Nixpkgs Reference Manual

Version 24.05pre-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
Multiple-output packages
Cross-compilation
Platform Notes
Build helpers
Fetchers
Trivial build helpers
Testers
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. lib.attrsets.attrByPath usage example
2. lib.attrsets.hasAttrByPath usage example
3. lib.attrsets.longestValidPathPrefix usage example
4. lib.attrsets.setAttrByPath usage example
5. lib.attrsets.getAttrFromPath usage example
6. lib.attrsets.concatMapAttrs usage example
7. lib.attrsets.updateManyAttrsByPath usage example
8. lib.attrsets.attrVals usage example
9. lib.attrsets.attrValues usage example
10. lib.attrsets.getAttrs usage example
11. lib.attrsets.catAttrs usage example
12. lib.attrsets.filterAttrs usage example
13. lib.attrsets.filterAttrsRecursive usage example
14. lib.attrsets.foldlAttrs usage example
15. lib.attrsets.foldAttrs usage example
16. lib.attrsets.collect usage example
17. lib.attrsets.cartesianProductOfSets usage example
18. lib.attrsets.nameValuePair usage example
19. lib.attrsets.mapAttrs usage example
20. lib.attrsets.mapAttrs' usage example
21. lib.attrsets.mapAttrsToList usage example
22. lib.attrsets.attrsToList usage example
23. Map over leaf attributes
24. Map over an leaf attributes defined by a condition
25. lib.attrsets.genAttrs usage example
26. lib.attrsets.isDerivation usage example
27. lib.attrsets.optionalAttrs usage example
28. lib.attrsets.zipAttrsWithNames usage example
29. lib.attrsets.zipAttrsWith usage example
30. lib.attrsets.zipAttrs usage example
31. lib.attrsets.mergeAttrsList usage example
32. lib.attrsets.recursiveUpdateUntil usage example
33. lib.attrsets.recursiveUpdate usage example
34. lib.attrsets.matchAttrs usage example
35. lib.attrsets.overrideExisting usage example
36. lib.attrsets.showAttrPath usage example
37. lib.attrsets.getOutput usage example
38. lib.attrsets.getBin usage example
39. lib.attrsets.getLib usage example
40. lib.attrsets.getDev usage example
41. lib.attrsets.getMan usage example
42. lib.attrsets.recurseIntoAttrs usage example
43. lib.strings.concatStrings usage example
44. lib.strings.concatMapStrings usage example
45. lib.strings.concatImapStrings usage example
46. lib.strings.intersperse usage example
47. lib.strings.concatStringsSep usage example
48. lib.strings.concatMapStringsSep usage example
49. lib.strings.concatImapStringsSep usage example
50. lib.strings.concatLines usage example
51. lib.strings.replicate usage example
52. lib.strings.makeSearchPath usage example
53. lib.strings.makeSearchPathOutput usage example
54. lib.strings.makeLibraryPath usage example
55. lib.strings.makeBinPath usage example
56. lib.strings.normalizePath usage example
57. lib.strings.optionalString usage example
58. lib.strings.hasPrefix usage example
59. lib.strings.hasSuffix usage example
60. lib.strings.hasInfix usage example
61. lib.strings.stringToCharacters usage example
62. lib.strings.stringAsChars usage example
63. lib.strings.charToInt usage example
64. lib.strings.escape usage example
65. lib.strings.escapeC usage example
66. lib.strings.escapeURL usage example
67. lib.strings.escapeShellArg usage example
68. lib.strings.escapeShellArgs usage example
69. lib.strings.isValidPosixName usage example
70. lib.strings.toShellVar usage example
71. lib.strings.toShellVars usage example
72. lib.strings.escapeNixString usage example
73. lib.strings.escapeRegex usage example
74. lib.strings.escapeNixIdentifier usage example
75. lib.strings.escapeXML usage example
76. lib.strings.toLower usage example
77. lib.strings.toUpper usage example
78. lib.strings.addContextFrom usage example
79. lib.strings.splitString usage example
80. lib.strings.removePrefix usage example
81. lib.strings.removeSuffix usage example
82. lib.strings.versionOlder usage example
83. lib.strings.versionAtLeast usage example
84. lib.strings.getName usage example
85. lib.strings.getVersion usage example
86. lib.strings.nameFromURL usage example
87. lib.strings.cmakeOptionType usage example
88. lib.strings.cmakeBool usage example
89. lib.strings.cmakeFeature usage example
90. lib.strings.mesonOption usage example
91. lib.strings.mesonBool usage example
92. lib.strings.mesonEnable usage example
93. lib.strings.enableFeature usage example
94. lib.strings.enableFeatureAs usage example
95. lib.strings.withFeature usage example
96. lib.strings.withFeatureAs usage example
97. lib.strings.fixedWidthString usage example
98. lib.strings.fixedWidthNumber usage example
99. lib.strings.floatToString usage example
100. lib.strings.isStorePath usage example
101. lib.strings.toInt usage example
102. lib.strings.toIntBase10 usage example
103. lib.strings.readPathsFromFile usage example
104. lib.strings.fileContents usage example
105. lib.strings.sanitizeDerivationName usage example
106. lib.strings.levenshtein usage example
107. lib.strings.levenshteinAtMost usage example
108. lib.versions.splitVersion usage example
109. lib.versions.major usage example
110. lib.versions.minor usage example
111. lib.versions.patch usage example
112. lib.versions.majorMinor usage example
113. lib.versions.pad usage example
114. lib.trivial.const usage example
115. lib.trivial.pipe usage example
116. lib.trivial.concat usage example
117. lib.trivial.mergeAttrs usage example
118. lib.trivial.flip usage example
119. lib.trivial.mapNullable usage example
120. lib.trivial.mod usage example
121. lib.trivial.splitByAndCompare usage example
122. lib.trivial.throwIfNot usage example
123. lib.trivial.checkListOfEnum usage example
124. lib.trivial.mirrorFunctionArgs usage example
125. lib.trivial.toFunction usage example
126. lib.fixedPoints.fix usage example
127. lib.fixedPoints.extends usage example
128. lib.debug.traceIf usage example
129. lib.debug.traceValFn usage example
130. lib.debug.traceVal usage example
131. lib.debug.traceSeq usage example
132. lib.debug.traceSeqN usage example
133. lib.debug.traceFnSeqN usage example
134. lib.debug.runTests usage example
135. lib.debug.testAllTrue usage example
136. lib.options.isOption usage example
137. lib.options.mkOption usage example
138. lib.options.mkEnableOption usage example
139. lib.options.mkPackageOption usage example
140. lib.options.getValues usage example
141. lib.options.getFiles usage example
142. lib.options.showOption usage example
143. lib.path.append usage example
144. lib.path.hasPrefix usage example
145. lib.path.removePrefix usage example
146. lib.path.splitRoot usage example
147. lib.path.hasStorePathPrefix usage example
148. lib.path.subpath.isValid usage example
149. lib.path.subpath.join usage example
150. lib.path.subpath.components usage example
151. lib.path.subpath.normalise usage example
152. lib.filesystem.pathType usage example
153. lib.filesystem.pathIsDirectory usage example
154. lib.filesystem.pathIsRegularFile usage example
155. lib.filesystem.packagesFromDirectoryRecursive usage example
156. lib.fileset.maybeMissing usage example
157. lib.fileset.trace usage example
158. lib.fileset.traceVal usage example
159. lib.fileset.toSource usage example
160. lib.fileset.toList usage example
161. lib.fileset.union usage example
162. lib.fileset.unions usage example
163. lib.fileset.intersection usage example
164. lib.fileset.difference usage example
165. lib.fileset.fileFilter usage example
166. lib.fileset.fromSource usage example
167. lib.fileset.gitTracked usage example
168. lib.fileset.gitTrackedWith usage example
169. lib.sources.commitIdFromGitRepo usage example
170. lib.sources.cleanSource usage example
171. lib.sources.cleanSourceWith usage example
172. lib.sources.sourceByRegex usage example
173. lib.sources.sourceFilesBySuffices usage example
174. lib.cli.toGNUCommandLineShell usage example
175. lib.gvariant.mkArray usage example
176. lib.gvariant.mkEmptyArray usage example
177. lib.gvariant.mkVariant usage example
178. lib.gvariant.mkDictionaryEntry usage example
179. lib.customisation.overrideDerivation usage example
180. lib.customisation.makeOverridable usage example
181. Create an interdependent package set on top of pkgs
182. Using callPackage from a scope
183. lib.meta.addMetaAttrs usage example
184. lib.meta.updateName usage example
185. lib.meta.platformMatch usage example
186. lib.meta.availableOn usage example
187. lib.meta.getLicenseFromSpdxId usage example
188. lib.meta.getExe usage example
189. lib.meta.getExe' usage example
190. lib.derivations.optionalDrvAttr usage example
191. Standard output of an update script using commit feature
192. Enable debug symbols for use with GDB
193. Invocation of runCommand
194. Usage 1 of writeTextFile
195. Usage 2 of writeTextFile
196. Usage 3 of writeTextFile
197. Usage of writeText
198. Usage of writeTextDir
199. Usage of writeScript
200. Usage of writeScriptBin
201. Usage of writeShellScript
202. Usage of writeShellScriptBin
203. Check that pkg-config modules are exposed using default values
204. Check that pkg-config modules are exposed using explicit module names
205. Check a program version using all the default values
206. Check the program version using a specified command and expected version string
207. Check that a build fails, and verify the changes made during build
208. Check that two paths have the same contents
209. Check that two packages produce the same derivation
210. Prevent nix from reusing the output of a fetcher
211. Run a NixOS test using runNixOSTest
212. Using fakeNss with dockerTools.buildImage
213. Using fakeNss with an override to add extra lines
214. Wrapping an AppImage from GitHub
215. Wrapping an AppImage with extra packages
216. Extracting an AppImage to install extra files
217. Extracting an AppImage to install extra files, using postExtract
218. Building a Docker image
219. Building a Docker image with runAsRoot
220. Building a Docker image with extraCommands
221. Building a Docker image with a creation date set to the current time
222. Building a layered Docker image
223. Streaming a layered Docker image
224. Exploring the layers in an image built with streamLayeredImage
225. Building a layered Docker image with packages directly in config
226. Pulling the nixos/nix Docker image from the default registry
227. Pulling the nixos/nix Docker image from a specific registry
228. Finding the digest and hash values to use for dockerTools.pullImage
229. Exporting a Docker image with dockerTools.exportImage
230. Importing an archive built with dockerTools.exportImage in Docker
231. Exploring output naming with dockerTools.exportImage
232. Using dockerTools.exportImage with a path as fromImage
233. Using dockerTools’s environment helpers with buildImage
234. Using dockerTools’s environment helpers with buildLayeredImage
235. Using dockerTools.shadowSetup with dockerTools.buildImage
236. Using dockerTools.shadowSetup with dockerTools.buildLayeredImage
237. Building a Docker image with buildNixShellImage with the build environment for the hello package
238. Building a Docker image with streamNixShellImage with the build environment for the hello package
239. Adding extra packages to a Docker image built with streamNixShellImage
240. Adding a shellHook to a Docker image built with streamNixShellImage
241. Creating an OCI runtime container that runs bash
242. Building a Portable Service image
243. Specifying symlinks when building a Portable Service image
244. Copying a package and its closure to another machine with mkBinaryCache
245. Ephemeral shell
246. Declarative shell
247. Using pkgs.zlib.override {}
248. Using pkgs.buildEmscriptenPackage {}
249. Overriding the kernel derivation
250. Using pkgs.linuxManualConfig with a specific source, version, and config file
251. 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 what 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.

Note that all this is checked during evaluation already, and the check includes any package that is evaluated. In particular, all build-time dependencies are checked. nix-env -qa will (attempt to) hide any packages that would be refused.

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

The nixpkgs configuration for a NixOS system is set in the configuration.nix, as in the following example:

{
  nixpkgs.config = {
    allowUnfree = true;
  };
}

However, this does not allow unfree software for individual users. Their configurations are managed separately.

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

{
  allowUnfree = true;
}

Note that we are not able to test or build unfree software on Hydra due to policy. Most unfree licenses prohibit us from 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 compatibity, 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

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

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

lib.attrsets: attribute set functions

Operations on attribute sets.

lib.attrsets.attrByPath

Type: attrByPath :: [String] -> Any -> AttrSet -> Any

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
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


Located at lib/attrsets.nix:38 in <nixpkgs>.

lib.attrsets.hasAttrByPath

Type: hasAttrByPath :: [String] -> AttrSet -> Bool

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

Laws:

  1. hasAttrByPath [] x == true
    
attrPath

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

e

The nested attribute set to check


Located at lib/attrsets.nix:87 in <nixpkgs>.

lib.attrsets.longestValidPathPrefix

Type: attrsets.longestValidPathPrefix :: [String] -> Value -> [String]

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
    
attrPath

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

v

The nested attribute set to check.


Located at lib/attrsets.nix:137 in <nixpkgs>.

lib.attrsets.setAttrByPath

Type: setAttrByPath :: [String] -> Any -> AttrSet

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

attrPath

A list of strings representing the attribute path to set

value

The value to set at the location described by attrPath


Located at lib/attrsets.nix:180 in <nixpkgs>.

lib.attrsets.getAttrFromPath

Type: getAttrFromPath :: [String] -> AttrSet -> Any

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
attrPath

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

set

The nested attribute set to find the value in.


Located at lib/attrsets.nix:214 in <nixpkgs>.

lib.attrsets.concatMapAttrs

Type: concatMapAttrs :: (String -> a -> AttrSet) -> AttrSet -> AttrSet

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

f

Function argument

v

Function argument


Located at lib/attrsets.nix:235 in <nixpkgs>.

lib.attrsets.updateManyAttrsByPath

Type: updateManyAttrsByPath :: [{ path :: [String]; update :: (Any -> Any); }] -> AttrSet -> AttrSet

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


Located at lib/attrsets.nix:286 in <nixpkgs>.

lib.attrsets.attrVals

Type: attrVals :: [String] -> AttrSet -> [Any]

Return the specified attributes from a set.

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


Located at lib/attrsets.nix:354 in <nixpkgs>.

lib.attrsets.attrValues

Type: attrValues :: AttrSet -> [Any]

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


Located at lib/attrsets.nix:371 in <nixpkgs>.

lib.attrsets.getAttrs

Type: getAttrs :: [String] -> AttrSet -> AttrSet

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

names

A list of attribute names to get out of set

attrs

The set to get the named attributes from


Located at lib/attrsets.nix:384 in <nixpkgs>.

lib.attrsets.catAttrs

Type: catAttrs :: String -> [AttrSet] -> [Any]

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


Located at lib/attrsets.nix:400 in <nixpkgs>.

lib.attrsets.filterAttrs

Type: filterAttrs :: (String -> Any -> Bool) -> AttrSet -> AttrSet

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

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


Located at lib/attrsets.nix:413 in <nixpkgs>.

lib.attrsets.filterAttrsRecursive

Type: filterAttrsRecursive :: (String -> Any -> Bool) -> AttrSet -> AttrSet

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

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


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

lib.attrsets.foldlAttrs

Type: foldlAttrs :: ( a -> String -> b -> a ) -> a -> { ... :: b } -> a

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.

f

Function argument

init

Function argument

set

Function argument


Located at lib/attrsets.nix:502 in <nixpkgs>.

lib.attrsets.foldAttrs

Type: foldAttrs :: (Any -> Any -> Any) -> Any -> [AttrSets] -> Any

Apply fold functions to values grouped by key.

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.


Located at lib/attrsets.nix:518 in <nixpkgs>.

lib.attrsets.collect

Type: collect :: (AttrSet -> Bool) -> AttrSet -> [x]

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

pred

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

attrs

The attribute set to recursively collect.


Located at lib/attrsets.nix:547 in <nixpkgs>.

lib.attrsets.cartesianProductOfSets

Type: cartesianProductOfSets :: AttrSet -> [AttrSet]

Return the cartesian product of attribute set value combinations.

attrsOfLists

Attribute set with attributes that are lists of values


Located at lib/attrsets.nix:572 in <nixpkgs>.

lib.attrsets.nameValuePair

Type: nameValuePair :: String -> Any -> { name :: String; value :: Any; }

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

name

Attribute name

value

Attribute value


Located at lib/attrsets.nix:591 in <nixpkgs>.

lib.attrsets.mapAttrs

Type: mapAttrs :: (String -> Any -> Any) -> AttrSet -> AttrSet

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


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

lib.attrsets.mapAttrs'

Type: mapAttrs' :: (String -> Any -> { name :: String; value :: Any; }) -> AttrSet -> AttrSet

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.

f

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

set

Attribute set to map over.


Located at lib/attrsets.nix:624 in <nixpkgs>.

lib.attrsets.mapAttrsToList

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

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

f

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

attrs

Attribute set to map over.


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

lib.attrsets.attrsToList

Type: attrsToList :: AttrSet -> [ { name :: String; value :: Any; } ]

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.


Located at lib/attrsets.nix:679 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.


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

Located at lib/attrsets.nix:707 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.


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

Located at lib/attrsets.nix:738 in <nixpkgs>.

lib.attrsets.genAttrs

Type: genAttrs :: [ String ] -> (String -> Any) -> AttrSet

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

names

Names of values in the resulting attribute set.

f

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


Located at lib/attrsets.nix:767 in <nixpkgs>.

lib.attrsets.isDerivation

Type: isDerivation :: Any -> Bool

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

value

Value to check.


Located at lib/attrsets.nix:788 in <nixpkgs>.

lib.attrsets.toDerivation

Type: toDerivation :: Path -> Derivation

Converts a store path to a fake derivation.

path

A store path to convert to a derivation.

Located at lib/attrsets.nix:797 in <nixpkgs>.

lib.attrsets.optionalAttrs

Type: optionalAttrs :: Bool -> AttrSet -> AttrSet

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

cond

Condition under which the as attribute set is returned.

as

The attribute set to return if cond is true.


Located at lib/attrsets.nix:825 in <nixpkgs>.

lib.attrsets.zipAttrsWithNames

Type: zipAttrsWithNames :: [ String ] -> (String -> [ Any ] -> Any) -> [ AttrSet ] -> AttrSet

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

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.


Located at lib/attrsets.nix:843 in <nixpkgs>.

lib.attrsets.zipAttrsWith

Type: zipAttrsWith :: (String -> [ Any ] -> Any) -> [ AttrSet ] -> AttrSet

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.


Located at lib/attrsets.nix:871 in <nixpkgs>.

lib.attrsets.zipAttrs

Type: zipAttrs :: [ AttrSet ] -> AttrSet

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

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


Located at lib/attrsets.nix:886 in <nixpkgs>.

lib.attrsets.mergeAttrsList

Type: mergeAttrsList :: [ Attrs ] -> Attrs

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).

list

Function argument


Located at lib/attrsets.nix:903 in <nixpkgs>.

lib.attrsets.recursiveUpdateUntil

Type: recursiveUpdateUntil :: ( [ String ] -> AttrSet -> AttrSet -> Bool ) -> AttrSet -> AttrSet -> AttrSet

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.

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.


Located at lib/attrsets.nix:955 in <nixpkgs>.

lib.attrsets.recursiveUpdate

Type: recursiveUpdate :: AttrSet -> AttrSet -> AttrSet

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.

lhs

Left attribute set of the merge.

rhs

Right attribute set of the merge.


Located at lib/attrsets.nix:995 in <nixpkgs>.

lib.attrsets.matchAttrs

Type: matchAttrs :: AttrSet -> AttrSet -> Bool

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.

pattern

Attribute set structure to match

attrs

Attribute set to check


Located at lib/attrsets.nix:1015 in <nixpkgs>.

lib.attrsets.overrideExisting

Type: overrideExisting :: AttrSet -> AttrSet -> AttrSet

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

old

Original attribute set

new

Attribute set with attributes to override in old.


Located at lib/attrsets.nix:1051 in <nixpkgs>.

lib.attrsets.showAttrPath

Type: showAttrPath :: [String] -> String

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.

path

Attribute path to render to a string


Located at lib/attrsets.nix:1073 in <nixpkgs>.

lib.attrsets.getOutput

Type: getOutput :: String -> Derivation -> String

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

output

Function argument

pkg

Function argument


Located at lib/attrsets.nix:1090 in <nixpkgs>.

lib.attrsets.getBin

Type: getBin :: Derivation -> String

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


Located at lib/attrsets.nix:1105 in <nixpkgs>.

lib.attrsets.getLib

Type: getLib :: Derivation -> String

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


Located at lib/attrsets.nix:1118 in <nixpkgs>.

lib.attrsets.getDev

Type: getDev :: Derivation -> String

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


Located at lib/attrsets.nix:1131 in <nixpkgs>.

lib.attrsets.getMan

Type: getMan :: Derivation -> String

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


Located at lib/attrsets.nix:1144 in <nixpkgs>.

lib.attrsets.chooseDevOutputs

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

Pick the outputs of packages to place in buildInputs

Located at lib/attrsets.nix:1151 in <nixpkgs>.

lib.attrsets.recurseIntoAttrs

Type: recurseIntoAttrs :: AttrSet -> AttrSet

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.

attrs

An attribute set to scan for derivations.


Located at lib/attrsets.nix:1171 in <nixpkgs>.

lib.attrsets.dontRecurseIntoAttrs

Type: dontRecurseIntoAttrs :: AttrSet -> AttrSet

Undo the effect of recurseIntoAttrs.

attrs

An attribute set to not scan for derivations.

Located at lib/attrsets.nix:1181 in <nixpkgs>.

lib.attrsets.unionOfDisjoint

Type: unionOfDisjoint :: AttrSet -> AttrSet -> AttrSet

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 (//).

x

Function argument

y

Function argument

Located at lib/attrsets.nix:1193 in <nixpkgs>.

lib.strings: string manipulation functions

String manipulation functions.

lib.strings.concatStrings

Type: concatStrings :: [string] -> string

Concatenate a list of strings.


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


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


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


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


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


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


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").


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


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


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


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

lib.strings.makeBinPath

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


Located at lib/strings.nix:216 in <nixpkgs>.

lib.strings.normalizePath

Type: normalizePath :: string -> string

Normalize path, removing extraneous /s

s

Function argument


Located at lib/strings.nix:226 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


Located at lib/strings.nix:252 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


Located at lib/strings.nix:268 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


Located at lib/strings.nix:295 in <nixpkgs>.

lib.strings.hasInfix

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

Determine whether a string contains the given infix

infix

Function argument

content

Function argument


Located at lib/strings.nix:332 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


Located at lib/strings.nix:362 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


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

lib.strings.charToInt

Type: charToInt :: string -> int

Convert char to ascii value, must be in printable range

c

Function argument


Located at lib/strings.nix:393 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


Located at lib/strings.nix:404 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


Located at lib/strings.nix:417 in <nixpkgs>.

lib.strings.escapeURL

Type: escapeURL :: string -> string

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


Located at lib/strings.nix:428 in <nixpkgs>.

lib.strings.escapeShellArg

Type: escapeShellArg :: string -> string

Quote string to be used safely within the Bourne shell.

arg

Function argument


Located at lib/strings.nix:442 in <nixpkgs>.

lib.strings.escapeShellArgs

Type: escapeShellArgs :: [string] -> string

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


Located at lib/strings.nix:452 in <nixpkgs>.

lib.strings.isValidPosixName

Type: string -> bool

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

name

Function argument


Located at lib/strings.nix:464 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


Located at lib/strings.nix:484 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


Located at lib/strings.nix:512 in <nixpkgs>.

lib.strings.escapeNixString

Type: string -> string

Turn a string into a Nix expression representing that string

s

Function argument


Located at lib/strings.nix:522 in <nixpkgs>.

lib.strings.escapeRegex

Type: string -> string

Turn a string into an exact regular expression


Located at lib/strings.nix:532 in <nixpkgs>.

lib.strings.escapeNixIdentifier

Type: string -> string

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

s

Function argument


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

lib.strings.escapeXML

Type: string -> string

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


Located at lib/strings.nix:558 in <nixpkgs>.

lib.strings.toLower

Type: toLower :: string -> string

Converts an ASCII string to lower-case.


Located at lib/strings.nix:577 in <nixpkgs>.

lib.strings.toUpper

Type: toUpper :: string -> string

Converts an ASCII string to upper-case.


Located at lib/strings.nix:587 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


Located at lib/strings.nix:602 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


Located at lib/strings.nix:613 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


Located at lib/strings.nix:629 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


Located at lib/strings.nix:662 in <nixpkgs>.

lib.strings.versionOlder

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

v1

Function argument

v2

Function argument


Located at lib/strings.nix:693 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


Located at lib/strings.nix:705 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.


Located at lib/strings.nix:717 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.


Located at lib/strings.nix:734 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.


Located at lib/strings.nix:773 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


Located at lib/strings.nix:793 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


Located at lib/strings.nix:811 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


Located at lib/strings.nix:828 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


Located at lib/strings.nix:847 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


Located at lib/strings.nix:866 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


Located at lib/strings.nix:880 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


Located at lib/strings.nix:894 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


Located at lib/strings.nix:906 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


Located at lib/strings.nix:919 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


Located at lib/strings.nix:934 in <nixpkgs>.

lib.strings.fixedWidthNumber

Format a number adding leading zeroes up to fixed width.

width

Function argument

n

Function argument


Located at lib/strings.nix:951 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


Located at lib/strings.nix:963 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:971 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:980 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:993 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.


Located at lib/strings.nix:1041 in <nixpkgs>.

lib.strings.fileContents

Type: fileContents :: path -> string

Read the contents of a file removing the trailing \n

file

Function argument


Located at lib/strings.nix:1162 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


Located at lib/strings.nix:1216 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:1237 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:1245 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


Located at lib/strings.nix:1269 in <nixpkgs>.

lib.versions: version string functions

Version string functions.

lib.versions.splitVersion

Break a version string into its component parts.


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

lib.versions.major

Get the major version string from a string.

v

Function argument


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

lib.versions.minor

Get the minor version string from a string.

v

Function argument


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

lib.versions.patch

Get the patch version string from a string.

v

Function argument


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


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


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

lib.trivial: miscellaneous functions

lib.trivial.id

Type: id :: a -> a

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

x

The value to return

Located at lib/trivial.nix:24 in <nixpkgs>.

lib.trivial.const

Type: const :: a -> b -> a

The constant function

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

x

Value to return

y

Value to ignore


Located at lib/trivial.nix:38 in <nixpkgs>.

lib.trivial.concat

Type: concat :: [a] -> [a] -> [a]

Concatenate two lists

x

Function argument

y

Function argument


Located at lib/trivial.nix:90 in <nixpkgs>.

lib.trivial.or

boolean “or”

x

Function argument

y

Function argument

Located at lib/trivial.nix:93 in <nixpkgs>.

lib.trivial.and

boolean “and”

x

Function argument

y

Function argument

Located at lib/trivial.nix:96 in <nixpkgs>.

lib.trivial.bitNot

bitwise “not”

Located at lib/trivial.nix:99 in <nixpkgs>.

lib.trivial.boolToString

Type: boolToString :: bool -> string

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!).

b

Function argument

Located at lib/trivial.nix:109 in <nixpkgs>.

lib.trivial.mergeAttrs

Merge two attribute sets shallowly, right side trumps left

mergeAttrs :: attrs -> attrs -> attrs

x

Left attribute set

y

Right attribute set (higher precedence for equal keys)


Located at lib/trivial.nix:119 in <nixpkgs>.

lib.trivial.flip

Type: flip :: (a -> b -> c) -> (b -> a -> c)

Flip the order of the arguments of a binary function.

f

Function argument

a

Function argument

b

Function argument


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

lib.trivial.mapNullable

Apply function if the supplied argument is non-null.

f

Function to call

a

Argument to check for null before passing it to f


Located at lib/trivial.nix:143 in <nixpkgs>.

lib.trivial.version

Returns the current full nixpkgs version number.

Located at lib/trivial.nix:159 in <nixpkgs>.

lib.trivial.release

Returns the current nixpkgs release number as string.

Located at lib/trivial.nix:162 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:175 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.

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:181 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:193 in <nixpkgs>.

lib.trivial.versionSuffix

Returns the current nixpkgs version suffix as string.

Located at lib/trivial.nix:196 in <nixpkgs>.

lib.trivial.revisionWithDefault

Type: revisionWithDefault :: string -> string

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

default

Default value to return if revision can not be determined

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

lib.trivial.inNixShell

Type: inNixShell :: bool

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

Located at lib/trivial.nix:225 in <nixpkgs>.

lib.trivial.inPureEvalMode

Type: inPureEvalMode :: bool

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.

Located at lib/trivial.nix:233 in <nixpkgs>.

lib.trivial.min

Return minimum of two numbers.

x

Function argument

y

Function argument

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

lib.trivial.max

Return maximum of two numbers.

x

Function argument

y

Function argument

Located at lib/trivial.nix:241 in <nixpkgs>.

lib.trivial.mod

Integer modulus

base

Function argument

int

Function argument


Located at lib/trivial.nix:251 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

a

Function argument

b

Function argument

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

lib.trivial.splitByAndCompare

Type: (a -> bool) -> (a -> a -> int) -> (a -> a -> int) -> (a -> a -> int)

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.

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


Located at lib/trivial.nix:287 in <nixpkgs>.

lib.trivial.importJSON

Type: importJSON :: path -> any

Reads a JSON file.

path

Function argument

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

lib.trivial.importTOML

Type: importTOML :: path -> any

Reads a TOML file.

path

Function argument

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

lib.trivial.warn

Type: string -> a -> a

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

Located at lib/trivial.nix:342 in <nixpkgs>.

lib.trivial.warnIf

Type: bool -> string -> a -> a

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

cond

Function argument

msg

Function argument

Located at lib/trivial.nix:352 in <nixpkgs>.

lib.trivial.warnIfNot

Type: bool -> string -> a -> a

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

cond

Function argument

msg

Function argument

Located at lib/trivial.nix:359 in <nixpkgs>.

lib.trivial.throwIfNot

Type: bool -> string -> a -> a

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.

cond

Function argument

msg

Function argument


Located at lib/trivial.nix:381 in <nixpkgs>.

lib.trivial.throwIf

Type: bool -> string -> a -> a

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

cond

Function argument

msg

Function argument

Located at lib/trivial.nix:388 in <nixpkgs>.

lib.trivial.checkListOfEnum

Type: String -> List ComparableVal -> List ComparableVal -> a -> a

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

msg

Function argument

valid

Function argument

given

Function argument


Located at lib/trivial.nix:400 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.

f

Function argument

args

Function argument

Located at lib/trivial.nix:423 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.

f

Function argument

Located at lib/trivial.nix:435 in <nixpkgs>.

lib.trivial.isFunction

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

f

Function argument

Located at lib/trivial.nix:443 in <nixpkgs>.

lib.trivial.mirrorFunctionArgs

Type: mirrorFunctionArgs :: (a -> b) -> (a -> c) -> (a -> c)

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).

f

Function to provide the argument metadata


Located at lib/trivial.nix:470 in <nixpkgs>.

lib.trivial.toFunction

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

v

Any value


Located at lib/trivial.nix:492 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:508 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 ]

base

Function argument

i

Function argument

Located at lib/trivial.nix:532 in <nixpkgs>.

lib.fixedPoints: explicit recursion functions

lib.fixedPoints.fix

Type: fix :: (a -> a) -> a

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.

f

Function argument


Located at lib/fixed-points.nix:75 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.

f

Function argument

Located at lib/fixed-points.nix:84 in <nixpkgs>.

lib.fixedPoints.converge

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

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

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

Function argument

x

Function argument

Located at lib/fixed-points.nix:97 in <nixpkgs>.

lib.fixedPoints.extends

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

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 130. 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; }

overlay

The overlay to apply to the fixed-point function

f

The fixed-point function


Located at lib/fixed-points.nix:240 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

f

Function argument

g

Function argument

final

Function argument

prev

Function argument

Located at lib/fixed-points.nix:260 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:276 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:299 in <nixpkgs>.

lib.fixedPoints.makeExtensibleWithCustomName

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

extenderName

Function argument

rattrs

Function argument

Located at lib/fixed-points.nix:305 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]

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]

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

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

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

list

The list to fold

Type
foldl' :: (acc -> x -> acc) -> acc -> [x] -> acc

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]

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]

lib.lists.concatMap

Map and concatenate the result.

Type
concatMap :: (a -> [b]) -> [a] -> [b]

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

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]

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

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)

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

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

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

lib.lists.count

Count how many elements of list match the supplied predicate function.

Inputs
pred

Predicate

Type
count :: (a -> bool) -> [a] -> int

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]

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]

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

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]

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]

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]; }

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]

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; }]

lib.lists.reverseList

Reverse the order of the elements of a list.

Inputs
xs

1. Function argument

Type
reverseList :: [a] -> [a]

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]

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

lib.lists.compareLists

Compare two lists element-by-element.

Inputs
cmp

1. Function argument

a

2. Function argument

b

3. Function argument

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]

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]

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

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]

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]

lib.lists.commonPrefix

The common prefix of two lists.

Inputs
list1

1. Function argument

list2

2. Function argument

Type
commonPrefix :: [a] -> [a] -> [a]

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

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]

lib.lists.crossLists

Return the image of the cross product of some lists by a function.

lib.lists.unique

Remove duplicate elements from the list. O(n^2) complexity.

Inputs
list

Input list

Type
unique :: [a] -> [a]

lib.lists.allUnique

Check if list contains only unique elements. O(n^2) complexity.

Inputs
list

1. Function argument

Type
allUnique :: [a] -> bool

lib.lists.intersectLists

Intersects list ‘list1’ and another list (list2).

O(nm) complexity.

Inputs
list1

First list

list2

Second list

lib.lists.subtractLists

Subtracts list ‘e’ from another list (list2).

O(nm) complexity.

Inputs
e

First list

list2

Second list

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:1829 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

Type: traceIf :: bool -> string -> a -> a

Conditionally trace the supplied message, based on a predicate.

pred

Predicate to check

msg

Message that should be traced

x

Value to return


Located at lib/debug.nix:44 in <nixpkgs>.

lib.debug.traceValFn

Type: traceValFn :: (a -> b) -> a -> a

Trace the supplied value after applying a function to it, and return the original value.

f

Function to apply

x

Value to trace and return


Located at lib/debug.nix:62 in <nixpkgs>.

lib.debug.traceVal

Type: traceVal :: a -> a

Trace the supplied value and return it.


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

lib.debug.traceSeq

Type: traceSeq :: a -> b -> b

builtins.trace, but the value is builtins.deepSeqed first.

x

The value to trace

y

The value to return


Located at lib/debug.nix:91 in <nixpkgs>.

lib.debug.traceSeqN

Type: traceSeqN :: Int -> a -> b -> b

Like traceSeq, but only evaluate down to depth n. This is very useful because lots of traceSeq usages lead to an infinite recursion.

depth

Function argument

x

Function argument

y

Function argument


Located at lib/debug.nix:108 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.

f

Function to apply

v

Value to trace

Located at lib/debug.nix:125 in <nixpkgs>.

lib.debug.traceValSeq

A combination of traceVal and traceSeq.

Located at lib/debug.nix:132 in <nixpkgs>.

lib.debug.traceValSeqNFn

A combination of traceVal and traceSeqN that applies a provided function to the value to be traced.

f

Function to apply

depth

Function argument

v

Value to trace

Located at lib/debug.nix:136 in <nixpkgs>.

lib.debug.traceValSeqN

A combination of traceVal and traceSeqN.

Located at lib/debug.nix:144 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.

depth

Function argument

name

Function argument

f

Function argument

v

Function argument


Located at lib/debug.nix:157 in <nixpkgs>.

lib.debug.runTests

Type:

runTests :: {
  tests = [ String ];
  ${testName} :: {
    expr :: a;
    expected :: a;
  };
}
->
[
  {
    name :: String;
    expected :: a;
    result :: a;
  }
]

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"];

tests

Tests to run


Located at lib/debug.nix:229 in <nixpkgs>.

lib.debug.testAllTrue

Create a test assuming that list elements are true.

expr

Function argument


Located at lib/debug.nix:245 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


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


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


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 188. 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

Alias of mkPackageOption. 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.


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


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. This is a no-op and no longer needed.

Located at lib/options.nix:405 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:411 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


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


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


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


Located at lib/path/default.nix:336 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


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


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


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


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

Type: pathType :: Path -> String

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.


Located at lib/filesystem.nix:46 in <nixpkgs>.

lib.filesystem.pathIsDirectory

Type: pathIsDirectory :: Path -> Bool

Whether a path exists and is a directory.

path

Function argument


Located at lib/filesystem.nix:78 in <nixpkgs>.

lib.filesystem.pathIsRegularFile

Type: pathIsRegularFile :: Path -> Bool

Whether a path exists and is a regular file, meaning not a symlink or any other special file type.

path

Function argument


Located at lib/filesystem.nix:97 in <nixpkgs>.

lib.filesystem.haskellPathsInDir

Type: Path -> Map String Path

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.

root

The directory within to search

Located at lib/filesystem.nix:107 in <nixpkgs>.

lib.filesystem.locateDominatingFile

Type: RegExp -> Path -> Nullable { path : Path; matches : [ MatchResults ]; }

Find the first directory containing a file matching ‘pattern’ upward from a given ‘file’. Returns ‘null’ if no directories contain a file matching ‘pattern’.

pattern

The pattern to search for

file

The file to start searching upward from

Located at lib/filesystem.nix:130 in <nixpkgs>.

lib.filesystem.listFilesRecursive

Type: Path -> [ Path ]

Given a directory, return a flattened list of all files within it recursively.

dir

The path to recursively list

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

lib.filesystem.packagesFromDirectoryRecursive

Type: packagesFromDirectoryRecursive :: AttrSet -> AttrSet

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.

structured function argument
callPackage

pkgs.callPackage

Type: Path -> AttrSet -> a

directory

The directory to read package files from

Type: Path


Located at lib/filesystem.nix:244 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

Type: maybeMissing :: Path -> FileSet

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.

path

Function argument


Located at lib/fileset/default.nix:172 in <nixpkgs>.

lib.fileset.trace

Type: trace :: FileSet -> Any -> Any

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.

fileset

The file set to trace.

This argument can also be a path, which gets implicitly coerced to a file set.


Located at lib/fileset/default.nix:210 in <nixpkgs>.

lib.fileset.traceVal

Type: traceVal :: FileSet -> FileSet

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.

fileset

The file set to trace and return.

This argument can also be a path, which gets implicitly coerced to a file set.


Located at lib/fileset/default.nix:257 in <nixpkgs>.

lib.fileset.toSource

Type:

toSource :: {
  root :: Path,
  fileset :: FileSet,
} -> SourceLike

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.

structured function argument
root

(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

(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.


Located at lib/fileset/default.nix:343 in <nixpkgs>.

lib.fileset.toList

Type: toList :: FileSet -> [ Path ]

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.

fileset

The file set whose file paths to return. This argument can also be a path, which gets implicitly coerced to a file set.


Located at lib/fileset/default.nix:445 in <nixpkgs>.

lib.fileset.union

Type: union :: FileSet -> FileSet -> FileSet

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.

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.


Located at lib/fileset/default.nix:473 in <nixpkgs>.

lib.fileset.unions

Type: unions :: [ FileSet ] -> FileSet

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.

filesets

A list of file sets. The elements can also be paths, which get implicitly coerced to file sets.


Located at lib/fileset/default.nix:525 in <nixpkgs>.

lib.fileset.intersection

Type: intersection :: FileSet -> FileSet -> FileSet

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.

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.


Located at lib/fileset/default.nix:558 in <nixpkgs>.

lib.fileset.difference

Type: union :: FileSet -> FileSet -> FileSet

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.

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.


Located at lib/fileset/default.nix:606 in <nixpkgs>.

lib.fileset.fileFilter

Type:

fileFilter ::
  ({
    name :: String,
    type :: String,
    hasExt :: String -> Bool,
    ...
  } -> Bool)
  -> Path
  -> FileSet

Filter a file set to only contain files matching some predicate.

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


Located at lib/fileset/default.nix:662 in <nixpkgs>.

lib.fileset.fromSource

Type: fromSource :: SourceLike -> FileSet

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.

source

Function argument


Located at lib/fileset/default.nix:744 in <nixpkgs>.

lib.fileset.gitTracked

Type: gitTracked :: Path -> FileSet

Create a file set containing all Git-tracked files in a repository.

This function behaves like gitTrackedWith { } - using the defaults.

path

The path to the working directory of a local Git repository. This directory must contain a .git file or subdirectory.


Located at lib/fileset/default.nix:787 in <nixpkgs>.

lib.fileset.gitTrackedWith

Type: gitTrackedWith :: { recurseSubmodules :: Bool ? false } -> Path -> FileSet

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.

structured function argument
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.


Located at lib/fileset/default.nix:831 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


Located at lib/sources.nix:271 in <nixpkgs>.

lib.sources.cleanSource

Filters a source tree removing version control files and directories using cleanSourceFilter.

src

Function argument


Located at lib/sources.nix:271 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".


Located at lib/sources.nix:271 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:271 in <nixpkgs>.

lib.sources.sourceByRegex

Filter sources by a list of regular expressions.

src

Function argument

regexes

Function argument


Located at lib/sources.nix:271 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


Located at lib/sources.nix:271 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:271 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

Type: isGVariant :: Any -> Bool

Check if a value is a GVariant value

v

Function argument

Located at lib/gvariant.nix:54 in <nixpkgs>.

lib.gvariant.mkValue

Type: mkValue :: Any -> gvariant

Returns the GVariant value that most closely matches the given Nix value. If no GVariant value can be found unambiguously then error is thrown.

v

Function argument

Located at lib/gvariant.nix:62 in <nixpkgs>.

lib.gvariant.mkArray

Type: mkArray :: [Any] -> gvariant

Returns the GVariant array from the given type of the elements and a Nix list.

elems

Function argument


Located at lib/gvariant.nix:85 in <nixpkgs>.

lib.gvariant.mkEmptyArray

Type: mkEmptyArray :: gvariant.type -> gvariant

Returns the GVariant array from the given empty Nix list.

elemType

Function argument


Located at lib/gvariant.nix:106 in <nixpkgs>.

lib.gvariant.mkVariant

Type: mkVariant :: Any -> gvariant

Returns the GVariant variant from the given Nix value. Variants are containers of different GVariant type.

elem

Function argument


Located at lib/gvariant.nix:123 in <nixpkgs>.

lib.gvariant.mkMaybe

Type: mkMaybe :: gvariant.type -> Any -> gvariant

Returns the GVariant maybe from the given element type.

elemType

Function argument

elem

Function argument

Located at lib/gvariant.nix:161 in <nixpkgs>.

lib.gvariant.mkNothing

Type: mkNothing :: gvariant.type -> gvariant

Returns the GVariant nothing from the given element type.

elemType

Function argument

Located at lib/gvariant.nix:175 in <nixpkgs>.

lib.gvariant.mkJust

Type: mkJust :: Any -> gvariant

Returns the GVariant just from the given Nix value.

elem

Function argument

Located at lib/gvariant.nix:182 in <nixpkgs>.

lib.gvariant.mkTuple

Type: mkTuple :: [Any] -> gvariant

Returns the GVariant tuple from the given Nix list.

elems

Function argument

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

lib.gvariant.mkBoolean

Type: mkBoolean :: Bool -> gvariant

Returns the GVariant boolean from the given Nix bool value.

v

Function argument

Located at lib/gvariant.nix:204 in <nixpkgs>.

lib.gvariant.mkString

Type: mkString :: String -> gvariant

Returns the GVariant string from the given Nix string value.

v

Function argument

Located at lib/gvariant.nix:214 in <nixpkgs>.

lib.gvariant.mkObjectpath

Type: mkObjectpath :: String -> gvariant

Returns the GVariant object path from the given Nix string value.

v

Function argument

Located at lib/gvariant.nix:225 in <nixpkgs>.

lib.gvariant.mkUchar

Type: mkUchar :: Int -> gvariant

Returns the GVariant uchar from the given Nix int value.

Located at lib/gvariant.nix:235 in <nixpkgs>.

lib.gvariant.mkInt16

Type: mkInt16 :: Int -> gvariant

Returns the GVariant int16 from the given Nix int value.

Located at lib/gvariant.nix:242 in <nixpkgs>.

lib.gvariant.mkUint16

Type: mkUint16 :: Int -> gvariant

Returns the GVariant uint16 from the given Nix int value.

Located at lib/gvariant.nix:249 in <nixpkgs>.

lib.gvariant.mkInt32

Type: mkInt32 :: Int -> gvariant

Returns the GVariant int32 from the given Nix int value.

v

Function argument

Located at lib/gvariant.nix:256 in <nixpkgs>.

lib.gvariant.mkUint32

Type: mkUint32 :: Int -> gvariant

Returns the GVariant uint32 from the given Nix int value.

Located at lib/gvariant.nix:266 in <nixpkgs>.

lib.gvariant.mkInt64

Type: mkInt64 :: Int -> gvariant

Returns the GVariant int64 from the given Nix int value.

Located at lib/gvariant.nix:273 in <nixpkgs>.

lib.gvariant.mkUint64

Type: mkUint64 :: Int -> gvariant

Returns the GVariant uint64 from the given Nix int value.

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

lib.gvariant.mkDouble

Type: mkDouble :: Float -> gvariant

Returns the GVariant double from the given Nix float value.

v

Function argument

Located at lib/gvariant.nix:287 in <nixpkgs>.

lib.customisation: Functions to customise (derivation-related) functions, derivatons, or attribute sets

lib.customisation.overrideDerivation

Type: overrideDerivation :: Derivation -> ( Derivation -> AttrSet ) -> Derivation

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.

drv

Function argument

f

Function argument


Located at lib/customisation.nix:55 in <nixpkgs>.

lib.customisation.makeOverridable

Type: makeOverridable :: (AttrSet -> a) -> AttrSet -> a

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.

f

Function argument


Located at lib/customisation.nix:91 in <nixpkgs>.

lib.customisation.callPackageWith

Type: callPackageWith :: AttrSet -> ((AttrSet -> a) | Path) -> AttrSet -> a

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;
};
autoArgs

Function argument

fn

Function argument

args

Function argument

Located at lib/customisation.nix:153 in <nixpkgs>.

lib.customisation.callPackagesWith

Type: callPackagesWith :: AttrSet -> ((AttrSet -> AttrSet) | Path) -> AttrSet -> AttrSet

Like callPackage, but for a function that returns an attribute set of derivations. The override function is added to the individual attributes.

autoArgs

Function argument

fn

Function argument

args

Function argument

Located at lib/customisation.nix:220 in <nixpkgs>.

lib.customisation.extendDerivation

Type: extendDerivation :: Bool -> Any -> Derivation -> Derivation

Add attributes to each output of a derivation without changing the derivation itself and check a given condition when evaluating.

condition

Function argument

passthru

Function argument

drv

Function argument

Located at lib/customisation.nix:242 in <nixpkgs>.

lib.customisation.hydraJob

Type: hydraJob :: (Derivation | Null) -> (Derivation | Null)

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.

drv

Function argument

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

lib.customisation.makeScope

Make an attribute set (a “scope”) from functions that take arguments from that same attribute set. See Example 230 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 230 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 231 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.

makeScope :: (AttrSet -> ((AttrSet -> a) | Path) -> AttrSet -> a) -> (AttrSet -> AttrSet) -> scope

Located at lib/customisation.nix:433 in <nixpkgs>.

lib.customisation.makeScopeWithSplicing

backward compatibility with old uncurried form; deprecated

splicePackages

Function argument

newScope

Function argument

otherSplices

Function argument

keep

Function argument

extra

Function argument

f

Function argument

Located at lib/customisation.nix:447 in <nixpkgs>.

lib.customisation.makeScopeWithSplicing'

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
  }

Like makeScope, but aims to support cross compilation. It’s still ugly, but hopefully it helps a little bit.

structured function argument
splicePackages

Function argument

newScope

Function argument

structured function argument
otherSplices

Function argument

keep

Function argument

extra

Function argument

f

Function argument

Located at lib/customisation.nix:473 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.

newAttrs

Function argument

drv

Function argument


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

lib.meta.dontDistribute

Disable Hydra builds of given derivation.

drv

Function argument

Located at lib/meta.nix:26 in <nixpkgs>.

lib.meta.setName

Change the symbolic name of a package for presentation purposes (i.e., so that nix-env users can tell them apart).

name

Function argument

drv

Function argument

Located at lib/meta.nix:32 in <nixpkgs>.

lib.meta.updateName

Like setName, but takes the previous name as an argument.

updater

Function argument

drv

Function argument


Located at lib/meta.nix:40 in <nixpkgs>.

lib.meta.appendToName

Append a suffix to the name of a package (before the version part).

suffix

Function argument

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

lib.meta.mapDerivationAttrset

Apply a function to each derivation and only to derivations in an attrset.

f

Function argument

set

Function argument

Located at lib/meta.nix:51 in <nixpkgs>.

lib.meta.setPrio

Set the nix-env priority of the package.

priority

Function argument

Located at lib/meta.nix:55 in <nixpkgs>.

lib.meta.lowPrio

Decrease the nix-env priority of the package, i.e., other versions/variants of the package will be preferred.

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

lib.meta.lowPrioSet

Apply lowPrio to an attrset with derivations

set

Function argument

Located at lib/meta.nix:64 in <nixpkgs>.

lib.meta.hiPrio

Increase the nix-env priority of the package, i.e., this version/variant of the package will be preferred.

Located at lib/meta.nix:70 in <nixpkgs>.

lib.meta.hiPrioSet

Apply hiPrio to an attrset with derivations

set

Function argument

Located at lib/meta.nix:74 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.

platform

Function argument

elem

Function argument


Located at lib/meta.nix:95 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.

platform

Function argument

pkg

Function argument


Located at lib/meta.nix:124 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
structured function argument
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:66 in <nixpkgs>.

lib.derivations.optionalDrvAttr

Type: optionalDrvAttr :: Bool -> a -> a | Null

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.

cond

Condition

value

Attribute value


Located at lib/derivations.nix:172 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 in lib/generators.nix.

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> {} }:

 nix-gitignore.gitignoreSource [] ./source
     # Simplest version

 nix-gitignore.gitignoreSource "supplemental-ignores\n" ./source
     # This one reads the ./source/.gitignore and concats the auxiliary ignores

 nix-gitignore.gitignoreSourcePure "ignore-this\nignore-that\n" ./source
     # Use this string as gitignore, don't read ./source/.gitignore.

 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.


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.

Special variables

passthru

This is an attribute set which can be filled with arbitrary values. For example:

passthru = {
  foo = "bar";
  baz = {
    value1 = 4;
    value2 = 5;
  };
}

Values inside it are not passed to the builder, so you can change them without triggering a rebuild. However, they can be accessed outside of a derivation directly, as if they were set inside a derivation itself, e.g. hello.baz.value1. We don’t specify any usage or schema of passthru - it is meant for values that would be useful outside the derivation in other parts of a Nix expression (e.g. in other derivations). An example would be to convey some specific dependency of your derivation which contains a program with plugins support. Later, others who make derivations with plugins can use passed-through dependency to ensure that their plugin would be binary-compatible with built program.

passthru.updateScript

A script to be run by maintainers/scripts/update.nix when the package is matched. The attribute can contain one of the following:

Tip

A common pattern is to use the nix-update-script attribute provided in Nixpkgs, which runs nix-update:

passthru.updateScript = nix-update-script { };

For simple packages, this is often enough, and will ensure that the package is updated automatically by nixpkgs-update when a new version is released. The update bot runs periodically to attempt to automatically update packages, and will run passthru.updateScript if set. While not strictly necessary if the project is listed on Repology, using nix-update-script allows the package to update via many more sources (e.g. GitHub releases).

How update scripts are executed?

Update scripts are to be invoked by maintainers/scripts/update.nix script. You can run nix-shell maintainers/scripts/update.nix in the root of Nixpkgs repository for information on how to use it. update.nix offers several modes for selecting packages to update (e.g. select by attribute path, traverse Nixpkgs and filter by maintainer, etc.), and it will execute update scripts for all matched packages that have an updateScript attribute.

Each update script will be passed the following environment variables:

Note

An update script will be usually run from the root of the Nixpkgs repository but you should not rely on that. Also note that update.nix executes update scripts in parallel by default so you should avoid running git commit or any other commands that cannot handle that.

Tip

While update scripts should not create commits themselves, maintainers/scripts/update.nix supports automatically creating commits when running it with --argstr commit true. If you need to customize commit message, you can have the update script implement commit feature.

Supported features
commit

This feature allows update scripts to ask update.nix to create Git commits.

When support of this feature is declared, whenever the update script exits with 0 return status, it is expected to print a JSON list containing an object described below for each updated attribute to standard output.

When update.nix is run with --argstr commit true arguments, it will create a separate commit for each of the objects. An empty list can be returned when the script did not update any files, for example, when the package is already at the latest version.

The commit object contains the following values:

If the returned array contains exactly one object (e.g. [{}]), all values are optional and will be determined automatically.


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, the flag --prefix=$prefix is added to the configure flags. If this is undesirable, set this variable 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 prefix. By default, this is set to --prefix= as that is used by the majority of packages.

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.

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.


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 “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.

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

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.



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

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.

Don’t include a period at the end. Don’t include newline characters. Capitalise the first character. For brevity, don’t repeat the name of package — just describe what it does.

Wrong: "libpng is a library that allows you to decode PNG images."

Right: "A 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.patterns.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.

tests

Warning

This attribute is special in that it is not actually under the meta attribute set but rather under the passthru attribute set. This is due to how meta attributes work, and the fact that they are supposed to contain only metadata, not derivations.

An attribute set with tests as values. A test is a derivation that builds when the test passes and fails to build otherwise.

You can run these tests with:

$ cd path/to/nixpkgs
$ nix-build -A your-package.tests

Package tests

Tests that are part of the source package are often executed in the installCheckPhase.

Prefer passthru.tests for tests that are introduced in nixpkgs because:

  • passthru.tests tests the ‘real’ package, independently from the environment in which it was built

  • we can run passthru.tests independently

  • installCheckPhase adds overhead to each build

For more on how to write and run package tests, see the section called “Package tests”.

NixOS tests

The NixOS tests are available as nixosTests in parameters of derivations. For instance, the OpenSMTPD derivation includes lines similar to:

{ /* ... */, nixosTests }:
{
  # ...
  passthru.tests = {
    basic-functionality-and-dovecot-integration = nixosTests.opensmtpd;
  };
}

NixOS tests run in a VM, so they are slower than regular package tests. For more information see NixOS module tests.

Alternatively, you can specify other derivations as tests. You can make use of the optional parameter to inject the correct package without relying on non-local definitions, even in the presence of overrideAttrs. Here that’s finalAttrs.finalPackage, but you could choose a different name if finalAttrs already exists in your scope.

(mypkg.overrideAttrs f).passthru.tests will be as expected, as long as the definition of tests does not rely on the original mypkg or overrides it in all places.

# my-package/default.nix
{ stdenv, callPackage }:
stdenv.mkDerivation (finalAttrs: {
  # ...
  passthru.tests.example = callPackage ./example.nix { my-package = finalAttrs.finalPackage; };
})
# my-package/example.nix
{ runCommand, lib, my-package, ... }:
runCommand "my-package-test" {
  nativeBuildInputs = [ my-package ];
  src = lib.sources.sourcesByRegex ./. [ ".*.in" ".*.expected" ];
} ''
  my-package --help
  my-package <example.in >example.actual
  diff -U3 --color=auto example.expected example.actual
  mkdir $out
''

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.

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, Nixpkgs provides fetchers: functions to obtain remote sources via various protocols and services.

Nixpkgs fetchers differ from built-in fetchers such as builtins.fetchTarball:

  • 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 otherwise 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 the entirety of Nixpkgs, and allows Hydra to retain and re-distribute sources used by Nixpkgs in the public binary cache. For these reasons, built-in fetchers are not allowed in Nixpkgs source code.

The following table shows an overview of 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

Caveats

The fact that the hash belongs to the Nix derivation output and not the file itself can lead to confusion. For example, consider the following fetcher:

fetchurl {
  url = "http://www.example.org/hello-1.0.tar.gz";
  hash = "sha256-lTeyxzJNQeMdu1IVdovNMtgn77jRIhSybLdMbTkf2Ww=";
};

A common mistake is to update a fetcher’s URL, or a version parameter, without updating the hash.

fetchurl {
  url = "http://www.example.org/hello-1.1.tar.gz";
  hash = "sha256-lTeyxzJNQeMdu1IVdovNMtgn77jRIhSybLdMbTkf2Ww=";
};

This will reuse the old contents. Remember to invalidate the hash argument, in this case by setting the hash attribute to an empty string.

fetchurl {
  url = "http://www.example.org/hello-1.1.tar.gz";
  hash = "";
};

Use the resulting error message to determine the correct hash.

error: hash mismatch in fixed-output derivation '/path/to/my.drv':
         specified: sha256-AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA=
            got:    sha256-lTeyxzJNQeMdu1IVdovNMtgn77jRIhSybLdMbTkf2Ww=

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.

fetchurl and fetchzip

Two basic fetchers are fetchurl and fetchzip. Both of these have two required arguments, a URL and a hash. The hash is typically hash, although many more hash algorithms are supported. Nixpkgs contributors are currently recommended to use hash. This hash will be used by Nix to identify your source. A typical usage of fetchurl is provided below.

{ stdenv, fetchurl }:

stdenv.mkDerivation {
  name = "hello";
  src = fetchurl {
    url = "http://www.example.org/hello.tar.gz";
    hash = "sha256-BBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBB=";
  };
}

The main difference between fetchurl and fetchzip is in how they store the contents. fetchurl will store the unaltered contents of the URL within the Nix store. fetchzip on the other hand, will decompress the archive for you, making files and directories directly accessible in the future. fetchzip can only be used with archives. Despite the name, fetchzip is not limited to .zip files and can also be used with any tarball.

Additional parameters to fetchurl:

  • downloadToTemp: Defaults to false. If true, saves the source to $downloadedFile, to be used in conjunction with postFetch

  • postFetch: Shell code executed after the file has been fetched successfully. Use it for postprocessing, to check or transform the file.

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 stdenv.mkDerivation, each of these build helpers creates a derivation, but the arguments passed are different (usually simpler) from those required by stdenv.mkDerivation.

runCommand

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

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

By default runCommand runs in a stdenv with no compiler environment, whereas runCommandCC uses the default stdenv, pkgs.stdenv.

name :: String

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

drvAttr :: AttrSet

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

buildCommand :: String

Shell commands to run in the derivation builder.

Note

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


runCommandCC

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

runCommandLocal

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

Note

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

Writing text files

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

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

Note

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

For example, if the file destination is a directory:

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

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

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

writeTextFile

Write a text file to the Nix store.

writeTextFile takes an attribute set with the following possible attributes:

name (String)

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

text (String)

The contents of the file.

executable (Bool, optional)

Make this file have the executable bit set.

Default: false

destination (String, optional)

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

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

Default: ""

checkPhase (String, optional)

Commands to run after generating the file.

Default: ""

meta (Attribute set, optional)

Additional metadata for the derivation.

Default: {}

allowSubstitutes (Bool, optional)

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

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

Default: false

preferLocalBuild (Bool, optional)

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

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

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

Default: true

derivationArgs (Attribute set, optional)

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

Default: {}

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



Example 246. Usage 3 of writeTextFile

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

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

writeText

Write a text file to the Nix store

writeText takes the following arguments: a string.

name (String)

The name used in the Nix store path.

text (String)

The contents of the file.

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


This is equivalent to:

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

writeTextDir

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

writeTextDir takes the following arguments:

path (String)

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

text (String)

The contents of the file.

The store path will be a directory.


This is equivalent to:

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

writeScript

Write an executable script file to the Nix store.

writeScript takes the following arguments:

name (String)

The name used in the Nix store path.

text (String)

The contents of the file.

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


This is equivalent to:

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

writeScriptBin

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

writeScriptBin takes the following arguments:

name (String)

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

text (String)

The contents of the file.

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


This is equivalent to:

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

writeShellScript

Write a Bash script to the store.

writeShellScript takes the following arguments:

name (String)

The name used in the Nix store path.

text (String)

The contents of the file.

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

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


This is equivalent to:

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

writeShellScriptBin

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

writeShellScriptBin takes the following arguments:

name (String)

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

text (String)

The contents of the file.

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

This function is a combination of the section called “writeShellScript and the section called “writeScriptBin.


This is equivalent to:

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

concatTextFile, concatText, concatScript

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

Here are a few examples:


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

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

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

writeShellApplication

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

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

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

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

  runtimeInputs = [ curl w3m ];

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

symlinkJoin

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

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

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

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

writeReferencesToFile

Writes the closure of transitive dependencies to a file.

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

For example,

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

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

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

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

writeDirectReferencesToFile

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

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

For example,

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

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

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

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

Testers

This chapter describes several testing builders which are available in the testers namespace.

testVersion

Checks that the output from running a command contains the specified version string in it as a whole word.

Although simplistic, this test assures that the main program can run. While there’s no substitute for a real test case, it does catch dynamic linking errors and such. It also provides some protection against accidentally building the wrong version, for example when using an “old” hash in a fixed-output derivation.

By default, the command to be run will be inferred from the given package attribute: it will check meta.mainProgram first, and fall back to pname or name. The default argument to the command is --version, and the version to be checked will be inferred from the given package attribute as well.



testBuildFailure

Make sure that a build does not succeed. This is useful for testing testers.

This returns a derivation with an override on the builder, with the following effects:

  • Fail the build when the original builder succeeds

  • Move $out to $out/result, if it exists (assuming out is the default output)

  • Save the build log to $out/testBuildFailure.log (same)

While testBuildFailure is designed to keep changes to the original builder’s environment to a minimum, some small changes are inevitable:

  • The file $TMPDIR/testBuildFailure.log is present. It should not be deleted.

  • stdout and stderr are a pipe instead of a tty. This could be improved.

  • One or two extra processes are present in the sandbox during the original builder’s execution.

  • The derivation and output hashes are different, but not unusual.

  • The derivation includes a dependency on buildPackages.bash and expect-failure.sh, which is built to include a transitive dependency on buildPackages.coreutils and possibly more. These are not added to PATH or any other environment variable, so they should be hard to observe.


testEqualDerivation

Checks that two packages produce the exact same build instructions.

This can be used to make sure that a certain difference of configuration, such as the presence of an overlay does not cause a cache miss.

When the derivations are equal, the return value is an empty file. Otherwise, the build log explains the difference via nix-diff.


invalidateFetcherByDrvHash

Use the derivation hash to invalidate the output via name, for testing.

Type: (a@{ name, ... } -> Derivation) -> a -> Derivation

Normally, fixed output derivations can and should be cached by their output hash only, but for testing we want to re-fetch everytime the fetcher changes.

Changes to the fetcher become apparent in the drvPath, which is a hash of how to fetch, rather than a fixed store path. By inserting this hash into the name, we can make sure to re-run the fetcher every time the fetcher changes.

This relies on the assumption that Nix isn’t clever enough to reuse its database of local store contents to optimize fetching.

You might notice that the “salted” name derives from the normal invocation, not the final derivation. invalidateFetcherByDrvHash has to invoke the fetcher function twice: once to get a derivation hash, and again to produce the final fixed output derivation.


runNixOSTest

A helper function that behaves exactly like the NixOS runTest, except it also assigns this Nixpkgs package set as the pkgs of the test and makes the nixpkgs.* options read-only.

If your test is part of the Nixpkgs repository, or if you need a more general entrypoint, see “Calling a test” in the NixOS manual.


nixosTest

Run a NixOS VM network test using this evaluation of Nixpkgs.

NOTE: This function is primarily for external use. NixOS itself uses make-test-python.nix directly. Packages defined in Nixpkgs reuse NixOS tests via nixosTests, plural.

It is mostly equivalent to the function import ./make-test-python.nix from the NixOS manual, except that the current application of Nixpkgs (pkgs) will be used, instead of letting NixOS invoke Nixpkgs anew.

If a test machine needs to set NixOS options under nixpkgs, it must set only the nixpkgs.pkgs option.

Parameter

A NixOS VM test network, or path to it. Example:

{
  name = "my-test";
  nodes = {
    machine1 = { lib, pkgs, nodes, ... }: {
      environment.systemPackages = [ pkgs.hello ];
      services.foo.enable = true;
    };
    # machine2 = ...;
  };
  testScript = ''
    start_all()
    machine1.wait_for_unit("foo.service")
    machine1.succeed("hello | foo-send")
  '';
}

Result

A derivation that runs the VM test.

Notable attributes:

  • nodes: the evaluated NixOS configurations. Useful for debugging and exploring the configuration.

  • driverInteractive: a script that launches an interactive Python session in the context of the testScript.

Special build helpers

This chapter describes several special build helpers.

fakeNss

Provides /etc/passwd and /etc/group files that contain root and nobody, allowing user/group lookups to work in binaries that insist on doing those. This might be a better choice than a custom script running useradd and related utilities if you only need those files to exist with some entries.

fakeNss also provides /etc/nsswitch.conf, configuring NSS host resolution to first check /etc/hosts before checking DNS, since the default in the absence of a config file (dns [!UNAVAIL=return] files) is quite unexpected.

It also creates an empty directory at /var/empty because it uses that as the home directory for the root and nobody users. The /var/empty directory can also be used as a chroot target to prevent file access in processes that do not need to access files, if your container runs such processes.

The user entries created by fakeNss use the /bin/sh shell, which is not provided by fakeNss because in most cases it won’t be used. If you need that to be available, see dockerTools.binSh or provide your own.

Inputs

fakeNss is made available in Nixpkgs as a package rather than a function, but it has two attributes that can be overridden and might be useful in particular cases. For more details on how overriding works, see Example 263 and the section called “<pkg>.override”.

extraPasswdLines (List of Strings; optional)

A list of lines that will be added to /etc/passwd. Useful if extra users need to exist in the output of fakeNss. If extraPasswdLines is specified, it will not override the root and nobody entries created by fakeNss. Those entries will always exist.

Lines specified here must follow the format in passwd(5).

Default value: [].

extraGroupLines (List of Strings; optional)

A list of lines that will be added to /etc/group. Useful if extra groups need to exist in the output of fakeNss. If extraGroupLines is specified, it will not override the root and nobody entries created by fakeNss. Those entries will always exist.

Lines specified here must follow the format in group(5).

Default value: [].

buildFHSEnv

buildFHSEnv provides a way to build and run FHS-compatible lightweight sandboxes. It creates an isolated root filesystem with the host’s /nix/store, so its footprint in terms of disk space is quite small. This allows you to run software which is hard or unfeasible to patch for NixOS; 3rd-party source trees with FHS assumptions, games distributed as tarballs, software with integrity checking and/or external self-updated binaries for instance. It uses Linux’ namespaces feature to create temporary lightweight environments which are destroyed after all child processes exit, without requiring elevated privileges. It works similar to containerisation technology such as Docker or FlatPak but provides no security-relevant separation from the host system.

Accepted arguments are:

  • name The name of the environment and the wrapper executable.

  • targetPkgs Packages to be installed for the main host’s architecture (i.e. x86_64 on x86_64 installations). Along with libraries binaries are also installed.

  • multiPkgs Packages to be installed for all architectures supported by a host (i.e. i686 and x86_64 on x86_64 installations). Only libraries are installed by default.

  • multiArch Whether to install 32bit multiPkgs into the FHSEnv in 64bit environments

  • extraBuildCommands Additional commands to be executed for finalizing the directory structure.

  • extraBuildCommandsMulti Like extraBuildCommands, but executed only on multilib architectures.

  • extraOutputsToInstall Additional derivation outputs to be linked for both target and multi-architecture packages.

  • extraInstallCommands Additional commands to be executed for finalizing the derivation with runner script.

  • runScript A shell command to be executed inside the sandbox. It defaults to bash. Command line arguments passed to the resulting wrapper are appended to this command by default. This command must be escaped; i.e. "foo app" --do-stuff --with "some file". See lib.escapeShellArgs.

  • profile Optional script for /etc/profile within the sandbox.

You can create a simple environment using a shell.nix like this:

{ pkgs ? import <nixpkgs> {} }:

(pkgs.buildFHSEnv {
  name = "simple-x11-env";
  targetPkgs = pkgs: (with pkgs; [
    udev
    alsa-lib
  ]) ++ (with pkgs.xorg; [
    libX11
    libXcursor
    libXrandr
  ]);
  multiPkgs = pkgs: (with pkgs; [
    udev
    alsa-lib
  ]);
  runScript = "bash";
}).env

Running nix-shell on it would drop you into a shell inside an FHS env where those libraries and binaries are available in FHS-compliant paths. Applications that expect an FHS structure (i.e. proprietary binaries) can run inside this environment without modification. You can build a wrapper by running your binary in runScript, e.g. ./bin/start.sh. Relative paths work as expected.

Additionally, the FHS builder links all relocated gsettings-schemas (the glib setup-hook moves them to share/gsettings-schemas/${name}/glib-2.0/schemas) to their standard FHS location. This means you don’t need to wrap binaries with wrapGAppsHook.

pkgs.makeSetupHook

pkgs.makeSetupHook is a build helper that produces hooks that go in to nativeBuildInputs

Usage

pkgs.makeSetupHook {
  name = "something-hook";
  propagatedBuildInputs = [ pkgs.commandsomething ];
  depsTargetTargetPropagated = [ pkgs.libsomething ];
} ./script.sh

setup hook that depends on the hello package and runs hello and @shell@ is substituted with path to bash

pkgs.makeSetupHook {
    name = "run-hello-hook";
    propagatedBuildInputs = [ pkgs.hello ];
    substitutions = { shell = "${pkgs.bash}/bin/bash"; };
    passthru.tests.greeting = callPackage ./test { };
    meta.platforms = lib.platforms.linux;
} (writeScript "run-hello-hook.sh" ''
    #!@shell@
    hello
'')

Attributes

  • name Set the name of the hook.

  • propagatedBuildInputs Runtime dependencies (such as binaries) of the hook.

  • depsTargetTargetPropagated Non-binary dependencies.

  • meta

  • passthru

  • substitutions Variables for substituteAll

pkgs.mkShell

pkgs.mkShell is a specialized stdenv.mkDerivation that removes some repetition when using it with nix-shell (or nix develop).

Usage

Here is a common usage example:

{ pkgs ? import <nixpkgs> {} }:
pkgs.mkShell {
  packages = [ pkgs.gnumake ];

  inputsFrom = [ pkgs.hello pkgs.gnutar ];

  shellHook = ''
    export DEBUG=1
  '';
}

Attributes

  • name (default: nix-shell). Set the name of the derivation.

  • packages (default: []). Add executable packages to the nix-shell environment.

  • inputsFrom (default: []). Add build dependencies of the listed derivations to the nix-shell environment.

  • shellHook (default: ""). Bash statements that are executed by nix-shell.

… all the attributes of stdenv.mkDerivation.

Variants

pkgs.mkShellNoCC is a variant that uses stdenvNoCC instead of stdenv as base environment. This is useful if no C compiler is needed in the shell environment.

Building the shell

This derivation output will contain a text file that contains a reference to all the build inputs. This is useful in CI where we want to make sure that every derivation, and its dependencies, build properly. Or when creating a GC root so that the build dependencies don’t get garbage-collected.

vmTools

A set of VM related utilities, that help in building some packages in more advanced scenarios.

vmTools.createEmptyImage

A bash script fragment that produces a disk image at destination.

Attributes

  • size. The disk size, in MiB.

  • fullName. Name that will be written to ${destination}/nix-support/full-name.

  • destination (optional, default $out). Where to write the image files.

vmTools.runInLinuxVM

Run a derivation in a Linux virtual machine (using Qemu/KVM). By default, there is no disk image; the root filesystem is a tmpfs, and the Nix store is shared with the host (via the 9P protocol). Thus, any pure Nix derivation should run unmodified.

If the build fails and Nix is run with the -K/--keep-failed option, a script run-vm will be left behind in the temporary build directory that allows you to boot into the VM and debug it interactively.

Attributes

  • preVM (optional). Shell command to be evaluated before the VM is started (i.e., on the host).

  • memSize (optional, default 512). The memory size of the VM in MiB.

  • diskImage (optional). A file system image to be attached to /dev/sda. Note that currently we expect the image to contain a filesystem, not a full disk image with a partition table etc.

Examples

Build the derivation hello inside a VM:

{ pkgs }: with pkgs; with vmTools;
runInLinuxVM hello

Build inside a VM with extra memory:

{ pkgs }: with pkgs; with vmTools;
runInLinuxVM (hello.overrideAttrs (_: { memSize = 1024; }))

Use VM with a disk image (implicitly sets diskImage, see vmTools.createEmptyImage):

{ pkgs }: with pkgs; with vmTools;
runInLinuxVM (hello.overrideAttrs (_: {
  preVM = createEmptyImage {
    size = 1024;
    fullName = "vm-image";
  };
}))

vmTools.extractFs

Takes a file, such as an ISO, and extracts its contents into the store.

Attributes

  • file. Path to the file to be extracted. Note that currently we expect the image to contain a filesystem, not a full disk image with a partition table etc.

  • fs (optional). Filesystem of the contents of the file.

Examples

Extract the contents of an ISO file:

{ pkgs }: with pkgs; with vmTools;
extractFs { file = ./image.iso; }

vmTools.extractMTDfs

Like the section called “vmTools.extractFs, but it makes use of a Memory Technology Device (MTD).

vmTools.runInLinuxImage

Like the section called “vmTools.runInLinuxVM, but instead of using stdenv from the Nix store, run the build using the tools provided by /bin, /usr/bin, etc. from the specified filesystem image, which typically is a filesystem containing a FHS-based Linux distribution.

vmTools.makeImageTestScript

Generate a script that can be used to run an interactive session in the given image.

Examples

Create a script for running a Fedora 27 VM:

{ pkgs }: with pkgs; with vmTools;
makeImageTestScript diskImages.fedora27x86_64

Create a script for running an Ubuntu 20.04 VM:

{ pkgs }: with pkgs; with vmTools;
makeImageTestScript diskImages.ubuntu2004x86_64

vmTools.diskImageFuns

A set of functions that build a predefined set of minimal Linux distributions images.

Images

  • Fedora

    • fedora26x86_64

    • fedora27x86_64

  • CentOS

    • centos6i386

    • centos6x86_64

    • centos7x86_64

  • Ubuntu

    • ubuntu1404i386

    • ubuntu1404x86_64

    • ubuntu1604i386

    • ubuntu1604x86_64

    • ubuntu1804i386

    • ubuntu1804x86_64

    • ubuntu2004i386

    • ubuntu2004x86_64

    • ubuntu2204i386

    • ubuntu2204x86_64

  • Debian

    • debian10i386

    • debian10x86_64

    • debian11i386

    • debian11x86_64

Attributes

  • size (optional, defaults to 4096). The size of the image, in MiB.

  • extraPackages (optional). A list names of additional packages from the distribution that should be included in the image.

Examples

8GiB image containing Firefox in addition to the default packages:

{ pkgs }: with pkgs; with vmTools;
diskImageFuns.ubuntu2004x86_64 { extraPackages = [ "firefox" ]; size = 8192; }

vmTools.diskImageExtraFuns

Shorthand for vmTools.diskImageFuns.<attr> { extraPackages = ... }.

vmTools.diskImages

Shorthand for vmTools.diskImageFuns.<attr> { }.

pkgs.checkpointBuildTools

pkgs.checkpointBuildTools provides a way to build derivations incrementally. It consists of two functions to make checkpoint builds using Nix possible.

For hermeticity, Nix derivations do not allow any state to be carried over between builds, making a transparent incremental build within a derivation impossible.

However, we can tell Nix explicitly what the previous build state was, by representing that previous state as a derivation output. This allows the passed build state to be used for an incremental build.

To change a normal derivation to a checkpoint based build, these steps must be taken:

  • apply prepareCheckpointBuild on the desired derivation, e.g.

checkpointArtifacts = (pkgs.checkpointBuildTools.prepareCheckpointBuild pkgs.virtualbox);
  • change something you want in the sources of the package, e.g. use a source override:

changedVBox = pkgs.virtualbox.overrideAttrs (old: {
  src = path/to/vbox/sources;
});
  • use mkCheckpointBuild changedVBox checkpointArtifacts

  • enjoy shorter build times

Example

{ pkgs ? import <nixpkgs> {} }:
let
  inherit (pkgs.checkpointBuildTools)
    prepareCheckpointBuild
    mkCheckpointBuild
    ;
  helloCheckpoint = prepareCheckpointBuild pkgs.hello;
  changedHello = pkgs.hello.overrideAttrs (_: {
    doCheck = false;
    patchPhase = ''
      sed -i 's/Hello, world!/Hello, Nix!/g' src/hello.c
    '';
  });
in mkCheckpointBuild changedHello helloCheckpoint

Images

This chapter describes tools for creating various types of images.

pkgs.appimageTools

pkgs.appimageTools is a set of functions for extracting and wrapping AppImage files. They are meant to be used if traditional packaging from source is infeasible, or if it would take too long. To quickly run an AppImage file, pkgs.appimage-run can be used as well.

Warning

The appimageTools API is unstable and may be subject to backwards-incompatible changes in the future.

Wrapping

Use wrapType2 to wrap any AppImage. This will create a FHS environment with many packages expected to exist for the AppImage to work. wrapType2 expects an argument with the src attribute, and either a name attribute or pname and version attributes.

It will eventually call into buildFHSEnv, and any extra attributes in the argument to wrapType2 will be passed through to it. This means that you can pass the extraInstallCommands attribute, for example, and it will have the same effect as described in buildFHSEnv.

Note

In the past, appimageTools provided both wrapType1 and wrapType2, to be used depending on the type of AppImage that was being wrapped. However, those were unified early 2020, meaning that both wrapType1 and wrapType2 have the same behaviour now.


The argument passed to wrapType2 can also contain an extraPkgs attribute, which allows you to include additional packages inside the FHS environment your AppImage is going to run in. extraPkgs must be a function that returns a list of packages. There are a few ways to learn which dependencies an application needs:

  • Looking through the extracted AppImage files, reading its scripts and running patchelf and ldd on its executables. This can also be done in appimage-run, by setting APPIMAGE_DEBUG_EXEC=bash.

  • Running strace -vfefile on the wrapped executable, looking for libraries that can’t be found.


Extracting

Use extract if you need to extract the contents of an AppImage. This is usually used in Nixpkgs to install extra files in addition to wrapping the AppImage. extract expects an argument with the src attribute, and either a name attribute or pname and version attributes.

Note

In the past, appimageTools provided both extractType1 and extractType2, to be used depending on the type of AppImage that was being extracted. However, those were unified early 2020, meaning that both extractType1 and extractType2 have the same behaviour as extract now.


The argument passed to extract can also contain a postExtract attribute, which allows you to execute additional commands after the files are extracted from the AppImage. postExtract must be a string with commands to run.

Example 267. Extracting an AppImage to install extra files, using postExtract

This is a rewrite of Example 266 to use postExtract.

{ appimageTools, fetchurl }:
let
  pname = "irccloud";
  version = "0.16.0";

  src = fetchurl {
    url = "https://github.com/irccloud/irccloud-desktop/releases/download/v${version}/IRCCloud-${version}-linux-x86_64.AppImage";
    sha256 = "sha256-/hMPvYdnVB1XjKgU2v47HnVvW4+uC3rhRjbucqin4iI=";
  };

  appimageContents = appimageTools.extract {
    inherit pname version src;
    postExtract = ''
      substituteInPlace $out/irccloud.desktop --replace 'Exec=AppRun' 'Exec=${pname}'
    '';
  };
in appimageTools.wrapType2 {
  inherit pname version src;

  extraPkgs = pkgs: [ pkgs.at-spi2-core ];

  extraInstallCommands = ''
    mv $out/bin/${pname}-${version} $out/bin/${pname}
    install -m 444 -D ${appimageContents}/irccloud.desktop $out/share/applications/irccloud.desktop
    install -m 444 -D ${appimageContents}/usr/share/icons/hicolor/512x512/apps/irccloud.png \
      $out/share/icons/hicolor/512x512/apps/irccloud.png
  '';
}

pkgs.dockerTools

pkgs.dockerTools is a set of functions for creating and manipulating Docker images according to the Docker Image Specification v1.3.0. Docker itself is not used to perform any of the operations done by these functions.

buildImage

This function builds a Docker-compatible repository tarball containing a single image. As such, the result is suitable for being loaded in Docker with docker image load (see Example 268 for how to do this).

This function will create a single layer for all files (and dependencies) that are specified in its argument. Only new dependencies that are not already in the existing layers will be copied. If you prefer to create multiple layers for the files and dependencies you want to add to the image, see the section called “buildLayeredImage” or the section called “streamLayeredImage” instead.

This function allows a script to be run during the layer generation process, allowing custom behaviour to affect the final results of the image (see the documentation of the runAsRoot and extraCommands attributes).

The resulting repository tarball will list a single image as specified by the name and tag attributes. By default, that image will use a static creation date (see documentation for the created attribute). This allows buildImage to produce reproducible images.

Tip

When running an image built with buildImage, you might encounter certain errors depending on what you included in the image, especially if you did not start with any base image.

If you encounter errors similar to getProtocolByName: does not exist (no such protocol name: tcp), you may need to add the contents of pkgs.iana-etc in the copyToRoot attribute. Similarly, if you encounter errors similar to Error_Protocol ("certificate has unknown CA",True,UnknownCa), you may need to add the contents of pkgs.cacert in the copyToRoot attribute.

Inputs

buildImage expects an argument with the following attributes:

name (String)

The name of the generated image.

tag (String or Null; optional)

Tag of the generated image. If null, the hash of the nix derivation will be used as the tag.

Default value: null.

fromImage (Path or Null; optional)

The repository tarball of an image to be used as the base for the generated image. It must be a valid Docker image, such as one exported by docker image save, or another image built with the dockerTools utility functions. This can be seen as an equivalent of FROM fromImage in a Dockerfile. A value of null can be seen as an equivalent of FROM scratch.

If specified, the layer created by buildImage will be appended to the layers defined in the base image, resulting in an image with at least two layers (one or more layers from the base image, and the layer created by buildImage). Otherwise, the resulting image with contain the single layer created by buildImage.

Default value: null.

fromImageName (String or Null; optional)

Used to specify the image within the repository tarball in case it contains multiple images. A value of null means that buildImage will use the first image available in the repository.

Note

This must be used with fromImageTag. Using only fromImageName without fromImageTag will make buildImage use the first image available in the repository.

Default value: null.

fromImageTag (String or Null; optional)

Used to specify the image within the repository tarball in case it contains multiple images. A value of null means that buildImage will use the first image available in the repository.

Note

This must be used with fromImageName. Using only fromImageTag without fromImageName will make buildImage use the first image available in the repository

Default value: null.

copyToRoot (Path, List of Paths, or Null; optional)

Files to add to the generated image. Anything that coerces to a path (e.g. a derivation) can also be used. This can be seen as an equivalent of ADD contents/ / in a Dockerfile.

Default value: null.

keepContentsDirlinks (Boolean; optional)

When adding files to the generated image (as specified by copyToRoot), this attribute controls whether to preserve symlinks to directories. If false, the symlinks will be transformed into directories. This behaves the same as rsync -k when keepContentsDirlinks is false, and the same as rsync -K when keepContentsDirlinks is true.

Default value: false.

runAsRoot (String or Null; optional)

A bash script that will run as root inside a VM that contains the existing layers of the base image and the new generated layer (including the files from copyToRoot). The script will be run with a working directory of /. This can be seen as an equivalent of RUN ... in a Dockerfile. A value of null means that this step in the image generation process will be skipped.

See Example 269 for how to work with this attribute.

Caution

Using this attribute requires the kvm device to be available, see system-features. If the kvm device isn’t available, you should consider using buildLayeredImage or streamLayeredImage instead. Those functions allow scripts to be run as root without access to the kvm device.

Note

At the time the script in runAsRoot is run, the files specified directly in copyToRoot will be present in the VM, but their dependencies might not be there yet. Copying their dependencies into the generated image is a step that happens after runAsRoot finishes running.

Default value: null.

extraCommands (String; optional)

A bash script that will run before the layer created by buildImage is finalised. The script will be run on some (opaque) working directory which will become / once the layer is created. This is similar to runAsRoot, but the script specified in extraCommands is not run as root, and does not involve creating a VM. It is simply run as part of building the derivation that outputs the layer created by buildImage.

See Example 270 for how to work with this attribute, and subtle differences compared to runAsRoot.

Default value: "".

config (Attribute Set or Null; optional)

Used to specify the configuration of the containers that will be started off the generated image. Must be an attribute set, with each attribute as listed in the Docker Image Specification v1.3.0.

Default value: null.

architecture (String; optional)

Used to specify the image architecture. This is useful for multi-architecture builds that don’t need cross compiling. If specified, its value should follow the OCI Image Configuration Specification, which should still be compatible with Docker. According to the linked specification, all possible values for $GOARCH in the Go docs should be valid, but will commonly be one of 386, amd64, arm, or arm64.

Default value: the same value from pkgs.go.GOARCH.

diskSize (Number; optional)

Controls the disk size (in megabytes) of the VM used to run the script specified in runAsRoot. This attribute is ignored if runAsRoot is null.

Default value: 1024.

buildVMMemorySize (Number; optional)

Controls the amount of memory (in megabytes) provisioned for the VM used to run the script specified in runAsRoot. This attribute is ignored if runAsRoot is null.

Default value: 512.

created (String; optional)

Specifies the time of creation of the generated image. This should be either a date and time formatted according to ISO-8601 or "now", in which case buildImage will use the current date.

See Example 271 for how to use "now".

Caution

Using "now" means that the generated image will not be reproducible anymore (because the date will always change whenever it’s built).

Default value: "1970-01-01T00:00:01Z".

uid (Number; optional)

The uid of the user that will own the files packed in the new layer built by buildImage.

Default value: 0.

gid (Number; optional)

The gid of the group that will own the files packed in the new layer built by buildImage.

Default value: 0.

compressor (String; optional)

Selects the algorithm used to compress the image.

Default value: "gz".
Possible values: "none", "gz", "zstd".

contents DEPRECATED

This attribute is deprecated, and users are encouraged to use copyToRoot instead.

Passthru outputs

buildImage defines a few passthru attributes:

buildArgs (Attribute Set)

The argument passed to buildImage itself. This allows you to inspect all attributes specified in the argument, as described above.

layer (Attribute Set)

The derivation with the layer created by buildImage. This allows easier inspection of the contents added by buildImage in the generated image.

imageTag (String)

The tag of the generated image. This is useful if no tag was specified in the attributes of the argument to buildImage, because an automatic tag will be used instead. imageTag allows you to retrieve the value of the tag used in this case.

Examples


Example 269. Building a Docker image with runAsRoot

The following package builds a Docker image with the hello executable from the hello package. It uses runAsRoot to create a directory and a file inside the image.

This works the same as Example 270, but uses runAsRoot instead of extraCommands.

{ dockerTools, buildEnv, hello }:
dockerTools.buildImage {
  name = "hello";
  tag = "latest";

  copyToRoot = buildEnv {
    name = "image-root";
    paths = [ hello ];
    pathsToLink = [ "/bin" ];
  };

  runAsRoot = ''
    mkdir -p /data
    echo "some content" > my-file
  '';

  config = {
    Cmd = [ "/bin/hello" ];
    WorkingDir = "/data";
  };
}

Example 270. Building a Docker image with extraCommands

The following package builds a Docker image with the hello executable from the hello package. It uses extraCommands to create a directory and a file inside the image.

This works the same as Example 269, but uses extraCommands instead of runAsRoot. Note that with extraCommands, we can’t directly reference / and must create files and directories as if we were already on /.

{ dockerTools, buildEnv, hello }:
dockerTools.buildImage {
  name = "hello";
  tag = "latest";

  copyToRoot = buildEnv {
    name = "image-root";
    paths = [ hello ];
    pathsToLink = [ "/bin" ];
  };

  extraCommands = ''
    mkdir -p data
    echo "some content" > my-file
  '';

  config = {
    Cmd = [ "/bin/hello" ];
    WorkingDir = "/data";
  };
}


buildLayeredImage

buildLayeredImage uses streamLayeredImage underneath to build a compressed Docker-compatible repository tarball. Basically, buildLayeredImage runs the script created by streamLayeredImage to save the compressed image in the Nix store. buildLayeredImage supports the same options as streamLayeredImage, see streamLayeredImage for details.

Note

Despite the similar name, buildImage works completely differently from buildLayeredImage and streamLayeredImage.

Even though some of the arguments may seem related, they cannot be interchanged.

You can load the result of this function in Docker with docker image load. See Example 272 to see how to do that.

streamLayeredImage

streamLayeredImage builds a script which, when run, will stream to stdout a Docker-compatible repository tarball containing a single image, using multiple layers to improve sharing between images. This means that streamLayeredImage does not output an image into the Nix store, but only a script that builds the image, saving on IO and disk/cache space, particularly with large images.

You can load the result of this function in Docker with docker image load. See Example 273 to see how to do that.

For this function, you specify a store path or a list of store paths to be added to the image, and the functions will automatically include any dependencies of those paths in the image. The function will attempt to create one layer per object in the Nix store that needs to be added to the image. In case there are more objects to include than available layers, the function will put the most “popular” objects in their own layers, and group all remaining objects into a single layer.

An additional layer will be created with symlinks to the store paths you specified to be included in the image. These symlinks are built with symlinkJoin, so they will be included in the root of the image. See Example 274 to understand how these symlinks are laid out in the generated image.

streamLayeredImage allows scripts to be run when creating the additional layer with symlinks, allowing custom behaviour to affect the final results of the image (see the documentation of the extraCommands and fakeRootCommands attributes).

The resulting repository tarball will list a single image as specified by the name and tag attributes. By default, that image will use a static creation date (see documentation for the created attribute). This allows the function to produce reproducible images.

Inputs

streamLayeredImage expects one argument with the following attributes:

name (String)

The name of the generated image.

tag (String or Null; optional)

Tag of the generated image. If null, the hash of the nix derivation will be used as the tag.

Default value: null.

fromImage(Path or Null; optional)

The repository tarball of an image to be used as the base for the generated image. It must be a valid Docker image, such as one exported by docker image save, or another image built with the dockerTools utility functions. This can be seen as an equivalent of FROM fromImage in a Dockerfile. A value of null can be seen as an equivalent of FROM scratch.

If specified, the created layers will be appended to the layers defined in the base image.

Default value: null.

contents (Path or List of Paths; optional)

Directories whose contents will be added to the generated image. Things that coerce to paths (e.g. a derivation) can also be used. This can be seen as an equivalent of ADD contents/ / in a Dockerfile.

All the contents specified by contents will be added as a final layer in the generated image. They will be added as links to the actual files (e.g. links to the store paths). The actual files will be added in previous layers.

Default value: []

config (Attribute Set or Null; optional)

Used to