.concatStrings

  # make that string into a nix derivation
  (pkgs.runCommand "copy-to-out" {})

]
=> <drv which copies all files to $out>

The output type of each function has to be the input type
of the next function, and the last function returns the
final value.

Located at lib/trivial.nix:128 in <nixpkgs>.

lib.trivial.concat

Concatenate two lists

Inputs
x

1. Function argument

y

2. Function argument

Type
concat :: [a] -> [a] -> [a]
Examples
Example 121. lib.trivial.concat usage example
concat [ 1 2 ] [ 3 4 ]
=> [ 1 2 3 4 ]

Located at lib/trivial.nix:168 in <nixpkgs>.

lib.trivial.or

boolean “or”

Inputs
x

1. Function argument

y

2. Function argument

Located at lib/trivial.nix:184 in <nixpkgs>.

lib.trivial.and

boolean “and”

Inputs
x

1. Function argument

y

2. Function argument

Located at lib/trivial.nix:200 in <nixpkgs>.

lib.trivial.xor

boolean “exclusive or”

Inputs
x

1. Function argument

y

2. Function argument

Located at lib/trivial.nix:218 in <nixpkgs>.

lib.trivial.bitNot

bitwise “not”

Located at lib/trivial.nix:223 in <nixpkgs>.

lib.trivial.boolToString

Convert a boolean to a string.

This function uses the strings “true” and “false” to represent boolean values. Calling toString on a bool instead returns “1” and “” (sic!).

Inputs
b

1. Function argument

Type
boolToString :: bool -> string

Located at lib/trivial.nix:245 in <nixpkgs>.

lib.trivial.mergeAttrs

Merge two attribute sets shallowly, right side trumps left

mergeAttrs :: attrs -> attrs -> attrs

Inputs
x

Left attribute set

y

Right attribute set (higher precedence for equal keys)

Examples
Example 122. lib.trivial.mergeAttrs usage example
mergeAttrs { a = 1; b = 2; } { b = 3; c = 4; }
=> { a = 1; b = 3; c = 4; }

Located at lib/trivial.nix:275 in <nixpkgs>.

lib.trivial.flip

Flip the order of the arguments of a binary function.

Inputs
f

1. Function argument

a

2. Function argument

b

3. Function argument

Type
flip :: (a -> b -> c) -> (b -> a -> c)
Examples
Example 123. lib.trivial.flip usage example
flip concat [1] [2]
=> [ 2 1 ]

Located at lib/trivial.nix:314 in <nixpkgs>.

lib.trivial.mapNullable

Apply function if the supplied argument is non-null.

Inputs
f

Function to call

a

Argument to check for null before passing it to f

Examples
Example 124. lib.trivial.mapNullable usage example
mapNullable (x: x+1) null
=> null
mapNullable (x: x+1) 22
=> 23

Located at lib/trivial.nix:344 in <nixpkgs>.

lib.trivial.version

Returns the current full nixpkgs version number.

Located at lib/trivial.nix:360 in <nixpkgs>.

lib.trivial.release

Returns the current nixpkgs release number as string.

Located at lib/trivial.nix:365 in <nixpkgs>.

lib.trivial.oldestSupportedRelease

The latest release that is supported, at the time of release branch-off, if applicable.

Ideally, out-of-tree modules should be able to evaluate cleanly with all supported Nixpkgs versions (master, release and old release until EOL). So if possible, deprecation warnings should take effect only when all out-of-tree expressions/libs/modules can upgrade to the new way without losing support for supported Nixpkgs versions.

This release number allows deprecation warnings to be implemented such that they take effect as soon as the oldest release reaches end of life.

Located at lib/trivial.nix:380 in <nixpkgs>.

lib.trivial.isInOldestRelease

Whether a feature is supported in all supported releases (at the time of release branch-off, if applicable). See oldestSupportedRelease.

Inputs
release

Release number of feature introduction as an integer, e.g. 2111 for 21.11. Set it to the upcoming release, matching the nixpkgs/.version file.

Located at lib/trivial.nix:396 in <nixpkgs>.

lib.trivial.codeName

Returns the current nixpkgs release code name.

On each release the first letter is bumped and a new animal is chosen starting with that new letter.

Located at lib/trivial.nix:406 in <nixpkgs>.

lib.trivial.versionSuffix

Returns the current nixpkgs version suffix as string.

Located at lib/trivial.nix:411 in <nixpkgs>.

lib.trivial.revisionWithDefault

Attempts to return the the current revision of nixpkgs and returns the supplied default value otherwise.

Inputs
default

Default value to return if revision can not be determined

Type
revisionWithDefault :: string -> string

Located at lib/trivial.nix:434 in <nixpkgs>.

lib.trivial.inNixShell

Determine whether the function is being called from inside a Nix shell.

Type
inNixShell :: bool

Located at lib/trivial.nix:456 in <nixpkgs>.

lib.trivial.inPureEvalMode

Determine whether the function is being called from inside pure-eval mode by seeing whether builtins contains currentSystem. If not, we must be in pure-eval mode.

Type
inPureEvalMode :: bool

Located at lib/trivial.nix:469 in <nixpkgs>.

lib.trivial.min

Return minimum of two numbers.

Inputs
x

1. Function argument

y

2. Function argument

Located at lib/trivial.nix:487 in <nixpkgs>.

lib.trivial.max

Return maximum of two numbers.

Inputs
x

1. Function argument

y

2. Function argument

Located at lib/trivial.nix:503 in <nixpkgs>.

lib.trivial.mod

Integer modulus

Inputs
base

1. Function argument

int

2. Function argument

Examples
Example 125. lib.trivial.mod usage example
mod 11 10
=> 1
mod 1 10
=> 1

Located at lib/trivial.nix:533 in <nixpkgs>.

lib.trivial.compare

C-style comparisons

a < b, compare a b => -1 a == b, compare a b => 0 a > b, compare a b => 1

Inputs
a

1. Function argument

b

2. Function argument

Located at lib/trivial.nix:556 in <nixpkgs>.

lib.trivial.splitByAndCompare

Split type into two subtypes by predicate p, take all elements of the first subtype to be less than all the elements of the second subtype, compare elements of a single subtype with yes and no respectively.

Inputs
p

Predicate

yes

Comparison function if predicate holds for both values

no

Comparison function if predicate holds for neither value

a

First value to compare

b

Second value to compare

Type
(a -> bool) -> (a -> a -> int) -> (a -> a -> int) -> (a -> a -> int)
Examples
Example 126. lib.trivial.splitByAndCompare usage example
let cmp = splitByAndCompare (hasPrefix "foo") compare compare; in

cmp "a" "z" => -1
cmp "fooa" "fooz" => -1

cmp "f" "a" => 1
cmp "fooa" "a" => -1
# while
compare "fooa" "a" => 1

Located at lib/trivial.nix:616 in <nixpkgs>.

lib.trivial.importJSON

Reads a JSON file.

Examples
Example 127. lib.trivial.importJSON usage example

example.json

{
  "title": "Example JSON",
  "hello": {
    "world": "foo",
    "bar": {
      "foobar": true
    }
  }
}
importJSON ./example.json
=> {
  title = "Example JSON";
  hello = {
    world = "foo";
    bar = {
      foobar = true;
    };
  };
}

Inputs
path

1. Function argument

Type
importJSON :: path -> any

Located at lib/trivial.nix:670 in <nixpkgs>.

lib.trivial.importTOML

Reads a TOML file.

Examples
Example 128. lib.trivial.importTOML usage example

example.toml

title = "TOML Example"

[hello]
world = "foo"

[hello.bar]
foobar = true
importTOML ./example.toml
=> {
  title = "TOML Example";
  hello = {
    world = "foo";
    bar = {
      foobar = true;
    };
  };
}

Inputs
path

1. Function argument

Type
importTOML :: path -> any

Located at lib/trivial.nix:718 in <nixpkgs>.

lib.trivial.warn

Print a warning before returning the second argument. This function behaves like builtins.trace, but requires a string message and formats it as a warning, including the warning: prefix.

To get a call stack trace and abort evaluation, set the environment variable NIX_ABORT_ON_WARN=true and set the Nix options --option pure-eval false --show-trace

Inputs
msg

Warning message to print.

val

Value to return as-is.

Type
string -> a -> a

Located at lib/trivial.nix:760 in <nixpkgs>.

lib.trivial.warnIf

Like warn, but only warn when the first argument is true.

Inputs
cond

1. Function argument

msg

2. Function argument

val

Value to return as-is.

Type
bool -> string -> a -> a

Located at lib/trivial.nix:789 in <nixpkgs>.

lib.trivial.warnIfNot

Like warnIf, but negated (warn if the first argument is false).

Inputs
cond

1. Function argument

msg

2. Function argument

val

Value to return as-is.

Type
bool -> string -> a -> a

Located at lib/trivial.nix:815 in <nixpkgs>.

lib.trivial.throwIfNot

Like the assert b; e expression, but with a custom error message and without the semicolon.

If true, return the identity function, r: r.

If false, throw the error message.

Calls can be juxtaposed using function application, as (r: r) a = a, so (r: r) (r: r) a = a, and so forth.

Inputs
cond

1. Function argument

msg

2. Function argument

Type
bool -> string -> a -> a
Examples
Example 129. lib.trivial.throwIfNot usage example
throwIfNot (lib.isList overlays) "The overlays argument to nixpkgs must be a list."
lib.foldr (x: throwIfNot (lib.isFunction x) "All overlays passed to nixpkgs must be functions.") (r: r) overlays
pkgs

Located at lib/trivial.nix:857 in <nixpkgs>.

lib.trivial.throwIf

Like throwIfNot, but negated (throw if the first argument is true).

Inputs
cond

1. Function argument

msg

2. Function argument

Type
bool -> string -> a -> a

Located at lib/trivial.nix:879 in <nixpkgs>.

lib.trivial.checkListOfEnum

Check if the elements in a list are valid values from a enum, returning the identity function, or throwing an error message otherwise.

Inputs
msg

1. Function argument

valid

2. Function argument

given

3. Function argument

Type
String -> List ComparableVal -> List ComparableVal -> a -> a
Examples
Example 130. lib.trivial.checkListOfEnum usage example
let colorVariants = ["bright" "dark" "black"]
in checkListOfEnum "color variants" [ "standard" "light" "dark" ] colorVariants;
=>
error: color variants: bright, black unexpected; valid ones: standard, light, dark

Located at lib/trivial.nix:918 in <nixpkgs>.

lib.trivial.setFunctionArgs

Add metadata about expected function arguments to a function. The metadata should match the format given by builtins.functionArgs, i.e. a set from expected argument to a bool representing whether that argument has a default or not. setFunctionArgs : (a → b) → Map String Bool → (a → b)

This function is necessary because you can’t dynamically create a function of the { a, b ? foo, … }: format, but some facilities like callPackage expect to be able to query expected arguments.

Inputs
f

1. Function argument

args

2. Function argument

Located at lib/trivial.nix:953 in <nixpkgs>.

lib.trivial.functionArgs

Extract the expected function arguments from a function. This works both with nix-native { a, b ? foo, … }: style functions and functions with args set with ‘setFunctionArgs’. It has the same return type and semantics as builtins.functionArgs. setFunctionArgs : (a → b) → Map String Bool.

Inputs
f

1. Function argument

Located at lib/trivial.nix:973 in <nixpkgs>.

lib.trivial.isFunction

Check whether something is a function or something annotated with function args.

Inputs
f

1. Function argument

Located at lib/trivial.nix:989 in <nixpkgs>.

lib.trivial.mirrorFunctionArgs

mirrorFunctionArgs f g creates a new function g' with the same behavior as g (g' x == g x) but its function arguments mirroring f (lib.functionArgs g' == lib.functionArgs f).

Inputs
f

Function to provide the argument metadata

g

Function to set the argument metadata to

Type
mirrorFunctionArgs :: (a -> b) -> (a -> c) -> (a -> c)
Examples
Example 131. lib.trivial.mirrorFunctionArgs usage example
addab = {a, b}: a + b
addab { a = 2; b = 4; }
=> 6
lib.functionArgs addab
=> { a = false; b = false; }
addab1 = attrs: addab attrs + 1
addab1 { a = 2; b = 4; }
=> 7
lib.functionArgs addab1
=> { }
addab1' = lib.mirrorFunctionArgs addab addab1
addab1' { a = 2; b = 4; }
=> 7
lib.functionArgs addab1'
=> { a = false; b = false; }

Located at lib/trivial.nix:1037 in <nixpkgs>.

lib.trivial.toFunction

Turns any non-callable values into constant functions. Returns callable values as is.

Inputs
v

Any value

Examples
Example 132. lib.trivial.toFunction usage example
nix-repl> lib.toFunction 1 2
1

nix-repl> lib.toFunction (x: x + 1) 2
3

Located at lib/trivial.nix:1071 in <nixpkgs>.

lib.trivial.toHexString

Convert the given positive integer to a string of its hexadecimal representation. For example:

toHexString 0 => “0”

toHexString 16 => “10”

toHexString 250 => “FA”

Located at lib/trivial.nix:1087 in <nixpkgs>.

lib.trivial.toBaseDigits

toBaseDigits base i converts the positive integer i to a list of its digits in the given base. For example:

toBaseDigits 10 123 => [ 1 2 3 ]

toBaseDigits 2 6 => [ 1 1 0 ]

toBaseDigits 16 250 => [ 15 10 ]

Inputs
base

1. Function argument

i

2. Function argument

Located at lib/trivial.nix:1123 in <nixpkgs>.

lib.fixedPoints: explicit recursion functions

lib.fixedPoints.fix

fix f computes the fixed point of the given function f. In other words, the return value is x in x = f x.

f must be a lazy function. This means that x must be a value that can be partially evaluated, such as an attribute set, a list, or a function. This way, f can use one part of x to compute another part.

Relation to syntactic recursion

This section explains fix by refactoring from syntactic recursion to a call of fix instead.

For context, Nix lets you define attributes in terms of other attributes syntactically using the rec { } syntax.

nix-repl> rec {
  foo = "foo";
  bar = "bar";
  foobar = foo + bar;
}
{ bar = "bar"; foo = "foo"; foobar = "foobar"; }

This is convenient when constructing a value to pass to a function for example, but an equivalent effect can be achieved with the let binding syntax:

nix-repl> let self = {
  foo = "foo";
  bar = "bar";
  foobar = self.foo + self.bar;
}; in self
{ bar = "bar"; foo = "foo"; foobar = "foobar"; }

But in general you can get more reuse out of let bindings by refactoring them to a function.

nix-repl> f = self: {
  foo = "foo";
  bar = "bar";
  foobar = self.foo + self.bar;
}

This is where fix comes in, it contains the syntactic recursion that’s not in f anymore.

nix-repl> fix = f:
  let self = f self; in self;

By applying fix we get the final result.

nix-repl> fix f
{ bar = "bar"; foo = "foo"; foobar = "foobar"; }

Such a refactored f using fix is not useful by itself. See extends for an example use case. There self is also often called final.

Inputs
f

1. Function argument

Type
fix :: (a -> a) -> a
Examples
Example 133. lib.fixedPoints.fix usage example
fix (self: { foo = "foo"; bar = "bar"; foobar = self.foo + self.bar; })
=> { bar = "bar"; foo = "foo"; foobar = "foobar"; }

fix (self: [ 1 2 (elemAt self 0 + elemAt self 1) ])
=> [ 1 2 3 ]

Located at lib/fixed-points.nix:93 in <nixpkgs>.

lib.fixedPoints.fix'

A variant of fix that records the original recursive attribute set in the result, in an attribute named __unfix__.

This is useful in combination with the extends function to implement deep overriding.

Inputs
f

1. Function argument

Located at lib/fixed-points.nix:109 in <nixpkgs>.

lib.fixedPoints.converge

Return the fixpoint that f converges to when called iteratively, starting with the input x.

nix-repl> converge (x: x / 2) 16
0
Inputs
f

1. Function argument

x

2. Function argument

Type
(a -> a) -> a -> a

Located at lib/fixed-points.nix:137 in <nixpkgs>.

lib.fixedPoints.extends

Extend a function using an overlay.

Overlays allow modifying and extending fixed-point functions, specifically ones returning attribute sets. A fixed-point function is a function which is intended to be evaluated by passing the result of itself as the argument. This is possible due to Nix’s lazy evaluation.

A fixed-point function returning an attribute set has the form

final: { # attributes }

where final refers to the lazily evaluated attribute set returned by the fixed-point function.

An overlay to such a fixed-point function has the form

final: prev: { # attributes }

where prev refers to the result of the original function to final, and final is the result of the composition of the overlay and the original function.

Applying an overlay is done with extends:

let
  f = final: { # attributes };
  overlay = final: prev: { # attributes };
in extends overlay f;

To get the value of final, use lib.fix:

let
  f = final: { # attributes };
  overlay = final: prev: { # attributes };
  g = extends overlay f;
in fix g

Note

The argument to the given fixed-point function after applying an overlay will not refer to its own return value, but rather to the value after evaluating the overlay function.

The given fixed-point function is called with a separate argument than if it was evaluated with lib.fix.

Example 134. Extend a fixed-point function with an overlay

Define a fixed-point function f that expects its own output as the argument final:

f = final: {
  # Constant value a
  a = 1;

  # b depends on the final value of a, available as final.a
  b = final.a + 2;
}

Evaluate this using lib.fix to get the final result:

fix f
=> { a = 1; b = 3; }

An overlay represents a modification or extension of such a fixed-point function. Here’s an example of an overlay:

overlay = final: prev: {
  # Modify the previous value of a, available as prev.a
  a = prev.a + 10;

  # Extend the attribute set with c, letting it depend on the final values of a and b
  c = final.a + final.b;
}

Use extends overlay f to apply the overlay to the fixed-point function f. This produces a new fixed-point function g with the combined behavior of f and overlay:

g = extends overlay f

The result is a function, so we can’t print it directly, but it’s the same as:

g' = final: {
  # The constant from f, but changed with the overlay
  a = 1 + 10;

  # Unchanged from f
  b = final.a + 2;

  # Extended in the overlay
  c = final.a + final.b;
}

Evaluate this using lib.fix again to get the final result:

fix g
=> { a = 11; b = 13; c = 24; }

Inputs
overlay

The overlay to apply to the fixed-point function

f

The fixed-point function

Type
extends :: (Attrs -> Attrs -> Attrs) # The overlay to apply to the fixed-point function
        -> (Attrs -> Attrs) # A fixed-point function
        -> (Attrs -> Attrs) # The resulting fixed-point function
Examples
Example 135. lib.fixedPoints.extends usage example
f = final: { a = 1; b = final.a + 2; }

fix f
=> { a = 1; b = 3; }

fix (extends (final: prev: { a = prev.a + 10; }) f)
=> { a = 11; b = 13; }

fix (extends (final: prev: { b = final.a + 5; }) f)
=> { a = 1; b = 6; }

fix (extends (final: prev: { c = final.a + final.b; }) f)
=> { a = 1; b = 3; c = 4; }

Located at lib/fixed-points.nix:301 in <nixpkgs>.

lib.fixedPoints.composeExtensions

Compose two extending functions of the type expected by ‘extends’ into one where changes made in the first are available in the ‘super’ of the second

Inputs
f

1. Function argument

g

2. Function argument

final

3. Function argument

prev

4. Function argument

Located at lib/fixed-points.nix:337 in <nixpkgs>.

lib.fixedPoints.composeManyExtensions

Compose several extending functions of the type expected by ‘extends’ into one where changes made in preceding functions are made available to subsequent ones.

composeManyExtensions : [packageSet -> packageSet -> packageSet] -> packageSet -> packageSet -> packageSet
                          ^final        ^prev         ^overrides     ^final        ^prev         ^overrides

Located at lib/fixed-points.nix:353 in <nixpkgs>.

lib.fixedPoints.makeExtensible

Create an overridable, recursive attribute set. For example:

nix-repl> obj = makeExtensible (self: { })

nix-repl> obj
{ __unfix__ = «lambda»; extend = «lambda»; }

nix-repl> obj = obj.extend (self: super: { foo = "foo"; })

nix-repl> obj
{ __unfix__ = «lambda»; extend = «lambda»; foo = "foo"; }

nix-repl> obj = obj.extend (self: super: { foo = super.foo + " + "; bar = "bar"; foobar = self.foo + self.bar; })

nix-repl> obj
{ __unfix__ = «lambda»; bar = "bar"; extend = «lambda»; foo = "foo + "; foobar = "foo + bar"; }

Located at lib/fixed-points.nix:376 in <nixpkgs>.

lib.fixedPoints.makeExtensibleWithCustomName

Same as makeExtensible but the name of the extending attribute is customized.

Inputs
extenderName

1. Function argument

rattrs

2. Function argument

Located at lib/fixed-points.nix:393 in <nixpkgs>.

lib.lists: list manipulation functions

General list operations.

lib.lists.singleton

Create a list consisting of a single element. singleton x is sometimes more convenient with respect to indentation than [x] when x spans multiple lines.

Inputs
x

1. Function argument

Type
singleton :: a -> [a]
Examples
Example 136. lib.lists.singleton usage example
singleton "foo"
=> [ "foo" ]

Located at lib/lists.nix:42 in <nixpkgs>.

lib.lists.forEach

Apply the function to each element in the list. Same as map, but arguments flipped.

Inputs
xs

1. Function argument

f

2. Function argument

Type
forEach :: [a] -> (a -> b) -> [b]
Examples
Example 137. lib.lists.forEach usage example
forEach [ 1 2 ] (x:
  toString x
)
=> [ "1" "2" ]

Located at lib/lists.nix:77 in <nixpkgs>.

lib.lists.foldr

“right fold” a binary function op between successive elements of list with nul as the starting value, i.e., foldr op nul [x_1 x_2 ... x_n] == op x_1 (op x_2 ... (op x_n nul)).

Inputs
op

1. Function argument

nul

2. Function argument

list

3. Function argument

Type
foldr :: (a -> b -> b) -> b -> [a] -> b
Examples
Example 138. lib.lists.foldr usage example
concat = foldr (a: b: a + b) "z"
concat [ "a" "b" "c" ]
=> "abcz"
# different types
strange = foldr (int: str: toString (int + 1) + str) "a"
strange [ 1 2 3 4 ]
=> "2345a"

Located at lib/lists.nix:121 in <nixpkgs>.

lib.lists.fold

fold is an alias of foldr for historic reasons

Located at lib/lists.nix:134 in <nixpkgs>.

lib.lists.foldl

“left fold”, like foldr, but from the left:

foldl op nul [x_1 x_2 ... x_n] == op (... (op (op nul x_1) x_2) ... x_n).

Inputs
op

1. Function argument

nul

2. Function argument

list

3. Function argument

Type
foldl :: (b -> a -> b) -> b -> [a] -> b
Examples
Example 139. lib.lists.foldl usage example
lconcat = foldl (a: b: a + b) "z"
lconcat [ "a" "b" "c" ]
=> "zabc"
# different types
lstrange = foldl (str: int: str + toString (int + 1)) "a"
lstrange [ 1 2 3 4 ]
=> "a2345"

Located at lib/lists.nix:178 in <nixpkgs>.

lib.lists.foldl'

Reduce a list by applying a binary operator from left to right, starting with an initial accumulator.

Before each application of the operator, the accumulator value is evaluated. This behavior makes this function stricter than foldl.

Unlike builtins.foldl', the initial accumulator argument is evaluated before the first iteration.

A call like

foldl' op acc₀ [ x₀ x₁ x₂ ... xₙ₋₁ xₙ ]

is (denotationally) equivalent to the following, but with the added benefit that foldl' itself will never overflow the stack.

let
  acc₁   = builtins.seq acc₀   (op acc₀   x₀  );
  acc₂   = builtins.seq acc₁   (op acc₁   x₁  );
  acc₃   = builtins.seq acc₂   (op acc₂   x₂  );
  ...
  accₙ   = builtins.seq accₙ₋₁ (op accₙ₋₁ xₙ₋₁);
  accₙ₊₁ = builtins.seq accₙ   (op accₙ   xₙ  );
in
accₙ₊₁

# Or ignoring builtins.seq
op (op (... (op (op (op acc₀ x₀) x₁) x₂) ...) xₙ₋₁) xₙ
Inputs
op

The binary operation to run, where the two arguments are:

  1. acc: The current accumulator value: Either the initial one for the first iteration, or the result of the previous iteration

  2. x: The corresponding list element for this iteration

acc

The initial accumulator value.

The accumulator value is evaluated in any case before the first iteration starts.

To avoid evaluation even before the list argument is given an eta expansion can be used:

list: lib.foldl' op acc list
list

The list to fold

Type
foldl' :: (acc -> x -> acc) -> acc -> [x] -> acc
Examples
Example 140. lib.lists.foldl' usage example
foldl' (acc: x: acc + x) 0 [1 2 3]
=> 6

Located at lib/lists.nix:262 in <nixpkgs>.

lib.lists.imap0

Map with index starting from 0

Inputs
f

1. Function argument

list

2. Function argument

Type
imap0 :: (int -> a -> b) -> [a] -> [b]
Examples
Example 141. lib.lists.imap0 usage example
imap0 (i: v: "${v}-${toString i}") ["a" "b"]
=> [ "a-0" "b-1" ]

Located at lib/lists.nix:301 in <nixpkgs>.

lib.lists.imap1

Map with index starting from 1

Inputs
f

1. Function argument

list

2. Function argument

Type
imap1 :: (int -> a -> b) -> [a] -> [b]
Examples
Example 142. lib.lists.imap1 usage example
imap1 (i: v: "${v}-${toString i}") ["a" "b"]
=> [ "a-1" "b-2" ]

Located at lib/lists.nix:334 in <nixpkgs>.

lib.lists.ifilter0

Filter a list for elements that satisfy a predicate function. The predicate function is called with both the index and value for each element. It must return true/false to include/exclude a given element in the result. This function is strict in the result of the predicate function for each element. This function has O(n) complexity.

Also see builtins.filter (available as lib.lists.filter), which can be used instead when the index isn’t needed.

Inputs
ipred

The predicate function, it takes two arguments:

    1. (int): the index of the element.

    1. (a): the value of the element.

It must return true/false to include/exclude a given element from the result.

list

The list to filter using the predicate.

Type
ifilter0 :: (int -> a -> bool) -> [a] -> [a]
Examples
Example 143. lib.lists.ifilter0 usage example
ifilter0 (i: v: i == 0 || v > 2) [ 1 2 3 ]
=> [ 1 3 ]

Located at lib/lists.nix:375 in <nixpkgs>.

lib.lists.concatMap

Map and concatenate the result.

Type
concatMap :: (a -> [b]) -> [a] -> [b]
Examples
Example 144. lib.lists.concatMap usage example
concatMap (x: [x] ++ ["z"]) ["a" "b"]
=> [ "a" "z" "b" "z" ]

Located at lib/lists.nix:404 in <nixpkgs>.

lib.lists.flatten

Flatten the argument into a single list; that is, nested lists are spliced into the top-level lists.

Inputs
x

1. Function argument

Examples
Example 145. lib.lists.flatten usage example
flatten [1 [2 [3] 4] 5]
=> [1 2 3 4 5]
flatten 1
=> [1]

Located at lib/lists.nix:431 in <nixpkgs>.

lib.lists.remove

Remove elements equal to ‘e’ from a list. Useful for buildInputs.

Inputs
e

Element to remove from list

list

The list

Type
remove :: a -> [a] -> [a]
Examples
Example 146. lib.lists.remove usage example
remove 3 [ 1 3 4 3 ]
=> [ 1 4 ]

Located at lib/lists.nix:467 in <nixpkgs>.

lib.lists.findSingle

Find the sole element in the list matching the specified predicate.

Returns default if no such element exists, or multiple if there are multiple matching elements.

Inputs
pred

Predicate

default

Default value to return if element was not found.

multiple

Default value to return if more than one element was found

list

Input list

Type
findSingle :: (a -> bool) -> a -> a -> [a] -> a
Examples
Example 147. lib.lists.findSingle usage example
findSingle (x: x == 3) "none" "multiple" [ 1 3 3 ]
=> "multiple"
findSingle (x: x == 3) "none" "multiple" [ 1 3 ]
=> 3
findSingle (x: x == 3) "none" "multiple" [ 1 9 ]
=> "none"

Located at lib/lists.nix:517 in <nixpkgs>.

lib.lists.findFirstIndex

Find the first index in the list matching the specified predicate or return default if no such element exists.

Inputs
pred

Predicate

default

Default value to return

list

Input list

Type
findFirstIndex :: (a -> Bool) -> b -> [a] -> (Int | b)
Examples
Example 148. lib.lists.findFirstIndex usage example
findFirstIndex (x: x > 3) null [ 0 6 4 ]
=> 1
findFirstIndex (x: x > 9) null [ 0 6 4 ]
=> null

Located at lib/lists.nix:564 in <nixpkgs>.

lib.lists.findFirst

Find the first element in the list matching the specified predicate or return default if no such element exists.

Inputs
pred

Predicate

default

Default value to return

list

Input list

Type
findFirst :: (a -> bool) -> a -> [a] -> a
Examples
Example 149. lib.lists.findFirst usage example
findFirst (x: x > 3) 7 [ 1 6 4 ]
=> 6
findFirst (x: x > 9) 7 [ 1 6 4 ]
=> 7

Located at lib/lists.nix:638 in <nixpkgs>.

lib.lists.any

Return true if function pred returns true for at least one element of list.

Inputs
pred

Predicate

list

Input list

Type
any :: (a -> bool) -> [a] -> bool
Examples
Example 150. lib.lists.any usage example
any isString [ 1 "a" { } ]
=> true
any isString [ 1 { } ]
=> false

Located at lib/lists.nix:683 in <nixpkgs>.

lib.lists.all

Return true if function pred returns true for all elements of list.

Inputs
pred

Predicate

list

Input list

Type
all :: (a -> bool) -> [a] -> bool
Examples
Example 151. lib.lists.all usage example
all (x: x < 3) [ 1 2 ]
=> true
all (x: x < 3) [ 1 2 3 ]
=> false

Located at lib/lists.nix:718 in <nixpkgs>.

lib.lists.count

Count how many elements of list match the supplied predicate function.

Inputs
pred

Predicate

Type
count :: (a -> bool) -> [a] -> int
Examples
Example 152. lib.lists.count usage example
count (x: x == 3) [ 3 2 3 4 6 ]
=> 2

Located at lib/lists.nix:747 in <nixpkgs>.

lib.lists.optional

Return a singleton list or an empty list, depending on a boolean value. Useful when building lists with optional elements (e.g. ++ optional (system == "i686-linux") firefox).

Inputs
cond

1. Function argument

elem

2. Function argument

Type
optional :: bool -> a -> [a]
Examples
Example 153. lib.lists.optional usage example
optional true "foo"
=> [ "foo" ]
optional false "foo"
=> [ ]

Located at lib/lists.nix:784 in <nixpkgs>.

lib.lists.optionals

Return a list or an empty list, depending on a boolean value.

Inputs
cond

Condition

elems

List to return if condition is true

Type
optionals :: bool -> [a] -> [a]
Examples
Example 154. lib.lists.optionals usage example
optionals true [ 2 3 ]
=> [ 2 3 ]
optionals false [ 2 3 ]
=> [ ]

Located at lib/lists.nix:818 in <nixpkgs>.

lib.lists.toList

If argument is a list, return it; else, wrap it in a singleton list. If you’re using this, you should almost certainly reconsider if there isn’t a more “well-typed” approach.

Inputs
x

1. Function argument

Examples
Example 155. lib.lists.toList usage example
toList [ 1 2 ]
=> [ 1 2 ]
toList "hi"
=> [ "hi "]

Located at lib/lists.nix:847 in <nixpkgs>.

lib.lists.range

Return a list of integers from first up to and including last.

Inputs
first

First integer in the range

last

Last integer in the range

Type
range :: int -> int -> [int]
Examples
Example 156. lib.lists.range usage example
range 2 4
=> [ 2 3 4 ]
range 3 2
=> [ ]

Located at lib/lists.nix:881 in <nixpkgs>.

lib.lists.replicate

Return a list with n copies of an element.

Inputs
n

1. Function argument

elem

2. Function argument

Type
replicate :: int -> a -> [a]
Examples
Example 157. lib.lists.replicate usage example
replicate 3 "a"
=> [ "a" "a" "a" ]
replicate 2 true
=> [ true true ]

Located at lib/lists.nix:921 in <nixpkgs>.

lib.lists.partition

Splits the elements of a list in two lists, right and wrong, depending on the evaluation of a predicate.

Inputs
pred

Predicate

list

Input list

Type
(a -> bool) -> [a] -> { right :: [a]; wrong :: [a]; }
Examples
Example 158. lib.lists.partition usage example
partition (x: x > 2) [ 5 1 2 3 4 ]
=> { right = [ 5 3 4 ]; wrong = [ 1 2 ]; }

Located at lib/lists.nix:954 in <nixpkgs>.

lib.lists.groupBy'

Splits the elements of a list into many lists, using the return value of a predicate. Predicate should return a string which becomes keys of attrset groupBy returns. groupBy' allows to customise the combining function and initial value

Inputs
op

1. Function argument

nul

2. Function argument

pred

3. Function argument

lst

4. Function argument

Examples
Example 159. lib.lists.groupBy' usage example
groupBy (x: boolToString (x > 2)) [ 5 1 2 3 4 ]
=> { true = [ 5 3 4 ]; false = [ 1 2 ]; }
groupBy (x: x.name) [ {name = "icewm"; script = "icewm &";}
                      {name = "xfce";  script = "xfce4-session &";}
                      {name = "icewm"; script = "icewmbg &";}
                      {name = "mate";  script = "gnome-session &";}
                    ]
=> { icewm = [ { name = "icewm"; script = "icewm &"; }
               { name = "icewm"; script = "icewmbg &"; } ];
     mate  = [ { name = "mate";  script = "gnome-session &"; } ];
     xfce  = [ { name = "xfce";  script = "xfce4-session &"; } ];
   }

groupBy' builtins.add 0 (x: boolToString (x > 2)) [ 5 1 2 3 4 ]
=> { true = 12; false = 3; }

Located at lib/lists.nix:1004 in <nixpkgs>.

lib.lists.zipListsWith

Merges two lists of the same size together. If the sizes aren’t the same the merging stops at the shortest. How both lists are merged is defined by the first argument.

Inputs
f

Function to zip elements of both lists

fst

First list

snd

Second list

Type
zipListsWith :: (a -> b -> c) -> [a] -> [b] -> [c]
Examples
Example 160. lib.lists.zipListsWith usage example
zipListsWith (a: b: a + b) ["h" "l"] ["e" "o"]
=> ["he" "lo"]

Located at lib/lists.nix:1050 in <nixpkgs>.

lib.lists.zipLists

Merges two lists of the same size together. If the sizes aren’t the same the merging stops at the shortest.

Inputs
fst

First list

snd

Second list

Type
zipLists :: [a] -> [b] -> [{ fst :: a; snd :: b; }]
Examples
Example 161. lib.lists.zipLists usage example
zipLists [ 1 2 ] [ "a" "b" ]
=> [ { fst = 1; snd = "a"; } { fst = 2; snd = "b"; } ]

Located at lib/lists.nix:1088 in <nixpkgs>.

lib.lists.reverseList

Reverse the order of the elements of a list.

Inputs
xs

1. Function argument

Type
reverseList :: [a] -> [a]
Examples
Example 162. lib.lists.reverseList usage example
reverseList [ "b" "o" "j" ]
=> [ "j" "o" "b" ]

Located at lib/lists.nix:1116 in <nixpkgs>.

lib.lists.listDfs

Depth-First Search (DFS) for lists list != [].

before a b == true means that b depends on a (there’s an edge from b to a).

Inputs
stopOnCycles

1. Function argument

before

2. Function argument

list

3. Function argument

Examples
Example 163. lib.lists.listDfs usage example
listDfs true hasPrefix [ "/home/user" "other" "/" "/home" ]
  == { minimal = "/";                  # minimal element
       visited = [ "/home/user" ];     # seen elements (in reverse order)
       rest    = [ "/home" "other" ];  # everything else
     }

listDfs true hasPrefix [ "/home/user" "other" "/" "/home" "/" ]
  == { cycle   = "/";                  # cycle encountered at this element
       loops   = [ "/" ];              # and continues to these elements
       visited = [ "/" "/home/user" ]; # elements leading to the cycle (in reverse order)
       rest    = [ "/home" "other" ];  # everything else

Located at lib/lists.nix:1161 in <nixpkgs>.

lib.lists.toposort

Sort a list based on a partial ordering using DFS. This implementation is O(N^2), if your ordering is linear, use sort instead.

before a b == true means that b should be after a in the result.

Inputs
before

1. Function argument

list

2. Function argument

Examples
Example 164. lib.lists.toposort usage example
toposort hasPrefix [ "/home/user" "other" "/" "/home" ]
  == { result = [ "/" "/home" "/home/user" "other" ]; }

toposort hasPrefix [ "/home/user" "other" "/" "/home" "/" ]
  == { cycle = [ "/home/user" "/" "/" ]; # path leading to a cycle
       loops = [ "/" ]; }                # loops back to these elements

toposort hasPrefix [ "other" "/home/user" "/home" "/" ]
  == { result = [ "other" "/" "/home" "/home/user" ]; }

toposort (a: b: a < b) [ 3 2 1 ] == { result = [ 1 2 3 ]; }

Located at lib/lists.nix:1218 in <nixpkgs>.

lib.lists.sort

Sort a list based on a comparator function which compares two elements and returns true if the first argument is strictly below the second argument. The returned list is sorted in an increasing order. The implementation does a quick-sort.

See also sortOn, which applies the default comparison on a function-derived property, and may be more efficient.

Inputs
comparator

1. Function argument

list

2. Function argument

Type
sort :: (a -> a -> Bool) -> [a] -> [a]
Examples
Example 165. lib.lists.sort usage example
sort (p: q: p < q) [ 5 3 7 ]
=> [ 3 5 7 ]

Located at lib/lists.nix:1273 in <nixpkgs>.

lib.lists.sortOn

Sort a list based on the default comparison of a derived property b.

The items are returned in b-increasing order.

Performance:

The passed function f is only evaluated once per item, unlike an unprepared sort using f p < f q.

Laws:

sortOn f == sort (p: q: f p < f q)
Inputs
f

1. Function argument

list

2. Function argument

Type
sortOn :: (a -> b) -> [a] -> [a], for comparable b
Examples
Example 166. lib.lists.sortOn usage example
sortOn stringLength [ "aa" "b" "cccc" ]
=> [ "b" "aa" "cccc" ]

Located at lib/lists.nix:1319 in <nixpkgs>.

lib.lists.compareLists

Compare two lists element-by-element.

Inputs
cmp

1. Function argument

a

2. Function argument

b

3. Function argument

Examples
Example 167. lib.lists.compareLists usage example
compareLists compare [] []
=> 0
compareLists compare [] [ "a" ]
=> -1
compareLists compare [ "a" ] []
=> 1
compareLists compare [ "a" "b" ] [ "a" "c" ]
=> -1

Located at lib/lists.nix:1367 in <nixpkgs>.

lib.lists.naturalSort

Sort list using “Natural sorting”. Numeric portions of strings are sorted in numeric order.

Inputs
lst

1. Function argument

Examples
Example 168. lib.lists.naturalSort usage example
naturalSort ["disk11" "disk8" "disk100" "disk9"]
=> ["disk8" "disk9" "disk11" "disk100"]
naturalSort ["10.46.133.149" "10.5.16.62" "10.54.16.25"]
=> ["10.5.16.62" "10.46.133.149" "10.54.16.25"]
naturalSort ["v0.2" "v0.15" "v0.0.9"]
=> [ "v0.0.9" "v0.2" "v0.15" ]

Located at lib/lists.nix:1406 in <nixpkgs>.

lib.lists.take

Return the first (at most) N elements of a list.

Inputs
count

Number of elements to take

list

Input list

Type
take :: int -> [a] -> [a]
Examples
Example 169. lib.lists.take usage example
take 2 [ "a" "b" "c" "d" ]
=> [ "a" "b" ]
take 2 [ ]
=> [ ]

Located at lib/lists.nix:1447 in <nixpkgs>.

lib.lists.drop

Remove the first (at most) N elements of a list.

Inputs
count

Number of elements to drop

list

Input list

Type
drop :: int -> [a] -> [a]
Examples
Example 170. lib.lists.drop usage example
drop 2 [ "a" "b" "c" "d" ]
=> [ "c" "d" ]
drop 2 [ ]
=> [ ]

Located at lib/lists.nix:1483 in <nixpkgs>.

lib.lists.hasPrefix

Whether the first list is a prefix of the second list.

Inputs
list1

1. Function argument

list2

2. Function argument

Type
hasPrefix :: [a] -> [a] -> bool
Examples
Example 171. lib.lists.hasPrefix usage example
hasPrefix [ 1 2 ] [ 1 2 3 4 ]
=> true
hasPrefix [ 0 1 ] [ 1 2 3 4 ]
=> false

Located at lib/lists.nix:1520 in <nixpkgs>.

lib.lists.removePrefix

Remove the first list as a prefix from the second list. Error if the first list isn’t a prefix of the second list.

Inputs
list1

1. Function argument

list2

2. Function argument

Type
removePrefix :: [a] -> [a] -> [a]
Examples
Example 172. lib.lists.removePrefix usage example
removePrefix [ 1 2 ] [ 1 2 3 4 ]
=> [ 3 4 ]
removePrefix [ 0 1 ] [ 1 2 3 4 ]
=> <error>

Located at lib/lists.nix:1558 in <nixpkgs>.

lib.lists.sublist

Return a list consisting of at most count elements of list, starting at index start.

Inputs
start

Index at which to start the sublist

count

Number of elements to take

list

Input list

Type
sublist :: int -> int -> [a] -> [a]
Examples
Example 173. lib.lists.sublist usage example
sublist 1 3 [ "a" "b" "c" "d" "e" ]
=> [ "b" "c" "d" ]
sublist 1 3 [ ]
=> [ ]

Located at lib/lists.nix:1603 in <nixpkgs>.

lib.lists.commonPrefix

The common prefix of two lists.

Inputs
list1

1. Function argument

list2

2. Function argument

Type
commonPrefix :: [a] -> [a] -> [a]
Examples
Example 174. lib.lists.commonPrefix usage example
commonPrefix [ 1 2 3 4 5 6 ] [ 1 2 4 8 ]
=> [ 1 2 ]
commonPrefix [ 1 2 3 ] [ 1 2 3 4 5 ]
=> [ 1 2 3 ]
commonPrefix [ 1 2 3 ] [ 4 5 6 ]
=> [ ]

Located at lib/lists.nix:1649 in <nixpkgs>.

lib.lists.last

Return the last element of a list.

This function throws an error if the list is empty.

Inputs
list

1. Function argument

Type
last :: [a] -> a
Examples
Example 175. lib.lists.last usage example
last [ 1 2 3 ]
=> 3

Located at lib/lists.nix:1692 in <nixpkgs>.

lib.lists.init

Return all elements but the last.

This function throws an error if the list is empty.

Inputs
list

1. Function argument

Type
init :: [a] -> [a]
Examples
Example 176. lib.lists.init usage example
init [ 1 2 3 ]
=> [ 1 2 ]

Located at lib/lists.nix:1725 in <nixpkgs>.

lib.lists.crossLists

Return the image of the cross product of some lists by a function.

Examples
Example 177. lib.lists.crossLists usage example
crossLists (x: y: "${toString x}${toString y}") [[1 2] [3 4]]
=> [ "13" "14" "23" "24" ]

The following function call is equivalent to the one deprecated above:

mapCartesianProduct (x: "${toString x.a}${toString x.b}") { a = [1 2]; b = [3 4]; }
=> [ "13" "14" "23" "24" ]

Located at lib/lists.nix:1751 in <nixpkgs>.

lib.lists.unique

Remove duplicate elements from the list. O(n^2) complexity.

Inputs
list

Input list

Type
unique :: [a] -> [a]
Examples
Example 178. lib.lists.unique usage example
unique [ 3 2 3 4 ]
=> [ 3 2 4 ]

Located at lib/lists.nix:1793 in <nixpkgs>.

lib.lists.allUnique

Check if list contains only unique elements. O(n^2) complexity.

Inputs
list

1. Function argument

Type
allUnique :: [a] -> bool
Examples
Example 179. lib.lists.allUnique usage example
allUnique [ 3 2 3 4 ]
=> false
allUnique [ 3 2 4 1 ]
=> true

Located at lib/lists.nix:1824 in <nixpkgs>.

lib.lists.intersectLists

Intersects list ‘list1’ and another list (list2).

O(nm) complexity.

Inputs
list1

First list

list2

Second list

Examples
Example 180. lib.lists.intersectLists usage example
intersectLists [ 1 2 3 ] [ 6 3 2 ]
=> [ 3 2 ]

Located at lib/lists.nix:1854 in <nixpkgs>.

lib.lists.subtractLists

Subtracts list ‘e’ from another list (list2).

O(nm) complexity.

Inputs
e

First list

list2

Second list

Examples
Example 181. lib.lists.subtractLists usage example
subtractLists [ 3 2 ] [ 1 2 3 4 5 3 ]
=> [ 1 4 5 ]

Located at lib/lists.nix:1883 in <nixpkgs>.

lib.lists.mutuallyExclusive

Test if two lists have no common element. It should be slightly more efficient than (intersectLists a b == [])

Inputs
a

1. Function argument

b

2. Function argument

Located at lib/lists.nix:1899 in <nixpkgs>.

lib.debug: debugging functions

Collection of functions useful for debugging broken nix expressions.

  • trace-like functions take two values, print the first to stderr and return the second.

  • traceVal-like functions take one argument which both printed and returned.

  • traceSeq-like functions fully evaluate their traced value before printing (not just to “weak head normal form” like trace does by default).

  • Functions that end in -Fn take an additional function as their first argument, which is applied to the traced value before it is printed.

lib.debug.traceIf

Conditionally trace the supplied message, based on a predicate.

Inputs
pred

Predicate to check

msg

Message that should be traced

x

Value to return

Type
traceIf :: bool -> string -> a -> a
Examples
Example 182. lib.debug.traceIf usage example
traceIf true "hello" 3
trace: hello
=> 3

Located at lib/debug.nix:72 in <nixpkgs>.

lib.debug.traceValFn

Trace the supplied value after applying a function to it, and return the original value.

Inputs
f

Function to apply

x

Value to trace and return

Type
traceValFn :: (a -> b) -> a -> a
Examples
Example 183. lib.debug.traceValFn usage example
traceValFn (v: "mystring ${v}") "foo"
trace: mystring foo
=> "foo"

Located at lib/debug.nix:110 in <nixpkgs>.

lib.debug.traceVal

Trace the supplied value and return it.

Inputs
x

Value to trace and return

Type
traceVal :: a -> a
Examples
Example 184. lib.debug.traceVal usage example
traceVal 42
# trace: 42
=> 42

Located at lib/debug.nix:141 in <nixpkgs>.

lib.debug.traceSeq

builtins.trace, but the value is builtins.deepSeqed first.

Inputs
x

The value to trace

y

The value to return

Type
traceSeq :: a -> b -> b
Examples
Example 185. lib.debug.traceSeq usage example
trace { a.b.c = 3; } null
trace: { a = <CODE>; }
=> null
traceSeq { a.b.c = 3; } null
trace: { a = { b = { c = 3; }; }; }
=> null

Located at lib/debug.nix:178 in <nixpkgs>.

lib.debug.traceSeqN

Like traceSeq, but only evaluate down to depth n. This is very useful because lots of traceSeq usages lead to an infinite recursion.

Inputs
depth

1. Function argument

x

2. Function argument

y

3. Function argument

Type
traceSeqN :: Int -> a -> b -> b
Examples
Example 186. lib.debug.traceSeqN usage example
traceSeqN 2 { a.b.c = 3; } null
trace: { a = { b = {…}; }; }
=> null

Located at lib/debug.nix:220 in <nixpkgs>.

lib.debug.traceValSeqFn

A combination of traceVal and traceSeq that applies a provided function to the value to be traced after deepSeqing it.

Inputs
f

Function to apply

v

Value to trace

Located at lib/debug.nix:249 in <nixpkgs>.

lib.debug.traceValSeq

A combination of traceVal and traceSeq.

Inputs
v

Value to trace

Located at lib/debug.nix:263 in <nixpkgs>.

lib.debug.traceValSeqNFn

A combination of traceVal and traceSeqN that applies a provided function to the value to be traced.

Inputs
f

Function to apply

depth

2. Function argument

v

Value to trace

Located at lib/debug.nix:284 in <nixpkgs>.

lib.debug.traceValSeqN

A combination of traceVal and traceSeqN.

Inputs
depth

1. Function argument

v

Value to trace

Located at lib/debug.nix:302 in <nixpkgs>.

lib.debug.traceFnSeqN

Trace the input and output of a function f named name, both down to depth.

This is useful for adding around a function call, to see the before/after of values as they are transformed.

Inputs
depth

1. Function argument

name

2. Function argument

f

3. Function argument

v

4. Function argument

Examples
Example 187. lib.debug.traceFnSeqN usage example
traceFnSeqN 2 "id" (x: x) { a.b.c = 3; }
trace: { fn = "id"; from = { a.b = {…}; }; to = { a.b = {…}; }; }
=> { a.b.c = 3; }

Located at lib/debug.nix:343 in <nixpkgs>.

lib.debug.runTests

Evaluates a set of tests.

A test is an attribute set {expr, expected}, denoting an expression and its expected result.

The result is a list of failed tests, each represented as {name, expected, result},

  • expected

    • What was passed as expected

  • result

    • The actual result of the test

Used for regression testing of the functions in lib; see tests.nix for more examples.

Important: Only attributes that start with test are executed.

  • If you want to run only a subset of the tests add the attribute tests = ["testName"];

Inputs
tests

Tests to run

Type
runTests :: {
  tests = [ String ];
  ${testName} :: {
    expr :: a;
    expected :: a;
  };
}
->
[
  {
    name :: String;
    expected :: a;
    result :: a;
  }
]
Examples
Example 188. lib.debug.runTests usage example
runTests {
  testAndOk = {
    expr = lib.and true false;
    expected = false;
  };
  testAndFail = {
    expr = lib.and true false;
    expected = true;
  };
}
->
[
  {
    name = "testAndFail";
    expected = true;
    result = false;
  }
]

Located at lib/debug.nix:432 in <nixpkgs>.

lib.debug.testAllTrue

Create a test assuming that list elements are true.

Inputs
expr

1. Function argument

Examples
Example 189. lib.debug.testAllTrue usage example
{ testX = allTrue [ true ]; }

Located at lib/debug.nix:463 in <nixpkgs>.

lib.options: NixOS / nixpkgs option handling

Nixpkgs/NixOS option handling.

lib.options.isOption

Type: isOption :: a -> bool

Returns true when the given argument is an option

Example 190. lib.options.isOption usage example
isOption 1             // => false
isOption (mkOption {}) // => true

Located at lib/options.nix:56 in <nixpkgs>.

lib.options.mkOption

Creates an Option attribute set. mkOption accepts an attribute set with the following keys:

All keys default to null when not given.

structured function argument
default

Default value used when no definition is given in the configuration.

defaultText

Textual representation of the default, for the manual.

example

Example value used in the manual.

description

String describing the option.

relatedPackages

Related packages used in the manual (see genRelatedPackages in …/nixos/lib/make-options-doc/default.nix).

type

Option type, providing type-checking and value merging.

apply

Function that converts the option value to something else.

internal

Whether the option is for NixOS developers only.

visible

Whether the option shows up in the manual. Default: true. Use false to hide the option and any sub-options from submodules. Use “shallow” to hide only sub-options.

readOnly

Whether the option can be set only once

Example 191. lib.options.mkOption usage example
mkOption { }  // => { _type = "option"; }
mkOption { default = "foo"; } // => { _type = "option"; default = "foo"; }

Located at lib/options.nix:66 in <nixpkgs>.

lib.options.mkEnableOption

Creates an Option attribute set for a boolean value option i.e an option to be toggled on or off:

name

Name for the created option

Example 192. lib.options.mkEnableOption usage example
mkEnableOption "foo"
=> { _type = "option"; default = false; description = "Whether to enable foo."; example = true; type = { ... }; }

Located at lib/options.nix:98 in <nixpkgs>.

lib.options.mkPackageOption

Type: mkPackageOption :: pkgs -> (string|[string]) -> { nullable? :: bool, default? :: string|[string], example? :: null|string|[string], extraDescription? :: string, pkgsText? :: string } -> option

Creates an Option attribute set for an option that specifies the package a module should use for some purpose.

The package is specified in the third argument under default as a list of strings representing its attribute path in nixpkgs (or another package set). Because of this, you need to pass nixpkgs itself (usually pkgs in a module; alternatively to nixpkgs itself, another package set) as the first argument.

If you pass another package set you should set the pkgsText option. This option is used to display the expression for the package set. It is "pkgs" by default. If your expression is complex you should parenthesize it, as the pkgsText argument is usually immediately followed by an attribute lookup (.).

The second argument may be either a string or a list of strings. It provides the display name of the package in the description of the generated option (using only the last element if the passed value is a list) and serves as the fallback value for the default argument.

To include extra information in the description, pass extraDescription to append arbitrary text to the generated description.

You can also pass an example value, either a literal string or an attribute path.

The default argument can be omitted if the provided name is an attribute of pkgs (if name is a string) or a valid attribute path in pkgs (if name is a list). You can also set default to just a string in which case it is interpreted as an attribute name (a singleton attribute path, if you will).

If you wish to explicitly provide no default, pass null as default.

If you want users to be able to set no package, pass nullable = true. In this mode a default = null will not be interpreted as no default and is interpreted literally.

pkgs

Package set (an instantiation of nixpkgs such as pkgs in modules or another package set)

name

Name for the package, shown in option description

structured function argument
nullable

Whether the package can be null, for example to disable installing a package altogether (defaults to false)

default

The attribute path where the default package is located (may be omitted, in which case it is copied from name)

example

A string or an attribute path to use as an example (may be omitted)

extraDescription

Additional text to include in the option description (may be omitted)

pkgsText

Representation of the package set passed as pkgs (defaults to "pkgs")

Example 193. lib.options.mkPackageOption usage example
mkPackageOption pkgs "hello" { }
=> { ...; default = pkgs.hello; defaultText = literalExpression "pkgs.hello"; description = "The hello package to use."; type = package; }


mkPackageOption pkgs "GHC" {
  default = [ "ghc" ];
  example = "pkgs.haskell.packages.ghc92.ghc.withPackages (hkgs: [ hkgs.primes ])";
}
=> { ...; default = pkgs.ghc; defaultText = literalExpression "pkgs.ghc"; description = "The GHC package to use."; example = literalExpression "pkgs.haskell.packages.ghc92.ghc.withPackages (hkgs: [ hkgs.primes ])"; type = package; }


mkPackageOption pkgs [ "python3Packages" "pytorch" ] {
  extraDescription = "This is an example and doesn't actually do anything.";
}
=> { ...; default = pkgs.python3Packages.pytorch; defaultText = literalExpression "pkgs.python3Packages.pytorch"; description = "The pytorch package to use. This is an example and doesn't actually do anything."; type = package; }


mkPackageOption pkgs "nushell" {
  nullable = true;
}
=> { ...; default = pkgs.nushell; defaultText = literalExpression "pkgs.nushell"; description = "The nushell package to use."; type = nullOr package; }


mkPackageOption pkgs "coreutils" {
  default = null;
}
=> { ...; description = "The coreutils package to use."; type = package; }


mkPackageOption pkgs "dbus" {
  nullable = true;
  default = null;
}
=> { ...; default = null; description = "The dbus package to use."; type = nullOr package; }


mkPackageOption pkgs.javaPackages "OpenJFX" {
  default = "openjfx20";
  pkgsText = "pkgs.javaPackages";
}
=> { ...; default = pkgs.javaPackages.openjfx20; defaultText = literalExpression "pkgs.javaPackages.openjfx20"; description = "The OpenJFX package to use."; type = package; }

Located at lib/options.nix:185 in <nixpkgs>.

lib.options.mkPackageOptionMD

Deprecated alias of mkPackageOption, to be removed in 25.05. Previously used to create options with markdown documentation, which is no longer required.

Located at lib/options.nix:226 in <nixpkgs>.

lib.options.mkSinkUndeclaredOptions

This option accepts anything, but it does not produce any result.

This is useful for sharing a module across different module sets without having to implement similar features as long as the values of the options are not accessed.

attrs

Function argument

Located at lib/options.nix:233 in <nixpkgs>.

lib.options.mergeOneOption

Require a single definition.

WARNING: Does not perform nested checks, as this does not run the merge function!

Located at lib/options.nix:262 in <nixpkgs>.

lib.options.mergeUniqueOption

Require a single definition.

NOTE: When the type is not checked completely by check, pass a merge function for further checking (of sub-attributes, etc).

structured function argument
message

Function argument

merge

WARNING: the default merge function assumes that the definition is a valid (option) value. You MUST pass a merge function if the return value needs to be - type checked beyond what .check does (which should be very litte; only on the value head; not attribute values, etc) - if you want attribute values to be checked, or list items - if you want coercedTo-like behavior to work

loc

Function argument

defs

Function argument

Located at lib/options.nix:269 in <nixpkgs>.

lib.options.mergeEqualOption

“Merge” option definitions by checking that they all have the same value.

loc

Function argument

defs

Function argument

Located at lib/options.nix:284 in <nixpkgs>.

lib.options.getValues

Type: getValues :: [ { value :: a; } ] -> [a]

Extracts values of all “value” keys of the given list.

Example 194. lib.options.getValues usage example
getValues [ { value = 1; } { value = 2; } ] // => [ 1 2 ]
getValues [ ]                               // => [ ]

Located at lib/options.nix:304 in <nixpkgs>.

lib.options.getFiles

Type: getFiles :: [ { file :: a; } ] -> [a]

Extracts values of all “file” keys of the given list

Example 195. lib.options.getFiles usage example
getFiles [ { file = "file1"; } { file = "file2"; } ] // => [ "file1" "file2" ]
getFiles [ ]                                         // => [ ]

Located at lib/options.nix:314 in <nixpkgs>.

lib.options.scrubOptionValue

This function recursively removes all derivation attributes from x except for the name attribute.

This is to make the generation of options.xml much more efficient: the XML representation of derivations is very large (on the order of megabytes) and is not actually used by the manual generator.

This function was made obsolete by renderOptionValue and is kept for compatibility with out-of-tree code.

x

Function argument

Located at lib/options.nix:372 in <nixpkgs>.

lib.options.renderOptionValue

Ensures that the given option value (default or example) is a _typed string by rendering Nix values to literalExpressions.

v

Function argument

Located at lib/options.nix:383 in <nixpkgs>.

lib.options.literalExpression

For use in the defaultText and example option attributes. Causes the given string to be rendered verbatim in the documentation as Nix code. This is necessary for complex values, e.g. functions, or values that depend on other values or packages.

text

Function argument

Located at lib/options.nix:396 in <nixpkgs>.

lib.options.mdDoc

Transition marker for documentation that’s already migrated to markdown syntax. Has been a no-op for some while and been removed from nixpkgs. Kept here to alert downstream users who may not be aware of the migration’s completion that it should be removed from modules.

Located at lib/options.nix:407 in <nixpkgs>.

lib.options.literalMD

For use in the defaultText and example option attributes. Causes the given MD text to be inserted verbatim in the documentation, for when a literalExpression would be too hard to read.

text

Function argument

Located at lib/options.nix:413 in <nixpkgs>.

lib.options.showOption

Convert an option, described as a list of the option parts to a human-readable version.

parts

Function argument

Example 196. lib.options.showOption usage example
(showOption ["foo" "bar" "baz"]) == "foo.bar.baz"
  (showOption ["foo" "bar.baz" "tux"]) == "foo.\"bar.baz\".tux"
  (showOption ["windowManager" "2bwm" "enable"]) == "windowManager.\"2bwm\".enable"

Placeholders will not be quoted as they are not actual values:
  (showOption ["foo" "*" "bar"]) == "foo.*.bar"
  (showOption ["foo" "<name>" "bar"]) == "foo.<name>.bar"

Located at lib/options.nix:431 in <nixpkgs>.

lib.path: path functions

Functions for working with path values.

lib.path.append

Type: append :: Path -> String -> Path

Append a subpath string to a path.

Like path + ("/" + string) but safer, because it errors instead of returning potentially surprising results. More specifically, it checks that the first argument is a path value type, and that the second argument is a valid subpath string.

Laws:

  • Not influenced by subpath normalisation:

    append p s == append p (subpath.normalise s)
    
path

The absolute path to append to

subpath

The subpath string to append

Example 197. lib.path.append usage example
append /foo "bar/baz"
=> /foo/bar/baz

# subpaths don't need to be normalised
append /foo "./bar//baz/./"
=> /foo/bar/baz

# can append to root directory
append /. "foo/bar"
=> /foo/bar

# first argument needs to be a path value type
append "/foo" "bar"
=> <error>

# second argument needs to be a valid subpath string
append /foo /bar
=> <error>
append /foo ""
=> <error>
append /foo "/bar"
=> <error>
append /foo "../bar"
=> <error>

Located at lib/path/default.nix:192 in <nixpkgs>.

lib.path.hasPrefix

Type: hasPrefix :: Path -> Path -> Bool

Whether the first path is a component-wise prefix of the second path.

Laws:

path1

Function argument

Example 198. lib.path.hasPrefix usage example
hasPrefix /foo /foo/bar
=> true
hasPrefix /foo /foo
=> true
hasPrefix /foo/bar /foo
=> false
hasPrefix /. /foo
=> true

Located at lib/path/default.nix:226 in <nixpkgs>.

lib.path.removePrefix

Type: removePrefix :: Path -> Path -> String

Remove the first path as a component-wise prefix from the second path. The result is a normalised subpath string.

Laws:

path1

Function argument

Example 199. lib.path.removePrefix usage example
removePrefix /foo /foo/bar/baz
=> "./bar/baz"
removePrefix /foo /foo
=> "./."
removePrefix /foo/bar /foo
=> <error>
removePrefix /. /foo
=> "./foo"

Located at lib/path/default.nix:271 in <nixpkgs>.

lib.path.splitRoot

Type: splitRoot :: Path -> { root :: Path, subpath :: String }

Split the filesystem root from a path. The result is an attribute set with these attributes:

  • root: The filesystem root of the path, meaning that this directory has no parent directory.

  • subpath: The normalised subpath string that when appended to root returns the original path.

Laws:

  • Appending the root and subpath gives the original path:

    p ==
      append
        (splitRoot p).root
        (splitRoot p).subpath
    
  • Trying to get the parent directory of root using readDir returns root itself:

    dirOf (splitRoot p).root == (splitRoot p).root
    
path

The path to split the root off of

Example 200. lib.path.splitRoot usage example
splitRoot /foo/bar
=> { root = /.; subpath = "./foo/bar"; }

splitRoot /.
=> { root = /.; subpath = "./."; }

# Nix neutralises `..` path components for all path values automatically
splitRoot /foo/../bar
=> { root = /.; subpath = "./bar"; }

splitRoot "/foo/bar"
=> <error>

Located at lib/path/default.nix:336 in <nixpkgs>.

lib.path.hasStorePathPrefix

Type: hasStorePathPrefix :: Path -> Bool

Whether a path has a store path as a prefix.

Note

As with all functions of this lib.path library, it does not work on paths in strings, which is how you’d typically get store paths.

Instead, this function only handles path values themselves, which occur when Nix files in the store use relative path expressions.

path

Function argument

Example 201. lib.path.hasStorePathPrefix usage example
# Subpaths of derivation outputs have a store path as a prefix
hasStorePathPrefix /nix/store/nvl9ic0pj1fpyln3zaqrf4cclbqdfn1j-foo/bar/baz
=> true

# The store directory itself is not a store path
hasStorePathPrefix /nix/store
=> false

# Derivation outputs are store paths themselves
hasStorePathPrefix /nix/store/nvl9ic0pj1fpyln3zaqrf4cclbqdfn1j-foo
=> true

# Paths outside the Nix store don't have a store path prefix
hasStorePathPrefix /home/user
=> false

# Not all paths under the Nix store are store paths
hasStorePathPrefix /nix/store/.links/10gg8k3rmbw8p7gszarbk7qyd9jwxhcfq9i6s5i0qikx8alkk4hq
=> false

# Store derivations are also store paths themselves
hasStorePathPrefix /nix/store/nvl9ic0pj1fpyln3zaqrf4cclbqdfn1j-foo.drv
=> true

Located at lib/path/default.nix:390 in <nixpkgs>.

lib.path.subpath.isValid

Type: subpath.isValid :: String -> Bool

Whether a value is a valid subpath string.

A subpath string points to a specific file or directory within an absolute base directory. It is a stricter form of a relative path that excludes .. components, since those could escape the base directory.

  • The value is a string.

  • The string is not empty.

  • The string doesn’t start with a /.

  • The string doesn’t contain any .. path components.

value

The value to check

Example 202. lib.path.subpath.isValid usage example
# Not a string
subpath.isValid null
=> false

# Empty string
subpath.isValid ""
=> false

# Absolute path
subpath.isValid "/foo"
=> false

# Contains a `..` path component
subpath.isValid "../foo"
=> false

# Valid subpath
subpath.isValid "foo/bar"
=> true

# Doesn't need to be normalised
subpath.isValid "./foo//bar/"
=> true

Located at lib/path/default.nix:447 in <nixpkgs>.

lib.path.subpath.join

Type: subpath.join :: [ String ] -> String

Join subpath strings together using /, returning a normalised subpath string.

Like concatStringsSep "/" but safer, specifically:

  • All elements must be valid subpath strings.

  • The result gets normalised.

  • The edge case of an empty list gets properly handled by returning the neutral subpath "./.".

Laws:

  • Associativity:

    subpath.join [ x (subpath.join [ y z ]) ] == subpath.join [ (subpath.join [ x y ]) z ]
    
  • Identity - "./." is the neutral element for normalised paths:

    subpath.join [ ] == "./."
    subpath.join [ (subpath.normalise p) "./." ] == subpath.normalise p
    subpath.join [ "./." (subpath.normalise p) ] == subpath.normalise p
    
  • Normalisation - the result is normalised:

    subpath.join ps == subpath.normalise (subpath.join ps)
    
  • For non-empty lists, the implementation is equivalent to normalising the result of concatStringsSep "/". Note that the above laws can be derived from this one:

    ps != [] -> subpath.join ps == subpath.normalise (concatStringsSep "/" ps)
    
subpaths

The list of subpaths to join together

Example 203. lib.path.subpath.join usage example
subpath.join [ "foo" "bar/baz" ]
=> "./foo/bar/baz"

# normalise the result
subpath.join [ "./foo" "." "bar//./baz/" ]
=> "./foo/bar/baz"

# passing an empty list results in the current directory
subpath.join [ ]
=> "./."

# elements must be valid subpath strings
subpath.join [ /foo ]
=> <error>
subpath.join [ "" ]
=> <error>
subpath.join [ "/foo" ]
=> <error>
subpath.join [ "../foo" ]
=> <error>

Located at lib/path/default.nix:510 in <nixpkgs>.

lib.path.subpath.components

Type: subpath.components :: String -> [ String ]

Split a subpath into its path component strings. Throw an error if the subpath isn’t valid. Note that the returned path components are also valid subpath strings, though they are intentionally not normalised.

Laws:

  • Splitting a subpath into components and joining the components gives the same subpath but normalised:

    subpath.join (subpath.components s) == subpath.normalise s
    
subpath

The subpath string to split into components

Example 204. lib.path.subpath.components usage example
subpath.components "."
=> [ ]

subpath.components "./foo//bar/./baz/"
=> [ "foo" "bar" "baz" ]

subpath.components "/foo"
=> <error>

Located at lib/path/default.nix:552 in <nixpkgs>.

lib.path.subpath.normalise

Type: subpath.normalise :: String -> String

Normalise a subpath. Throw an error if the subpath isn’t valid.

  • Limit repeating / to a single one.

  • Remove redundant . components.

  • Remove trailing / and /..

  • Add leading ./.

Laws:

  • Idempotency - normalising multiple times gives the same result:

    subpath.normalise (subpath.normalise p) == subpath.normalise p
    
  • Uniqueness - there’s only a single normalisation for the paths that lead to the same file system node:

    subpath.normalise p != subpath.normalise q -> $(realpath ${p}) != $(realpath ${q})
    
  • Don’t change the result when appended to a Nix path value:

    append base p == append base (subpath.normalise p)
    
  • Don’t change the path according to realpath:

    $(realpath ${p}) == $(realpath ${subpath.normalise p})
    
  • Only error on invalid subpaths:

    builtins.tryEval (subpath.normalise p)).success == subpath.isValid p
    
subpath

The subpath string to normalise

Example 205. lib.path.subpath.normalise usage example
# limit repeating `/` to a single one
subpath.normalise "foo//bar"
=> "./foo/bar"

# remove redundant `.` components
subpath.normalise "foo/./bar"
=> "./foo/bar"

# add leading `./`
subpath.normalise "foo/bar"
=> "./foo/bar"

# remove trailing `/`
subpath.normalise "foo/bar/"
=> "./foo/bar"

# remove trailing `/.`
subpath.normalise "foo/bar/."
=> "./foo/bar"

# Return the current directory as `./.`
subpath.normalise "."
=> "./."

# error on `..` path components
subpath.normalise "foo/../bar"
=> <error>

# error on empty string
subpath.normalise ""
=> <error>

# error on absolute path
subpath.normalise "/foo"
=> <error>

Located at lib/path/default.nix:633 in <nixpkgs>.

lib.filesystem: filesystem functions

Functions for querying information about the filesystem without copying any files to the Nix store.

lib.filesystem.pathType

The type of a path. The path needs to exist and be accessible. The result is either “directory” for a directory, “regular” for a regular file, “symlink” for a symlink, or “unknown” for anything else.

Inputs
path

The path to query

Type
pathType :: Path -> String
Examples
Example 206. lib.filesystem.pathType usage example
pathType /.
=> "directory"

pathType /some/file.nix
=> "regular"

Located at lib/filesystem.nix:62 in <nixpkgs>.

lib.filesystem.pathIsDirectory

Whether a path exists and is a directory.

Inputs
path

1. Function argument

Type
pathIsDirectory :: Path -> Bool
Examples
Example 207. lib.filesystem.pathIsDirectory usage example
pathIsDirectory /.
=> true

pathIsDirectory /this/does/not/exist
=> false

pathIsDirectory /some/file.nix
=> false

Located at lib/filesystem.nix:111 in <nixpkgs>.

lib.filesystem.pathIsRegularFile

Whether a path exists and is a regular file, meaning not a symlink or any other special file type.

Inputs
path

1. Function argument

Type
pathIsRegularFile :: Path -> Bool
Examples
Example 208. lib.filesystem.pathIsRegularFile usage example
pathIsRegularFile /.
=> false

pathIsRegularFile /this/does/not/exist
=> false

pathIsRegularFile /some/file.nix
=> true

Located at lib/filesystem.nix:147 in <nixpkgs>.

lib.filesystem.haskellPathsInDir

A map of all haskell packages defined in the given path, identified by having a cabal file with the same name as the directory itself.

Inputs
root

The directory within to search

Type
Path -> Map String Path

Located at lib/filesystem.nix:168 in <nixpkgs>.

lib.filesystem.locateDominatingFile

Find the first directory containing a file matching ‘pattern’ upward from a given ‘file’. Returns ‘null’ if no directories contain a file matching ‘pattern’.

Inputs
pattern

The pattern to search for

file

The file to start searching upward from

Type
RegExp -> Path -> Nullable { path : Path; matches : [ MatchResults ]; }

Located at lib/filesystem.nix:205 in <nixpkgs>.

lib.filesystem.listFilesRecursive

Given a directory, return a flattened list of all files within it recursively.

Inputs
dir

The path to recursively list

Type
Path -> [ Path ]

Located at lib/filesystem.nix:242 in <nixpkgs>.

lib.filesystem.packagesFromDirectoryRecursive

Transform a directory tree containing package files suitable for callPackage into a matching nested attribute set of derivations.

For a directory tree like this:

my-packages
├── a.nix
├── b.nix
├── c
│  ├── my-extra-feature.patch
│  ├── package.nix
│  └── support-definitions.nix
└── my-namespace
   ├── d.nix
   ├── e.nix
   └── f
      └── package.nix

packagesFromDirectoryRecursive will produce an attribute set like this:

# packagesFromDirectoryRecursive {
#   callPackage = pkgs.callPackage;
#   directory = ./my-packages;
# }
{
  a = pkgs.callPackage ./my-packages/a.nix { };
  b = pkgs.callPackage ./my-packages/b.nix { };
  c = pkgs.callPackage ./my-packages/c/package.nix { };
  my-namespace = {
    d = pkgs.callPackage ./my-packages/my-namespace/d.nix { };
    e = pkgs.callPackage ./my-packages/my-namespace/e.nix { };
    f = pkgs.callPackage ./my-packages/my-namespace/f/package.nix { };
  };
}

In particular:

  • If the input directory contains a package.nix file, then callPackage <directory>/package.nix { } is returned.

  • Otherwise, the input directory’s contents are listed and transformed into an attribute set.

    • If a file name has the .nix extension, it is turned into attribute where:

      • The attribute name is the file name without the .nix extension

      • The attribute value is callPackage <file path> { }

    • Other files are ignored.

    • Directories are turned into an attribute where:

      • The attribute name is the name of the directory

      • The attribute value is the result of calling packagesFromDirectoryRecursive { ... } on the directory.

      As a result, directories with no .nix files (including empty directories) will be transformed into empty attribute sets.

Inputs
Structured function argument

Attribute set containing the following attributes. Additional attributes are ignored.

callPackage

pkgs.callPackage

Type: Path -> AttrSet -> a

directory

The directory to read package files from

Type: Path

Type
packagesFromDirectoryRecursive :: AttrSet -> AttrSet
Examples
Example 209. lib.filesystem.packagesFromDirectoryRecursive usage example
packagesFromDirectoryRecursive {
  inherit (pkgs) callPackage;
  directory = ./my-packages;
}
=> { ... }

lib.makeScope pkgs.newScope (
  self: packagesFromDirectoryRecursive {
    callPackage = self.callPackage;
    directory = ./my-packages;
  }
)
=> { ... }

Located at lib/filesystem.nix:357 in <nixpkgs>.

lib.fileset: file set functions

The lib.fileset library allows you to work with file sets. A file set is a (mathematical) set of local files that can be added to the Nix store for use in Nix derivations. File sets are easy and safe to use, providing obvious and composable semantics with good error messages to prevent mistakes.

Overview

Basics:

Combinators:

Filtering:

Utilities:

If you need more file set functions, see this issue to request it.

Implicit coercion from paths to file sets

All functions accepting file sets as arguments can also accept paths as arguments. Such path arguments are implicitly coerced to file sets containing all files under that path:

  • A path to a file turns into a file set containing that single file.

  • A path to a directory turns into a file set containing all files recursively in that directory.

If the path points to a non-existent location, an error is thrown.

Note

Just like in Git, file sets cannot represent empty directories. Because of this, a path to a directory that contains no files (recursively) will turn into a file set containing no files.

Note

File set coercion does not add any of the files under the coerced paths to the store. Only the toSource function adds files to the Nix store, and only those files contained in the fileset argument. This is in contrast to using paths in string interpolation, which does add the entire referenced path to the store.

Example

Assume we are in a local directory with a file hierarchy like this:

├─ a/
│  ├─ x (file)
│  └─ b/
│     └─ y (file)
└─ c/
   └─ d/

Here’s a listing of which files get included when different path expressions get coerced to file sets:

  • ./. as a file set contains both a/x and a/b/y (c/ does not contain any files and is therefore omitted).

  • ./a as a file set contains both a/x and a/b/y.

  • ./a/x as a file set contains only a/x.

  • ./a/b as a file set contains only a/b/y.

  • ./c as a file set is empty, since neither c nor c/d contain any files.

lib.fileset.maybeMissing

Create a file set from a path that may or may not exist:

  • If the path does exist, the path is coerced to a file set.

  • If the path does not exist, a file set containing no files is returned.

Inputs
path

1. Function argument

Type
maybeMissing :: Path -> FileSet
Examples
Example 210. lib.fileset.maybeMissing usage example
# All files in the current directory, but excluding main.o if it exists
difference ./. (maybeMissing ./main.o)

Located at lib/fileset/default.nix:189 in <nixpkgs>.

lib.fileset.trace

Incrementally evaluate and trace a file set in a pretty way. This function is only intended for debugging purposes. The exact tracing format is unspecified and may change.

This function takes a final argument to return. In comparison, traceVal returns the given file set argument.

This variant is useful for tracing file sets in the Nix repl.

Inputs
fileset

The file set to trace.

This argument can also be a path, which gets implicitly coerced to a file set.

val

The value to return.

Type
trace :: FileSet -> Any -> Any
Examples
Example 211. lib.fileset.trace usage example
trace (unions [ ./Makefile ./src ./tests/run.sh ]) null
=>
trace: /home/user/src/myProject
trace: - Makefile (regular)
trace: - src (all files in directory)
trace: - tests
trace:   - run.sh (regular)
null

Located at lib/fileset/default.nix:251 in <nixpkgs>.

lib.fileset.traceVal

Incrementally evaluate and trace a file set in a pretty way. This function is only intended for debugging purposes. The exact tracing format is unspecified and may change.

This function returns the given file set. In comparison, trace takes another argument to return.

This variant is useful for tracing file sets passed as arguments to other functions.

Inputs
fileset

The file set to trace and return.

This argument can also be a path, which gets implicitly coerced to a file set.

Type
traceVal :: FileSet -> FileSet
Examples
Example 212. lib.fileset.traceVal usage example
toSource {
  root = ./.;
  fileset = traceVal (unions [
    ./Makefile
    ./src
    ./tests/run.sh
  ]);
}
=>
trace: /home/user/src/myProject
trace: - Makefile (regular)
trace: - src (all files in directory)
trace: - tests
trace:   - run.sh (regular)
"/nix/store/...-source"

Located at lib/fileset/default.nix:311 in <nixpkgs>.

lib.fileset.toSource

Add the local files contained in fileset to the store as a single store path rooted at root.

The result is the store path as a string-like value, making it usable e.g. as the src of a derivation, or in string interpolation:

stdenv.mkDerivation {
  src = lib.fileset.toSource { ... };
  # ...
}

The name of the store path is always source.

Inputs

Takes an attribute set with the following attributes

root (Path; required)

The local directory path that will correspond to the root of the resulting store path. Paths in strings, including Nix store paths, cannot be passed as root. root has to be a directory.

Note

Changing root only affects the directory structure of the resulting store path, it does not change which files are added to the store. The only way to change which files get added to the store is by changing the fileset attribute.

fileset (FileSet; required)

The file set whose files to import into the store. File sets can be created using other functions in this library. This argument can also be a path, which gets implicitly coerced to a file set.

Note

If a directory does not recursively contain any file, it is omitted from the store path contents.

Type
toSource :: {
  root :: Path,
  fileset :: FileSet,
} -> SourceLike
Examples
Example 213. lib.fileset.toSource usage example
# Import the current directory into the store
# but only include files under ./src
toSource {
  root = ./.;
  fileset = ./src;
}
=> "/nix/store/...-source"

# Import the current directory into the store
# but only include ./Makefile and all files under ./src
toSource {
  root = ./.;
  fileset = union
    ./Makefile
    ./src;
}
=> "/nix/store/...-source"

# Trying to include a file outside the root will fail
toSource {
  root = ./.;
  fileset = unions [
    ./Makefile
    ./src
    ../LICENSE
  ];
}
=> <error>

# The root needs to point to a directory that contains all the files
toSource {
  root = ../.;
  fileset = unions [
    ./Makefile
    ./src
    ../LICENSE
  ];
}
=> "/nix/store/...-source"

# The root has to be a local filesystem path
toSource {
  root = "/nix/store/...-source";
  fileset = ./.;
}
=> <error>

Located at lib/fileset/default.nix:426 in <nixpkgs>.

lib.fileset.toList

The list of file paths contained in the given file set.

Note

This function is strict in the entire file set. This is in contrast with combinators lib.fileset.union, lib.fileset.intersection and lib.fileset.difference.

Thus it is recommended to call toList on file sets created using the combinators, instead of doing list processing on the result of toList.

The resulting list of files can be turned back into a file set using lib.fileset.unions.

Inputs
fileset

The file set whose file paths to return. This argument can also be a path, which gets implicitly coerced to a file set.

Type
toList :: FileSet -> [ Path ]
Examples
Example 214. lib.fileset.toList usage example
toList ./.
[ ./README.md ./Makefile ./src/main.c ./src/main.h ]

toList (difference ./. ./src)
[ ./README.md ./Makefile ]

Located at lib/fileset/default.nix:524 in <nixpkgs>.

lib.fileset.union

The file set containing all files that are in either of two given file sets. This is the same as unions, but takes just two file sets instead of a list. See also Union (set theory).

The given file sets are evaluated as lazily as possible, with the first argument being evaluated first if needed.

Inputs
fileset1

The first file set. This argument can also be a path, which gets implicitly coerced to a file set.

fileset2

The second file set. This argument can also be a path, which gets implicitly coerced to a file set.

Type
union :: FileSet -> FileSet -> FileSet
Examples
Example 215. lib.fileset.union usage example
# Create a file set containing the file `Makefile`
# and all files recursively in the `src` directory
union ./Makefile ./src

# Create a file set containing the file `Makefile`
# and the LICENSE file from the parent directory
union ./Makefile ../LICENSE

Located at lib/fileset/default.nix:569 in <nixpkgs>.

lib.fileset.unions

The file set containing all files that are in any of the given file sets. This is the same as union, but takes a list of file sets instead of just two. See also Union (set theory).

The given file sets are evaluated as lazily as possible, with earlier elements being evaluated first if needed.

Inputs
filesets

A list of file sets. The elements can also be paths, which get implicitly coerced to file sets.

Type
unions :: [ FileSet ] -> FileSet
Examples
Example 216. lib.fileset.unions usage example
# Create a file set containing selected files
unions [
  # Include the single file `Makefile` in the current directory
  # This errors if the file doesn't exist
  ./Makefile

  # Recursively include all files in the `src/code` directory
  # If this directory is empty this has no effect
  ./src/code

  # Include the files `run.sh` and `unit.c` from the `tests` directory
  ./tests/run.sh
  ./tests/unit.c

  # Include the `LICENSE` file from the parent directory
  ../LICENSE
]

Located at lib/fileset/default.nix:632 in <nixpkgs>.

lib.fileset.intersection

The file set containing all files that are in both of two given file sets. See also Intersection (set theory).

The given file sets are evaluated as lazily as possible, with the first argument being evaluated first if needed.

Inputs
fileset1

The first file set. This argument can also be a path, which gets implicitly coerced to a file set.

fileset2

The second file set. This argument can also be a path, which gets implicitly coerced to a file set.

Type
intersection :: FileSet -> FileSet -> FileSet
Examples
Example 217. lib.fileset.intersection usage example
# Limit the selected files to the ones in ./., so only ./src and ./Makefile
intersection ./. (unions [ ../LICENSE ./src ./Makefile ])

Located at lib/fileset/default.nix:683 in <nixpkgs>.

lib.fileset.difference

The file set containing all files from the first file set that are not in the second file set. See also Difference (set theory).

The given file sets are evaluated as lazily as possible, with the first argument being evaluated first if needed.

Inputs
positive

The positive file set. The result can only contain files that are also in this file set. This argument can also be a path, which gets implicitly coerced to a file set.

negative

The negative file set. The result will never contain files that are also in this file set. This argument can also be a path, which gets implicitly coerced to a file set.

Type
union :: FileSet -> FileSet -> FileSet
Examples
Example 218. lib.fileset.difference usage example
# Create a file set containing all files from the current directory,
# except ones under ./tests
difference ./. ./tests

let
  # A set of Nix-related files
  nixFiles = unions [ ./default.nix ./nix ./tests/default.nix ];
in
# Create a file set containing all files under ./tests, except ones in `nixFiles`,
# meaning only without ./tests/default.nix
difference ./tests nixFiles

Located at lib/fileset/default.nix:746 in <nixpkgs>.

lib.fileset.fileFilter

Filter a file set to only contain files matching some predicate.

Inputs
predicate

The predicate function to call on all files contained in given file set. A file is included in the resulting file set if this function returns true for it.

This function is called with an attribute set containing these attributes:

  • name (String): The name of the file

  • type (String, one of "regular", "symlink" or "unknown"): The type of the file. This matches result of calling builtins.readFileType on the file’s path.

  • hasExt (String -> Bool): Whether the file has a certain file extension. hasExt ext is true only if hasSuffix ".${ext}" name.

    This also means that e.g. for a file with name .gitignore, hasExt "gitignore" is true.

Other attributes may be added in the future.

path

The path whose files to filter

Type
fileFilter ::
  ({
    name :: String,
    type :: String,
    hasExt :: String -> Bool,
    ...
  } -> Bool)
  -> Path
  -> FileSet
Examples
Example 219. lib.fileset.fileFilter usage example
# Include all regular `default.nix` files in the current directory
fileFilter (file: file.name == "default.nix") ./.

# Include all non-Nix files from the current directory
fileFilter (file: ! file.hasExt "nix") ./.

# Include all files that start with a "." in the current directory
fileFilter (file: hasPrefix "." file.name) ./.

# Include all regular files (not symlinks or others) in the current directory
fileFilter (file: file.type == "regular") ./.

Located at lib/fileset/default.nix:829 in <nixpkgs>.

lib.fileset.fromSource

Create a file set with the same files as a lib.sources-based value. This does not import any of the files into the store.

This can be used to gradually migrate from lib.sources-based filtering to lib.fileset.

A file set can be turned back into a source using toSource.

Note

File sets cannot represent empty directories. Turning the result of this function back into a source using toSource will therefore not preserve empty directories.

Inputs
source

1. Function argument

Type
fromSource :: SourceLike -> FileSet
Examples
Example 220. lib.fileset.fromSource usage example
# There's no cleanSource-like function for file sets yet,
# but we can just convert cleanSource to a file set and use it that way
toSource {
  root = ./.;
  fileset = fromSource (lib.sources.cleanSource ./.);
}

# Keeping a previous sourceByRegex (which could be migrated to `lib.fileset.unions`),
# but removing a subdirectory using file set functions
difference
  (fromSource (lib.sources.sourceByRegex ./. [
    "^README\.md$"
    # This regex includes everything in ./doc
    "^doc(/.*)?$"
  ])
  ./doc/generated

# Use cleanSource, but limit it to only include ./Makefile and files under ./src
intersection
  (fromSource (lib.sources.cleanSource ./.))
  (unions [
    ./Makefile
    ./src
  ]);

Located at lib/fileset/default.nix:908 in <nixpkgs>.

lib.fileset.gitTracked

Create a file set containing all Git-tracked files in a repository.

This function behaves like gitTrackedWith { } - using the defaults.

Inputs
path

The path to the working directory of a local Git repository. This directory must contain a .git file or subdirectory.

Type
gitTracked :: Path -> FileSet
Examples
Example 221. lib.fileset.gitTracked usage example
# Include all files tracked by the Git repository in the current directory
gitTracked ./.

# Include only files tracked by the Git repository in the parent directory
# that are also in the current directory
intersection ./. (gitTracked ../.)

Located at lib/fileset/default.nix:969 in <nixpkgs>.

lib.fileset.gitTrackedWith

Create a file set containing all Git-tracked files in a repository. The first argument allows configuration with an attribute set, while the second argument is the path to the Git working tree.

gitTrackedWith does not perform any filtering when the path is a Nix store path and not a repository. In this way, it accommodates the use case where the expression that makes the gitTracked call does not reside in an actual git repository anymore, and has presumably already been fetched in a way that excludes untracked files. Fetchers with such equivalent behavior include builtins.fetchGit, builtins.fetchTree (experimental), and pkgs.fetchgit when used without leaveDotGit.

If you don’t need the configuration, you can use gitTracked instead.

This is equivalent to the result of unions on all files returned by git ls-files (which uses --cached by default).

Warning

Currently this function is based on builtins.fetchGit As such, this function causes all Git-tracked files to be unnecessarily added to the Nix store, without being re-usable by toSource.

This may change in the future.

Inputs
options (attribute set)
recurseSubmodules (optional, default: false)

Whether to recurse into Git submodules to also include their tracked files. If true, this is equivalent to passing the –recurse-submodules flag to git ls-files.

path

The path to the working directory of a local Git repository. This directory must contain a .git file or subdirectory.

Type
gitTrackedWith :: { recurseSubmodules :: Bool ? false } -> Path -> FileSet
Examples
Example 222. lib.fileset.gitTrackedWith usage example
# Include all files tracked by the Git repository in the current directory
# and any submodules under it
gitTracked { recurseSubmodules = true; } ./.

Located at lib/fileset/default.nix:1031 in <nixpkgs>.

lib.sources: source filtering functions

Functions for copying sources to the Nix store.

lib.sources.commitIdFromGitRepo

Get the commit id of a git repo.

path

Function argument

Example 223. lib.sources.commitIdFromGitRepo usage example
commitIdFromGitRepo <nixpkgs/.git>

Located at lib/sources.nix:273 in <nixpkgs>.

lib.sources.cleanSource

Filters a source tree removing version control files and directories using cleanSourceFilter.

src

Function argument

Example 224. lib.sources.cleanSource usage example
cleanSource ./.

Located at lib/sources.nix:275 in <nixpkgs>.

lib.sources.cleanSourceWith

Like builtins.filterSource, except it will compose with itself, allowing you to chain multiple calls together without any intermediate copies being put in the nix store.

structured function argument
src

A path or cleanSourceWith result to filter and/or rename.

filter

Optional with default value: constant true (include everything) The function will be combined with the && operator such that src.filter is called lazily. For implementing a filter, see https://nixos.org/nix/manual/#builtin-filterSource Type: A function (path -> type -> bool)

name

Optional name to use as part of the store path. This defaults to src.name or otherwise "source".

Example 225. lib.sources.cleanSourceWith usage example
lib.cleanSourceWith {
  filter = f;
  src = lib.cleanSourceWith {
    filter = g;
    src = ./.;
  };
}
# Succeeds!

builtins.filterSource f (builtins.filterSource g ./.)
# Fails!

Located at lib/sources.nix:276 in <nixpkgs>.

lib.sources.cleanSourceFilter

A basic filter for cleanSourceWith that removes directories of version control system, backup files (*~) and some generated files.

name

Function argument

type

Function argument

Located at lib/sources.nix:277 in <nixpkgs>.

lib.sources.sourceByRegex

Filter sources by a list of regular expressions.

src

Function argument

regexes

Function argument

Example 226. lib.sources.sourceByRegex usage example
src = sourceByRegex ./my-subproject [".*\.py$" "^database.sql$"]

Located at lib/sources.nix:281 in <nixpkgs>.

lib.sources.sourceFilesBySuffices

Type: sourceLike -> [String] -> Source

Get all files ending with the specified suffices from the given source directory or its descendants, omitting files that do not match any suffix. The result of the example below will include files like ./dir/module.c and ./dir/subdir/doc.xml if present.

src

Path or source containing the files to be returned

exts

A list of file suffix strings

Example 227. lib.sources.sourceFilesBySuffices usage example
sourceFilesBySuffices ./. [ ".xml" ".c" ]

Located at lib/sources.nix:282 in <nixpkgs>.

lib.sources.trace

Type: sources.trace :: sourceLike -> Source

Add logging to a source, for troubleshooting the filtering behavior.

src

Source to debug. The returned source will behave like this source, but also log its filter invocations.

Located at lib/sources.nix:284 in <nixpkgs>.

lib.cli: command-line serialization functions

lib.cli.toGNUCommandLineShell

Automatically convert an attribute set to command-line options.

This helps protect against malformed command lines and also to reduce boilerplate related to command-line construction for simple use cases.

toGNUCommandLineShell returns an escaped shell string.

Inputs
options

How to format the arguments, see toGNUCommandLine

attrs

The attributes to transform into arguments.

Examples
Example 228. lib.cli.toGNUCommandLineShell usage example
cli.toGNUCommandLineShell {} {
  data = builtins.toJSON { id = 0; };
  X = "PUT";
  retry = 3;
  retry-delay = null;
  url = [ "https://example.com/foo" "https://example.com/bar" ];
  silent = false;
  verbose = true;
}
=> "'-X' 'PUT' '--data' '{\"id\":0}' '--retry' '3' '--url' 'https://example.com/foo' '--url' 'https://example.com/bar' '--verbose'";

Located at lib/cli.nix:43 in <nixpkgs>.

lib.cli.toGNUCommandLine

Automatically convert an attribute set to a list of command-line options.

toGNUCommandLine returns a list of string arguments.

Inputs
options

How to format the arguments, see below.

attrs

The attributes to transform into arguments.

Options
mkOptionName

How to string-format the option name; By default one character is a short option (-), more than one characters a long option (--).

mkBool

How to format a boolean value to a command list; By default it’s a flag option (only the option name if true, left out completely if false).

mkList

How to format a list value to a command list; By default the option name is repeated for each value and mkOption is applied to the values themselves.

mkOption

How to format any remaining value to a command list; On the toplevel, booleans and lists are handled by mkBool and mkList, though they can still appear as values of a list. By default, everything is printed verbatim and complex types are forbidden (lists, attrsets, functions). null values are omitted.

optionValueSeparator

How to separate an option from its flag; By default, there is no separator, so option -c and value 5 would become [“-c” “5”]. This is useful if the command requires equals, for example, -c=5.

Examples
Example 229. lib.cli.toGNUCommandLine usage example
cli.toGNUCommandLine {} {
  data = builtins.toJSON { id = 0; };
  X = "PUT";
  retry = 3;
  retry-delay = null;
  url = [ "https://example.com/foo" "https://example.com/bar" ];
  silent = false;
  verbose = true;
}
=> [
  "-X" "PUT"
  "--data" "{\"id\":0}"
  "--retry" "3"
  "--url" "https://example.com/foo"
  "--url" "https://example.com/bar"
  "--verbose"
]

Located at lib/cli.nix:119 in <nixpkgs>.

lib.generators: functions that create file formats from nix data structures

Functions that generate widespread file formats from nix data structures.

They all follow a similar interface:

generator { config-attrs } data

config-attrs are “holes” in the generators with sensible default implementations that can be overwritten. The default implementations are mostly generators themselves, called with their respective default values; they can be reused.

Tests can be found in ./tests/misc.nix

Further Documentation can be found here.

lib.generators.mkValueStringDefault

Convert a value to a sensible default string representation. The builtin toString function has some strange defaults, suitable for bash scripts but not much else.

Inputs
Options

Empty set, there may be configuration options in the future

v

2. Function argument

Located at lib/generators.nix:90 in <nixpkgs>.

lib.generators.mkKeyValueDefault

Generate a line of key k and value v, separated by character sep. If sep appears in k, it is escaped. Helper for synaxes with different separators.

mkValueString specifies how values should be formatted.

mkKeyValueDefault {} ":" "f:oo" "bar"
> "f\:oo:bar"
Inputs
Structured function argument
mkValueString (optional, default: mkValueStringDefault {})

Function to convert values to strings

sep

2. Function argument

k

3. Function argument

v

4. Function argument

Located at lib/generators.nix:147 in <nixpkgs>.

lib.generators.toKeyValue

Generate a key-value-style config file from an attrset.

Inputs
Structured function argument
mkKeyValue (optional, default: mkKeyValueDefault {} "=")

format a setting line from key and value

listsAsDuplicateKeys (optional, default: false)

allow lists as values for duplicate keys

indent (optional, default: "")

Initial indentation level

Located at lib/generators.nix:173 in <nixpkgs>.

lib.generators.toINI

Generate an INI-style config file from an attrset of sections to an attrset of key-value pairs.

Inputs
Structured function argument
mkSectionName (optional, default: (name: escape [ "[" "]" ] name))

apply transformations (e.g. escapes) to section names

mkKeyValue (optional, default: {} "=")

format a setting line from key and value

listsAsDuplicateKeys (optional, default: false)

allow lists as values for duplicate keys

Examples
Example 230. lib.generators.toINI usage example
generators.toINI {} {
  foo = { hi = "${pkgs.hello}"; ciao = "bar"; };
  baz = { "also, integers" = 42; };
}

> [baz]
> also, integers=42
>
> [foo]
> ciao=bar
> hi=/nix/store/y93qql1p5ggfnaqjjqhxcw0vqw95rlz0-hello-2.10

The mk* configuration attributes can generically change the way sections and key-value strings are generated.

For more examples see the test cases in ./tests/misc.nix.


Located at lib/generators.nix:227 in <nixpkgs>.

lib.generators.toINIWithGlobalSection

Generate an INI-style config file from an attrset specifying the global section (no header), and an attrset of sections to an attrset of key-value pairs.

Inputs
1. Structured function argument
mkSectionName (optional, default: (name: escape [ "[" "]" ] name))

apply transformations (e.g. escapes) to section names

mkKeyValue (optional, default: {} "=")

format a setting line from key and value

listsAsDuplicateKeys (optional, default: false)

allow lists as values for duplicate keys

2. Structured function argument
globalSection (required)

global section key-value pairs

sections (optional, default: {})

attrset of sections to key-value pairs

Examples
Example 231. lib.generators.toINIWithGlobalSection usage example
generators.toINIWithGlobalSection {} {
  globalSection = {
    someGlobalKey = "hi";
  };
  sections = {
    foo = { hi = "${pkgs.hello}"; ciao = "bar"; };
    baz = { "also, integers" = 42; };
}

> someGlobalKey=hi
>
> [baz]
> also, integers=42
>
> [foo]
> ciao=bar
> hi=/nix/store/y93qql1p5ggfnaqjjqhxcw0vqw95rlz0-hello-2.10

The mk* configuration attributes can generically change the way sections and key-value strings are generated.

For more examples see the test cases in ./tests/misc.nix.


If you don’t need a global section, you can also use generators.toINI directly, which only takes the part in sections.

Located at lib/generators.nix:305 in <nixpkgs>.

lib.generators.toGitINI

Generate a git-config file from an attrset.

It has two major differences from the regular INI format:

  1. values are indented with tabs

  2. sections can have sub-sections

Further: https://git-scm.com/docs/git-config#EXAMPLES

Examples
Example 232. lib.generators.toGitINI usage example
generators.toGitINI {
  url."ssh://git@github.com/".insteadOf = "https://github.com";
  user.name = "edolstra";
}

> [url "ssh://git@github.com/"]
>   insteadOf = "https://github.com"
>
> [user]
>   name = "edolstra"

Inputs
attrs

Key-value pairs to be converted to a git-config file. See: https://git-scm.com/docs/git-config#_variables for possible values.

Located at lib/generators.nix:353 in <nixpkgs>.

lib.generators.mkDconfKeyValue

mkKeyValueDefault wrapper that handles dconf INI quirks. The main differences of the format is that it requires strings to be quoted.

Located at lib/generators.nix:400 in <nixpkgs>.

lib.generators.toDconfINI

Generates INI in dconf keyfile style. See https://help.gnome.org/admin/system-admin-guide/stable/dconf-keyfiles.html.en for details.

Located at lib/generators.nix:406 in <nixpkgs>.

lib.generators.withRecursion

Recurses through a Value limited to a certain depth. (depthLimit)

If the depth is exceeded, an error is thrown, unless throwOnDepthLimit is set to false.

Inputs
Structured function argument
depthLimit (required)

If this option is not null, the given value will stop evaluating at a certain depth

throwOnDepthLimit (optional, default: true)

If this option is true, an error will be thrown, if a certain given depth is exceeded

Value

The value to be evaluated recursively

Located at lib/generators.nix:426 in <nixpkgs>.

lib.generators.toPretty

Pretty print a value, akin to builtins.trace.

Should probably be a builtin as well.

The pretty-printed string should be suitable for rendering default values in the NixOS manual. In particular, it should be as close to a valid Nix expression as possible.

Inputs
Structured function argument
allowPrettyValues

If this option is true, attrsets like { __pretty = fn; val = …; } will use fn to convert val to a pretty printed representation. (This means fn is type Val -> String.)

multiline

If this option is true, the output is indented with newlines for attribute sets and lists

indent

Initial indentation level

Value

The value to be pretty printed

Located at lib/generators.nix:483 in <nixpkgs>.

lib.generators.toPlist

Translate a simple Nix expression to Plist notation.

Inputs
Options

Empty set, there may be configuration options in the future

Value

The value to be converted to Plist

Located at lib/generators.nix:557 in <nixpkgs>.

lib.generators.toDhall

Translate a simple Nix expression to Dhall notation.

Note that integers are translated to Integer and never the Natural type.

Inputs
Options

Empty set, there may be configuration options in the future

Value

The value to be converted to Dhall

Located at lib/generators.nix:622 in <nixpkgs>.

lib.generators.toLua

Translate a simple Nix expression to Lua representation with occasional Lua-inlines that can be constructed by mkLuaInline function.

Configuration:

  • multiline - by default is true which results in indented block-like view.

  • indent - initial indent.

  • asBindings - by default generate single value, but with this use attrset to set global vars.

Attention:

Regardless of multiline parameter there is no trailing newline.

Inputs
Structured function argument
multiline (optional, default: true)

If this option is true, the output is indented with newlines for attribute sets and lists

indent (optional, default: "")

Initial indentation level

asBindings (optional, default: false)

Interpret as variable bindings

Value

The value to be converted to Lua

Type
toLua :: AttrSet -> Any -> String
Examples
Example 233. lib.generators.toLua usage example
generators.toLua {}
  {
    cmd = [ "typescript-language-server" "--stdio" ];
    settings.workspace.library = mkLuaInline ''vim.api.nvim_get_runtime_file("", true)'';
  }
->
 {
   ["cmd"] = {
     "typescript-language-server",
     "--stdio"
   },
   ["settings"] = {
     ["workspace"] = {
       ["library"] = (vim.api.nvim_get_runtime_file("", true))
     }
   }
 }

Located at lib/generators.nix:704 in <nixpkgs>.

lib.generators.mkLuaInline

Mark string as Lua expression to be inlined when processed by toLua.

Inputs
expr

1. Function argument

Type
mkLuaInline :: String -> AttrSet

Located at lib/generators.nix:771 in <nixpkgs>.

lib.gvariant: GVariant formatted string serialization functions

A partial and basic implementation of GVariant formatted strings. See GVariant Format Strings for details.

Warning

This API is not considered fully stable and it might therefore change in backwards incompatible ways without prior notice.

lib.gvariant.isGVariant

Check if a value is a GVariant value

Inputs
v

value to check

Type
isGVariant :: Any -> Bool

Located at lib/gvariant.nix:65 in <nixpkgs>.

lib.gvariant.mkValue

Returns the GVariant value that most closely matches the given Nix value. If no GVariant value can be found unambiguously then error is thrown.

Inputs
v

1. Function argument

Type
mkValue :: Any -> gvariant

Located at lib/gvariant.nix:131 in <nixpkgs>.

lib.gvariant.mkArray

Returns the GVariant array from the given type of the elements and a Nix list.

Inputs
elems

1. Function argument

Type
mkArray :: [Any] -> gvariant
Examples
Example 234. lib.gvariant.mkArray usage example
# Creating a string array
lib.gvariant.mkArray [ "a" "b" "c" ]

Located at lib/gvariant.nix:184 in <nixpkgs>.

lib.gvariant.mkEmptyArray

Returns the GVariant array from the given empty Nix list.

Inputs
elemType

1. Function argument

Type
mkEmptyArray :: gvariant.type -> gvariant
Examples
Example 235. lib.gvariant.mkEmptyArray usage example
# Creating an empty string array
lib.gvariant.mkEmptyArray (lib.gvariant.type.string)

Located at lib/gvariant.nix:223 in <nixpkgs>.

lib.gvariant.mkVariant

Returns the GVariant variant from the given Nix value. Variants are containers of different GVariant type.

Inputs
elem

1. Function argument

Type
mkVariant :: Any -> gvariant
Examples
Example 236. lib.gvariant.mkVariant usage example
lib.gvariant.mkArray [
  (lib.gvariant.mkVariant "a string")
  (lib.gvariant.mkVariant (lib.gvariant.mkInt32 1))
]

Located at lib/gvariant.nix:258 in <nixpkgs>.

lib.gvariant.mkDictionaryEntry

Returns the GVariant dictionary entry from the given key and value.

Inputs
name

The key of the entry

value

The value of the entry

Type
mkDictionaryEntry :: String -> Any -> gvariant
Examples
Example 237. lib.gvariant.mkDictionaryEntry usage example
# A dictionary describing an Epiphany’s search provider
[
  (lib.gvariant.mkDictionaryEntry "url" (lib.gvariant.mkVariant "https://duckduckgo.com/?q=%s&t=epiphany"))
  (lib.gvariant.mkDictionaryEntry "bang" (lib.gvariant.mkVariant "!d"))
  (lib.gvariant.mkDictionaryEntry "name" (lib.gvariant.mkVariant "DuckDuckGo"))
]

Located at lib/gvariant.nix:299 in <nixpkgs>.

lib.gvariant.mkMaybe

Returns the GVariant maybe from the given element type.

Inputs
elemType

1. Function argument

elem

2. Function argument

Type
mkMaybe :: gvariant.type -> Any -> gvariant

Located at lib/gvariant.nix:331 in <nixpkgs>.

lib.gvariant.mkNothing

Returns the GVariant nothing from the given element type.

Inputs
elemType

1. Function argument

Type
mkNothing :: gvariant.type -> gvariant

Located at lib/gvariant.nix:356 in <nixpkgs>.

lib.gvariant.mkJust

Returns the GVariant just from the given Nix value.

Inputs
elem

1. Function argument

Type
mkJust :: Any -> gvariant

Located at lib/gvariant.nix:374 in <nixpkgs>.

lib.gvariant.mkTuple

Returns the GVariant tuple from the given Nix list.

Inputs
elems

1. Function argument

Type
mkTuple :: [Any] -> gvariant

Located at lib/gvariant.nix:392 in <nixpkgs>.

lib.gvariant.mkBoolean

Returns the GVariant boolean from the given Nix bool value.

Inputs
v

1. Function argument

Type
mkBoolean :: Bool -> gvariant

Located at lib/gvariant.nix:418 in <nixpkgs>.

lib.gvariant.mkString

Returns the GVariant string from the given Nix string value.

Inputs
v

1. Function argument

Type
mkString :: String -> gvariant

Located at lib/gvariant.nix:439 in <nixpkgs>.

lib.gvariant.mkObjectpath

Returns the GVariant object path from the given Nix string value.

Inputs
v

1. Function argument

Type
mkObjectpath :: String -> gvariant

Located at lib/gvariant.nix:461 in <nixpkgs>.

lib.gvariant.mkUchar

Returns the GVariant uchar from the given Nix int value.

Type
mkUchar :: Int -> gvariant

Located at lib/gvariant.nix:475 in <nixpkgs>.

lib.gvariant.mkInt16

Returns the GVariant int16 from the given Nix int value.

Type
mkInt16 :: Int -> gvariant

Located at lib/gvariant.nix:486 in <nixpkgs>.

lib.gvariant.mkUint16

Returns the GVariant uint16 from the given Nix int value.

Type
mkUint16 :: Int -> gvariant

Located at lib/gvariant.nix:497 in <nixpkgs>.

lib.gvariant.mkInt32

Returns the GVariant int32 from the given Nix int value.

Inputs
v

1. Function argument

Type
mkInt32 :: Int -> gvariant

Located at lib/gvariant.nix:515 in <nixpkgs>.

lib.gvariant.mkUint32

Returns the GVariant uint32 from the given Nix int value.

Type
mkUint32 :: Int -> gvariant

Located at lib/gvariant.nix:529 in <nixpkgs>.

lib.gvariant.mkInt64

Returns the GVariant int64 from the given Nix int value.

Type
mkInt64 :: Int -> gvariant

Located at lib/gvariant.nix:540 in <nixpkgs>.

lib.gvariant.mkUint64

Returns the GVariant uint64 from the given Nix int value.

Type
mkUint64 :: Int -> gvariant

Located at lib/gvariant.nix:551 in <nixpkgs>.

lib.gvariant.mkDouble

Returns the GVariant double from the given Nix float value.

Inputs
v

1. Function argument

Type
mkDouble :: Float -> gvariant

Located at lib/gvariant.nix:569 in <nixpkgs>.

lib.customisation: Functions to customise (derivation-related) functions, derivatons, or attribute sets

lib.customisation.overrideDerivation

overrideDerivation drv f takes a derivation (i.e., the result of a call to the builtin function derivation) and returns a new derivation in which the attributes of the original are overridden according to the function f. The function f is called with the original derivation attributes.

overrideDerivation allows certain “ad-hoc” customisation scenarios (e.g. in ~/.config/nixpkgs/config.nix). For instance, if you want to “patch” the derivation returned by a package function in Nixpkgs to build another version than what the function itself provides.

For another application, see build-support/vm, where this function is used to build arbitrary derivations inside a QEMU virtual machine.

Note that in order to preserve evaluation errors, the new derivation’s outPath depends on the old one’s, which means that this function cannot be used in circular situations when the old derivation also depends on the new one.

You should in general prefer drv.overrideAttrs over this function; see the nixpkgs manual for more information on overriding.

Inputs
drv

1. Function argument

f

2. Function argument

Type
overrideDerivation :: Derivation -> ( Derivation -> AttrSet ) -> Derivation
Examples
Example 238. lib.customisation.overrideDerivation usage example
mySed = overrideDerivation pkgs.gnused (oldAttrs: {
  name = "sed-4.2.2-pre";
  src = fetchurl {
    url = ftp://alpha.gnu.org/gnu/sed/sed-4.2.2-pre.tar.bz2;
    hash = "sha256-MxBJRcM2rYzQYwJ5XKxhXTQByvSg5jZc5cSHEZoB2IY=";
  };
  patches = [];
});

Located at lib/customisation.nix:77 in <nixpkgs>.

lib.customisation.makeOverridable

makeOverridable takes a function from attribute set to attribute set and injects override attribute which can be used to override arguments of the function.

Please refer to documentation on <pkg>.overrideDerivation to learn about overrideDerivation and caveats related to its use.

Inputs
f

1. Function argument

Type
makeOverridable :: (AttrSet -> a) -> AttrSet -> a
Examples
Example 239. lib.customisation.makeOverridable usage example
nix-repl> x = {a, b}: { result = a + b; }

nix-repl> y = lib.makeOverridable x { a = 1; b = 2; }

nix-repl> y
{ override = «lambda»; overrideDerivation = «lambda»; result = 3; }

nix-repl> y.override { a = 10; }
{ override = «lambda»; overrideDerivation = «lambda»; result = 12; }

Located at lib/customisation.nix:131 in <nixpkgs>.

lib.customisation.callPackageWith

Call the package function in the file fn with the required arguments automatically. The function is called with the arguments args, but any missing arguments are obtained from autoArgs. This function is intended to be partially parameterised, e.g.,

callPackage = callPackageWith pkgs;
pkgs = {
  libfoo = callPackage ./foo.nix { };
  libbar = callPackage ./bar.nix { };
};

If the libbar function expects an argument named libfoo, it is automatically passed as an argument. Overrides or missing arguments can be supplied in args, e.g.

libbar = callPackage ./bar.nix {
  libfoo = null;
  enableX11 = true;
};
Inputs
autoArgs

1. Function argument

fn

2. Function argument

args

3. Function argument

Type
callPackageWith :: AttrSet -> ((AttrSet -> a) | Path) -> AttrSet -> a

Located at lib/customisation.nix:212 in <nixpkgs>.

lib.customisation.callPackagesWith

Like callPackage, but for a function that returns an attribute set of derivations. The override function is added to the individual attributes.

Inputs
autoArgs

1. Function argument

fn

2. Function argument

args

3. Function argument

Type
callPackagesWith :: AttrSet -> ((AttrSet -> AttrSet) | Path) -> AttrSet -> AttrSet

Located at lib/customisation.nix:298 in <nixpkgs>.

lib.customisation.extendDerivation

Add attributes to each output of a derivation without changing the derivation itself and check a given condition when evaluating.

Inputs
condition

1. Function argument

passthru

2. Function argument

drv

3. Function argument

Type
extendDerivation :: Bool -> Any -> Derivation -> Derivation

Located at lib/customisation.nix:339 in <nixpkgs>.

lib.customisation.hydraJob

Strip a derivation of all non-essential attributes, returning only those needed by hydra-eval-jobs. Also strictly evaluate the result to ensure that there are no thunks kept alive to prevent garbage collection.

Inputs
drv

1. Function argument

Type
hydraJob :: (Derivation | Null) -> (Derivation | Null)

Located at lib/customisation.nix:388 in <nixpkgs>.

lib.customisation.makeScope

Make an attribute set (a “scope”) from functions that take arguments from that same attribute set. See Example 240 for how to use it.

Inputs
  1. newScope (AttrSet -> ((AttrSet -> a) | Path) -> AttrSet -> a)

    A function that takes an attribute set attrs and returns what ends up as callPackage in the output.

    Typical values are callPackageWith or the output attribute newScope.

  2. f (AttrSet -> AttrSet)

    A function that takes an attribute set as returned by makeScope newScope f (a “scope”) and returns any attribute set.

    This function is used to compute the fixpoint of the resulting scope using callPackage. Its argument is the lazily evaluated reference to the value of that fixpoint, and is typically called self or final.

    See Example 240 for how to use it. See the section called “lib.fixedPoints: explicit recursion functions” for details on fixpoint computation.

Output

makeScope returns an attribute set of a form called scope, which also contains the final attributes produced by f:

scope :: {
  callPackage :: ((AttrSet -> a) | Path) -> AttrSet -> a
  newScope = AttrSet -> scope
  overrideScope = (scope -> scope -> AttrSet) -> scope
  packages :: AttrSet -> AttrSet
}
  • callPackage (((AttrSet -> a) | Path) -> AttrSet -> a)

    A function that

    1. Takes a function p, or a path to a Nix file that contains a function p, which takes an attribute set and returns value of arbitrary type a,

    2. Takes an attribute set args with explicit attributes to pass to p,

    3. Calls f with attributes from the original attribute set attrs passed to newScope updated with args, i.e. attrs // args, if they match the attributes in the argument of p`.

    All such functions p will be called with the same value for attrs.

    See Example 241 for how to use it.

  • newScope (AttrSet -> scope)

    Takes an attribute set attrs and returns a scope that extends the original scope.

  • overrideScope ((scope -> scope -> AttrSet) -> scope)

    Takes a function g of the form final: prev: { # attributes } to act as an overlay on f, and returns a new scope with values determined by extends g f. See for details.

    This allows subsequent modification of the final attribute set in a consistent way, i.e. all functions p invoked with callPackage will be called with the modified values.

  • packages (AttrSet -> AttrSet)

    The value of the argument f to makeScope.

  • final attributes

    The final values returned by f.

Examples
Example 240. Create an interdependent package set on top of pkgs

The functions in foo.nix and bar.nix can depend on each other, in the sense that foo.nix can contain a function that expects bar as an attribute in its argument.

let
  pkgs = import <nixpkgs> { };
in
pkgs.lib.makeScope pkgs.newScope (self: {
  foo = self.callPackage ./foo.nix { };
  bar = self.callPackage ./bar.nix { };
})

evaluates to

{
  callPackage = «lambda»;
  newScope = «lambda»;
  overrideScope = «lambda»;
  packages = «lambda»;
  foo = «derivation»;
  bar = «derivation»;
}

Example 241. Using callPackage from a scope
let
  pkgs = import <nixpkgs> { };
  inherit (pkgs) lib;
  scope = lib.makeScope lib.callPackageWith (self: { a = 1; b = 2; });
  three = scope.callPackage ({ a, b }: a + b) { };
  four = scope.callPackage ({ a, b }: a + b) { a = 2; };
in
[ three four ]

evaluates to

[ 3 4 ]

Type
makeScope :: (AttrSet -> ((AttrSet -> a) | Path) -> AttrSet -> a) -> (AttrSet -> AttrSet) -> scope

Located at lib/customisation.nix:541 in <nixpkgs>.

lib.customisation.makeScopeWithSplicing

backward compatibility with old uncurried form; deprecated

Inputs
splicePackages

1. Function argument

newScope

2. Function argument

otherSplices

3. Function argument

keep

4. Function argument

extra

5. Function argument

f

6. Function argument

Located at lib/customisation.nix:584 in <nixpkgs>.

lib.customisation.makeScopeWithSplicing'

Like makeScope, but aims to support cross compilation. It’s still ugly, but hopefully it helps a little bit.

Type
makeScopeWithSplicing' ::
  { splicePackages :: Splice -> AttrSet
  , newScope :: AttrSet -> ((AttrSet -> a) | Path) -> AttrSet -> a
  }
  -> { otherSplices :: Splice, keep :: AttrSet -> AttrSet, extra :: AttrSet -> AttrSet }
  -> AttrSet

Splice ::
  { pkgsBuildBuild :: AttrSet
  , pkgsBuildHost :: AttrSet
  , pkgsBuildTarget :: AttrSet
  , pkgsHostHost :: AttrSet
  , pkgsHostTarget :: AttrSet
  , pkgsTargetTarget :: AttrSet
  }

Located at lib/customisation.nix:614 in <nixpkgs>.

lib.meta: functions for derivation metadata

Some functions for manipulating meta attributes, as well as the name attribute.

lib.meta.addMetaAttrs

Add to or override the meta attributes of the given derivation.

Inputs
newAttrs

1. Function argument

drv

2. Function argument

Examples
Example 242. lib.meta.addMetaAttrs usage example
addMetaAttrs {description = "Bla blah";} somePkg

Located at lib/meta.nix:41 in <nixpkgs>.

lib.meta.dontDistribute

Disable Hydra builds of given derivation.

Inputs
drv

1. Function argument

Located at lib/meta.nix:54 in <nixpkgs>.

lib.meta.setName

Change the symbolic name of a derivation.

Warning

Dependent derivations will be rebuilt when the symbolic name is changed.

Inputs
name

1. Function argument

drv

2. Function argument

Located at lib/meta.nix:74 in <nixpkgs>.

lib.meta.updateName

Like setName, but takes the previous name as an argument.

Inputs
updater

1. Function argument

drv

2. Function argument

Examples
Example 243. lib.meta.updateName usage example
updateName (oldName: oldName + "-experimental") somePkg

Located at lib/meta.nix:101 in <nixpkgs>.

lib.meta.appendToName

Append a suffix to the name of a package (before the version part).

Inputs
suffix

1. Function argument

Located at lib/meta.nix:114 in <nixpkgs>.

lib.meta.mapDerivationAttrset

Apply a function to each derivation and only to derivations in an attrset.

Inputs
f

1. Function argument

set

2. Function argument

Located at lib/meta.nix:132 in <nixpkgs>.

lib.meta.setPrio

Set the nix-env priority of the package.

Inputs
priority

1. Function argument

drv

2. Function argument

Located at lib/meta.nix:145 in <nixpkgs>.

lib.meta.lowPrio

Decrease the nix-env priority of the package, i.e., other versions/variants of the package will be preferred.

Inputs
drv

1. Function argument

Located at lib/meta.nix:158 in <nixpkgs>.

lib.meta.lowPrioSet

Apply lowPrio to an attrset with derivations

Inputs
set

1. Function argument

Located at lib/meta.nix:170 in <nixpkgs>.

lib.meta.hiPrio

Increase the nix-env priority of the package, i.e., this version/variant of the package will be preferred.

Inputs
drv

1. Function argument

Located at lib/meta.nix:183 in <nixpkgs>.

lib.meta.hiPrioSet

Apply hiPrio to an attrset with derivations

Inputs
set

1. Function argument

Located at lib/meta.nix:195 in <nixpkgs>.

lib.meta.platformMatch

Check to see if a platform is matched by the given meta.platforms element.

A meta.platform pattern is either

  1. (legacy) a system string.

  2. (modern) a pattern for the entire platform structure (see lib.systems.inspect.platformPatterns).

  3. (modern) a pattern for the platform parsed field (see lib.systems.inspect.patterns).

We can inject these into a pattern for the whole of a structured platform, and then match that.

Inputs
platform

1. Function argument

elem

2. Function argument

Examples
Example 244. lib.meta.platformMatch usage example
lib.meta.platformMatch { system = "aarch64-darwin"; } "aarch64-darwin"
=> true

Located at lib/meta.nix:236 in <nixpkgs>.

lib.meta.availableOn

Check if a package is available on a given platform.

A package is available on a platform if both

  1. One of meta.platforms pattern matches the given platform, or meta.platforms is not present.

  2. None of meta.badPlatforms pattern matches the given platform.

Inputs
platform

1. Function argument

pkg

2. Function argument

Examples
Example 245. lib.meta.availableOn usage example
lib.meta.availableOn { system = "aarch64-darwin"; } pkg.zsh
=> true

Located at lib/meta.nix:285 in <nixpkgs>.

lib.meta.getLicenseFromSpdxId

Get the corresponding attribute in lib.licenses from the SPDX ID. For SPDX IDs, see https://spdx.org/licenses

Type
getLicenseFromSpdxId :: str -> AttrSet
Examples
Example 246. lib.meta.getLicenseFromSpdxId usage example
lib.getLicenseFromSpdxId "MIT" == lib.licenses.mit
=> true
lib.getLicenseFromSpdxId "mIt" == lib.licenses.mit
=> true
lib.getLicenseFromSpdxId "MY LICENSE"
=> trace: warning: getLicenseFromSpdxId: No license matches the given SPDX ID: MY LICENSE
=> { shortName = "MY LICENSE"; }

Located at lib/meta.nix:317 in <nixpkgs>.

lib.meta.getExe

Get the path to the main program of a package based on meta.mainProgram

Inputs
x

1. Function argument

Type
getExe :: package -> string
Examples
Example 247. lib.meta.getExe usage example
getExe pkgs.hello
=> "/nix/store/g124820p9hlv4lj8qplzxw1c44dxaw1k-hello-2.12/bin/hello"
getExe pkgs.mustache-go
=> "/nix/store/am9ml4f4ywvivxnkiaqwr0hyxka1xjsf-mustache-go-1.3.0/bin/mustache"

Located at lib/meta.nix:356 in <nixpkgs>.

lib.meta.getExe'

Get the path of a program of a derivation.

Inputs
x

1. Function argument

y

2. Function argument

Type
getExe' :: derivation -> string -> string
Examples
Example 248. lib.meta.getExe' usage example
getExe' pkgs.hello "hello"
=> "/nix/store/g124820p9hlv4lj8qplzxw1c44dxaw1k-hello-2.12/bin/hello"
getExe' pkgs.imagemagick "convert"
=> "/nix/store/5rs48jamq7k6sal98ymj9l4k2bnwq515-imagemagick-7.1.1-15/bin/convert"

Located at lib/meta.nix:396 in <nixpkgs>.

lib.derivations: miscellaneous derivation-specific functions

lib.derivations.lazyDerivation

Restrict a derivation to a predictable set of attribute names, so that the returned attrset is not strict in the actual derivation, saving a lot of computation when the derivation is non-trivial.

This is useful in situations where a derivation might only be used for its passthru attributes, improving evaluation performance.

The returned attribute set is lazy in derivation. Specifically, this means that the derivation will not be evaluated in at least the situations below.

For illustration and/or testing, we define derivation such that its evaluation is very noticeable.

let derivation = throw "This won't be evaluated.";

In the following expressions, derivation will not be evaluated:

(lazyDerivation { inherit derivation; }).type

attrNames (lazyDerivation { inherit derivation; })

(lazyDerivation { inherit derivation; } // { foo = true; }).foo

(lazyDerivation { inherit derivation; meta.foo = true; }).meta

In these expressions, derivation will be evaluated:

"${lazyDerivation { inherit derivation }}"

(lazyDerivation { inherit derivation }).outPath

(lazyDerivation { inherit derivation }).meta

And the following expressions are not valid, because the refer to implementation details and/or attributes that may not be present on some derivations:

(lazyDerivation { inherit derivation }).buildInputs

(lazyDerivation { inherit derivation }).passthru

(lazyDerivation { inherit derivation }).pythonPath
Inputs

Takes an attribute set with the following attributes

derivation

The derivation to be wrapped.

meta

Optional meta attribute.

While this function is primarily about derivations, it can improve the meta package attribute, which is usually specified through mkDerivation.

passthru

Optional extra values to add to the returned attrset.

This can be used for adding package attributes, such as tests.

outputs

Optional list of assumed outputs. Default: [“out”]

This must match the set of outputs that the returned derivation has. You must use this when the derivation has multiple outputs.

Located at lib/derivations.nix:90 in <nixpkgs>.

lib.derivations.optionalDrvAttr

Conditionally set a derivation attribute.

Because mkDerivation sets __ignoreNulls = true, a derivation attribute set to null will not impact the derivation output hash. Thus, this function passes through its value argument if the cond is true, but returns null if not.

Inputs
cond

Condition

value

Attribute value

Type
optionalDrvAttr :: Bool -> a -> a | Null
Examples
Example 249. lib.derivations.optionalDrvAttr usage example
(stdenv.mkDerivation {
  name = "foo";
  x = optionalDrvAttr true 1;
  y = optionalDrvAttr false 1;
}).drvPath == (stdenv.mkDerivation {
  name = "foo";
  x = 1;
}).drvPath
=> true

Located at lib/derivations.nix:206 in <nixpkgs>.

Generators

Generators are functions that create file formats from nix data structures, e. g. for configuration files. There are generators available for: INI, JSON and YAML

All generators follow a similar call interface: generatorName configFunctions data, where configFunctions is an attrset of user-defined functions that format nested parts of the content. They each have common defaults, so often they do not need to be set manually. An example is mkSectionName ? (name: libStr.escape [ "[" "]" ] name) from the INI generator. It receives the name of a section and sanitizes it. The default mkSectionName escapes [ and ] with a backslash.

Generators can be fine-tuned to produce exactly the file format required by your application/service. One example is an INI-file format which uses : as separator, the strings "yes"/"no" as boolean values and requires all string values to be quoted:

let
  inherit (lib) generators isString;

  customToINI = generators.toINI {
    # specifies how to format a key/value pair
    mkKeyValue = generators.mkKeyValueDefault {
      # specifies the generated string for a subset of nix values
      mkValueString = v:
             if v == true then ''"yes"''
        else if v == false then ''"no"''
        else if isString v then ''"${v}"''
        # and delegates all other values to the default generator
        else generators.mkValueStringDefault {} v;
    } ":";
  };

# the INI file can now be given as plain old nix values
in customToINI {
  main = {
    pushinfo = true;
    autopush = false;
    host = "localhost";
    port = 42;
  };
  mergetool = {
    merge = "diff3";
  };
}

This will produce the following INI file as nix string:

[main]
autopush:"no"
host:"localhost"
port:42
pushinfo:"yes"
str\:ange:"very::strange"

[mergetool]
merge:"diff3"

Note

Nix store paths can be converted to strings by enclosing a derivation attribute like so: "${drv}".

Detailed documentation for each generator can be found here

Debugging Nix Expressions

Nix is a unityped, dynamic language, this means every value can potentially appear anywhere. Since it is also non-strict, evaluation order and what ultimately is evaluated might surprise you. Therefore it is important to be able to debug nix expressions.

In the lib/debug.nix file you will find a number of functions that help (pretty-)printing values while evaluation is running. You can even specify how deep these values should be printed recursively, and transform them on the fly. Please consult the docstrings in lib/debug.nix for usage information.

prefer-remote-fetch overlay

prefer-remote-fetch is an overlay that download sources on remote builder. This is useful when the evaluating machine has a slow upload while the builder can fetch faster directly from the source. To use it, put the following snippet as a new overlay:

self: super:
  (super.prefer-remote-fetch self super)

A full configuration example for that sets the overlay up for your own account, could look like this

$ mkdir ~/.config/nixpkgs/overlays/
$ cat > ~/.config/nixpkgs/overlays/prefer-remote-fetch.nix <<EOF
  self: super: super.prefer-remote-fetch self super
EOF

pkgs.nix-gitignore

pkgs.nix-gitignore is a function that acts similarly to builtins.filterSource but also allows filtering with the help of the gitignore format.

Usage

pkgs.nix-gitignore exports a number of functions, but you’ll most likely need either gitignoreSource or gitignoreSourcePure. As their first argument, they both accept either 1. a file with gitignore lines or 2. a string with gitignore lines, or 3. a list of either of the two. They will be concatenated into a single big string.

{ pkgs ? import <nixpkgs> {} }: {

 src = nix-gitignore.gitignoreSource [] ./source;
     # Simplest version

 src = nix-gitignore.gitignoreSource "supplemental-ignores\n" ./source;
     # This one reads the ./source/.gitignore and concats the auxiliary ignores

 src = nix-gitignore.gitignoreSourcePure "ignore-this\nignore-that\n" ./source;
     # Use this string as gitignore, don't read ./source/.gitignore.

 src = nix-gitignore.gitignoreSourcePure ["ignore-this\nignore-that\n" ~/.gitignore] ./source;
     # It also accepts a list (of strings and paths) that will be concatenated
     # once the paths are turned to strings via readFile.
}

These functions are derived from the Filter functions by setting the first filter argument to (_: _: true):

{
  gitignoreSourcePure = gitignoreFilterSourcePure (_: _: true);
  gitignoreSource = gitignoreFilterSource (_: _: true);
}

Those filter functions accept the same arguments the builtins.filterSource function would pass to its filters, thus fn: gitignoreFilterSourcePure fn "" should be extensionally equivalent to filterSource. The file is blacklisted if it’s blacklisted by either your filter or the gitignoreFilter.

If you want to make your own filter from scratch, you may use

{
  gitignoreFilter = ign: root: filterPattern (gitignoreToPatterns ign) root;
}

gitignore files in subdirectories

If you wish to use a filter that would search for .gitignore files in subdirectories, just like git does by default, use this function:

{
  # gitignoreFilterRecursiveSource = filter: patterns: root:
  # OR
  gitignoreRecursiveSource = gitignoreFilterSourcePure (_: _: true);
}

Module System

Table of Contents

Introduction
lib.evalModules

Introduction

The module system is a language for handling configuration, implemented as a Nix library.

Compared to plain Nix, it adds documentation, type checking and composition or extensibility.

Note

This chapter is new and not complete yet. For a gentle introduction to the module system, in the context of NixOS, see Writing NixOS Modules in the NixOS manual.

lib.evalModules

Evaluate a set of modules. This function is typically only used once per application (e.g. once in NixOS, once in Home Manager, …).

Parameters

modules

A list of modules. These are merged together to form the final configuration.

specialArgs

An attribute set of module arguments that can be used in imports.

This is in contrast to config._module.args, which is only available after all imports have been resolved.

class

If the class attribute is set and non-null, the module system will reject imports with a different _class declaration.

The class value should be a string in lower camel case.

If applicable, the class should match the “prefix” of the attributes used in (experimental) flakes. Some examples are:

  • nixos as in flake.nixosModules

  • nixosTest: modules that constitute a NixOS VM test

prefix

A list of strings representing the location at or below which all options are evaluated. This is used by types.submodule to improve error reporting and find the implicit name module argument.

Return value

The result is an attribute set with the following attributes:

options

The nested attribute set of all option declarations.

config

The nested attribute set of all option values.

type

A module system type. This type is an instance of types.submoduleWith containing the current modules.

The option definitions that are typed with this type will extend the current set of modules, like extendModules.

However, the value returned from the type is just the config, like any submodule.

If you’re familiar with prototype inheritance, you can think of this evalModules invocation as the prototype, and usages of this type as the instances.

This type is also available to the modules as the module argument moduleType.

extendModules

A function similar to evalModules but building on top of the already passed modules. Its arguments, modules and specialArgs are added to the existing values.

If you’re familiar with prototype inheritance, you can think of the current, actual evalModules invocation as the prototype, and the return value of extendModules as the instance.

This functionality is also available to modules as the extendModules module argument.

Note

Evaluation Performance

extendModules returns a configuration that shares very little with the original evalModules invocation, because the module arguments may be different.

So if you have a configuration that has been (or will be) largely evaluated, almost none of the computation is shared with the configuration returned by extendModules.

The real work of module evaluation happens while computing the values in config and options, so multiple invocations of extendModules have a particularly small cost, as long as only the final config and options are evaluated.

If you do reference multiple config (or options) from before and after extendModules, evaluation performance is the same as with multiple evalModules invocations, because the new modules’ ability to override existing configuration fundamentally requires constructing a new config and options fixpoint.

_module

A portion of the configuration tree which is elided from config.

_type

A nominal type marker, always "configuration".

class

The class argument.

Standard environment

The Standard Environment

The standard build environment in the Nix Packages collection provides an environment for building Unix packages that does a lot of common build tasks automatically. In fact, for Unix packages that use the standard ./configure; make; make install build interface, you don’t need to write a build script at all; the standard environment does everything automatically. If stdenv doesn’t do what you need automatically, you can easily customise or override the various build phases.

Using stdenv

To build a package with the standard environment, you use the function stdenv.mkDerivation, instead of the primitive built-in function derivation, e.g.

stdenv.mkDerivation {
  name = "libfoo-1.2.3";
  src = fetchurl {
    url = "http://example.org/libfoo-1.2.3.tar.bz2";
    hash = "sha256-tWxU/LANbQE32my+9AXyt3nCT7NBVfJ45CX757EMT3Q=";
  };
}

(stdenv needs to be in scope, so if you write this in a separate Nix expression from pkgs/all-packages.nix, you need to pass it as a function argument.) Specifying a name and a src is the absolute minimum Nix requires. For convenience, you can also use pname and version attributes and mkDerivation will automatically set name to "${pname}-${version}" by default. Since RFC 0035, this is preferred for packages in Nixpkgs, as it allows us to reuse the version easily:

stdenv.mkDerivation rec {
  pname = "libfoo";
  version = "1.2.3";
  src = fetchurl {
    url = "http://example.org/libfoo-source-${version}.tar.bz2";
    hash = "sha256-tWxU/LANbQE32my+9AXyt3nCT7NBVfJ45CX757EMT3Q=";
  };
}

Many packages have dependencies that are not provided in the standard environment. It’s usually sufficient to specify those dependencies in the buildInputs attribute:

stdenv.mkDerivation {
  pname = "libfoo";
  version = "1.2.3";
  # ...
  buildInputs = [libbar perl ncurses];
}

This attribute ensures that the bin subdirectories of these packages appear in the PATH environment variable during the build, that their include subdirectories are searched by the C compiler, and so on. (See the section called “Package setup hooks” for details.)

Often it is necessary to override or modify some aspect of the build. To make this easier, the standard environment breaks the package build into a number of phases, all of which can be overridden or modified individually: unpacking the sources, applying patches, configuring, building, and installing. (There are some others; see the section called “Phases”.) For instance, a package that doesn’t supply a makefile but instead has to be compiled “manually” could be handled like this:

stdenv.mkDerivation {
  pname = "fnord";
  version = "4.5";
  # ...
  buildPhase = ''
    gcc foo.c -o foo
  '';
  installPhase = ''
    mkdir -p $out/bin
    cp foo $out/bin
  '';
}

(Note the use of ''-style string literals, which are very convenient for large multi-line script fragments because they don’t need escaping of " and \, and because indentation is intelligently removed.)

There are many other attributes to customise the build. These are listed in the section called “Attributes”.

While the standard environment provides a generic builder, you can still supply your own build script:

stdenv.mkDerivation {
  pname = "libfoo";
  version = "1.2.3";
  # ...
  builder = ./builder.sh;
}

where the builder can do anything it wants, but typically starts with

source $stdenv/setup

to let stdenv set up the environment (e.g. by resetting PATH and populating it from build inputs). If you want, you can still use stdenv’s generic builder:

source $stdenv/setup

buildPhase() {
  echo "... this is my custom build phase ..."
  gcc foo.c -o foo
}

installPhase() {
  mkdir -p $out/bin
  cp foo $out/bin
}

genericBuild

Building a stdenv package in nix-shell

To build a stdenv package in a nix-shell, enter a shell, find the phases you wish to build, then invoke genericBuild manually:

Go to an empty directory, invoke nix-shell with the desired package, and from inside the shell, set the output variables to a writable directory:

cd "$(mktemp -d)"
nix-shell '<nixpkgs>' -A some_package
export out=$(pwd)/out

Next, invoke the desired parts of the build. First, run the phases that generate a working copy of the sources, which will change directory to the sources for you:

phases="${prePhases[*]:-} unpackPhase patchPhase" genericBuild

Then, run more phases up until the failure is reached. If the failure is in the build or check phase, the following phases would be required:

phases="${preConfigurePhases[*]:-} configurePhase ${preBuildPhases[*]:-} buildPhase checkPhase" genericBuild

Use this command to run all install phases:

phases="${preInstallPhases[*]:-} installPhase ${preFixupPhases[*]:-} fixupPhase installCheckPhase" genericBuild

Single phase can be re-run as many times as necessary to examine the failure like so:

phases="buildPhase" genericBuild

To modify a phase, first print it with

echo "$buildPhase"

Or, if that is empty, for instance, if it is using a function:

type buildPhase

then change it in a text editor, and paste it back to the terminal.

Note

This method may have some inconsistencies in environment variables and behaviour compared to a normal build within the Nix build sandbox. The following is a non-exhaustive list of such differences:

  • TMP, TMPDIR, and similar variables likely point to non-empty directories that the build might conflict with files in.

  • Output store paths are not writable, so the variables for outputs need to be overridden to writable paths.

  • Other environment variables may be inconsistent with a nix-build either due to nix-shell’s initialization script or due to the use of nix-shell without the --pure option.

If the build fails differently inside the shell than in the sandbox, consider using breakpointHook and invoking nix-build instead. The --keep-failed option for nix-build may also be useful to examine the build directory of a failed build.

Tools provided by stdenv

The standard environment provides the following packages:

  • The GNU C Compiler, configured with C and C++ support.

  • GNU coreutils (contains a few dozen standard Unix commands).

  • GNU findutils (contains find).

  • GNU diffutils (contains diff, cmp).

  • GNU sed.

  • GNU grep.

  • GNU awk.

  • GNU tar.

  • gzip, bzip2 and xz.

  • GNU Make.

  • Bash. This is the shell used for all builders in the Nix Packages collection. Not using /bin/sh removes a large source of portability problems.

  • The patch command.

On Linux, stdenv also includes the patchelf utility.

Specifying dependencies

Build systems often require more dependencies than just what stdenv provides. This section describes attributes accepted by stdenv.mkDerivation that can be used to make these dependencies available to the build system.

Overview

A full reference of the different kinds of dependencies is provided in the section called “Reference”, but here is an overview of the most common ones. It should cover most use cases.

Add dependencies to nativeBuildInputs if they are executed during the build:

  • those which are needed on $PATH during the build, for example cmake and pkg-config

  • setup hooks, for example makeWrapper

  • interpreters needed by patchShebangs for build scripts (with the --build flag), which can be the case for e.g. perl

Add dependencies to buildInputs if they will end up copied or linked into the final output or otherwise used at runtime:

  • libraries used by compilers, for example zlib,

  • interpreters needed by patchShebangs for scripts which are installed, which can be the case for e.g. perl

Note

These criteria are independent.

For example, software using Wayland usually needs the wayland library at runtime, so wayland should be added to buildInputs. But it also executes the wayland-scanner program as part of the build to generate code, so wayland should also be added to nativeBuildInputs.

Dependencies needed only to run tests are similarly classified between native (executed during build) and non-native (executed at runtime):

  • nativeCheckInputs for test tools needed on $PATH (such as ctest) and setup hooks (for example pytestCheckHook)

  • checkInputs for libraries linked into test executables (for example the qcheck OCaml package)

These dependencies are only injected when doCheck is set to true.

Example

Consider for example this simplified derivation for solo5, a sandboxing tool:

stdenv.mkDerivation rec {
  pname = "solo5";
  version = "0.7.5";

  src = fetchurl {
    url = "https://github.com/Solo5/solo5/releases/download/v${version}/solo5-v${version}.tar.gz";
    hash = "sha256-viwrS9lnaU8sTGuzK/+L/PlMM/xRRtgVuK5pixVeDEw=";
  };

  nativeBuildInputs = [ makeWrapper pkg-config ];
  buildInputs = [ libseccomp ];

  postInstall = ''
    substituteInPlace $out/bin/solo5-virtio-mkimage \
      --replace-fail "/usr/lib/syslinux" "${syslinux}/share/syslinux" \
      --replace-fail "/usr/share/syslinux" "${syslinux}/share/syslinux" \
      --replace-fail "cp " "cp --no-preserve=mode "

    wrapProgram $out/bin/solo5-virtio-mkimage \
      --prefix PATH : ${lib.makeBinPath [ dosfstools mtools parted syslinux ]}
  '';

  doCheck = true;
  nativeCheckInputs = [ util-linux qemu ];
  checkPhase = '' [elided] '';
}
  • makeWrapper is a setup hook, i.e., a shell script sourced by the generic builder of stdenv. It is thus executed during the build and must be added to nativeBuildInputs.

  • pkg-config is a build tool which the configure script of solo5 expects to be on $PATH during the build: therefore, it must be added to nativeBuildInputs.

  • libseccomp is a library linked into $out/bin/solo5-elftool. As it is used at runtime, it must be added to buildInputs.

  • Tests need qemu and getopt (from util-linux) on $PATH, these must be added to nativeCheckInputs.

  • Some dependencies are injected directly in the shell code of phases: syslinux, dosfstools, mtools, and parted. In this specific case, they will end up in the output of the derivation ($out here). As Nix marks dependencies whose absolute path is present in the output as runtime dependencies, adding them to buildInputs is not required.

For more complex cases, like libraries linked into an executable which is then executed as part of the build system, see the section called “Reference”.

Reference

As described in the Nix manual, almost any *.drv store path in a derivation’s attribute set will induce a dependency on that derivation. mkDerivation, however, takes a few attributes intended to include all the dependencies of a package. This is done both for structure and consistency, but also so that certain other setup can take place. For example, certain dependencies need their bin directories added to the PATH. That is built-in, but other setup is done via a pluggable mechanism that works in conjunction with these dependency attributes. See the section called “Package setup hooks” for details.

Dependencies can be broken down along these axes: their host and target platforms relative to the new derivation’s. The platform distinctions are motivated by cross compilation; see Cross-compilation for exactly what each platform means. [1] But even if one is not cross compiling, the platforms imply whether a dependency is needed at run-time or build-time.

The extension of PATH with dependencies, alluded to above, proceeds according to the relative platforms alone. The process is carried out only for dependencies whose host platform matches the new derivation’s build platform i.e. dependencies which run on the platform where the new derivation will be built. [2] For each dependency <dep> of those dependencies, dep/bin, if present, is added to the PATH environment variable.

Dependency propagation

Propagated dependencies are made available to all downstream dependencies. This is particularly useful for interpreted languages, where all transitive dependencies have to be present in the same environment. Therefore it is used for the Python infrastructure in Nixpkgs.

Note

Propagated dependencies should be used with care, because they obscure the actual build inputs of dependent derivations and cause side effects through setup hooks. This can lead to conflicting dependencies that cannot easily be resolved.

Example 250. A propagated dependency
with import <nixpkgs> {};
let
  bar = stdenv.mkDerivation {
    name = "bar";
    dontUnpack = true;
    # `hello` is also made available to dependents, such as `foo`
    propagatedBuildInputs = [ hello ];
    postInstall = "mkdir $out";
  };
  foo = stdenv.mkDerivation {
    name = "foo";
    dontUnpack = true;
    # `bar` is a direct dependency, which implicitly includes the propagated `hello`
    buildInputs = [ bar ];
    # The `hello` binary is available!
    postInstall = "hello > $out";
  };
in
foo

Dependency propagation takes cross compilation into account, meaning that dependencies that cross platform boundaries are properly adjusted.

To determine the exact rules for dependency propagation, we start by assigning to each dependency a couple of ternary numbers (-1 for build, 0 for host, and 1 for target) representing its dependency type, which captures how its host and target platforms are each “offset” from the depending derivation’s host and target platforms. The following table summarize the different combinations that can be obtained:

host → targetattribute nameoffset
build --> builddepsBuildBuild-1, -1
build --> hostnativeBuildInputs-1, 0
build --> targetdepsBuildTarget-1, 1
host --> hostdepsHostHost0, 0
host --> targetbuildInputs0, 1
target --> targetdepsTargetTarget1, 1

Algorithmically, we traverse propagated inputs, accumulating every propagated dependency’s propagated dependencies and adjusting them to account for the “shift in perspective” described by the current dependency’s platform offsets. This results is sort of a transitive closure of the dependency relation, with the offsets being approximately summed when two dependency links are combined. We also prune transitive dependencies whose combined offsets go out-of-bounds, which can be viewed as a filter over that transitive closure removing dependencies that are blatantly absurd.

We can define the process precisely with Natural Deduction using the inference rules. This probably seems a bit obtuse, but so is the bash code that actually implements it! [3] They’re confusing in very different ways so… hopefully if something doesn’t make sense in one presentation, it will in the other!

let mapOffset(h, t, i) = i + (if i <= 0 then h else t - 1)

propagated-dep(h0, t0, A, B)
propagated-dep(h1, t1, B, C)
h0 + h1 in {-1, 0, 1}
h0 + t1 in {-1, 0, 1}
-------------------------------------- Transitive property
propagated-dep(mapOffset(h0, t0, h1),
               mapOffset(h0, t0, t1),
               A, C)
let mapOffset(h, t, i) = i + (if i <= 0 then h else t - 1)

dep(h0, t0, A, B)
propagated-dep(h1, t1, B, C)
h0 + h1 in {-1, 0, 1}
h0 + t1 in {-1, 0, -1}
----------------------------- Take immediate dependencies' propagated dependencies
propagated-dep(mapOffset(h0, t0, h1),
               mapOffset(h0, t0, t1),
               A, C)
propagated-dep(h, t, A, B)
----------------------------- Propagated dependencies count as dependencies
dep(h, t, A, B)

Some explanation of this monstrosity is in order. In the common case, the target offset of a dependency is the successor to the target offset: t = h + 1. That means that:

let f(h, t, i) = i + (if i <= 0 then h else t - 1)
let f(h, h + 1, i) = i + (if i <= 0 then h else (h + 1) - 1)
let f(h, h + 1, i) = i + (if i <= 0 then h else h)
let f(h, h + 1, i) = i + h

This is where “sum-like” comes in from above: We can just sum all of the host offsets to get the host offset of the transitive dependency. The target offset is the transitive dependency is the host offset + 1, just as it was with the dependencies composed to make this transitive one; it can be ignored as it doesn’t add any new information.

Because of the bounds checks, the uncommon cases are h = t and h + 2 = t. In the former case, the motivation for mapOffset is that since its host and target platforms are the same, no transitive dependency of it should be able to “discover” an offset greater than its reduced target offsets. mapOffset effectively “squashes” all its transitive dependencies’ offsets so that none will ever be greater than the target offset of the original h = t package. In the other case, h + 1 is skipped over between the host and target offsets. Instead of squashing the offsets, we need to “rip” them apart so no transitive dependencies’ offset is that one.

Overall, the unifying theme here is that propagation shouldn’t be introducing transitive dependencies involving platforms the depending package is unaware of. [One can imagine the depending package asking for dependencies with the platforms it knows about; other platforms it doesn’t know how to ask for. The platform description in that scenario is a kind of unforgeable capability.] The offset bounds checking and definition of mapOffset together ensure that this is the case. Discovering a new offset is discovering a new platform, and since those platforms weren’t in the derivation “spec” of the needing package, they cannot be relevant. From a capability perspective, we can imagine that the host and target platforms of a package are the capabilities a package requires, and the depending package must provide the capability to the dependency.

Variables specifying dependencies

depsBuildBuild

A list of dependencies whose host and target platforms are the new derivation’s build platform. These are programs and libraries used at build time that produce programs and libraries also used at build time. If the dependency doesn’t care about the target platform (i.e. isn’t a compiler or similar tool), put it in nativeBuildInputs instead. The most common use of this buildPackages.stdenv.cc, the default C compiler for this role. That example crops up more than one might think in old commonly used C libraries.

Since these packages are able to be run at build-time, they are always added to the PATH, as described above. But since these packages are only guaranteed to be able to run then, they shouldn’t persist as run-time dependencies. This isn’t currently enforced, but could be in the future.

nativeBuildInputs

A list of dependencies whose host platform is the new derivation’s build platform, and target platform is the new derivation’s host platform. These are programs and libraries used at build-time that, if they are a compiler or similar tool, produce code to run at run-time—i.e. tools used to build the new derivation. If the dependency doesn’t care about the target platform (i.e. isn’t a compiler or similar tool), put it here, rather than in depsBuildBuild or depsBuildTarget. This could be called depsBuildHost but nativeBuildInputs is used for historical continuity.

Since these packages are able to be run at build-time, they are added to the PATH, as described above. But since these packages are only guaranteed to be able to run then, they shouldn’t persist as run-time dependencies. This isn’t currently enforced, but could be in the future.

depsBuildTarget

A list of dependencies whose host platform is the new derivation’s build platform, and target platform is the new derivation’s target platform. These are programs used at build time that produce code to run with code produced by the depending package. Most commonly, these are tools used to build the runtime or standard library that the currently-being-built compiler will inject into any code it compiles. In many cases, the currently-being-built-compiler is itself employed for that task, but when that compiler won’t run (i.e. its build and host platform differ) this is not possible. Other times, the compiler relies on some other tool, like binutils, that is always built separately so that the dependency is unconditional.

This is a somewhat confusing concept to wrap one’s head around, and for good reason. As the only dependency type where the platform offsets, -1 and 1, are not adjacent integers, it requires thinking of a bootstrapping stage two away from the current one. It and its use-case go hand in hand and are both considered poor form: try to not need this sort of dependency, and try to avoid building standard libraries and runtimes in the same derivation as the compiler produces code using them. Instead strive to build those like a normal library, using the newly-built compiler just as a normal library would. In short, do not use this attribute unless you are packaging a compiler and are sure it is needed.

Since these packages are able to run at build time, they are added to the PATH, as described above. But since these packages are only guaranteed to be able to run then, they shouldn’t persist as run-time dependencies. This isn’t currently enforced, but could be in the future.

depsHostHost

A list of dependencies whose host and target platforms match the new derivation’s host platform. In practice, this would usually be tools used by compilers for macros or a metaprogramming system, or libraries used by the macros or metaprogramming code itself. It’s always preferable to use a depsBuildBuild dependency in the derivation being built over a depsHostHost on the tool doing the building for this purpose.

buildInputs

A list of dependencies whose host platform and target platform match the new derivation’s. This would be called depsHostTarget but for historical continuity. If the dependency doesn’t care about the target platform (i.e. isn’t a compiler or similar tool), put it here, rather than in depsBuildBuild.

These are often programs and libraries used by the new derivation at run-time, but that isn’t always the case. For example, the machine code in a statically-linked library is only used at run-time, but the derivation containing the library is only needed at build-time. Even in the dynamic case, the library may also be needed at build-time to appease the linker.

depsTargetTarget

A list of dependencies whose host platform matches the new derivation’s target platform. These are packages that run on the target platform, e.g. the standard library or run-time deps of standard library that a compiler insists on knowing about. It’s poor form in almost all cases for a package to depend on another from a future stage [future stage corresponding to positive offset]. Do not use this attribute unless you are packaging a compiler and are sure it is needed.

depsBuildBuildPropagated

The propagated equivalent of depsBuildBuild. This perhaps never ought to be used, but it is included for consistency [see below for the others].

propagatedNativeBuildInputs

The propagated equivalent of nativeBuildInputs. This would be called depsBuildHostPropagated but for historical continuity. For example, if package Y has propagatedNativeBuildInputs = [X], and package Z has buildInputs = [Y], then package Z will be built as if it included package X in its nativeBuildInputs. If instead, package Z has nativeBuildInputs = [Y], then Z will be built as if it included X in the depsBuildBuild of package Z, because of the sum of the two -1 host offsets.

depsBuildTargetPropagated

The propagated equivalent of depsBuildTarget. This is prefixed for the same reason of alerting potential users.

depsHostHostPropagated

The propagated equivalent of depsHostHost.

propagatedBuildInputs

The propagated equivalent of buildInputs. This would be called depsHostTargetPropagated but for historical continuity.

depsTargetTargetPropagated

The propagated equivalent of depsTargetTarget. This is prefixed for the same reason of alerting potential users.

Attributes

Variables affecting stdenv initialisation

NIX_DEBUG

A number between 0 and 7 indicating how much information to log. If set to 1 or higher, stdenv will print moderate debugging information during the build. In particular, the gcc and ld wrapper scripts will print out the complete command line passed to the wrapped tools. If set to 6 or higher, the stdenv setup script will be run with set -x tracing. If set to 7 or higher, the gcc and ld wrapper scripts will also be run with set -x tracing.

Attributes affecting build properties

enableParallelBuilding

If set to true, stdenv will pass specific flags to make and other build tools to enable parallel building with up to build-cores workers.

Unless set to false, some build systems with good support for parallel building including cmake, meson, and qmake will set it to true.

Fixed-point arguments of mkDerivation

If you pass a function to mkDerivation, it will receive as its argument the final arguments, including the overrides when reinvoked via overrideAttrs. For example:

mkDerivation (finalAttrs: {
  pname = "hello";
  withFeature = true;
  configureFlags =
    lib.optionals finalAttrs.withFeature ["--with-feature"];
})

Note that this does not use the rec keyword to reuse withFeature in configureFlags. The rec keyword works at the syntax level and is unaware of overriding.

Instead, the definition references finalAttrs, allowing users to change withFeature consistently with overrideAttrs.

finalAttrs also contains the attribute finalPackage, which includes the output paths, etc.

Let’s look at a more elaborate example to understand the differences between various bindings:

# `pkg` is the _original_ definition (for illustration purposes)
let pkg =
  mkDerivation (finalAttrs: {
    # ...

    # An example attribute
    packages = [];

    # `passthru.tests` is a commonly defined attribute.
    passthru.tests.simple = f finalAttrs.finalPackage;

    # An example of an attribute containing a function
    passthru.appendPackages = packages':
      finalAttrs.finalPackage.overrideAttrs (newSelf: super: {
        packages = super.packages ++ packages';
      });

    # For illustration purposes; referenced as
    # `(pkg.overrideAttrs(x)).finalAttrs` etc in the text below.
    passthru.finalAttrs = finalAttrs;
    passthru.original = pkg;
  });
in pkg

Unlike the pkg binding in the above example, the finalAttrs parameter always references the final attributes. For instance (pkg.overrideAttrs(x)).finalAttrs.finalPackage is identical to pkg.overrideAttrs(x), whereas (pkg.overrideAttrs(x)).original is the same as the original pkg.

See also the section about passthru.tests.

Phases

stdenv.mkDerivation sets the Nix derivation’s builder to a script that loads the stdenv setup.sh bash library and calls genericBuild. Most packaging functions rely on this default builder.

This generic command either invokes a script at buildCommandPath, or a buildCommand, or a number of phases. Package builds are split into phases to make it easier to override specific parts of the build (e.g., unpacking the sources or installing the binaries).

Each phase can be overridden in its entirety either by setting the environment variable namePhase to a string containing some shell commands to be executed, or by redefining the shell function namePhase. The former is convenient to override a phase from the derivation, while the latter is convenient from a build script. However, typically one only wants to add some commands to a phase, e.g. by defining postInstall or preFixup, as skipping some of the default actions may have unexpected consequences. The default script for each phase is defined in the file pkgs/stdenv/generic/setup.sh.

When overriding a phase, for example installPhase, it is important to start with runHook preInstall and end it with runHook postInstall, otherwise preInstall and postInstall will not be run. Even if you don’t use them directly, it is good practice to do so anyways for downstream users who would want to add a postInstall by overriding your derivation.

While inside an interactive nix-shell, if you wanted to run all phases in the order they would be run in an actual build, you can invoke genericBuild yourself.

Controlling phases

There are a number of variables that control what phases are executed and in what order:

Variables affecting phase control

phases

Specifies the phases. You can change the order in which phases are executed, or add new phases, by setting this variable. If it’s not set, the default value is used, which is $prePhases unpackPhase patchPhase $preConfigurePhases configurePhase $preBuildPhases buildPhase checkPhase $preInstallPhases installPhase fixupPhase installCheckPhase $preDistPhases distPhase $postPhases.

It is discouraged to set this variable, as it is easy to miss some important functionality hidden in some of the less obviously needed phases (like fixupPhase which patches the shebang of scripts). Usually, if you just want to add a few phases, it’s more convenient to set one of the variables below (such as preInstallPhases).

prePhases

Additional phases executed before any of the default phases.

preConfigurePhases

Additional phases executed just before the configure phase.

preBuildPhases

Additional phases executed just before the build phase.

preInstallPhases

Additional phases executed just before the install phase.

preFixupPhases

Additional phases executed just before the fixup phase.

preDistPhases

Additional phases executed just before the distribution phase.

postPhases

Additional phases executed after any of the default phases.

The unpack phase

The unpack phase is responsible for unpacking the source code of the package. The default implementation of unpackPhase unpacks the source files listed in the src environment variable to the current directory. It supports the following files by default:

Tar files

These can optionally be compressed using gzip (.tar.gz, .tgz or .tar.Z), bzip2 (.tar.bz2, .tbz2 or .tbz) or xz (.tar.xz, .tar.lzma or .txz).

Zip files

Zip files are unpacked using unzip. However, unzip is not in the standard environment, so you should add it to nativeBuildInputs yourself.

Directories in the Nix store

These are copied to the current directory. The hash part of the file name is stripped, e.g. /nix/store/1wydxgby13cz...-my-sources would be copied to my-sources.

Additional file types can be supported by setting the unpackCmd variable (see below).

Variables controlling the unpack phase

srcs / src

The list of source files or directories to be unpacked or copied. One of these must be set. Note that if you use srcs, you should also set sourceRoot or setSourceRoot.

sourceRoot

After unpacking all of src and srcs, if neither of sourceRoot and setSourceRoot are set, unpackPhase of the generic builder checks that the unpacking produced a single directory and moves the current working directory into it.

If unpackPhase produces multiple source directories, you should set sourceRoot to the name of the intended directory. You can also set sourceRoot = "."; if you want to control it yourself in a later phase.

For example, if your want your build to start in a sub-directory inside your sources, and you are using fetchzip-derived src (like fetchFromGitHub or similar), you need to set sourceRoot = "${src.name}/my-sub-directory".

setSourceRoot

Alternatively to setting sourceRoot, you can set setSourceRoot to a shell command to be evaluated by the unpack phase after the sources have been unpacked. This command must set sourceRoot.

For example, if you are using fetchurl on an archive file that gets unpacked into a single directory the name of which changes between package versions, and you want your build to start in its sub-directory, you need to set setSourceRoot = "sourceRoot=$(echo */my-sub-directory)";, or in the case of multiple sources, you could use something more specific, like setSourceRoot = "sourceRoot=$(echo ${pname}-*/my-sub-directory)";.

preUnpack

Hook executed at the start of the unpack phase.

postUnpack

Hook executed at the end of the unpack phase.

dontUnpack

Set to true to skip the unpack phase.

dontMakeSourcesWritable

If set to 1, the unpacked sources are not made writable. By default, they are made writable to prevent problems with read-only sources. For example, copied store directories would be read-only without this.

unpackCmd

The unpack phase evaluates the string $unpackCmd for any unrecognised file. The path to the current source file is contained in the curSrc variable.

The patch phase

The patch phase applies the list of patches defined in the patches variable.

Variables controlling the patch phase

dontPatch

Set to true to skip the patch phase.

patches

The list of patches. They must be in the format accepted by the patch command, and may optionally be compressed using gzip (.gz), bzip2 (.bz2) or xz (.xz).

patchFlags

Flags to be passed to patch. If not set, the argument -p1 is used, which causes the leading directory component to be stripped from the file names in each patch.

prePatch

Hook executed at the start of the patch phase.

postPatch

Hook executed at the end of the patch phase.

The configure phase

The configure phase prepares the source tree for building. The default configurePhase runs ./configure (typically an Autoconf-generated script) if it exists.

Variables controlling the configure phase

configureScript

The name of the configure script. It defaults to ./configure if it exists; otherwise, the configure phase is skipped. This can actually be a command (like perl ./Configure.pl).

configureFlags

A list of strings passed as additional arguments to the configure script.

dontConfigure

Set to true to skip the configure phase.

configureFlagsArray

A shell array containing additional arguments passed to the configure script. You must use this instead of configureFlags if the arguments contain spaces.

dontAddPrefix

By default, ./configure is passed the concatenation of prefixKey and prefix on the command line. Disable this by setting dontAddPrefix to true.

prefix

The prefix under which the package must be installed, passed via the --prefix option to the configure script. It defaults to $out.

prefixKey

The key to use when specifying the installation prefix. By default, this is set to --prefix= as that is used by the majority of packages. Other packages may need --prefix (with a trailing space) or PREFIX=.

dontAddStaticConfigureFlags

By default, when building statically, stdenv will try to add build system appropriate configure flags to try to enable static builds.

If this is undesirable, set this variable to true.

dontAddDisableDepTrack

By default, the flag --disable-dependency-tracking is added to the configure flags to speed up Automake-based builds. If this is undesirable, set this variable to true.

dontFixLibtool

By default, the configure phase applies some special hackery to all files called ltmain.sh before running the configure script in order to improve the purity of Libtool-based packages [4] . If this is undesirable, set this variable to true.

dontDisableStatic

By default, when the configure script has --enable-static, the option --disable-static is added to the configure flags.

If this is undesirable, set this variable to true. It is automatically set to true when building statically, for example through pkgsStatic.

configurePlatforms

By default, when cross compiling, the configure script has --build=... and --host=... passed. Packages can instead pass [ "build" "host" "target" ] or a subset to control exactly which platform flags are passed. Compilers and other tools can use this to also pass the target platform. [5]

preConfigure

Hook executed at the start of the configure phase.

postConfigure

Hook executed at the end of the configure phase.

The build phase

The build phase is responsible for actually building the package (e.g. compiling it). The default buildPhase calls make if a file named Makefile, makefile or GNUmakefile exists in the current directory (or the makefile is explicitly set); otherwise it does nothing.

Variables controlling the build phase

dontBuild

Set to true to skip the build phase.

makefile

The file name of the Makefile.

makeFlags

A list of strings passed as additional flags to make. These flags are also used by the default install and check phase. For setting make flags specific to the build phase, use buildFlags (see below).

{
  makeFlags = [ "PREFIX=$(out)" ];
}

Note

The flags are quoted in bash, but environment variables can be specified by using the make syntax.

makeFlagsArray

A shell array containing additional arguments passed to make. You must use this instead of makeFlags if the arguments contain spaces, e.g.

{
  preBuild = ''
    makeFlagsArray+=(CFLAGS="-O0 -g" LDFLAGS="-lfoo -lbar")
  '';
}

Note that shell arrays cannot be passed through environment variables, so you cannot set makeFlagsArray in a derivation attribute (because those are passed through environment variables): you have to define them in shell code.

buildFlags / buildFlagsArray

A list of strings passed as additional flags to make. Like makeFlags and makeFlagsArray, but only used by the build phase. Any build targets should be specified as part of the buildFlags.

preBuild

Hook executed at the start of the build phase.

postBuild

Hook executed at the end of the build phase.

You can set flags for make through the makeFlags variable.

Before and after running make, the hooks preBuild and postBuild are called, respectively.

The check phase

The check phase checks whether the package was built correctly by running its test suite. The default checkPhase calls make $checkTarget, but only if the doCheck variable is enabled.

It is highly recommended, for packages’ sources that are not distributed with any tests, to at least use versionCheckHook to test that the resulting executable is basically functional.

Variables controlling the check phase

doCheck

Controls whether the check phase is executed. By default it is skipped, but if doCheck is set to true, the check phase is usually executed. Thus you should set

{
  doCheck = true;
}

in the derivation to enable checks. The exception is cross compilation. Cross compiled builds never run tests, no matter how doCheck is set, as the newly-built program won’t run on the platform used to build it.

makeFlags / makeFlagsArray / makefile

See the build phase for details.

checkTarget

The make target that runs the tests. If unset, use check if it exists, otherwise test; if neither is found, do nothing.

checkFlags / checkFlagsArray

A list of strings passed as additional flags to make. Like makeFlags and makeFlagsArray, but only used by the check phase. Unlike with buildFlags, the checkTarget is automatically added to the make invocation in addition to any checkFlags specified.

checkInputs

A list of host dependencies used by the phase, usually libraries linked into executables built during tests. This gets included in buildInputs when doCheck is set.

nativeCheckInputs

A list of native dependencies used by the phase, notably tools needed on $PATH. This gets included in nativeBuildInputs when doCheck is set.

preCheck

Hook executed at the start of the check phase.

postCheck

Hook executed at the end of the check phase.

The install phase

The install phase is responsible for installing the package in the Nix store under out. The default installPhase creates the directory $out and calls make install.

Variables controlling the install phase

dontInstall

Set to true to skip the install phase.

makeFlags / makeFlagsArray / makefile

See the build phase for details.

installTargets

The make targets that perform the installation. Defaults to install. Example:

{
  installTargets = "install-bin install-doc";
}
installFlags / installFlagsArray

A list of strings passed as additional flags to make. Like makeFlags and makeFlagsArray, but only used by the install phase. Unlike with buildFlags, the installTargets are automatically added to the make invocation in addition to any installFlags specified.

preInstall

Hook executed at the start of the install phase.

postInstall

Hook executed at the end of the install phase.

The fixup phase

The fixup phase performs (Nix-specific) post-processing actions on the files installed under $out by the install phase. The default fixupPhase does the following:

  • It moves the man/, doc/ and info/ subdirectories of $out to share/.

  • It strips libraries and executables of debug information.

  • On Linux, it applies the patchelf command to ELF executables and libraries to remove unused directories from the RPATH in order to prevent unnecessary runtime dependencies.

  • It rewrites the interpreter paths of shell scripts to paths found in PATH. E.g., /usr/bin/perl will be rewritten to /nix/store/some-perl/bin/perl found in PATH. See the section called “patch-shebangs.sh for details.

Variables controlling the fixup phase

dontFixup

Set to true to skip the fixup phase.

dontStrip

If set, libraries and executables are not stripped. By default, they are.

dontStripHost

Like dontStrip, but only affects the strip command targeting the package’s host platform. Useful when supporting cross compilation, but otherwise feel free to ignore.

dontStripTarget

Like dontStrip, but only affects the strip command targeting the packages’ target platform. Useful when supporting cross compilation, but otherwise feel free to ignore.

dontMoveSbin

If set, files in $out/sbin are not moved to $out/bin. By default, they are.

stripAllList

List of directories to search for libraries and executables from which all symbols should be stripped. By default, it’s empty. Stripping all symbols is risky, since it may remove not just debug symbols but also ELF information necessary for normal execution.

stripAllListTarget

Like stripAllList, but only applies to packages’ target platform. By default, it’s empty. Useful when supporting cross compilation.

stripAllFlags

Flags passed to the strip command applied to the files in the directories listed in stripAllList. Defaults to -s (i.e. --strip-all).

stripDebugList

List of directories to search for libraries and executables from which only debugging-related symbols should be stripped. It defaults to lib lib32 lib64 libexec bin sbin.

stripDebugListTarget

Like stripDebugList, but only applies to packages’ target platform. By default, it’s empty. Useful when supporting cross compilation.

stripDebugFlags

Flags passed to the strip command applied to the files in the directories listed in stripDebugList. Defaults to -S (i.e. --strip-debug).

stripExclude

A list of filenames or path patterns to avoid stripping. A file is excluded if its name or path (from the derivation root) matches.

This example prevents all *.rlib files from being stripped:

stdenv.mkDerivation {
  # ...
  stripExclude = [ "*.rlib" ];
}

This example prevents files within certain paths from being stripped:

stdenv.mkDerivation {
  # ...
  stripExclude = [ "lib/modules/*/build/*" ];
}
dontPatchELF

If set, the patchelf command is not used to remove unnecessary RPATH entries. Only applies to Linux.

dontPatchShebangs

If set, scripts starting with #! do not have their interpreter paths rewritten to paths in the Nix store. See the section called “patch-shebangs.sh on how patching shebangs works.

dontPruneLibtoolFiles

If set, libtool .la files associated with shared libraries won’t have their dependency_libs field cleared.

forceShare

The list of directories that must be moved from $out to $out/share. Defaults to man doc info.

setupHook

A package can export a setup hook by setting this variable. The setup hook, if defined, is copied to $out/nix-support/setup-hook. Environment variables are then substituted in it using substituteAll.

preFixup

Hook executed at the start of the fixup phase.

postFixup

Hook executed at the end of the fixup phase.

separateDebugInfo

If set to true, the standard environment will enable debug information in C/C++ builds. After installation, the debug information will be separated from the executables and stored in the output named debug. (This output is enabled automatically; you don’t need to set the outputs attribute explicitly.) To be precise, the debug information is stored in debug/lib/debug/.build-id/XX/YYYY…, where <XXYYYY…> is the <build ID> of the binary — a SHA-1 hash of the contents of the binary. Debuggers like GDB use the build ID to look up the separated debug information.

Example 251. Enable debug symbols for use with GDB

To make GDB find debug information for the socat package and its dependencies, you can use the following shell.nix:

let
  pkgs = import ./. {
    config = {};
    overlays = [
      (final: prev: {
        ncurses = prev.ncurses.overrideAttrs { separateDebugInfo = true; };
        readline = prev.readline.overrideAttrs { separateDebugInfo = true; };
      })
    ];
  };

  myDebugInfoDirs = pkgs.symlinkJoin {
    name = "myDebugInfoDirs";
    paths = with pkgs; [
      glibc.debug
      ncurses.debug
      openssl.debug
      readline.debug
    ];
  };
in
  pkgs.mkShell {

    NIX_DEBUG_INFO_DIRS = "${pkgs.lib.getLib myDebugInfoDirs}/lib/debug";

    packages = [
      pkgs.gdb
      pkgs.socat
    ];

    shellHook = ''
      ${pkgs.lib.getBin pkgs.gdb}/bin/gdb ${pkgs.lib.getBin pkgs.socat}/bin/socat
    '';
  }

This setup works as follows:

  • Add overlays to the package set, since debug symbols are disabled for ncurses and readline by default.

  • Create a derivation to combine all required debug symbols under one path with symlinkJoin.

  • Set the environment variable NIX_DEBUG_INFO_DIRS in the shell. Nixpkgs patches gdb to use it for looking up debug symbols.

  • Run gdb on the socat binary on shell startup in the shellHook. Here we use lib.getBin to ensure that the correct derivation output is selected rather than the default one.


The installCheck phase

The installCheck phase checks whether the package was installed correctly by running its test suite against the installed directories. The default installCheck calls make installcheck.

It is often better to add tests that are not part of the source distribution to passthru.tests (see the section called “passthru.tests). This avoids adding overhead to every build and enables us to run them independently.

Variables controlling the installCheck phase

doInstallCheck

Controls whether the installCheck phase is executed. By default it is skipped, but if doInstallCheck is set to true, the installCheck phase is usually executed. Thus you should set

{
  doInstallCheck = true;
}

in the derivation to enable install checks. The exception is cross compilation. Cross compiled builds never run tests, no matter how doInstallCheck is set, as the newly-built program won’t run on the platform used to build it.

installCheckTarget

The make target that runs the install tests. Defaults to installcheck.

installCheckFlags / installCheckFlagsArray

A list of strings passed as additional flags to make. Like makeFlags and makeFlagsArray, but only used by the installCheck phase.

installCheckInputs

A list of host dependencies used by the phase, usually libraries linked into executables built during tests. This gets included in buildInputs when doInstallCheck is set.

nativeInstallCheckInputs

A list of native dependencies used by the phase, notably tools needed on $PATH. This gets included in nativeBuildInputs when doInstallCheck is set.

preInstallCheck

Hook executed at the start of the installCheck phase.

postInstallCheck

Hook executed at the end of the installCheck phase.

The distribution phase

The distribution phase is intended to produce a source distribution of the package. The default distPhase first calls make dist, then it copies the resulting source tarballs to $out/tarballs/. This phase is only executed if the attribute doDist is set.

Variables controlling the distribution phase

doDist

If set, the distribution phase is executed.

distTarget

The make target that produces the distribution. Defaults to dist.

distFlags / distFlagsArray

Additional flags passed to make.

tarballs

The names of the source distribution files to be copied to $out/tarballs/. It can contain shell wildcards. The default is *.tar.gz.

dontCopyDist

If set, no files are copied to $out/tarballs/.

preDist

Hook executed at the start of the distribution phase.

postDist

Hook executed at the end of the distribution phase.

Shell functions and utilities

The standard environment provides a number of useful functions.

makeWrapper <executable> <wrapperfile> <args>

Constructs a wrapper for a program with various possible arguments. It is defined as part of 2 setup-hooks named makeWrapper and makeBinaryWrapper that implement the same bash functions. Hence, to use it you have to add makeWrapper to your nativeBuildInputs. Here’s an example usage:

# adds `FOOBAR=baz` to `$out/bin/foo`’s environment
makeWrapper $out/bin/foo $wrapperfile --set FOOBAR baz

# Prefixes the binary paths of `hello` and `git`
# and suffixes the binary path of `xdg-utils`.
# Be advised that paths often should be patched in directly
# (via string replacements or in `configurePhase`).
makeWrapper $out/bin/foo $wrapperfile \
  --prefix PATH : ${lib.makeBinPath [ hello git ]} \
  --suffix PATH : ${lib.makeBinPath [ xdg-utils ]}

Packages may expect or require other utilities to be available at runtime. makeWrapper can be used to add packages to a PATH environment variable local to a wrapper.

Use --prefix to explicitly set dependencies in PATH.

Note

--prefix essentially hard-codes dependencies into the wrapper. They cannot be overridden without rebuilding the package.

If dependencies should be resolved at runtime, use --suffix to append fallback values to PATH.

There’s many more kinds of arguments, they are documented in nixpkgs/pkgs/build-support/setup-hooks/make-wrapper.sh for the makeWrapper implementation and in nixpkgs/pkgs/build-support/setup-hooks/make-binary-wrapper/make-binary-wrapper.sh for the makeBinaryWrapper implementation.

wrapProgram is a convenience function you probably want to use most of the time, implemented by both makeWrapper and makeBinaryWrapper.

Using the makeBinaryWrapper implementation is usually preferred, as it creates a tiny compiled wrapper executable, that can be used as a shebang interpreter. This is needed mostly on Darwin, where shebangs cannot point to scripts, due to a limitation with the execve-syscall. Compiled wrappers generated by makeBinaryWrapper can be inspected with less <path-to-wrapper> - by scrolling past the binary data you should be able to see the shell command that generated the executable and there see the environment variables that were injected into the wrapper.

remove-references-to -t <storepath> [ -t <storepath> … ] <file> …

Removes the references of the specified files to the specified store files. This is done without changing the size of the file by replacing the hash by eeeeeeeeeeeeeeeeeeeeeeeeeeeeeeee, and should work on compiled executables. This is meant to be used to remove the dependency of the output on inputs that are known to be unnecessary at runtime. Of course, reckless usage will break the patched programs. To use this, add removeReferencesTo to nativeBuildInputs.

As remove-references-to is an actual executable and not a shell function, it can be used with find. Example removing all references to the compiler in the output:

{
  postInstall = ''
    find "$out" -type f -exec remove-references-to -t ${stdenv.cc} '{}' +
  '';
}

substitute <infile> <outfile> <subs>

Performs string substitution on the contents of <infile>, writing the result to <outfile>. The substitutions in <subs> are of the following form:

--replace-fail <s1> <s2>

Replace every occurrence of the string <s1> by <s2>. Will error if no change is made.

--replace-warn <s1> <s2>

Replace every occurrence of the string <s1> by <s2>. Will print a warning if no change is made.

--replace-quiet <s1> <s2>

Replace every occurrence of the string <s1> by <s2>. Will do nothing if no change can be made.

--subst-var <varName>

Replace every occurrence of @varName@ by the contents of the environment variable <varName>. This is useful for generating files from templates, using @...@ in the template as placeholders.

--subst-var-by <varName> <s>

Replace every occurrence of @varName@ by the string <s>.

Example:

substitute ./foo.in ./foo.out \
    --replace-fail /usr/bin/bar $bar/bin/bar \
    --replace-fail "a string containing spaces" "some other text" \
    --subst-var someVar

substituteInPlace <multiple files> <subs>

Like substitute, but performs the substitutions in place on the files passed.

substituteAll <infile> <outfile>

Replaces every occurrence of @varName@, where <varName> is any environment variable, in <infile>, writing the result to <outfile>. For instance, if <infile> has the contents

#! @bash@/bin/sh
PATH=@coreutils@/bin
echo @foo@

and the environment contains bash=/nix/store/bmwp0q28cf21...-bash-3.2-p39 and coreutils=/nix/store/68afga4khv0w...-coreutils-6.12, but does not contain the variable foo, then the output will be

#! /nix/store/bmwp0q28cf21...-bash-3.2-p39/bin/sh
PATH=/nix/store/68afga4khv0w...-coreutils-6.12/bin
echo @foo@

That is, no substitution is performed for undefined variables.

Environment variables that start with an uppercase letter or an underscore are filtered out, to prevent global variables (like HOME) or private variables (like __ETC_PROFILE_DONE) from accidentally getting substituted. The variables also have to be valid bash “names”, as defined in the bash manpage (alphanumeric or _, must not start with a number).

substituteAllInPlace <file>

Like substituteAll, but performs the substitutions in place on the file <file>.

stripHash <path>

Strips the directory and hash part of a store path, outputting the name part to stdout. For example:

# prints coreutils-8.24
stripHash "/nix/store/9s9r019176g7cvn2nvcw41gsp862y6b4-coreutils-8.24"

If you wish to store the result in another variable, then the following idiom may be useful:

name="/nix/store/9s9r019176g7cvn2nvcw41gsp862y6b4-coreutils-8.24"
someVar=$(stripHash $name)

wrapProgram <executable> <makeWrapperArgs>

Convenience function for makeWrapper that replaces <executable> with a wrapper that executes the original program. It takes all the same arguments as makeWrapper, except for --inherit-argv0 (used by the makeBinaryWrapper implementation) and --argv0 (used by both makeWrapper and makeBinaryWrapper wrapper implementations).

If you will apply it multiple times, it will overwrite the wrapper file and you will end up with double wrapping, which should be avoided.

prependToVar <variableName> <elements…>

Prepend elements to a variable.

Example:

$ configureFlags="--disable-static"
$ prependToVar configureFlags --disable-dependency-tracking --enable-foo
$ echo $configureFlags
--disable-dependency-tracking --enable-foo --disable-static

appendToVar <variableName> <elements…>

Append elements to a variable.

Example:

$ configureFlags="--disable-static"
$ appendToVar configureFlags --disable-dependency-tracking --enable-foo
$ echo $configureFlags
--disable-static --disable-dependency-tracking --enable-foo

Package setup hooks

Nix itself considers a build-time dependency as merely something that should previously be built and accessible at build time—packages themselves are on their own to perform any additional setup. In most cases, that is fine, and the downstream derivation can deal with its own dependencies. But for a few common tasks, that would result in almost every package doing the same sort of setup work—depending not on the package itself, but entirely on which dependencies were used.

In order to alleviate this burden, the setup hook mechanism was written, where any package can include a shell script that [by convention rather than enforcement by Nix], any downstream reverse-dependency will source as part of its build process. That allows the downstream dependency to merely specify its dependencies, and lets those dependencies effectively initialize themselves. No boilerplate mirroring the list of dependencies is needed.

The setup hook mechanism is a bit of a sledgehammer though: a powerful feature with a broad and indiscriminate area of effect. The combination of its power and implicit use may be expedient, but isn’t without costs. Nix itself is unchanged, but the spirit of added dependencies being effect-free is violated even if the latter isn’t. For example, if a derivation path is mentioned more than once, Nix itself doesn’t care and makes sure the dependency derivation is already built just the same—depending is just needing something to exist, and needing is idempotent. However, a dependency specified twice will have its setup hook run twice, and that could easily change the build environment (though a well-written setup hook will therefore strive to be idempotent so this is in fact not observable). More broadly, setup hooks are anti-modular in that multiple dependencies, whether the same or different, should not interfere and yet their setup hooks may well do so.

The most typical use of the setup hook is actually to add other hooks which are then run (i.e. after all the setup hooks) on each dependency. For example, the C compiler wrapper’s setup hook feeds itself flags for each dependency that contains relevant libraries and headers. This is done by defining a bash function, and appending its name to one of envBuildBuildHooks, envBuildHostHooks, envBuildTargetHooks, envHostHostHooks, envHostTargetHooks, or envTargetTargetHooks. These 6 bash variables correspond to the 6 sorts of dependencies by platform (there’s 12 total but we ignore the propagated/non-propagated axis).

Packages adding a hook should not hard code a specific hook, but rather choose a variable relative to how they are included. Returning to the C compiler wrapper example, if the wrapper itself is an n dependency, then it only wants to accumulate flags from n + 1 dependencies, as only those ones match the compiler’s target platform. The hostOffset variable is defined with the current dependency’s host offset targetOffset with its target offset, before its setup hook is sourced. Additionally, since most environment hooks don’t care about the target platform, that means the setup hook can append to the right bash array by doing something like

addEnvHooks "$hostOffset" myBashFunction

The existence of setups hooks has long been documented and packages inside Nixpkgs are free to use this mechanism. Other packages, however, should not rely on these mechanisms not changing between Nixpkgs versions. Because of the existing issues with this system, there’s little benefit from mandating it be stable for any period of time.

First, let’s cover some setup hooks that are part of Nixpkgs default stdenv. This means that they are run for every package built using stdenv.mkDerivation or when using a custom builder that has source $stdenv/setup. Some of these are platform specific, so they may run on Linux but not Darwin or vice-versa.

move-docs.sh

This setup hook moves any installed documentation to the /share subdirectory directory. This includes the man, doc and info directories. This is needed for legacy programs that do not know how to use the share subdirectory.

compress-man-pages.sh

This setup hook compresses any man pages that have been installed. The compression is done using the gzip program. This helps to reduce the installed size of packages.

strip.sh

This runs the strip command on installed binaries and libraries. This removes unnecessary information like debug symbols when they are not needed. This also helps to reduce the installed size of packages.

patch-shebangs.sh

This setup hook patches installed scripts to add Nix store paths to their shebang interpreter as found in the build environment. The shebang line tells a Unix-like operating system which interpreter to use to execute the script’s contents.

Note

The generic builder populates PATH from inputs of the derivation.

Invocation

Multiple paths can be specified.

patchShebangs [--build | --host] PATH...
Flags
--build

Look up commands available at build time

--host

Look up commands available at run time

Examples
patchShebangs --host /nix/store/<hash>-hello-1.0/bin
patchShebangs --build configure

#!/bin/sh will be rewritten to #!/nix/store/<hash>-some-bash/bin/sh.

#!/usr/bin/env gets special treatment: #!/usr/bin/env python is rewritten to /nix/store/<hash>/bin/python.

Interpreter paths that point to a valid Nix store location are not changed.

Note

A script file must be marked as executable, otherwise it will not be considered.

This mechanism ensures that the interpreter for a given script is always found and is exactly the one specified by the build.

It can be disabled by setting dontPatchShebangs:

stdenv.mkDerivation {
  # ...
  dontPatchShebangs = true;
  # ...
}

The file patch-shebangs.sh defines the patchShebangs function. It is used to implement patchShebangsAuto, the setup hook that is registered to run during the fixup phase by default.

If you need to run patchShebangs at build time, it must be called explicitly within one of the build phases.

audit-tmpdir.sh

This verifies that no references are left from the install binaries to the directory used to build those binaries. This ensures that the binaries do not need things outside the Nix store. This is currently supported in Linux only.

multiple-outputs.sh

This setup hook adds configure flags that tell packages to install files into any one of the proper outputs listed in outputs. This behavior can be turned off by setting setOutputFlags to false in the derivation environment. See Multiple-output packages for more information.

move-sbin.sh

This setup hook moves any binaries installed in the sbin/ subdirectory into bin/. In addition, a link is provided from sbin/ to bin/ for compatibility.

move-lib64.sh

This setup hook moves any libraries installed in the lib64/ subdirectory into lib/. In addition, a link is provided from lib64/ to lib/ for compatibility.

move-systemd-user-units.sh

This setup hook moves any systemd user units installed in the lib/ subdirectory into share/. In addition, a link is provided from share/ to lib/ for compatibility. This is needed for systemd to find user services when installed into the user profile.

This hook only runs when compiling for Linux.

set-source-date-epoch-to-latest.sh

This sets SOURCE_DATE_EPOCH to the modification time of the most recent file.

Bintools Wrapper and hook

The Bintools Wrapper wraps the binary utilities for a bunch of miscellaneous purposes. These are GNU Binutils when targeting Linux, and a mix of cctools and GNU binutils for Darwin. [The “Bintools” name is supposed to be a compromise between “Binutils” and “cctools” not denoting any specific implementation.] Specifically, the underlying bintools package, and a C standard library (glibc or Darwin’s libSystem, just for the dynamic loader) are all fed in, and dependency finding, hardening (see below), and purity checks for each are handled by the Bintools Wrapper. Packages typically depend on CC Wrapper, which in turn (at run time) depends on the Bintools Wrapper.

The Bintools Wrapper was only just recently split off from CC Wrapper, so the division of labor is still being worked out. For example, it shouldn’t care about the C standard library, but just take a derivation with the dynamic loader (which happens to be the glibc on linux). Dependency finding however is a task both wrappers will continue to need to share, and probably the most important to understand. It is currently accomplished by collecting directories of host-platform dependencies (i.e. buildInputs and nativeBuildInputs) in environment variables. The Bintools Wrapper’s setup hook causes any lib and lib64 subdirectories to be added to NIX_LDFLAGS. Since the CC Wrapper and the Bintools Wrapper use the same strategy, most of the Bintools Wrapper code is sparsely commented and refers to the CC Wrapper. But the CC Wrapper’s code, by contrast, has quite lengthy comments. The Bintools Wrapper merely cites those, rather than repeating them, to avoid falling out of sync.

A final task of the setup hook is defining a number of standard environment variables to tell build systems which executables fulfill which purpose. They are defined to just be the base name of the tools, under the assumption that the Bintools Wrapper’s binaries will be on the path. Firstly, this helps poorly-written packages, e.g. ones that look for just gcc when CC isn’t defined yet clang is to be used. Secondly, this helps packages not get confused when cross-compiling, in which case multiple Bintools Wrappers may simultaneously be in use. [6] BUILD_- and TARGET_-prefixed versions of the normal environment variable are defined for additional Bintools Wrappers, properly disambiguating them.

A problem with this final task is that the Bintools Wrapper is honest and defines LD as ld. Most packages, however, firstly use the C compiler for linking, secondly use LD anyways, defining it as the C compiler, and thirdly, only so define LD when it is undefined as a fallback. This triple-threat means Bintools Wrapper will break those packages, as LD is already defined as the actual linker which the package won’t override yet doesn’t want to use. The workaround is to define, just for the problematic package, LD as the C compiler. A good way to do this would be preConfigure = "LD=$CC".

CC Wrapper and hook

The CC Wrapper wraps a C toolchain for a bunch of miscellaneous purposes. Specifically, a C compiler (GCC or Clang), wrapped binary tools, and a C standard library (glibc or Darwin’s libSystem, just for the dynamic loader) are all fed in, and dependency finding, hardening (see below), and purity checks for each are handled by the CC Wrapper. Packages typically depend on the CC Wrapper, which in turn (at run-time) depends on the Bintools Wrapper.

Dependency finding is undoubtedly the main task of the CC Wrapper. This works just like the Bintools Wrapper, except that any include subdirectory of any relevant dependency is added to NIX_CFLAGS_COMPILE. The setup hook itself contains elaborate comments describing the exact mechanism by which this is accomplished.

Similarly, the CC Wrapper follows the Bintools Wrapper in defining standard environment variables with the names of the tools it wraps, for the same reasons described above. Importantly, while it includes a cc symlink to the c compiler for portability, the CC will be defined using the compiler’s “real name” (i.e. gcc or clang). This helps lousy build systems that inspect on the name of the compiler rather than run it.

Here are some more packages that provide a setup hook. Since the list of hooks is extensible, this is not an exhaustive list. The mechanism is only to be used as a last resort, so it might cover most uses.

Other hooks

Many other packages provide hooks, that are not part of stdenv. You can find these in the Hooks Reference.

Compiler and Linker wrapper hooks

If the file ${cc}/nix-support/cc-wrapper-hook exists, it will be run at the end of the compiler wrapper. If the file ${binutils}/nix-support/post-link-hook exists, it will be run at the end of the linker wrapper. These hooks allow a user to inject code into the wrappers. As an example, these hooks can be used to extract extraBefore, params and extraAfter which store all the command line arguments passed to the compiler and linker respectively.

Purity in Nixpkgs

Measures taken to prevent dependencies on packages outside the store, and what you can do to prevent them.

GCC doesn’t search in locations such as /usr/include. In fact, attempts to add such directories through the -I flag are filtered out. Likewise, the linker (from GNU binutils) doesn’t search in standard locations such as /usr/lib. Programs built on Linux are linked against a GNU C Library that likewise doesn’t search in the default system locations.

Hardening in Nixpkgs

There are flags available to harden packages at compile or link-time. These can be toggled using the stdenv.mkDerivation parameters hardeningDisable and hardeningEnable.

Both parameters take a list of flags as strings. The special "all" flag can be passed to hardeningDisable to turn off all hardening. These flags can also be used as environment variables for testing or development purposes.

For more in-depth information on these hardening flags and hardening in general, refer to the Debian Wiki, Ubuntu Wiki, Gentoo Wiki, and the Arch Wiki.

Note that support for some hardening flags varies by compiler, CPU architecture, target OS and libc. Combinations of these that don’t support a particular hardening flag will silently ignore attempts to enable it. To see exactly which hardening flags are being employed in any invocation, the NIX_DEBUG environment variable can be used.

Hardening flags enabled by default

The following flags are enabled by default and might require disabling with hardeningDisable if the program to package is incompatible.

format

Adds the -Wformat -Wformat-security -Werror=format-security compiler options. At present, this warns about calls to printf and scanf functions where the format string is not a string literal and there are no format arguments, as in printf(foo);. This may be a security hole if the format string came from untrusted input and contains %n.

This needs to be turned off or fixed for errors similar to:

/tmp/nix-build-zynaddsubfx-2.5.2.drv-0/zynaddsubfx-2.5.2/src/UI/guimain.cpp:571:28: error: format not a string literal and no format arguments [-Werror=format-security]
         printf(help_message);
                            ^
cc1plus: some warnings being treated as errors

stackprotector

Adds the -fstack-protector-strong --param ssp-buffer-size=4 compiler options. This adds safety checks against stack overwrites rendering many potential code injection attacks into aborting situations. In the best case this turns code injection vulnerabilities into denial of service or into non-issues (depending on the application).

This needs to be turned off or fixed for errors similar to:

bin/blib.a(bios_console.o): In function `bios_handle_cup':
/tmp/nix-build-ipxe-20141124-5cbdc41.drv-0/ipxe-5cbdc41/src/arch/i386/firmware/pcbios/bios_console.c:86: undefined reference to `__stack_chk_fail'

fortify

Adds the -O2 -D_FORTIFY_SOURCE=2 compiler options. During code generation the compiler knows a great deal of information about buffer sizes (where possible), and attempts to replace insecure unlimited length buffer function calls with length-limited ones. This is especially useful for old, crufty code. Additionally, format strings in writable memory that contain %n are blocked. If an application depends on such a format string, it will need to be worked around.

Additionally, some warnings are enabled which might trigger build failures if compiler warnings are treated as errors in the package build. In this case, set env.NIX_CFLAGS_COMPILE to -Wno-error=warning-type.

This needs to be turned off or fixed for errors similar to:

malloc.c:404:15: error: return type is an incomplete type
malloc.c:410:19: error: storage size of 'ms' isn't known

strdup.h:22:1: error: expected identifier or '(' before '__extension__'

strsep.c:65:23: error: register name not specified for 'delim'

installwatch.c:3751:5: error: conflicting types for '__open_2'

fcntl2.h:50:4: error: call to '__open_missing_mode' declared with attribute error: open with O_CREAT or O_TMPFILE in second argument needs 3 arguments

Disabling fortify implies disablement of fortify3

fortify3

Adds the -O2 -D_FORTIFY_SOURCE=3 compiler options. This expands the cases that can be protected by fortify-checks to include some situations with dynamic-length buffers whose length can be inferred at runtime using compiler hints.

Enabling this flag implies enablement of fortify. Disabling this flag does not imply disablement of fortify.

This flag can sometimes conflict with a build-system’s own attempts at enabling fortify support and result in errors complaining about redefinition of _FORTIFY_SOURCE.

pic

Adds the -fPIC compiler options. This options adds support for position independent code in shared libraries and thus making ASLR possible.

Most notably, the Linux kernel, kernel modules and other code not running in an operating system environment like boot loaders won’t build with PIC enabled. The compiler will is most cases complain that PIC is not supported for a specific build.

This needs to be turned off or fixed for assembler errors similar to:

ccbLfRgg.s: Assembler messages:
ccbLfRgg.s:33: Error: missing or invalid displacement expression `private_key_len@GOTOFF'

strictoverflow

Signed integer overflow is undefined behaviour according to the C standard. If it happens, it is an error in the program as it should check for overflow before it can happen, not afterwards. GCC provides built-in functions to perform arithmetic with overflow checking, which are correct and faster than any custom implementation. As a workaround, the option -fno-strict-overflow makes gcc behave as if signed integer overflows were defined.

This flag should not trigger any build or runtime errors.

relro

Adds the -z relro linker option. During program load, several ELF memory sections need to be written to by the linker, but can be turned read-only before turning over control to the program. This prevents some GOT (and .dtors) overwrite attacks, but at least the part of the GOT used by the dynamic linker (.got.plt) is still vulnerable.

This flag can break dynamic shared object loading. For instance, the module systems of Xorg and OpenCV are incompatible with this flag. In almost all cases the bindnow flag must also be disabled and incompatible programs typically fail with similar errors at runtime.

bindnow

Adds the -z now linker option. During program load, all dynamic symbols are resolved, allowing for the complete GOT to be marked read-only (due to relro). This prevents GOT overwrite attacks. For very large applications, this can incur some performance loss during initial load while symbols are resolved, but this shouldn’t be an issue for daemons.

This flag can break dynamic shared object loading. For instance, the module systems of Xorg and PHP are incompatible with this flag. Programs incompatible with this flag often fail at runtime due to missing symbols, like:

intel_drv.so: undefined symbol: vgaHWFreeHWRec

Hardening flags disabled by default

The following flags are disabled by default and should be enabled with hardeningEnable for packages that take untrusted input like network services.

pie

This flag is disabled by default for normal glibc based NixOS package builds, but enabled by default for musl based package builds.

Adds the -fPIE compiler and -pie linker options. Position Independent Executables are needed to take advantage of Address Space Layout Randomization, supported by modern kernel versions. While ASLR can already be enforced for data areas in the stack and heap (brk and mmap), the code areas must be compiled as position-independent. Shared libraries already do this with the pic flag, so they gain ASLR automatically, but binary .text regions need to be build with pie to gain ASLR. When this happens, ROP attacks are much harder since there are no static locations to bounce off of during a memory corruption attack.

Static libraries need to be compiled with -fPIE so that executables can link them in with the -pie linker option. If the libraries lack -fPIE, you will get the error recompile with -fPIE.

zerocallusedregs

Adds the -fzero-call-used-regs=used-gpr compiler option. This causes the general-purpose registers that an architecture’s calling convention considers “call-used” to be zeroed on return from the function. This can make it harder for attackers to construct useful ROP gadgets and also reduces the chance of data leakage from a function call.

trivialautovarinit

Adds the -ftrivial-auto-var-init=pattern compiler option. This causes “trivially-initializable” uninitialized stack variables to be forcibly initialized with a nonzero value that is likely to cause a crash (and therefore be noticed). Uninitialized variables generally take on their values based on fragments of previous program state, and attackers can carefully manipulate that state to craft malicious initial values for these variables.

Use of this flag is controversial as it can prevent tools that detect uninitialized variable use (such as valgrind) from operating correctly.



The build platform is ignored because it is a mere implementation detail of the package satisfying the dependency: As a general programming principle, dependencies are always specified as interfaces, not concrete implementation.[1]

Currently, this means for native builds all dependencies are put on the PATH. But in the future that may not be the case for sake of matching cross: the platforms would be assumed to be unique for native and cross builds alike, so only the depsBuild* and nativeBuildInputs would be added to the PATH.[2]

The findInputs function, currently residing in pkgs/stdenv/generic/setup.sh, implements the propagation logic.[3]

It clears the sys_lib_*search_path variables in the Libtool script to prevent Libtool from using libraries in /usr/lib and such.[4]

Eventually these will be passed building natively as well, to improve determinism: build-time guessing, as is done today, is a risk of impurity.[5]

Each wrapper targets a single platform, so if binaries for multiple platforms are needed, the underlying binaries must be wrapped multiple times. As this is a property of the wrapper itself, the multiple wrappings are needed whether or not the same underlying binaries can target multiple platforms.[6]

Meta-attributes

Nix packages can declare meta-attributes that contain information about a package such as a description, its homepage, its license, and so on. For instance, the GNU Hello package has a meta declaration like this:

{
  meta = {
    description = "A program that produces a familiar, friendly greeting";
    longDescription = ''
      GNU Hello is a program that prints "Hello, world!" when you run it.
      It is fully customizable.
    '';
    homepage = "https://www.gnu.org/software/hello/manual/";
    license = lib.licenses.gpl3Plus;
    maintainers = with lib.maintainers; [ eelco ];
    platforms = lib.platforms.all;
  };
}

Meta-attributes are not passed to the builder of the package. Thus, a change to a meta-attribute doesn’t trigger a recompilation of the package.

Standard meta-attributes

If the package is to be submitted to Nixpkgs, please check out the requirements for meta attributes in the contributing documentation.

It is expected that each meta-attribute is one of the following:

description

A short (one-line) description of the package. This is displayed on search.nixos.org.

The general requirements of a description are:

  • Be short, just one sentence.

  • Be capitalized.

  • Not start with definite (“The”) or indefinite (“A”/“An”) article.

  • Not start with the package name.

    • More generally, it should not refer to the package name.

  • Not end with a period (or any punctuation for that matter).

  • Provide factual information.

    • Avoid subjective language.

Wrong: "libpng is a library that allows you to decode PNG images."

Right: "Library for decoding PNG images"

longDescription

An arbitrarily long description of the package in CommonMark Markdown.

branch

Release branch. Used to specify that a package is not going to receive updates that are not in this branch; for example, Linux kernel 3.0 is supposed to be updated to 3.0.X, not 3.1.

homepage

The package’s homepage. Example: https://www.gnu.org/software/hello/manual/

downloadPage

The page where a link to the current version can be found. Example: https://ftp.gnu.org/gnu/hello/

changelog

A link or a list of links to the location of Changelog for a package. A link may use expansion to refer to the correct changelog version. Example: "https://git.savannah.gnu.org/cgit/hello.git/plain/NEWS?h=v${version}"

license

The license, or licenses, for the package. One from the attribute set defined in nixpkgs/lib/licenses.nix. At this moment using both a list of licenses and a single license is valid. If the license field is in the form of a list representation, then it means that parts of the package are licensed differently. Each license should preferably be referenced by their attribute. The non-list attribute value can also be a space delimited string representation of the contained attribute shortNames or spdxIds. The following are all valid examples:

  • Single license referenced by attribute (preferred) lib.licenses.gpl3Only.

  • Single license referenced by its attribute shortName (frowned upon) "gpl3Only".

  • Single license referenced by its attribute spdxId (frowned upon) "GPL-3.0-only".

  • Multiple licenses referenced by attribute (preferred) with lib.licenses; [ asl20 free ofl ].

  • Multiple licenses referenced as a space delimited string of attribute shortNames (frowned upon) "asl20 free ofl".

For details, see Licenses.

maintainers

A list of the maintainers of this Nix expression. Maintainers are defined in nixpkgs/maintainers/maintainer-list.nix. There is no restriction to becoming a maintainer, just add yourself to that list in a separate commit titled “maintainers: add alice” in the same pull request, and reference maintainers with maintainers = with lib.maintainers; [ alice bob ].

mainProgram

The name of the main binary for the package. This affects the binary nix run executes. Example: "rg"

priority

The priority of the package, used by nix-env to resolve file name conflicts between packages. See the manual page for nix-env for details. Example: "10" (a low-priority package).

platforms

The list of Nix platform types on which the package is supported. Hydra builds packages according to the platform specified. If no platform is specified, the package does not have prebuilt binaries. An example is:

{
  meta.platforms = lib.platforms.linux;
}

Attribute Set lib.platforms defines various common lists of platforms types.

badPlatforms

The list of Nix platform types on which the package is known not to be buildable. Hydra will never create prebuilt binaries for these platform types, even if they are in meta.platforms. In general it is preferable to set meta.platforms = lib.platforms.all and then exclude any platforms on which the package is known not to build. For example, a package which requires dynamic linking and cannot be linked statically could use this:

{
  meta.platforms = lib.platforms.all;
  meta.badPlatforms = [ lib.systems.inspect.platformPatterns.isStatic ];
}

The lib.meta.availableOn function can be used to test whether or not a package is available (i.e. buildable) on a given platform. Some packages use this to automatically detect the maximum set of features with which they can be built. For example, systemd requires dynamic linking, and has a meta.badPlatforms setting similar to the one above. Packages which can be built with or without systemd support will use lib.meta.availableOn to detect whether or not systemd is available on the hostPlatform for which they are being built; if it is not available (e.g. due to a statically-linked host platform like pkgsStatic) this support will be disabled by default.

timeout

A timeout (in seconds) for building the derivation. If the derivation takes longer than this time to build, Hydra will fail it due to breaking the timeout. However, all computers do not have the same computing power, hence some builders may decide to apply a multiplicative factor to this value. When filling this value in, try to keep it approximately consistent with other values already present in nixpkgs.

meta attributes are not stored in the instantiated derivation. Therefore, this setting may be lost when the package is used as a dependency. To be effective, it must be presented directly to an evaluation process that handles the meta.timeout attribute.

hydraPlatforms

The list of Nix platform types for which the Hydra instance at hydra.nixos.org will build the package. (Hydra is the Nix-based continuous build system.) It defaults to the value of meta.platforms. Thus, the only reason to set meta.hydraPlatforms is if you want hydra.nixos.org to build the package on a subset of meta.platforms, or not at all, e.g.

{
  meta.platforms = lib.platforms.linux;
  meta.hydraPlatforms = [];
}

broken

If set to true, the package is marked as “broken”, meaning that it won’t show up in search.nixos.org, and cannot be built or installed unless the environment variable NIXPKGS_ALLOW_BROKEN is set. Such unconditionally-broken packages should be removed from Nixpkgs eventually unless they are fixed.

The value of this attribute can depend on a package’s arguments, including stdenv. This means that broken can be used to express constraints, for example:

  • Does not cross compile

    {
      meta.broken = !(stdenv.buildPlatform.canExecute stdenv.hostPlatform);
    }
    
  • Broken if all of a certain set of its dependencies are broken

    {
      meta.broken = lib.all (map (p: p.meta.broken) [ glibc musl ]);
    }
    

This makes broken strictly more powerful than meta.badPlatforms. However meta.availableOn currently examines only meta.platforms and meta.badPlatforms, so meta.broken does not influence the default values for optional dependencies.

Licenses

The meta.license attribute should preferably contain a value from lib.licenses defined in nixpkgs/lib/licenses.nix, or in-place license description of the same format if the license is unlikely to be useful in another expression.

Although it’s typically better to indicate the specific license, a few generic options are available:

lib.licenses.free, "free"

Catch-all for free software licenses not listed above.

lib.licenses.unfreeRedistributable, "unfree-redistributable"

Unfree package that can be redistributed in binary form. That is, it’s legal to redistribute the output of the derivation. This means that the package can be included in the Nixpkgs channel.

Sometimes proprietary software can only be redistributed unmodified. Make sure the builder doesn’t actually modify the original binaries; otherwise we’re breaking the license. For instance, the NVIDIA X11 drivers can be redistributed unmodified, but our builder applies patchelf to make them work. Thus, its license is "unfree" and it cannot be included in the Nixpkgs channel.

lib.licenses.unfree, "unfree"

Unfree package that cannot be redistributed. You can build it yourself, but you cannot redistribute the output of the derivation. Thus it cannot be included in the Nixpkgs channel.

lib.licenses.unfreeRedistributableFirmware, "unfree-redistributable-firmware"

This package supplies unfree, redistributable firmware. This is a separate value from unfree-redistributable because not everybody cares whether firmware is free.

Source provenance

The value of a package’s meta.sourceProvenance attribute specifies the provenance of the package’s derivation outputs.

If a package contains elements that are not built from the original source by a nixpkgs derivation, the meta.sourceProvenance attribute should be a list containing one or more value from lib.sourceTypes defined in nixpkgs/lib/source-types.nix.

Adding this information helps users who have needs related to build transparency and supply-chain security to gain some visibility into their installed software or set policy to allow or disallow installation based on source provenance.

The presence of a particular sourceType in a package’s meta.sourceProvenance list indicates that the package contains some components falling into that category, though the absence of that sourceType does not guarantee the absence of that category of sourceType in the package’s contents. A package with no meta.sourceProvenance set implies it has no known sourceTypes other than fromSource.

The meaning of the meta.sourceProvenance attribute does not depend on the value of the meta.license attribute.

lib.sourceTypes.fromSource

Package elements which are produced by a nixpkgs derivation which builds them from source code.

lib.sourceTypes.binaryNativeCode

Native code to be executed on the target system’s CPU, built by a third party. This includes packages which wrap a downloaded AppImage or Debian package.

lib.sourceTypes.binaryFirmware

Code to be executed on a peripheral device or embedded controller, built by a third party.

lib.sourceTypes.binaryBytecode

Code to run on a VM interpreter or JIT compiled into bytecode by a third party. This includes packages which download Java .jar files from another source.

Passthru-attributes

Table of Contents

Common passthru-attributes

As opposed to most other mkDerivation input attributes, passthru is not passed to the derivation’s builder executable. Changing it will not trigger a rebuild – it is “passed through”. Its value can be accessed as if it was set inside a derivation.

Note

passthru attributes follow no particular schema, but there are a few conventional patterns.

Example 252. Setting and accessing passthru attributes
{ stdenv, fetchGit }:
let
  hello = stdenv.mkDerivation {
    pname = "hello";
    src = fetchGit { /* ... */ };

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

Common passthru-attributes

Many passthru attributes are situational, so this section only lists recurring patterns. They fall in one of these categories:

  • Global conventions, which are applied almost universally in Nixpkgs.

    Generally these don’t entail any special support built into the derivation they belong to. Common examples of this type are passthru.tests and passthru.updateScript.

  • Conventions for adding extra functionality to a derivation.

    These tend to entail support from the derivation or the passthru attribute in question. Common examples of this type are passthru.optional-dependencies, passthru.withPlugins, and passthru.withPackages. All of those allow associating the package with a set of components built for that specific package, such as when building Python runtime environments using (python.withPackages)[#python.withpackages-function].

Attributes that apply only to particular build helpers or language ecosystems are documented there.

passthru.tests

An attribute set with tests as values. A test is a derivation that builds when the test passes and fails to build otherwise.

Run these tests with:

$ cd path/to/nixpkgs
$ nix-build -A your-package.tests

Note

The Nixpkgs systems for continuous integration Hydra and nixpkgs-review don’t build these derivations by default, and (@ofborg) only builds them when evaluating pull requests for that particular package, or when manually instructed.

Package tests

Besides tests provided by upstream, that you run in the checkPhase, you may want to define tests derivations in the passthru.tests attribute, which won’t change the build. passthru.tests have several advantages over running tests during any of the standard phases:

  • They access the package as consumers would, independently from the environment in which it was built

  • They can be run and debugged without rebuilding the package, which is useful if that takes a long time

  • They don’t add overhead to each build, as opposed checks added to the distPhase, such as versionCheckHook.

It is also possible to use passthru.tests to test the version with testVersion, but since that is pretty trivial and recommended thing to do, we recommend using versionCheckHook for that, which has the following advantages over passthru.tests:

  • If the versionCheckPhase (the phase defined by versionCheckHook) fails, it triggers a failure which can’t be ignored if you use the package, or if you find out about it in a nixpkgs-review report.

  • Sometimes packages become silently broken - meaning they fail to launch but their build passes because they don’t perform any tests in the checkPhase. If you use this tool infrequently, such a silent breakage may rot in your system / profile configuration, and you will not notice the failure until you will want to use this package. Testing such basic functionality ensures you have to deal with the failure when you update your system / profile.

  • When you open a PR, ofborg’s CI will run passthru.tests of packages that are directly changed by your PR (according to your commits’ messages), but if you’d want to use the @ofborg build command for dependent packages, you won’t have to specify in addition the .tests attribute of the packages you want to build, and no body will be able to avoid these tests.

For more on how to write and run package tests for Nixpkgs, see the testing section in the package contributor guide.

NixOS tests

Tests written for NixOS are available as the nixosTests argument to package recipes. For instance, the OpenSMTPD derivation includes lines similar to:

{ nixosTests, ... }:
{
  # ...
  passthru.tests = {
    basic-functionality-and-dovecot-integration = nixosTests.opensmtpd;
  };
}

NixOS tests run in a virtual machine (VM), so they are slower than regular package tests. For more information see the NixOS manual on NixOS module tests.

passthru.updateScript

Nixpkgs tries to automatically update all packages that have an passthru.updateScript attribute. See the section on automatic package updates in the package contributor guide for details.

Multiple-output packages

The Nix language allows a derivation to produce multiple outputs, which is similar to what is utilized by other Linux distribution packaging systems. The outputs reside in separate Nix store paths, so they can be mostly handled independently of each other, including passing to build inputs, garbage collection or binary substitution. The exception is that building from source always produces all the outputs.

The main motivation is to save disk space by reducing runtime closure sizes; consequently also sizes of substituted binaries get reduced. Splitting can be used to have more granular runtime dependencies, for example the typical reduction is to split away development-only files, as those are typically not needed during runtime. As a result, closure sizes of many packages can get reduced to a half or even much less.

Note

The reduction effects could be instead achieved by building the parts in completely separate derivations. That would often additionally reduce build-time closures, but it tends to be much harder to write such derivations, as build systems typically assume all parts are being built at once. This compromise approach of single source package producing multiple binary packages is also utilized often by rpm and deb.

A number of attributes can be used to work with a derivation with multiple outputs. The attribute outputs is a list of strings, which are the names of the outputs. For each of these names, an identically named attribute is created, corresponding to that output.

The attribute meta.outputsToInstall is used to determine the default set of outputs to install when using the derivation name unqualified: bin, or out, or the first specified output; as well as man if that is specified.

Using a split package

In the Nix language the individual outputs can be reached explicitly as attributes, e.g. coreutils.info, but the typical case is just using packages as build inputs.

When a multiple-output derivation gets into a build input of another derivation, the dev output is added if it exists, otherwise the first output is added. In addition to that, propagatedBuildOutputs of that package which by default contain $outputBin and $outputLib are also added. (See the section called “File type groups”.)

In some cases it may be desirable to combine different outputs under a single store path. A function symlinkJoin can be used to do this. (Note that it may negate some closure size benefits of using a multiple-output package.)

Writing a split derivation

Here you find how to write a derivation that produces multiple outputs.

In nixpkgs there is a framework supporting multiple-output derivations. It tries to cover most cases by default behavior. You can find the source separated in <nixpkgs/pkgs/build-support/setup-hooks/multiple-outputs.sh>; it’s relatively well-readable. The whole machinery is triggered by defining the outputs attribute to contain the list of desired output names (strings).

{
  outputs = [ "bin" "dev" "out" "doc" ];
}

Often such a single line is enough. For each output an equally named environment variable is passed to the builder and contains the path in nix store for that output. Typically you also want to have the main out output, as it catches any files that didn’t get elsewhere.

Note

There is a special handling of the debug output, described at the section called “separateDebugInfo.

“Binaries first”

A commonly adopted convention in nixpkgs is that executables provided by the package are contained within its first output. This convention allows the dependent packages to reference the executables provided by packages in a uniform manner. For instance, provided with the knowledge that the perl package contains a perl executable it can be referenced as ${pkgs.perl}/bin/perl within a Nix derivation that needs to execute a Perl script.

The glibc package is a deliberate single exception to the “binaries first” convention. The glibc has libs as its first output allowing the libraries provided by glibc to be referenced directly (e.g. ${glibc}/lib/ld-linux-x86-64.so.2). The executables provided by glibc can be accessed via its bin attribute (e.g. ${lib.getBin stdenv.cc.libc}/bin/ldd).

The reason for why glibc deviates from the convention is because referencing a library provided by glibc is a very common operation among Nix packages. For instance, third-party executables packaged by Nix are typically patched and relinked with the relevant version of glibc libraries from Nix packages (please see the documentation on patchelf for more details).

File type groups

The support code currently recognizes some particular kinds of outputs and either instructs the build system of the package to put files into their desired outputs or it moves the files during the fixup phase. Each group of file types has an outputFoo variable specifying the output name where they should go. If that variable isn’t defined by the derivation writer, it is guessed – a default output name is defined, falling back to other possibilities if the output isn’t defined.

$outputDev

is for development-only files. These include C(++) headers (include/), pkg-config (lib/pkgconfig/), cmake (lib/cmake/) and aclocal files (share/aclocal/). They go to dev or out by default.

$outputBin

is meant for user-facing binaries, typically residing in bin/. They go to bin or out by default.

$outputLib

is meant for libraries, typically residing in lib/ and libexec/. They go to lib or out by default.

$outputDoc

is for user documentation, typically residing in share/doc/. It goes to doc or out by default.

$outputDevdoc

is for developer documentation. Currently we count gtk-doc and devhelp books, typically residing in share/gtk-doc/ and share/devhelp/, in there. It goes to devdoc or is removed (!) by default. This is because e.g. gtk-doc tends to be rather large and completely unused by nixpkgs users.

$outputMan

is for man pages (except for section 3), typically residing in share/man/man[0-9]/. They go to man or $outputBin by default.

$outputDevman

is for section 3 man pages, typically residing in share/man/man[0-9]/. They go to devman or $outputMan by default.

$outputInfo

is for info pages, typically residing in share/info/. They go to info or $outputBin by default.

Common caveats

  • Some configure scripts don’t like some of the parameters passed by default by the framework, e.g. --docdir=/foo/bar. You can disable this by setting setOutputFlags = false;.

  • The outputs of a single derivation can retain references to each other, but note that circular references are not allowed. (And each strongly-connected component would act as a single output anyway.)

  • Most of split packages contain their core functionality in libraries. These libraries tend to refer to various kind of data that typically gets into out, e.g. locale strings, so there is often no advantage in separating the libraries into lib, as keeping them in out is easier.

  • Some packages have hidden assumptions on install paths, which complicates splitting.

Cross-compilation

Introduction

“Cross-compilation” means compiling a program on one machine for another type of machine. For example, a typical use of cross-compilation is to compile programs for embedded devices. These devices often don’t have the computing power and memory to compile their own programs. One might think that cross-compilation is a fairly niche concern. However, there are significant advantages to rigorously distinguishing between build-time and run-time environments! Significant, because the benefits apply even when one is developing and deploying on the same machine. Nixpkgs is increasingly adopting the opinion that packages should be written with cross-compilation in mind, and Nixpkgs should evaluate in a similar way (by minimizing cross-compilation-specific special cases) whether or not one is cross-compiling.

This chapter will be organized in three parts. First, it will describe the basics of how to package software in a way that supports cross-compilation. Second, it will describe how to use Nixpkgs when cross-compiling. Third, it will describe the internal infrastructure supporting cross-compilation.

Packaging in a cross-friendly manner

Platform parameters

Nixpkgs follows the conventions of GNU autoconf. We distinguish between 3 types of platforms when building a derivation: build, host, and target. In summary, build is the platform on which a package is being built, host is the platform on which it will run. The third attribute, target, is relevant only for certain specific compilers and build tools.

In Nixpkgs, these three platforms are defined as attribute sets under the names buildPlatform, hostPlatform, and targetPlatform. They are always defined as attributes in the standard environment. That means one can access them like:

{ stdenv, fooDep, barDep, ... }: {
  # ...stdenv.buildPlatform...
}
buildPlatform

The “build platform” is the platform on which a package is built. Once someone has a built package, or pre-built binary package, the build platform should not matter and can be ignored.

hostPlatform

The “host platform” is the platform on which a package will be run. This is the simplest platform to understand, but also the one with the worst name.

targetPlatform

The “target platform” attribute is, unlike the other two attributes, not actually fundamental to the process of building software. Instead, it is only relevant for compatibility with building certain specific compilers and build tools. It can be safely ignored for all other packages.

The build process of certain compilers is written in such a way that the compiler resulting from a single build can itself only produce binaries for a single platform. The task of specifying this single “target platform” is thus pushed to build time of the compiler. The root cause of this is that the compiler (which will be run on the host) and the standard library/runtime (which will be run on the target) are built by a single build process.

There is no fundamental need to think about a single target ahead of time like this. If the tool supports modular or pluggable backends, both the need to specify the target at build time and the constraint of having only a single target disappear. An example of such a tool is LLVM.

Although the existence of a “target platform” is arguably a historical mistake, it is a common one: examples of tools that suffer from it are GCC, Binutils, GHC and Autoconf. Nixpkgs tries to avoid sharing in the mistake where possible. Still, because the concept of a target platform is so ingrained, it is best to support it as is.

The exact schema these fields follow is a bit ill-defined due to a long and convoluted evolution, but this is slowly being cleaned up. You can see examples of ones used in practice in lib.systems.examples; note how they are not all very consistent. For now, here are few fields can count on them containing:

system

This is a two-component shorthand for the platform. Examples of this would be “x86_64-darwin” and “i686-linux”; see lib.systems.doubles for more. The first component corresponds to the CPU architecture of the platform and the second to the operating system of the platform ([cpu]-[os]). This format has built-in support in Nix, such as the builtins.currentSystem impure string.

config

This is a 3- or 4- component shorthand for the platform. Examples of this would be x86_64-unknown-linux-gnu and aarch64-apple-darwin14. This is a standard format called the “LLVM target triple”, as they are pioneered by LLVM. In the 4-part form, this corresponds to [cpu]-[vendor]-[os]-[abi]. This format is strictly more informative than the “Nix host double”, as the previous format could analogously be termed. This needs a better name than config!

parsed

This is a Nix representation of a parsed LLVM target triple with white-listed components. This can be specified directly, or actually parsed from the config. See lib.systems.parse for the exact representation.

libc

This is a string identifying the standard C library used. Valid identifiers include “glibc” for GNU libc, “libSystem” for Darwin’s Libsystem, and “uclibc” for µClibc. It should probably be refactored to use the module system, like parse.

is*

These predicates are defined in lib.systems.inspect, and slapped onto every platform. They are superior to the ones in stdenv as they force the user to be explicit about which platform they are inspecting. Please use these instead of those.

platform

This is, quite frankly, a dumping ground of ad-hoc settings (it’s an attribute set). See lib.systems.platforms for examples—there’s hopefully one in there that will work verbatim for each platform that is working. Please help us triage these flags and give them better homes!

Theory of dependency categorization

Note

This is a rather philosophical description that isn’t very Nixpkgs-specific. For an overview of all the relevant attributes given to mkDerivation, see the section called “Specifying dependencies”. For a description of how everything is implemented, see the section called “Implementation of dependencies”.

In this section we explore the relationship between both runtime and build-time dependencies and the 3 Autoconf platforms.

A run time dependency between two packages requires that their host platforms match. This is directly implied by the meaning of “host platform” and “runtime dependency”: The package dependency exists while both packages are running on a single host platform.

A build time dependency, however, has a shift in platforms between the depending package and the depended-on package. “build time dependency” means that to build the depending package we need to be able to run the depended-on’s package. The depending package’s build platform is therefore equal to the depended-on package’s host platform.

If both the dependency and depending packages aren’t compilers or other machine-code-producing tools, we’re done. And indeed buildInputs and nativeBuildInputs have covered these simpler cases for many years. But if the dependency does produce machine code, we might need to worry about its target platform too. In principle, that target platform might be any of the depending package’s build, host, or target platforms, but we prohibit dependencies from a “later” platform to an earlier platform to limit confusion because we’ve never seen a legitimate use for them.

Finally, if the depending package is a compiler or other machine-code-producing tool, it might need dependencies that run at “emit time”. This is for compilers that (regrettably) insist on being built together with their source languages’ standard libraries. Assuming build != host != target, a run-time dependency of the standard library cannot be run at the compiler’s build time or run time, but only at the run time of code emitted by the compiler.

Putting this all together, that means that we have dependency types of the form “X→ E”, which means that the dependency executes on X and emits code for E; each of X and E can be build, host, or target, and E can be * to indicate that the dependency is not a compiler-like package.

Dependency types describe the relationships that a package has with each of its transitive dependencies. You could think of attaching one or more dependency types to each of the formal parameters at the top of a package’s .nix file, as well as to all of their formal parameters, and so on. Triples like (foo, bar, baz), on the other hand, are a property of an instantiated derivation – you could would attach a triple (mips-linux, mips-linux, sparc-solaris) to a .drv file in /nix/store.

Only nine dependency types matter in practice:

Possible dependency types

Dependency typeDependency’s host platformDependency’s target platform
build → *build(none)
build → buildbuildbuild
build → hostbuildhost
build → targetbuildtarget
host → *host(none)
host → hosthosthost
host → targethosttarget
target → *target(none)
target → targettargettarget

Let’s use g++ as an example to make this table clearer. g++ is a C++ compiler written in C. Suppose we are building g++ with a (build, host, target) platform triple of (foo, bar, baz). This means we are using a foo-machine to build a copy of g++ which will run on a bar-machine and emit binaries for the baz-machine.

  • g++ links against the host platform’s glibc C library, which is a “host→ *” dependency with a triple of (bar, bar, *). Since it is a library, not a compiler, it has no “target”.

  • Since g++ is written in C, the gcc compiler used to compile it is a “build→ host” dependency of g++ with a triple of (foo, foo, bar). This compiler runs on the build platform and emits code for the host platform.

  • gcc links against the build platform’s glibc C library, which is a “build→ *” dependency with a triple of (foo, foo, *). Since it is a library, not a compiler, it has no “target”.

  • This gcc is itself compiled by an earlier copy of gcc. This earlier copy of gcc is a “build→ build” dependency of g++ with a triple of (foo, foo, foo). This “early gcc” runs on the build platform and emits code for the build platform.

  • g++ is bundled with libgcc, which includes a collection of target-machine routines for exception handling and software floating point emulation. libgcc would be a “target→ *” dependency with triple (foo, baz, *), because it consists of machine code which gets linked against the output of the compiler that we are building. It is a library, not a compiler, so it has no target of its own.

  • libgcc is written in C and compiled with gcc. The gcc that compiles it will be a “build→ target” dependency with triple (foo, foo, baz). It gets compiled and run at g++-build-time (on platform foo), but must emit code for the baz-platform.

  • g++ allows inline assembler code, so it depends on access to a copy of the gas assembler. This would be a “host→ target” dependency with triple (foo, bar, baz).

  • g++ (and gcc) include a library libgccjit.so, which wrap the compiler in a library to create a just-in-time compiler. In nixpkgs, this library is in the libgccjit package; if C++ required that programs have access to a JIT, g++ would need to add a “target→ target” dependency for libgccjit with triple (foo, baz, baz). This would ensure that the compiler ships with a copy of libgccjit which both executes on and generates code for the baz-platform.

  • If g++ itself linked against libgccjit.so (for example, to allow compile-time-evaluated C++ expressions), then the libgccjit package used to provide this functionality would be a “host→ host” dependency of g++: it is code which runs on the host and emits code for execution on the host.

Cross packaging cookbook

Some frequently encountered problems when packaging for cross-compilation should be answered here. Ideally, the information above is exhaustive, so this section cannot provide any new information, but it is ludicrous and cruel to expect everyone to spend effort working through the interaction of many features just to figure out the same answer to the same common problem. Feel free to add to this list!

My package fails to find a binutils command (cc/ar/ld etc.)

Many packages assume that an unprefixed binutils (cc/ar/ld etc.) is available, but Nix doesn’t provide one. It only provides a prefixed one, just as it only does for all the other binutils programs. It may be necessary to patch the package to fix the build system to use a prefix. For instance, instead of cc, use ${stdenv.cc.targetPrefix}cc.

{
  makeFlags = [ "CC=${stdenv.cc.targetPrefix}cc" ];
}

How do I avoid compiling a GCC cross-compiler from source?

On less powerful machines, it can be inconvenient to cross-compile a package only to find out that GCC has to be compiled from source, which could take up to several hours. Nixpkgs maintains a limited cross-related jobset on Hydra, which tests cross-compilation to various platforms from build platforms “x86_64-darwin”, “x86_64-linux”, and “aarch64-linux”. See pkgs/top-level/release-cross.nix for the full list of target platforms and packages. For instance, the following invocation fetches the pre-built cross-compiled GCC for armv6l-unknown-linux-gnueabihf and builds GNU Hello from source.

$ nix-build '<nixpkgs>' -A pkgsCross.raspberryPi.hello

What if my package’s build system needs to build a C program to be run under the build environment?

Add the following to your mkDerivation invocation.

{
  depsBuildBuild = [ buildPackages.stdenv.cc ];
}

My package’s testsuite needs to run host platform code.

Add the following to your mkDerivation invocation.

{
  doCheck = stdenv.buildPlatform.canExecute stdenv.hostPlatform;
}

Package using Meson needs to run binaries for the host platform during build.

Add mesonEmulatorHook to nativeBuildInputs conditionally on if the target binaries can be executed.

e.g.

{
  nativeBuildInputs = [
    meson
  ] ++ lib.optionals (!stdenv.buildPlatform.canExecute stdenv.hostPlatform) [
    mesonEmulatorHook
  ];
}

Example of an error which this fixes.

[Errno 8] Exec format error: './gdk3-scan'

Cross-building packages

Nixpkgs can be instantiated with localSystem alone, in which case there is no cross-compiling and everything is built by and for that system, or also with crossSystem, in which case packages run on the latter, but all building happens on the former. Both parameters take the same schema as the 3 (build, host, and target) platforms defined in the previous section. As mentioned above, lib.systems.examples has some platforms which are used as arguments for these parameters in practice. You can use them programmatically, or on the command line:

$ nix-build '<nixpkgs>' --arg crossSystem '(import <nixpkgs/lib>).systems.examples.fooBarBaz' -A whatever

Note

Eventually we would like to make these platform examples an unnecessary convenience so that

$ nix-build '<nixpkgs>' --arg crossSystem '{ config = "<arch>-<os>-<vendor>-<abi>"; }' -A whatever

works in the vast majority of cases. The problem today is dependencies on other sorts of configuration which aren’t given proper defaults. We rely on the examples to crudely to set those configuration parameters in some vaguely sane manner on the users behalf. Issue #34274 tracks this inconvenience along with its root cause in crufty configuration options.

While one is free to pass both parameters in full, there’s a lot of logic to fill in missing fields. As discussed in the previous section, only one of system, config, and parsed is needed to infer the other two. Additionally, libc will be inferred from parse. Finally, localSystem.system is also impurely inferred based on the platform evaluation occurs. This means it is often not necessary to pass localSystem at all, as in the command-line example in the previous paragraph.

Note

Many sources (manual, wiki, etc) probably mention passing system, platform, along with the optional crossSystem to Nixpkgs: import <nixpkgs> { system = ..; platform = ..; crossSystem = ..; }. Passing those two instead of localSystem is still supported for compatibility, but is discouraged. Indeed, much of the inference we do for these parameters is motivated by compatibility as much as convenience.

One would think that localSystem and crossSystem overlap horribly with the three *Platforms (buildPlatform, hostPlatform, and targetPlatform; see stage.nix or the manual). Actually, those identifiers are purposefully not used here to draw a subtle but important distinction: While the granularity of having 3 platforms is necessary to properly build packages, it is overkill for specifying the user’s intent when making a build plan or package set. A simple “build vs deploy” dichotomy is adequate: the sliding window principle described in the previous section shows how to interpolate between the these two “end points” to get the 3 platform triple for each bootstrapping stage. That means for any package a given package set, even those not bound on the top level but only reachable via dependencies or buildPackages, the three platforms will be defined as one of localSystem or crossSystem, with the former replacing the latter as one traverses build-time dependencies. A last simple difference is that crossSystem should be null when one doesn’t want to cross-compile, while the *Platforms are always non-null. localSystem is always non-null.

Cross-compilation infrastructure

Implementation of dependencies

The categories of dependencies developed in the section called “Theory of dependency categorization” are specified as lists of derivations given to mkDerivation, as documented in the section called “Specifying dependencies”. In short, each list of dependencies for “host → target” is called deps<host><target> (where host, and target values are either build, host, or target), with exceptions for backwards compatibility that depsBuildHost is instead called nativeBuildInputs and depsHostTarget is instead called buildInputs. Nixpkgs is now structured so that each deps<host><target> is automatically taken from pkgs<host><target>. (These pkgs<host><target>s are quite new, so there is no special case for nativeBuildInputs and buildInputs.) For example, pkgsBuildHost.gcc should be used at build-time, while pkgsHostTarget.gcc should be used at run-time.

Now, for most of Nixpkgs’s history, there were no pkgs<host><target> attributes, and most packages have not been refactored to use it explicitly. Prior to those, there were just buildPackages, pkgs, and targetPackages. Those are now redefined as aliases to pkgsBuildHost, pkgsHostTarget, and pkgsTargetTarget. It is acceptable, even recommended, to use them for libraries to show that the host platform is irrelevant.

But before that, there was just pkgs, even though both buildInputs and nativeBuildInputs existed. [Cross barely worked, and those were implemented with some hacks on mkDerivation to override dependencies.] What this means is the vast majority of packages do not use any explicit package set to populate their dependencies, just using whatever callPackage gives them even if they do correctly sort their dependencies into the multiple lists described above. And indeed, asking that users both sort their dependencies, and take them from the right attribute set, is both too onerous and redundant, so the recommended approach (for now) is to continue just categorizing by list and not using an explicit package set.

To make this work, we “splice” together the six pkgsFooBar package sets and have callPackage actually take its arguments from that. This is currently implemented in pkgs/top-level/splice.nix. mkDerivation then, for each dependency attribute, pulls the right derivation out from the splice. This splicing can be skipped when not cross-compiling as the package sets are the same, but still is a bit slow for cross-compiling. We’d like to do something better, but haven’t come up with anything yet.

Bootstrapping

Each of the package sets described above come from a single bootstrapping stage. While pkgs/top-level/default.nix, coordinates the composition of stages at a high level, pkgs/top-level/stage.nix “ties the knot” (creates the fixed point) of each stage. The package sets are defined per-stage however, so they can be thought of as edges between stages (the nodes) in a graph. Compositions like pkgsBuildTarget.targetPackages can be thought of as paths to this graph.

While there are many package sets, and thus many edges, the stages can also be arranged in a linear chain. In other words, many of the edges are redundant as far as connectivity is concerned. This hinges on the type of bootstrapping we do. Currently for cross it is:

  1. (native, native, native)

  2. (native, native, foreign)

  3. (native, foreign, foreign)

In each stage, pkgsBuildHost refers to the previous stage, pkgsBuildBuild refers to the one before that, and pkgsHostTarget refers to the current one, and pkgsTargetTarget refers to the next one. When there is no previous or next stage, they instead refer to the current stage. Note how all the invariants regarding the mapping between dependency and depending packages’ build host and target platforms are preserved. pkgsBuildTarget and pkgsHostHost are more complex in that the stage fitting the requirements isn’t always a fixed chain of “prevs” and “nexts” away (modulo the “saturating” self-references at the ends). We just special case each instead. All the primary edges are implemented is in pkgs/stdenv/booter.nix, and secondarily aliases in pkgs/top-level/stage.nix.

Note

The native stages are bootstrapped in legacy ways that predate the current cross implementation. This is why the bootstrapping stages leading up to the final stages are ignored in the previous paragraph.

If one looks at the 3 platform triples, one can see that they overlap such that one could put them together into a chain like:

(native, native, native, foreign, foreign)

If one imagines the saturating self references at the end being replaced with infinite stages, and then overlays those platform triples, one ends up with the infinite tuple:

(native..., native, native, native, foreign, foreign, foreign...)

One can then imagine any sequence of platforms such that there are bootstrap stages with their 3 platforms determined by “sliding a window” that is the 3 tuple through the sequence. This was the original model for bootstrapping. Without a target platform (assume a better world where all compilers are multi-target and all standard libraries are built in their own derivation), this is sufficient. Conversely if one wishes to cross compile “faster”, with a “Canadian Cross” bootstrapping stage where build != host != target, more bootstrapping stages are needed since no sliding window provides the pesky pkgsBuildTarget package set since it skips the Canadian cross stage’s “host”.

Note

It is much better to refer to buildPackages than targetPackages, or more broadly package sets that do not mention “target”. There are three reasons for this.

First, it is because bootstrapping stages do not have a unique targetPackages. For example a (x86-linux, x86-linux, arm-linux) and (x86-linux, x86-linux, x86-windows) package set both have a (x86-linux, x86-linux, x86-linux) package set. Because there is no canonical targetPackages for such a native (build == host == target) package set, we set their targetPackages

Second, it is because this is a frequent source of hard-to-follow “infinite recursions” / cycles. When only package sets that don’t mention target are used, the package set forms a directed acyclic graph. This means that all cycles that exist are confined to one stage. This means they are a lot smaller, and easier to follow in the code or a backtrace. It also means they are present in native and cross builds alike, and so more likely to be caught by CI and other users.

Thirdly, it is because everything target-mentioning only exists to accommodate compilers with lousy build systems that insist on the compiler itself and standard library being built together. Of course that is bad because bigger derivations means longer rebuilds. It is also problematic because it tends to make the standard libraries less like other libraries than they could be, complicating code and build systems alike. Because of the other problems, and because of these innate disadvantages, compilers ought to be packaged another way where possible.

Note

If one explores Nixpkgs, they will see derivations with names like gccCross. Such *Cross derivations is a holdover from before we properly distinguished between the host and target platforms—the derivation with “Cross” in the name covered the build = host != target case, while the other covered the host = target, with build platform the same or not based on whether one was using its .__spliced.buildHost or .__spliced.hostTarget.

Platform Notes

Table of Contents

Darwin (macOS)

Darwin (macOS)

Some common issues when packaging software for Darwin:

  • The Darwin stdenv uses clang instead of gcc. When referring to the compiler $CC or cc will work in both cases. Some builds hardcode gcc/g++ in their build scripts, that can usually be fixed with using something like makeFlags = [ "CC=cc" ]; or by patching the build scripts.

    stdenv.mkDerivation {
      name = "libfoo-1.2.3";
      # ...
      buildPhase = ''
        $CC -o hello hello.c
      '';
    }
    
  • On Darwin, libraries are linked using absolute paths, libraries are resolved by their install_name at link time. Sometimes packages won’t set this correctly causing the library lookups to fail at runtime. This can be fixed by adding extra linker flags or by running install_name_tool -id during the fixupPhase.

    stdenv.mkDerivation {
      name = "libfoo-1.2.3";
      # ...
      makeFlags = lib.optional stdenv.isDarwin "LDFLAGS=-Wl,-install_name,$(out)/lib/libfoo.dylib";
    }
    
  • Even if the libraries are linked using absolute paths and resolved via their install_name correctly, tests can sometimes fail to run binaries. This happens because the checkPhase runs before the libraries are installed.

    This can usually be solved by running the tests after the installPhase or alternatively by using DYLD_LIBRARY_PATH. More information about this variable can be found in the dyld(1) manpage.

    dyld: Library not loaded: /nix/store/7hnmbscpayxzxrixrgxvvlifzlxdsdir-jq-1.5-lib/lib/libjq.1.dylib
    Referenced from: /private/tmp/nix-build-jq-1.5.drv-0/jq-1.5/tests/../jq
    Reason: image not found
    ./tests/jqtest: line 5: 75779 Abort trap: 6
    
    stdenv.mkDerivation {
      name = "libfoo-1.2.3";
      # ...
      doInstallCheck = true;
      installCheckTarget = "check";
    }
    
  • Some packages assume xcode is available and use xcrun to resolve build tools like clang, etc. This causes errors like xcode-select: error: no developer tools were found at '/Applications/Xcode.app' while the build doesn’t actually depend on xcode.

    stdenv.mkDerivation {
      name = "libfoo-1.2.3";
      # ...
      prePatch = ''
        substituteInPlace Makefile \
            --replace-fail '/usr/bin/xcrun clang' clang
      '';
    }
    

    The package xcbuild can be used to build projects that really depend on Xcode. However, this replacement is not 100% compatible with Xcode and can occasionally cause issues.

  • x86_64-darwin uses the 10.12 SDK by default, but some software is not compatible with that version of the SDK. In that case, the 11.0 SDK used by aarch64-darwin is available for use on x86_64-darwin. To use it, reference apple_sdk_11_0 instead of apple_sdk in your derivation and use pkgs.darwin.apple_sdk_11_0.callPackage instead of pkgs.callPackage. On Linux, this will have the same effect as pkgs.callPackage, so you can use pkgs.darwin.apple_sdk_11_0.callPackage regardless of platform.

Build helpers

A build helper is a function that produces derivations.

Warning

This is not to be confused with the builder argument of the Nix derivation primitive, which refers to the executable that produces the build result, or remote builder, which refers to a remote machine that could run such an executable.

Such a function is usually designed to abstract over a typical workflow for a given programming language or framework. This allows declaring a build recipe by setting a limited number of options relevant to the particular use case instead of using the derivation function directly.

stdenv.mkDerivation is the most widely used build helper, and serves as a basis for many others. In addition, it offers various options to customize parts of the builds.

There is no uniform interface for build helpers. Trivial build helpers and fetchers have various input types for convenience. Language- or framework-specific build helpers usually follow the style of stdenv.mkDerivation, which accepts an attribute set or a fixed-point function taking an attribute set.

Fetchers

Building software with Nix often requires downloading source code and other files from the internet. To this end, we use functions that we call fetchers, which obtain remote sources via various protocols and services.

Nix provides built-in fetchers such as builtins.fetchTarball. Nixpkgs provides its own fetchers, which work differently:

  • A built-in fetcher will download and cache files at evaluation time and produce a store path. A Nixpkgs fetcher will create a (fixed-output) derivation, and files are downloaded at build time.

  • Built-in fetchers will invalidate their cache after tarball-ttl expires, and will require network activity to check if the cache entry is up to date. Nixpkgs fetchers only re-download if the specified hash changes or the store object is not available.

  • Built-in fetchers do not use substituters. Derivations produced by Nixpkgs fetchers will use any configured binary cache transparently.

This significantly reduces the time needed to evaluate Nixpkgs, and allows Hydra to retain and re-distribute sources used by Nixpkgs in the public binary cache. For these reasons, Nix’s built-in fetchers are not allowed in Nixpkgs.

The following table summarises the differences:

FetchersDownloadOutputCacheRe-download when
builtins.fetch*evaluation timestore path/nix/store, ~/.cache/nixtarball-ttl expires, cache miss in ~/.cache/nix, output store object not in local store
pkgs.fetch*build timederivation/nix/store, substitutersoutput store object not available

Tip

pkgs.fetchFrom* helpers retrieve snapshots of version-controlled sources, as opposed to the entire version history, which is more efficient. pkgs.fetchgit by default also has the same behaviour, but can be changed through specific attributes given to it.

Caveats

Because Nixpkgs fetchers are fixed-output derivations, an output hash has to be specified, usually indirectly through a hash attribute. This hash refers to the derivation output, which can be different from the remote source itself!

This has the following implications that you should be aware of:

  • Use Nix (or Nix-aware) tooling to produce the output hash.

  • When changing any fetcher parameters, always update the output hash. Use one of the methods from the section called “Updating source hashes”. Otherwise, existing store objects that match the output hash will be re-used rather than fetching new content.

    Note

    A similar problem arises while testing changes to a fetcher’s implementation. If the output of the derivation already exists in the Nix store, test failures can go undetected. The invalidateFetcherByDrvHash function helps prevent reusing cached derivations.

Updating source hashes

There are several ways to obtain the hash corresponding to a remote source. Unless you understand how the fetcher you’re using calculates the hash from the downloaded contents, you should use the fake hash method.

  1. The fake hash method: In your package recipe, set the hash to one of

    • ""

    • lib.fakeHash

    • lib.fakeSha256

    • lib.fakeSha512

    Attempt to build, extract the calculated hashes from error messages, and put them into the recipe.

    Warning

    You must use one of these four fake hashes and not some arbitrarily-chosen hash. See the section called “Obtaining hashes securely” for details.

    Example 253. Update source hash with the fake hash method

    Consider the following recipe that produces a plain file:

    { fetchurl }:
    fetchurl {
      url = "https://raw.githubusercontent.com/NixOS/nixpkgs/23.05/.version";
      hash = "sha256-ZHl1emidXVojm83LCVrwULpwIzKE/mYwfztVkvpruOM=";
    }
    

    A common mistake is to update a fetcher parameter, such as url, without updating the hash:

    { fetchurl }:
    fetchurl {
      url = "https://raw.githubusercontent.com/NixOS/nixpkgs/23.11/.version";
      hash = "sha256-ZHl1emidXVojm83LCVrwULpwIzKE/mYwfztVkvpruOM=";
    }
    

    This will produce the same output as before! Set the hash to an empty string:

    { fetchurl }:
    fetchurl {
      url = "https://raw.githubusercontent.com/NixOS/nixpkgs/23.11/.version";
      hash = "";
    }
    

    When building the package, use the error message to determine the correct hash:

    $ nix-build
    (some output removed for clarity)
    error: hash mismatch in fixed-output derivation '/nix/store/7yynn53jpc93l76z9zdjj4xdxgynawcw-version.drv':
            specified: sha256-AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA=
                got:    sha256-BZqI7r0MNP29yGH5+yW2tjU9OOpOCEvwWKrWCv5CQ0I=
    error: build of '/nix/store/bqdjcw5ij5ymfbm41dq230chk9hdhqff-version.drv' failed
    

  2. Prefetch the source with nix-prefetch-<type> <URL>, where <type> is one of

    • url

    • git

    • hg

    • cvs

    • bzr

    • svn

    The hash is printed to stdout.

  3. Prefetch by package source (with nix-prefetch-url '<nixpkgs>' -A <package>.src, where <package> is package attribute name). The hash is printed to stdout.

    This works well when you’ve upgraded the existing package version and want to find out new hash, but is useless if the package can’t be accessed by attribute or the package has multiple sources (.srcs, architecture-dependent sources, etc).

  4. Upstream hash: use it when upstream provides sha256 or sha512. Don’t use it when upstream provides md5, compute sha256 instead.

    A little nuance is that nix-prefetch-* tools produce hashes with the nix32 encoding (a Nix-specific base32 adaptation), but upstream usually provides hexadecimal (base16) encoding. Fetchers understand both formats. Nixpkgs does not standardise on any one format.

    You can convert between hash formats with nix-hash.

  5. Extract the hash from a local source archive with sha256sum. Use nix-prefetch-url file:///path/to/archive if you want the custom Nix base32 hash.

Obtaining hashes securely

It’s always a good idea to avoid Man-in-the-Middle (MITM) attacks when downloading source contents. Otherwise, you could unknowingly download malware instead of the intended source, and instead of the actual source hash, you’ll end up using the hash of malware. Here are security considerations for this scenario:

  • http:// URLs are not secure to prefetch hashes.

  • Upstream hashes should be obtained via a secure protocol.

  • https:// URLs give you more protections when using nix-prefetch-* or for upstream hashes.

  • https:// URLs are secure when using the fake hash method only if you use one of the listed fake hashes. If you use any other hash, the download will be exposed to MITM attacks even if you use HTTPS URLs.

    In more concrete terms, if you use any other hash, the --insecure flag will be passed to the underlying call to curl when downloading content.

fetchurl

fetchurl returns a fixed-output derivation which downloads content from a given URL and stores the unaltered contents within the Nix store.

It uses curl(1) internally, and allows its behaviour to be modified by specifying a few attributes in the argument to fetchurl (see the documentation for attributes curlOpts, curlOptsList, and netrcPhase).

The resulting store path is determined by the hash given to fetchurl, and also the name (or pname and version) values.

If neither name nor pname and version are specified when calling fetchurl, it will default to using the basename of url or the first element of urls. If pname and version are specified, fetchurl will use those values and will ignore name, even if it is also specified.

Inputs

fetchurl requires an attribute set with the following attributes:

url (String; optional)

The URL to download from.

Note

Either url or urls must be specified, but not both.

All URLs of the format specified here are supported.

Default value: "".

urls (List of String; optional)

A list of URLs, specifying download locations for the same content. Each URL will be tried in order until one of them succeeds with some content or all of them fail. See Example 255 to understand how this attribute affects the behaviour of fetchurl.

Note

Either url or urls must be specified, but not both.

Default value: [].

hash (String; optional)

Hash of the derivation output of fetchurl, following the format for integrity metadata as defined by SRI. For more information, see the section called “Caveats”.

Note

It is recommended that you use the hash attribute instead of the other hash-specific attributes that exist for backwards compatibility.

If hash is not specified, you must specify outputHash and outputHashAlgo, or one of sha512, sha256, or sha1.

Default value: "".

outputHash (String; optional)

Hash of the derivation output of fetchurl in the format expected by Nix. See the documentation on the Nix manual for more information about its format.

Note

It is recommended that you use the hash attribute instead.

If outputHash is specified, you must also specify outputHashAlgo.

Default value: "".

outputHashAlgo (String; optional)

Algorithm used to generate the value specified in outputHash. See the documentation on the Nix manual for more information about the values it supports.

Note

It is recommended that you use the hash attribute instead.

The value specified in outputHashAlgo will be ignored if outputHash isn’t also specified.

Default value: "".

sha1 (String; optional)

SHA-1 hash of the derivation output of fetchurl in the format expected by Nix. See the documentation on the Nix manual for more information about its format.

Note

It is recommended that you use the hash attribute instead.

Default value: "".

sha256 (String; optional)

SHA-256 hash of the derivation output of fetchurl in the format expected by Nix. See the documentation on the Nix manual for more information about its format.

Note

It is recommended that you use the hash attribute instead.

Default value: "".

sha512 (String; optional)

SHA-512 hash of the derivation output of fetchurl in the format expected by Nix. See the documentation on the Nix manual for more information about its format.

Note

It is recommended that you use the hash attribute instead.

Default value: "".

name (String; optional)

The symbolic name of the downloaded file when saved in the Nix store. See the fetchurl overview for details on how the name of the file is decided.

Default value: "".

pname (String; optional)

A base name, which will be combined with version to form the symbolic name of the downloaded file when saved in the Nix store. See the fetchurl overview for details on how the name of the file is decided.

Note

If pname is specified, you must also specify version, otherwise fetchurl will ignore the value of pname.

Default value: "".

version (String; optional)

A version, which will be combined with pname to form the symbolic name of the downloaded file when saved in the Nix store. See the fetchurl overview for details on how the name of the file is decided.

Default value: "".

recursiveHash (Boolean; optional)

If set to true, will signal to Nix that the hash given to fetchurl was calculated using the "recursive" mode. See the documentation on the Nix manual for more information about the existing modes.

By default, fetchurl uses "recursive" mode when the executable attribute is set to true, so you don’t need to specify recursiveHash in this case.

Default value: false.

executable (Boolean; optional)

If true, sets the executable bit on the downloaded file.

Default value: false.

downloadToTemp (Boolean; optional)

If true, saves the downloaded file to a temporary location instead of the expected Nix store location. This is useful when used in conjunction with postFetch attribute, otherwise fetchurl will not produce any meaningful output.

The location of the downloaded file will be set in the $downloadedFile variable, which should be used by the script in the postFetch attribute. See Example 256 to understand how to work with this attribute.

Default value: false.

postFetch (String; optional)

Script executed after the file has been downloaded successfully, and before fetchurl finishes running. Useful for post-processing, to check or transform the file in some way. See Example 256 to understand how to work with this attribute.

Default value: "".

netrcPhase (String or Null; optional)

Script executed to create a netrc(5) file to be used with curl(1). The script should create the netrc file (note that it does not begin with a “.”) in the directory it’s currently running in ($PWD).

The script is executed during the setup done by fetchurl before it runs any of its code to download the specified content.

Note

If specified, fetchurl will automatically alter its invocation of curl(1) to use the netrc file, so you don’t need to add anything to curlOpts or curlOptsList.

Caution

Since netrcPhase needs to be specified in your source Nix code, any secrets that you put directly in it will be world-readable by design (both in your source code, and when the derivation gets created in the Nix store).

If you want to avoid this behaviour, see the documentation of netrcImpureEnvVars for an alternative way of dealing with these secrets.

Default value: null.

netrcImpureEnvVars (List of String; optional)

If specified, fetchurl will add these environment variable names to the list of impure environment variables, which will be passed from the environment of the calling user to the builder running the fetchurl code.

This is useful when used with netrcPhase to hide any secrets that are used in it, because the script in netrcPhase only needs to reference the environment variables with the secrets in them instead. However, note that these are called impure variables for a reason: the environment that starts the build needs to have these variables declared for everything to work properly, which means that additional setup is required outside what Nix controls.

Default value: [].

curlOpts (String; optional)

If specified, this value will be appended to the invocation of curl(1) when downloading the URL(s) given to fetchurl. Multiple arguments can be separated by spaces normally, but values with whitespaces will be interpreted as multiple arguments (instead of a single value), even if the value is escaped. See curlOptsList for a way to pass values with whitespaces in them.

Default value: "".

curlOptsList (List of String; optional)

If specified, each element of this list will be passed as an argument to the invocation of curl(1) when downloading the URL(s) given to fetchurl. This allows passing values that contain spaces, with no escaping needed.

Default value: [].

showURLs (Boolean; optional)

If set to true, this will stop fetchurl from downloading anything at all. Instead, it will output a list of all the URLs it would’ve used to download the content (after resolving mirror:// URLs, for example). This is useful for debugging.

Default value: false.

meta (Attribute Set; optional)

Specifies any meta-attributes for the derivation returned by fetchurl.

Default value: {}.

passthru (Attribute Set; optional)

Specifies any extra passthru attributes for the derivation returned by fetchurl. Note that fetchurl defines passthru attributes of its own. Attributes specified in passthru can override the default attributes returned by fetchurl.

Default value: {}.

preferLocalBuild (Boolean; optional)

This is the same attribute as defined in the Nix manual. It is true by default because making a remote machine download the content just duplicates network traffic (since the local machine might download the results from the derivation anyway), but this could be useful in cases where network access is restricted on local machines.

Default value: true.

nativeBuildInputs (List of Attribute Set; optional)

Additional packages needed to download the content. This is useful if you need extra packages for postFetch or netrcPhase, for example. Has the same semantics as in the section called “nativeBuildInputs. See Example 256 to understand how this can be used with postFetch.

Default value: [].

Passthru outputs

fetchurl also defines its own passthru attributes:

url (String)

The same url attribute passed in the argument to fetchurl.

Examples

Example 254. Using fetchurl to download a file

The following package downloads a small file from a URL and shows the most common way to use fetchurl:

{ fetchurl }:
fetchurl {
  url = "https://raw.githubusercontent.com/NixOS/nixpkgs/23.11/.version";
  hash = "sha256-BZqI7r0MNP29yGH5+yW2tjU9OOpOCEvwWKrWCv5CQ0I=";
}

After building the package, the file will be downloaded and place into the Nix store:

$ nix-build
(output removed for clarity)
/nix/store/4g9y3x851wqrvim4zcz5x2v3zivmsq8n-version

$ cat /nix/store/4g9y3x851wqrvim4zcz5x2v3zivmsq8n-version
23.11

Example 255. Using fetchurl to download a file with multiple possible URLs

The following package adapts Example 254 to use multiple URLs. The first URL was crafted to intentionally return an error to illustrate how fetchurl will try multiple URLs until it finds one that works (or all URLs fail).

{ fetchurl }:
fetchurl {
  urls = [
    "https://raw.githubusercontent.com/NixOS/nixpkgs/23.11/does-not-exist"
    "https://raw.githubusercontent.com/NixOS/nixpkgs/23.11/.version"
  ];
  hash = "sha256-BZqI7r0MNP29yGH5+yW2tjU9OOpOCEvwWKrWCv5CQ0I=";
}

After building the package, both URLs will be used to download the file:

$ nix-build
(some output removed for clarity)
trying https://raw.githubusercontent.com/NixOS/nixpkgs/23.11/does-not-exist
(some output removed for clarity)
curl: (22) The requested URL returned error: 404

trying https://raw.githubusercontent.com/NixOS/nixpkgs/23.11/.version
(some output removed for clarity)
/nix/store/n9asny31z32q7sdw6a8r1gllrsfy53kl-does-not-exist

$ cat /nix/store/n9asny31z32q7sdw6a8r1gllrsfy53kl-does-not-exist
23.11

However, note that the name of the file was derived from the first URL (this is further explained in the fetchurl overview). To ensure the result will have the same name regardless of which URLs are used, we can modify the package:

{ fetchurl }:
fetchurl {
  name = "nixpkgs-version";
  urls = [
    "https://raw.githubusercontent.com/NixOS/nixpkgs/23.11/does-not-exist"
    "https://raw.githubusercontent.com/NixOS/nixpkgs/23.11/.version"
  ];
  hash = "sha256-BZqI7r0MNP29yGH5+yW2tjU9OOpOCEvwWKrWCv5CQ0I=";
}

After building the package, the result will have the name we specified:

$ nix-build
(output removed for clarity)
/nix/store/zczb6wl3al6jm9sm5h3pr6nqn0i5ji9z-nixpkgs-version

Example 256. Manipulating the content downloaded by fetchurl

It might be useful to manipulate the content downloaded by fetchurl directly in its derivation. In this example, we’ll adapt Example 254 to append the result of running the hello package to the contents we download, purely to illustrate how to manipulate the content.

{ fetchurl, hello, lib }:
fetchurl {
  url = "https://raw.githubusercontent.com/NixOS/nixpkgs/23.11/.version";

  nativeBuildInputs = [ hello ];

  downloadToTemp = true;
  postFetch = ''
    ${lib.getExe hello} >> $downloadedFile
    mv $downloadedFile $out
  '';

  hash = "sha256-ceooQQYmDx5+0nfg40uU3NNI2yKrixP7HZ/xLZUNv+w=";
}

After building the package, the resulting file will have “Hello, world!” appended to it:

$ nix-build
(output removed for clarity)
/nix/store/ifi6pp7q0ag5h7c5v9h1c1c7bhd10c7f-version

$ cat /nix/store/ifi6pp7q0ag5h7c5v9h1c1c7bhd10c7f-version
23.11
Hello, world!

Note that the hash specified in the package is different than the hash specified in Example 254, because the contents of the output have changed (even though the actual file that was downloaded is the same). See the section called “Caveats” for more details on how to work with the hash attribute when the output changes.


fetchzip

Returns a fixed-output derivation which downloads an archive from a given URL and decompresses it.

Despite its name, fetchzip is not limited to .zip files but can also be used with various compressed tarball formats by default. This can extended by specifying additional attributes, see Example 258 to understand how to do that.

Inputs

fetchzip requires an attribute set, and most attributes are passed to the underlying call to fetchurl.

The attributes below are treated differently by fetchzip when compared to what fetchurl expects:

name (String; optional)

Works as defined in fetchurl, but has a different default value than fetchurl.

Default value: "source".

nativeBuildInputs (List of Attribute Set; optional)

Works as defined in fetchurl, but it is also augmented by fetchzip to include packages to deal with additional archives (such as .zip).

Default value: [].

postFetch (String; optional)

Works as defined in fetchurl, but it is also augmented with the code needed to make fetchzip work.

Caution

It is only safe to modify files in $out in postFetch. Consult the implementation of fetchzip for anything more involved.

Default value: "".

stripRoot (Boolean; optional)

If true, the decompressed contents are moved one level up the directory tree.

This is useful for archives that decompress into a single directory which commonly includes some values that change with time, such as version numbers. When this is the case (and stripRoot is true), fetchzip will remove this directory and make the decompressed contents available in the top-level directory.

Example 257 shows what this attribute does.

This attribute is not passed through to fetchurl.

Default value: true.

extension (String or Null; optional)

If set, the archive downloaded by fetchzip will be renamed to a filename with the extension specified in this attribute.

This is useful when making fetchzip support additional types of archives, because the implementation may use the extension of an archive to determine whether they can decompress it. If the URL you’re using to download the contents doesn’t end with the extension associated with the archive, use this attribute to fix the filename of the archive.

This attribute is not passed through to fetchurl.

Default value: null.

recursiveHash (Boolean; optional)

Works as defined in fetchurl, but its default value is different than for fetchurl.

Default value: true.

downloadToTemp (Boolean; optional)

Works as defined in fetchurl, but its default value is different than for fetchurl.

Default value: true.

extraPostFetch DEPRECATED

This attribute is deprecated. Please use postFetch instead.

This attribute is not passed through to fetchurl.

Examples

Example 257. Using fetchzip to output contents directly

The following recipe shows how to use fetchzip to decompress a .tar.gz archive:

{ fetchzip }:
fetchzip {
  url = "https://github.com/NixOS/patchelf/releases/download/0.18.0/patchelf-0.18.0.tar.gz";
  hash = "sha256-3ABYlME9R8klcpJ7MQpyFEFwHmxDDEzIYBqu/CpDYmg=";
}

This archive has all its contents in a directory named patchelf-0.18.0. This means that after decompressing, you’d have to enter this directory to see the contents of the archive. However, fetchzip makes this easier through the attribute stripRoot (enabled by default).

After building the recipe, the derivation output will show all the files in the archive at the top level:

$ nix-build
(output removed for clarity)
/nix/store/1b7h3fvmgrcddvs0m299hnqxlgli1yjw-source

$ ls /nix/store/1b7h3fvmgrcddvs0m299hnqxlgli1yjw-source
aclocal.m4  completions  configure.ac  m4           Makefile.in  patchelf.spec     README.md  tests
build-aux   configure    COPYING       Makefile.am  patchelf.1   patchelf.spec.in  src        version

If stripRoot is set to false, the derivation output will be the decompressed archive as-is:

{ fetchzip }:
fetchzip {
  url = "https://github.com/NixOS/patchelf/releases/download/0.18.0/patchelf-0.18.0.tar.gz";
  hash = "sha256-uv3FuKE4DqpHT3yfE0qcnq0gYjDNQNKZEZt2+PUAneg=";
  stripRoot = false;
}

Caution

The hash changed! Whenever changing attributes of a Nixpkgs fetcher, remember to invalidate the hash, otherwise you won’t get the results you’re expecting!

After building the recipe:

$ nix-build
(output removed for clarity)
/nix/store/2hy5bxw7xgbgxkn0i4x6hjr8w3dbx16c-source

$ ls /nix/store/2hy5bxw7xgbgxkn0i4x6hjr8w3dbx16c-source
patchelf-0.18.0

Example 258. Using fetchzip to decompress a .rar file

The unrar package provides a setup hook to decompress .rar archives during the unpack phase, which can be used with fetchzip to decompress those archives:

{ fetchzip, unrar }:
fetchzip {
  url = "https://archive.org/download/SpaceCadet_Plus95/Space_Cadet.rar";
  hash = "sha256-fC+zsR8BY6vXpUkVd6i1jF0IZZxVKVvNi6VWCKT+pA4=";
  stripRoot = false;
  nativeBuildInputs = [ unrar ];
}

Since this particular .rar file doesn’t put its contents in a directory inside the archive, stripRoot must be set to false.

After building the recipe, the derivation output will show the decompressed files:

$ nix-build
(output removed for clarity)
/nix/store/zpn7knxfva6rfjja2gbb4p3l9w1f0d36-source

$ ls /nix/store/zpn7knxfva6rfjja2gbb4p3l9w1f0d36-source
FONT.DAT      PINBALL.DAT  PINBALL.EXE	PINBALL2.MID  TABLE.BMP    WMCONFIG.EXE
MSCREATE.DIR  PINBALL.DOC  PINBALL.MID	Sounds	     WAVEMIX.INF

fetchpatch

fetchpatch works very similarly to fetchurl with the same arguments expected. It expects patch files as a source and performs normalization on them before computing the checksum. For example, it will remove comments or other unstable parts that are sometimes added by version control systems and can change over time.

  • relative: Similar to using git-diff’s --relative flag, only keep changes inside the specified directory, making paths relative to it.

  • stripLen: Remove the first stripLen components of pathnames in the patch.

  • decode: Pipe the downloaded data through this command before processing it as a patch.

  • extraPrefix: Prefix pathnames by this string.

  • excludes: Exclude files matching these patterns (applies after the above arguments).

  • includes: Include only files matching these patterns (applies after the above arguments).

  • revert: Revert the patch.

Note that because the checksum is computed after applying these effects, using or modifying these arguments will have no effect unless the hash argument is changed as well.

Most other fetchers return a directory rather than a single file.

fetchDebianPatch

A wrapper around fetchpatch, which takes:

  • patch and hash: the patch’s filename, and its hash after normalization by fetchpatch ;

  • pname: the Debian source package’s name ;

  • version: the upstream version number ;

  • debianRevision: the Debian revision number if applicable ;

  • the area of the Debian archive: main (default), contrib, or non-free.

Here is an example of fetchDebianPatch in action:

{ lib
, fetchDebianPatch
, buildPythonPackage
}:

buildPythonPackage rec {
  pname = "pysimplesoap";
  version = "1.16.2";
  src = <...>;

  patches = [
    (fetchDebianPatch {
      inherit pname version;
      debianRevision = "5";
      name = "Add-quotes-to-SOAPAction-header-in-SoapClient.patch";
      hash = "sha256-xA8Wnrpr31H8wy3zHSNfezFNjUJt1HbSXn3qUMzeKc0=";
    })
  ];

  # ...
}

Patches are fetched from sources.debian.org, and so must come from a package version that was uploaded to the Debian archive. Packages may be removed from there once that specific version isn’t in any suite anymore (stable, testing, unstable, etc.), so maintainers should use copy-tarballs.pl to archive the patch if it needs to be available longer-term.

fetchsvn

Used with Subversion. Expects url to a Subversion directory, rev, and hash.

fetchgit

Used with Git. Expects url to a Git repo, rev, and hash. rev in this case can be full the git commit id (SHA1 hash) or a tag name like refs/tags/v1.0.

Additionally, the following optional arguments can be given: fetchSubmodules = true makes fetchgit also fetch the submodules of a repository. If deepClone is set to true, the entire repository is cloned as opposing to just creating a shallow clone. deepClone = true also implies leaveDotGit = true which means that the .git directory of the clone won’t be removed after checkout.

If only parts of the repository are needed, sparseCheckout can be used. This will prevent git from fetching unnecessary blobs from server, see git sparse-checkout for more information:

{ stdenv, fetchgit }:

stdenv.mkDerivation {
  name = "hello";
  src = fetchgit {
    url = "https://...";
    sparseCheckout = [
      "directory/to/be/included"
      "another/directory"
    ];
    hash = "sha256-AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA=";
  };
}

fetchfossil

Used with Fossil. Expects url to a Fossil archive, rev, and hash.

fetchcvs

Used with CVS. Expects cvsRoot, tag, and hash.

fetchhg

Used with Mercurial. Expects url, rev, and hash.

A number of fetcher functions wrap part of fetchurl and fetchzip. They are mainly convenience functions intended for commonly used destinations of source code in Nixpkgs. These wrapper fetchers are listed below.

fetchFromGitea

fetchFromGitea expects five arguments. domain is the gitea server name. owner is a string corresponding to the Gitea user or organization that controls this repository. repo corresponds to the name of the software repository. These are located at the top of every Gitea HTML page as owner/repo. rev corresponds to the Git commit hash or tag (e.g v1.0) that will be downloaded from Git. Finally, hash corresponds to the hash of the extracted directory. Again, other hash algorithms are also available but hash is currently preferred.

fetchFromGitHub

fetchFromGitHub expects four arguments. owner is a string corresponding to the GitHub user or organization that controls this repository. repo corresponds to the name of the software repository. These are located at the top of every GitHub HTML page as owner/repo. rev corresponds to the Git commit hash or tag (e.g v1.0) that will be downloaded from Git. Finally, hash corresponds to the hash of the extracted directory. Again, other hash algorithms are also available, but hash is currently preferred.

To use a different GitHub instance, use githubBase (defaults to "github.com").

fetchFromGitHub uses fetchzip to download the source archive generated by GitHub for the specified revision. If leaveDotGit, deepClone or fetchSubmodules are set to true, fetchFromGitHub will use fetchgit instead. Refer to its section for documentation of these options.

fetchFromGitLab

This is used with GitLab repositories. It behaves similarly to fetchFromGitHub, and expects owner, repo, rev, and hash.

To use a specific GitLab instance, use domain (defaults to "gitlab.com").

fetchFromGitiles

This is used with Gitiles repositories. The arguments expected are similar to fetchgit.

fetchFromBitbucket

This is used with BitBucket repositories. The arguments expected are very similar to fetchFromGitHub above.

fetchFromSavannah

This is used with Savannah repositories. The arguments expected are very similar to fetchFromGitHub above.

fetchFromRepoOrCz

This is used with repo.or.cz repositories. The arguments expected are very similar to fetchFromGitHub above.

fetchFromSourcehut

This is used with sourcehut repositories. Similar to fetchFromGitHub above, it expects owner, repo, rev and hash, but don’t forget the tilde (~) in front of the username! Expected arguments also include vc (“git” (default) or “hg”), domain and fetchSubmodules.

If fetchSubmodules is true, fetchFromSourcehut uses fetchgit or fetchhg with fetchSubmodules or fetchSubrepos set to true, respectively. Otherwise, the fetcher uses fetchzip.

requireFile

requireFile allows requesting files that cannot be fetched automatically, but whose content is known. This is a useful last-resort workaround for license restrictions that prohibit redistribution, or for downloads that are only accessible after authenticating interactively in a browser. If the requested file is present in the Nix store, the resulting derivation will not be built, because its expected output is already available. Otherwise, the builder will run, but fail with a message explaining to the user how to provide the file. The following code, for example:

requireFile {
  name = "jdk-${version}_linux-x64_bin.tar.gz";
  url = "https://www.oracle.com/java/technologies/javase-jdk11-downloads.html";
  hash = "sha256-lL00+F7jjT71nlKJ7HRQuUQ7kkxVYlZh//5msD8sjeI=";
}

results in this error message:

***
Unfortunately, we cannot download file jdk-11.0.10_linux-x64_bin.tar.gz automatically.
Please go to https://www.oracle.com/java/technologies/javase-jdk11-downloads.html to download it yourself, and add it to the Nix store
using either
  nix-store --add-fixed sha256 jdk-11.0.10_linux-x64_bin.tar.gz
or
  nix-prefetch-url --type sha256 file:///path/to/jdk-11.0.10_linux-x64_bin.tar.gz

***

This function should only be used by non-redistributable software with an unfree license that we need to require the user to download manually. It produces packages that cannot be built automatically.

fetchtorrent

fetchtorrent expects two arguments. url which can either be a Magnet URI (Magnet Link) such as magnet:?xt=urn:btih:dd8255ecdc7ca55fb0bbf81323d87062db1f6d1c or an HTTP URL pointing to a .torrent file. It can also take a config argument which will craft a settings.json configuration file and give it to transmission, the underlying program that is performing the fetch. The available config options for transmission can be found here

{ fetchtorrent }:

fetchtorrent {
  config = { peer-limit-global = 100; };
  url = "magnet:?xt=urn:btih:dd8255ecdc7ca55fb0bbf81323d87062db1f6d1c";
  sha256 = "";
}

Parameters

  • url: Magnet URI (Magnet Link) such as magnet:?xt=urn:btih:dd8255ecdc7ca55fb0bbf81323d87062db1f6d1c or an HTTP URL pointing to a .torrent file.

  • backend: Which bittorrent program to use. Default: "transmission". Valid values are "rqbit" or "transmission". These are the two most suitable torrent clients for fetching in a fixed-output derivation at the time of writing, as they can be easily exited after usage. rqbit is written in Rust and has a smaller closure size than transmission, and the performance and peer discovery properties differs between these clients, requiring experimentation to decide upon which is the best.

  • config: When using transmission as the backend, a json configuration can be supplied to transmission. Refer to the upstream documentation for information on how to configure.

Trivial build helpers

Nixpkgs provides a variety of wrapper functions that help build commonly useful derivations. Like