Nixpkgs Manual

Version 23.11pre-git

Table of Contents

Using Nixpkgs
Platform Support
Global configuration
Nixpkgs lib
Functions reference
Module System
Standard environment
The Standard Environment
Multiple-output packages
Platform Notes
Build helpers
Trivial build helpers
Special build helpers
Hooks reference
Languages and frameworks
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.asserts.assertMsg usage example
2. lib.asserts.assertOneOf usage example
3. lib.asserts.assertEachOneOf usage example
4. lib.attrsets.attrByPath usage example
5. lib.attrsets.hasAttrByPath usage example
6. lib.attrsets.longestValidPathPrefix usage example
7. lib.attrsets.setAttrByPath usage example
8. lib.attrsets.getAttrFromPath usage example
9. lib.attrsets.concatMapAttrs usage example
10. lib.attrsets.updateManyAttrsByPath usage example
11. lib.attrsets.attrVals usage example
12. lib.attrsets.attrValues usage example
13. lib.attrsets.getAttrs usage example
14. lib.attrsets.catAttrs usage example
15. lib.attrsets.filterAttrs usage example
16. lib.attrsets.filterAttrsRecursive usage example
17. lib.attrsets.foldlAttrs usage example
18. lib.attrsets.foldAttrs usage example
19. lib.attrsets.collect usage example
20. lib.attrsets.cartesianProductOfSets usage example
21. lib.attrsets.nameValuePair usage example
22. lib.attrsets.mapAttrs usage example
23. lib.attrsets.mapAttrs' usage example
24. lib.attrsets.mapAttrsToList usage example
25. lib.attrsets.attrsToList usage example
26. lib.attrsets.mapAttrsRecursive usage example
27. lib.attrsets.mapAttrsRecursiveCond usage example
28. lib.attrsets.genAttrs usage example
29. lib.attrsets.isDerivation usage example
30. lib.attrsets.optionalAttrs usage example
31. lib.attrsets.zipAttrsWithNames usage example
32. lib.attrsets.zipAttrsWith usage example
33. lib.attrsets.zipAttrs usage example
34. lib.attrsets.mergeAttrsList usage example
35. lib.attrsets.recursiveUpdateUntil usage example
36. lib.attrsets.recursiveUpdate usage example
37. lib.attrsets.matchAttrs usage example
38. lib.attrsets.overrideExisting usage example
39. lib.attrsets.showAttrPath usage example
40. lib.attrsets.getOutput usage example
41. lib.attrsets.getBin usage example
42. lib.attrsets.getLib usage example
43. lib.attrsets.getDev usage example
44. lib.attrsets.getMan usage example
45. lib.attrsets.recurseIntoAttrs usage example
46. lib.strings.concatStrings usage example
47. lib.strings.concatMapStrings usage example
48. lib.strings.concatImapStrings usage example
49. lib.strings.intersperse usage example
50. lib.strings.concatStringsSep usage example
51. lib.strings.concatMapStringsSep usage example
52. lib.strings.concatImapStringsSep usage example
53. lib.strings.concatLines usage example
54. lib.strings.replicate usage example
55. lib.strings.makeSearchPath usage example
56. lib.strings.makeSearchPathOutput usage example
57. lib.strings.makeLibraryPath usage example
58. lib.strings.makeBinPath usage example
59. lib.strings.normalizePath usage example
60. lib.strings.optionalString usage example
61. lib.strings.hasPrefix usage example
62. lib.strings.hasSuffix usage example
63. lib.strings.hasInfix usage example
64. lib.strings.stringToCharacters usage example
65. lib.strings.stringAsChars usage example
66. lib.strings.charToInt usage example
67. lib.strings.escape usage example
68. lib.strings.escapeC usage example
69. lib.strings.escapeURL usage example
70. lib.strings.escapeShellArg usage example
71. lib.strings.escapeShellArgs usage example
72. lib.strings.isValidPosixName usage example
73. lib.strings.toShellVar usage example
74. lib.strings.toShellVars usage example
75. lib.strings.escapeNixString usage example
76. lib.strings.escapeRegex usage example
77. lib.strings.escapeNixIdentifier usage example
78. lib.strings.escapeXML usage example
79. lib.strings.toLower usage example
80. lib.strings.toUpper usage example
81. lib.strings.addContextFrom usage example
82. lib.strings.splitString usage example
83. lib.strings.removePrefix usage example
84. lib.strings.removeSuffix usage example
85. lib.strings.versionOlder usage example
86. lib.strings.versionAtLeast usage example
87. lib.strings.getName usage example
88. lib.strings.getVersion usage example
89. lib.strings.nameFromURL usage example
90. lib.strings.cmakeOptionType usage example
91. lib.strings.cmakeBool usage example
92. lib.strings.cmakeFeature usage example
93. lib.strings.mesonOption usage example
94. lib.strings.mesonBool usage example
95. lib.strings.mesonEnable usage example
96. lib.strings.enableFeature usage example
97. lib.strings.enableFeatureAs usage example
98. lib.strings.withFeature usage example
99. lib.strings.withFeatureAs usage example
100. lib.strings.fixedWidthString usage example
101. lib.strings.fixedWidthNumber usage example
102. lib.strings.floatToString usage example
103. lib.strings.isStorePath usage example
104. lib.strings.toInt usage example
105. lib.strings.toIntBase10 usage example
106. lib.strings.readPathsFromFile usage example
107. lib.strings.fileContents usage example
108. lib.strings.sanitizeDerivationName usage example
109. lib.strings.levenshtein usage example
110. lib.strings.levenshteinAtMost usage example
111. lib.versions.splitVersion usage example
112. lib.versions.major usage example
113. lib.versions.minor usage example
114. lib.versions.patch usage example
115. lib.versions.majorMinor usage example
116. lib.versions.pad usage example
117. lib.trivial.const usage example
118. lib.trivial.pipe usage example
119. lib.trivial.concat usage example
120. lib.trivial.mergeAttrs usage example
121. lib.trivial.flip usage example
122. lib.trivial.mapNullable usage example
123. lib.trivial.mod usage example
124. lib.trivial.splitByAndCompare usage example
125. lib.trivial.throwIfNot usage example
126. lib.trivial.checkListOfEnum usage example
127. lib.trivial.mirrorFunctionArgs usage example
128. lib.trivial.toFunction usage example
129. lib.fixedPoints.fix usage example
130. lib.lists.singleton usage example
131. lib.lists.forEach usage example
132. lib.lists.foldr usage example
133. lib.lists.foldl usage example
134. lib.lists.foldl' usage example
135. lib.lists.imap0 usage example
136. lib.lists.imap1 usage example
137. lib.lists.concatMap usage example
138. lib.lists.flatten usage example
139. lib.lists.remove usage example
140. lib.lists.findSingle usage example
141. lib.lists.findFirstIndex usage example
142. lib.lists.findFirst usage example
143. lib.lists.any usage example
144. lib.lists.all usage example
145. lib.lists.count usage example
146. lib.lists.optional usage example
147. lib.lists.optionals usage example
148. lib.lists.toList usage example
149. lib.lists.range usage example
150. lib.lists.replicate usage example
151. lib.lists.partition usage example
152. lib.lists.groupBy' usage example
153. lib.lists.zipListsWith usage example
154. lib.lists.zipLists usage example
155. lib.lists.reverseList usage example
156. lib.lists.listDfs usage example
157. lib.lists.toposort usage example
158. lib.lists.sort usage example
159. lib.lists.sortOn usage example
160. lib.lists.compareLists usage example
161. lib.lists.naturalSort usage example
162. lib.lists.take usage example
163. lib.lists.drop usage example
164. lib.lists.hasPrefix usage example
165. lib.lists.removePrefix usage example
166. lib.lists.sublist usage example
167. lib.lists.commonPrefix usage example
168. lib.lists.last usage example
169. lib.lists.init usage example
170. lib.lists.crossLists usage example
171. lib.lists.unique usage example
172. lib.lists.allUnique usage example
173. lib.lists.intersectLists usage example
174. lib.lists.subtractLists usage example
175. lib.debug.traceIf usage example
176. lib.debug.traceValFn usage example
177. lib.debug.traceVal usage example
178. lib.debug.traceSeq usage example
179. lib.debug.traceSeqN usage example
180. lib.debug.traceFnSeqN usage example
181. lib.debug.runTests usage example
182. lib.debug.testAllTrue usage example
183. lib.options.isOption usage example
184. lib.options.mkOption usage example
185. lib.options.mkEnableOption usage example
186. lib.options.mkPackageOption usage example
187. lib.options.getValues usage example
188. lib.options.getFiles usage example
189. lib.options.showOption usage example
190. lib.path.append usage example
191. lib.path.hasPrefix usage example
192. lib.path.removePrefix usage example
193. lib.path.splitRoot usage example
194. lib.path.hasStorePathPrefix usage example
195. lib.path.subpath.isValid usage example
196. lib.path.subpath.join usage example
197. lib.path.subpath.components usage example
198. lib.path.subpath.normalise usage example
199. lib.filesystem.pathType usage example
200. lib.filesystem.pathIsDirectory usage example
201. lib.filesystem.pathIsRegularFile usage example
202. lib.fileset.maybeMissing usage example
203. lib.fileset.trace usage example
204. lib.fileset.traceVal usage example
205. lib.fileset.toSource usage example
206. lib.fileset.toList usage example
207. lib.fileset.union usage example
208. lib.fileset.unions usage example
209. lib.fileset.intersection usage example
210. lib.fileset.difference usage example
211. lib.fileset.fileFilter usage example
212. lib.fileset.fromSource usage example
213. lib.fileset.gitTracked usage example
214. lib.fileset.gitTrackedWith usage example
215. lib.sources.commitIdFromGitRepo usage example
216. lib.sources.cleanSource usage example
217. lib.sources.cleanSourceWith usage example
218. lib.sources.sourceByRegex usage example
219. lib.sources.sourceFilesBySuffices usage example
220. lib.cli.toGNUCommandLineShell usage example
221. lib.gvariant.mkArray usage example
222. lib.gvariant.mkEmptyArray usage example
223. lib.gvariant.mkVariant usage example
224. lib.gvariant.mkDictionaryEntry usage example
225. lib.customisation.overrideDerivation usage example
226. lib.customisation.makeOverridable usage example
227. lib.meta.addMetaAttrs usage example
228. lib.meta.updateName usage example
229. lib.meta.getLicenseFromSpdxId usage example
230. lib.meta.getExe usage example
231. lib.meta.getExe' usage example
232. Standard output of an update script using commit feature
233. Enable debug symbols for use with GDB
234. Invocation of runCommand
235. Ephemeral shell
236. Declarative shell
237. Using pkgs.zlib.override {}
238. Using pkgs.buildEmscriptenPackage {}


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 manual primarily describes how to write packages for the Nix Packages collection (Nixpkgs). Thus it’s mainly for packagers and developers who want to add packages to Nixpkgs. If you like to learn more about the Nix package manager and the Nix expression language, then you are kindly referred to the Nix manual. The NixOS distribution is documented in the NixOS manual.

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. 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 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 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:

  • 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:

  • 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:

  • 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) [
  • 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:

  • 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 = [
  • 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 = { ... };

config Options Reference

The following attributes can be passed in config.


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:


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:


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:


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:


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:


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

Type: boolean

Default: false

Declared by:


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:


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:


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:


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

Type: boolean

Default: false

Declared by:


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:


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:


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:


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:


Whether to warn when config contains an unrecognized attribute.

Type: boolean

Default: false

Declared by:


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 = [

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 = [
      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 = [
      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/
      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:

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"

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/
      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

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.


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.


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/ and $out/lib/ to their respective BLAS libraries. Likewise, LAPACK providers will have symlinks in $out/lib/ and $out/lib/ 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 and 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;


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.


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: { arg1 = val1; arg2 = val2; ... }

It’s also possible to access the previous arguments. (previous: { arg1 = previous.arg1; ... })
import pkgs.path { overlays = [ (self: super: {
  foo = { barSupport = true ; };
mypkg = pkgs.callPackage ./mypkg.nix {
  mydep = pkgs.mydep.override { ... };

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


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 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).



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.


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 = "";
    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.


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.


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


Type: assertMsg :: Bool -> String -> Bool

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


Predicate that needs to succeed, otherwise msg is thrown


Message to throw in case pred fails

Example 1. lib.asserts.assertMsg usage example

assertMsg false "nope"
stderr> error: nope

assert assertMsg ("foo" == "bar") "foo is not bar, silly"; ""
stderr> error: foo is not bar, silly

Located at lib/asserts.nix:19 in <nixpkgs>.


Type: assertOneOf :: String -> ComparableVal -> List ComparableVal -> Bool

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


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


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


The list of valid values

Example 2. lib.asserts.assertOneOf usage example

let sslLibrary = "libressl";
in assertOneOf "sslLibrary" sslLibrary [ "openssl" "bearssl" ]
stderr> error: sslLibrary must be one of [
stderr>   "openssl"
stderr>   "bearssl"
stderr> ], but is: "libressl"

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


Type: assertEachOneOf :: String -> List ComparableVal -> List ComparableVal -> Bool

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.


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


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


The list of valid values

Example 3. lib.asserts.assertEachOneOf usage example

let sslLibraries = [ "libressl" "bearssl" ];
in assertEachOneOf "sslLibraries" sslLibraries [ "openssl" "bearssl" ]
stderr> error: each element in sslLibraries must be one of [
stderr>   "openssl"
stderr>   "bearssl"
stderr> ], but is: [
stderr>   "libressl"
stderr>   "bearssl"
stderr> ]

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

lib.attrsets: attribute set functions

Operations on attribute sets.


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

Return an attribute from nested attribute sets.


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


Default value if attrPath does not resolve to an existing value


The nested attribute set to select values from

Example 4. lib.attrsets.attrByPath usage example

x = { a = { b = 3; }; }
# ["a" "b"] is equivalent to x.a.b
# 6 is a default value to return if the path does not exist in attrset
attrByPath ["a" "b"] 6 x
=> 3
attrByPath ["z" "z"] 6 x
=> 6

Located at lib/attrsets.nix:30 in <nixpkgs>.


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

Return if an attribute from nested attribute set exists.


  1. hasAttrByPath [] x == true

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


The nested attribute set to check

Example 5. lib.attrsets.hasAttrByPath usage example

x = { a = { b = 3; }; }
hasAttrByPath ["a" "b"] x
=> true
hasAttrByPath ["z" "z"] x
=> false
hasAttrByPath [] (throw "no need")
=> true

Located at lib/attrsets.nix:63 in <nixpkgs>.


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.


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

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


The nested attribute set to check.

Example 6. lib.attrsets.longestValidPathPrefix usage example

x = { a = { b = 3; }; }
attrsets.longestValidPathPrefix ["a" "b" "c"] x
=> ["a" "b"]
attrsets.longestValidPathPrefix ["a"] x
=> ["a"]
attrsets.longestValidPathPrefix ["z" "z"] x
=> []
attrsets.longestValidPathPrefix ["z" "z"] (throw "no need")
=> []

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


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

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


A list of strings representing the attribute path to set


The value to set at the location described by attrPath

Example 7. lib.attrsets.setAttrByPath usage example

setAttrByPath ["a" "b"] 3
=> { a = { b = 3; }; }

Located at lib/attrsets.nix:150 in <nixpkgs>.


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

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


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


The nested attribute set to find the value in.

Example 8. lib.attrsets.getAttrFromPath usage example

x = { a = { b = 3; }; }
getAttrFromPath ["a" "b"] x
=> 3
getAttrFromPath ["z" "z"] x
=> error: cannot find attribute `z.z'

Located at lib/attrsets.nix:176 in <nixpkgs>.


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

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


Function argument


Function argument

Example 9. lib.attrsets.concatMapAttrs usage example

  (name: value: {
    ${name} = value;
    ${name + value} = value;
  { x = "a"; y = "b"; }
=> { x = "a"; xa = "a"; y = "b"; yb = "b"; }

Located at lib/attrsets.nix:198 in <nixpkgs>.


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.


  • 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

Example 10. lib.attrsets.updateManyAttrsByPath usage example

updateManyAttrsByPath [
    path = [ "a" "b" ];
    update = old: { d = old.c; };
    path = [ "a" "b" "c" ];
    update = old: old + 1;
    path = [ "x" "y" ];
    update = old: "xy";
] { a.b.c = 0; }
=> { a = { b = { d = 1; }; }; x = { y = "xy"; }; }

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


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

Return the specified attributes from a set.


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


The set to get attribute values from

Example 11. lib.attrsets.attrVals usage example

attrVals ["a" "b" "c"] as
=> [as.a as.b as.c]

Located at lib/attrsets.nix:317 in <nixpkgs>.


Type: attrValues :: AttrSet -> [Any]

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

Example 12. lib.attrsets.attrValues usage example

attrValues {c = 3; a = 1; b = 2;}
=> [1 2 3]

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


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

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


A list of attribute names to get out of set


The set to get the named attributes from

Example 13. lib.attrsets.getAttrs usage example

getAttrs [ "a" "b" ] { a = 1; b = 2; c = 3; }
=> { a = 1; b = 2; }

Located at lib/attrsets.nix:347 in <nixpkgs>.


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.

Example 14. lib.attrsets.catAttrs usage example

catAttrs "a" [{a = 1;} {b = 0;} {a = 2;}]
=> [1 2]

Located at lib/attrsets.nix:363 in <nixpkgs>.


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

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


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


The attribute set to filter

Example 15. lib.attrsets.filterAttrs usage example

filterAttrs (n: v: n == "foo") { foo = 1; bar = 2; }
=> { foo = 1; }

Located at lib/attrsets.nix:377 in <nixpkgs>.


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

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


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


The attribute set to filter

Example 16. lib.attrsets.filterAttrsRecursive usage example

filterAttrsRecursive (n: v: v != null) { foo = { bar = null; }; }
=> { foo = {}; }

Located at lib/attrsets.nix:395 in <nixpkgs>.


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.


Function argument


Function argument


Function argument

Example 17. lib.attrsets.foldlAttrs usage example

  (acc: name: value: {
    sum = acc.sum + value;
    names = acc.names ++ [name];
  { sum = 0; names = []; }
    foo = 1;
    bar = 10;
    sum = 11;
    names = ["bar" "foo"];

  (throw "function not needed")

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

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

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

Located at lib/attrsets.nix:466 in <nixpkgs>.


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

Apply fold functions to values grouped by key.


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


The starting value.


A list of attribute sets to fold together by key.

Example 18. lib.attrsets.foldAttrs usage example

foldAttrs (item: acc: [item] ++ acc) [] [{ a = 2; } { a = 3; }]
=> { a = [ 2 3 ]; }

Located at lib/attrsets.nix:482 in <nixpkgs>.


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.


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


The attribute set to recursively collect.

Example 19. lib.attrsets.collect usage example

collect isList { a = { b = ["b"]; }; c = [1]; }
=> [["b"] [1]]

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

Located at lib/attrsets.nix:511 in <nixpkgs>.


Type: cartesianProductOfSets :: AttrSet -> [AttrSet]

Return the cartesian product of attribute set value combinations.


Attribute set with attributes that are lists of values

Example 20. lib.attrsets.cartesianProductOfSets usage example

cartesianProductOfSets { a = [ 1 2 ]; b = [ 10 20 ]; }
=> [
     { a = 1; b = 10; }
     { a = 1; b = 20; }
     { a = 2; b = 10; }
     { a = 2; b = 20; }

Located at lib/attrsets.nix:536 in <nixpkgs>.


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

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


Attribute name


Attribute value

Example 21. lib.attrsets.nameValuePair usage example

nameValuePair "some" 6
=> { name = "some"; value = 6; }

Located at lib/attrsets.nix:555 in <nixpkgs>.


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

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

Example 22. lib.attrsets.mapAttrs usage example

mapAttrs (name: value: name + "-" + value)
   { x = "foo"; y = "bar"; }
=> { x = "x-foo"; y = "y-bar"; }

Located at lib/attrsets.nix:573 in <nixpkgs>.


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.


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


Attribute set to map over.

Example 23. lib.attrsets.mapAttrs' usage example

mapAttrs' (name: value: nameValuePair ("foo_" + name) ("bar-" + value))
   { x = "a"; y = "b"; }
=> { foo_x = "bar-a"; foo_y = "bar-b"; }

Located at lib/attrsets.nix:590 in <nixpkgs>.


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

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


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


Attribute set to map over.

Example 24. lib.attrsets.mapAttrsToList usage example

mapAttrsToList (name: value: name + value)
   { x = "a"; y = "b"; }
=> [ "xa" "yb" ]

Located at lib/attrsets.nix:610 in <nixpkgs>.


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


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.

Example 25. lib.attrsets.attrsToList usage example

attrsToList { foo = 1; bar = "asdf"; }
=> [ { name = "bar"; value = "asdf"; } { name = "foo"; value = 1; } ]

Located at lib/attrsets.nix:645 in <nixpkgs>.


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

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 argument 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.


A function, given a list of attribute names and a value, returns a new value.


Set to recursively map over.

Example 26. lib.attrsets.mapAttrsRecursive usage example

mapAttrsRecursive (path: value: concatStringsSep "-" (path ++ [value]))
  { n = { a = "A"; m = { b = "B"; c = "C"; }; }; d = "D"; }
=> { n = { a = "n-a-A"; m = { b = "n-m-b-B"; c = "n-m-c-C"; }; }; d = "d-D"; }

Located at lib/attrsets.nix:665 in <nixpkgs>.


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

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


A function, given the attribute set the recursion is currently at, determine if to recurse deeper into that attribute set.


A function, given a list of attribute names and a value, returns a new value.


Attribute set to recursively map over.

Example 27. lib.attrsets.mapAttrsRecursiveCond usage example

# To prevent recursing into derivations (which are attribute
# sets with the attribute "type" equal to "derivation"):
  (as: !(as ? "type" && as.type == "derivation"))
  (x: ... do something ...)

Located at lib/attrsets.nix:690 in <nixpkgs>.


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

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


Names of values in the resulting attribute set.


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

Example 28. lib.attrsets.genAttrs usage example

genAttrs [ "foo" "bar" ] (name: "x_" + name)
=> { foo = "x_foo"; bar = "x_bar"; }

Located at lib/attrsets.nix:719 in <nixpkgs>.


Type: isDerivation :: Any -> Bool

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


Value to check.

Example 29. lib.attrsets.isDerivation usage example

nixpkgs = import <nixpkgs> {}
isDerivation nixpkgs.ruby
=> true
isDerivation "foobar"
=> false

Located at lib/attrsets.nix:740 in <nixpkgs>.


Type: toDerivation :: Path -> Derivation

Converts a store path to a fake derivation.


A store path to convert to a derivation.

Located at lib/attrsets.nix:749 in <nixpkgs>.


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

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


Condition under which the as attribute set is returned.


The attribute set to return if cond is true.

Example 30. lib.attrsets.optionalAttrs usage example

optionalAttrs (true) { my = "set"; }
=> { my = "set"; }
optionalAttrs (false) { my = "set"; }
=> { }

Located at lib/attrsets.nix:777 in <nixpkgs>.


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

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


List of attribute names to zip.


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


List of values from the list of attribute sets.

Example 31. lib.attrsets.zipAttrsWithNames usage example

zipAttrsWithNames ["a"] (name: vs: vs) [{a = "x";} {a = "y"; b = "z";}]
=> { a = ["x" "y"]; }

Located at lib/attrsets.nix:795 in <nixpkgs>.


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.

Example 32. lib.attrsets.zipAttrsWith usage example

zipAttrsWith (name: values: values) [{a = "x";} {a = "y"; b = "z";}]
=> { a = ["x" "y"]; b = ["z"]; }

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


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.


List of attribute sets to zip together.

Example 33. lib.attrsets.zipAttrs usage example

zipAttrs [{a = "x";} {a = "y"; b = "z";}]
=> { a = ["x" "y"]; b = ["z"]; }

Located at lib/attrsets.nix:838 in <nixpkgs>.


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).


Function argument

Example 34. lib.attrsets.mergeAttrsList usage example

mergeAttrsList [ { a = 0; b = 1; } { c = 2; d = 3; } ]
=> { a = 0; b = 1; c = 2; d = 3; }
mergeAttrsList [ { a = 0; } { a = 1; } ]
=> { a = 1; }

Located at lib/attrsets.nix:858 in <nixpkgs>.


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.


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.


Left attribute set of the merge.


Right attribute set of the merge.

Example 35. lib.attrsets.recursiveUpdateUntil usage example

recursiveUpdateUntil (path: l: r: path == ["foo"]) {
  # first attribute set = 1;
  foo.baz = 2;
  bar = 3;
} {
  #second attribute set = 1;
  foo.quz = 2;
  baz = 4;

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

Located at lib/attrsets.nix:910 in <nixpkgs>.


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.


Left attribute set of the merge.


Right attribute set of the merge.

Example 36. lib.attrsets.recursiveUpdate usage example

recursiveUpdate {
  boot.loader.grub.enable = true;
  boot.loader.grub.device = "/dev/hda";
} {
  boot.loader.grub.device = "";

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

Located at lib/attrsets.nix:950 in <nixpkgs>.


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.


Attribute set structure to match


Attribute set to check

Example 37. lib.attrsets.matchAttrs usage example

matchAttrs { cpu = {}; } { cpu = { bits = 64; }; }
=> true

Located at lib/attrsets.nix:970 in <nixpkgs>.


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

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


Original attribute set


Attribute set with attributes to override in old.

Example 38. lib.attrsets.overrideExisting usage example

overrideExisting {} { a = 1; }
=> {}
overrideExisting { b = 2; } { a = 1; }
=> { b = 2; }
overrideExisting { a = 3; b = 2; } { a = 1; }
=> { a = 1; b = 2; }

Located at lib/attrsets.nix:1006 in <nixpkgs>.


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.


Attribute path to render to a string

Example 39. lib.attrsets.showAttrPath usage example

showAttrPath [ "foo" "10" "bar" ]
=> "foo.\"10\".bar"
showAttrPath []
=> "<root attribute path>"

Located at lib/attrsets.nix:1028 in <nixpkgs>.


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

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


Function argument


Function argument

Example 40. lib.attrsets.getOutput usage example

getOutput "dev" pkgs.openssl
=> "/nix/store/9rz8gxhzf8sw4kf2j2f1grr49w8zx5vj-openssl-1.0.1r-dev"

Located at lib/attrsets.nix:1045 in <nixpkgs>.


Type: getBin :: Derivation -> String

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

Example 41. lib.attrsets.getBin usage example

getBin pkgs.openssl
=> "/nix/store/9rz8gxhzf8sw4kf2j2f1grr49w8zx5vj-openssl-1.0.1r"

Located at lib/attrsets.nix:1060 in <nixpkgs>.


Type: getLib :: Derivation -> String

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

Example 42. lib.attrsets.getLib usage example

getLib pkgs.openssl
=> "/nix/store/9rz8gxhzf8sw4kf2j2f1grr49w8zx5vj-openssl-1.0.1r-lib"

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


Type: getDev :: Derivation -> String

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

Example 43. lib.attrsets.getDev usage example

getDev pkgs.openssl
=> "/nix/store/9rz8gxhzf8sw4kf2j2f1grr49w8zx5vj-openssl-1.0.1r-dev"

Located at lib/attrsets.nix:1086 in <nixpkgs>.


Type: getMan :: Derivation -> String

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

Example 44. lib.attrsets.getMan usage example

getMan pkgs.openssl
=> "/nix/store/9rz8gxhzf8sw4kf2j2f1grr49w8zx5vj-openssl-1.0.1r-man"

Located at lib/attrsets.nix:1099 in <nixpkgs>.


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

Pick the outputs of packages to place in buildInputs


List of packages to pick dev outputs from

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


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.


An attribute set to scan for derivations.

Example 45. lib.attrsets.recurseIntoAttrs usage example

{ pkgs ? import <nixpkgs> {} }:
  myTools = pkgs.lib.recurseIntoAttrs {
    inherit (pkgs) hello figlet;

Located at lib/attrsets.nix:1129 in <nixpkgs>.


Type: dontRecurseIntoAttrs :: AttrSet -> AttrSet

Undo the effect of recurseIntoAttrs.


An attribute set to not scan for derivations.

Located at lib/attrsets.nix:1139 in <nixpkgs>.


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 (//).


Function argument


Function argument

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

lib.strings: string manipulation functions

String manipulation functions.


Type: concatStrings :: [string] -> string

Concatenate a list of strings.

Example 46. lib.strings.concatStrings usage example

concatStrings ["foo" "bar"]
=> "foobar"

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


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

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


Function argument


Function argument

Example 47. lib.strings.concatMapStrings usage example

concatMapStrings (x: "a" + x) ["foo" "bar"]
=> "afooabar"

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


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

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


Function argument


Function argument

Example 48. lib.strings.concatImapStrings usage example

concatImapStrings (pos: x: "${toString pos}-${x}") ["foo" "bar"]
=> "1-foo2-bar"

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


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

Place an element between each element of a list


Separator to add between elements


Input list

Example 49. lib.strings.intersperse usage example

intersperse "/" ["usr" "local" "bin"]
=> ["usr" "/" "local" "/" "bin"].

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


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

Concatenate a list of strings with a separator between each element

Example 50. lib.strings.concatStringsSep usage example

concatStringsSep "/" ["usr" "local" "bin"]
=> "usr/local/bin"

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


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.


Separator to add between elements


Function to map over the list


List of input strings

Example 51. lib.strings.concatMapStringsSep usage example

concatMapStringsSep "-" (x: toUpper x)  ["foo" "bar" "baz"]

Located at lib/strings.nix:111 in <nixpkgs>.


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

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


Separator to add between elements


Function that receives elements and their positions


List of input strings

Example 52. lib.strings.concatImapStringsSep usage example

concatImapStringsSep "-" (pos: x: toString (x / pos)) [ 6 6 6 ]
=> "6-3-2"

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


Type: concatLines :: [string] -> string

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

Example 53. lib.strings.concatLines usage example

concatLines [ "foo" "bar" ]
=> "foo\nbar\n"

Located at lib/strings.nix:145 in <nixpkgs>.


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

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


Function argument


Function argument

Example 54. lib.strings.replicate usage example

replicate 3 "v"
=> "vvv"
replicate 5 "hello"
=> "hellohellohellohellohello"

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


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.


Directory name to append


List of base paths

Example 55. lib.strings.makeSearchPath usage example

makeSearchPath "bin" ["/root" "/usr" "/usr/local"]
=> "/root/bin:/usr/bin:/usr/local/bin"
makeSearchPath "bin" [""]
=> "/bin"

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


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.


Package output to use


Directory name to append


List of packages

Example 56. lib.strings.makeSearchPathOutput usage example

makeSearchPathOutput "dev" "bin" [ pkgs.openssl pkgs.zlib ]
=> "/nix/store/9rz8gxhzf8sw4kf2j2f1grr49w8zx5vj-openssl-1.0.1r-dev/bin:/nix/store/wwh7mhwh269sfjkm6k5665b5kgp7jrk2-zlib-1.2.8/bin"

Located at lib/strings.nix:190 in <nixpkgs>.


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

Example 57. lib.strings.makeLibraryPath usage example

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

Located at lib/strings.nix:208 in <nixpkgs>.


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

Example 58. lib.strings.makeBinPath usage example

makeBinPath ["/root" "/usr" "/usr/local"]
=> "/root/bin:/usr/bin:/usr/local/bin"

Located at lib/strings.nix:217 in <nixpkgs>.


Type: normalizePath :: string -> string

Normalize path, removing extraneous /s


Function argument

Example 59. lib.strings.normalizePath usage example

normalizePath "/a//b///c/"
=> "/a/b/c/"

Located at lib/strings.nix:227 in <nixpkgs>.


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.




String to return if condition is true

Example 60. lib.strings.optionalString usage example

optionalString true "some-string"
=> "some-string"
optionalString false "some-string"
=> ""

Located at lib/strings.nix:253 in <nixpkgs>.


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

Determine whether a string has given prefix.


Prefix to check for


Input string

Example 61. lib.strings.hasPrefix usage example

hasPrefix "foo" "foobar"
=> true
hasPrefix "foo" "barfoo"
=> false

Located at lib/strings.nix:269 in <nixpkgs>.


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

Determine whether a string has given suffix.


Suffix to check for


Input string

Example 62. lib.strings.hasSuffix usage example

hasSuffix "foo" "foobar"
=> false
hasSuffix "foo" "barfoo"
=> true

Located at lib/strings.nix:296 in <nixpkgs>.


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

Determine whether a string contains the given infix


Function argument


Function argument

Example 63. lib.strings.hasInfix usage example

hasInfix "bc" "abcd"
=> true
hasInfix "ab" "abcd"
=> true
hasInfix "cd" "abcd"
=> true
hasInfix "foo" "abcd"
=> false

Located at lib/strings.nix:333 in <nixpkgs>.


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.


Function argument

Example 64. lib.strings.stringToCharacters usage example

stringToCharacters ""
=> [ ]
stringToCharacters "abc"
=> [ "a" "b" "c" ]
stringToCharacters "🦄"
=> [ "�" "�" "�" "�" ]

Located at lib/strings.nix:363 in <nixpkgs>.


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

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


Function to map over each individual character


Input string

Example 65. lib.strings.stringAsChars usage example

stringAsChars (x: if x == "a" then "i" else x) "nax"
=> "nix"

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


Type: charToInt :: string -> int

Convert char to ascii value, must be in printable range


Function argument

Example 66. lib.strings.charToInt usage example

charToInt "A"
=> 65
charToInt "("
=> 40

Located at lib/strings.nix:394 in <nixpkgs>.


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

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


Function argument

Example 67. lib.strings.escape usage example

escape ["(" ")"] "(foo)"
=> "\\(foo\\)"

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


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.


Function argument

Example 68. lib.strings.escapeC usage example

escapeC [" "] "foo bar"
=> "foo\\x20bar"

Located at lib/strings.nix:418 in <nixpkgs>.


Type: escapeURL :: string -> string

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

Example 69. lib.strings.escapeURL usage example

escapeURL "foo/bar baz"
=> "foo%2Fbar%20baz"

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


Type: escapeShellArg :: string -> string

Quote string to be used safely within the Bourne shell.


Function argument

Example 70. lib.strings.escapeShellArg usage example

escapeShellArg "esc'ape\nme"
=> "'esc'\\''ape\nme'"

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


Type: escapeShellArgs :: [string] -> string

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

Example 71. lib.strings.escapeShellArgs usage example

escapeShellArgs ["one" "two three" "four'five"]
=> "'one' 'two three' 'four'\\''five'"

Located at lib/strings.nix:453 in <nixpkgs>.


Type: string -> bool

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


Function argument

Example 72. lib.strings.isValidPosixName usage example

isValidPosixName "foo_bar000"
=> true
isValidPosixName "0-bad.jpg"
=> false

Located at lib/strings.nix:465 in <nixpkgs>.


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).


Function argument


Function argument

Example 73. lib.strings.toShellVar usage example

  ${toShellVar "foo" "some string"}
  [[ "$foo" == "some string" ]]

Located at lib/strings.nix:485 in <nixpkgs>.


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

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


Function argument

Example 74. lib.strings.toShellVars usage example

  foo = "value";
  bar = foo;
in ''
  ${toShellVars { inherit foo bar; }}
  [[ "$foo" == "$bar" ]]

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


Type: string -> string

Turn a string into a Nix expression representing that string


Function argument

Example 75. lib.strings.escapeNixString usage example

escapeNixString "hello\${}\n"
=> "\"hello\\\${}\\n\""

Located at lib/strings.nix:523 in <nixpkgs>.


Type: string -> string

Turn a string into an exact regular expression

Example 76. lib.strings.escapeRegex usage example

escapeRegex "[^a-z]*"
=> "\\[\\^a-z]\\*"

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


Type: string -> string

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


Function argument

Example 77. lib.strings.escapeNixIdentifier usage example

escapeNixIdentifier "hello"
=> "hello"
escapeNixIdentifier "0abc"
=> "\"0abc\""

Located at lib/strings.nix:545 in <nixpkgs>.


Type: string -> string

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

Example 78. lib.strings.escapeXML usage example

escapeXML ''"test" 'test' < & >''
=> "&quot;test&quot; &apos;test&apos; &lt; &amp; &gt;"

Located at lib/strings.nix:559 in <nixpkgs>.


Type: toLower :: string -> string

Converts an ASCII string to lower-case.

Example 79. lib.strings.toLower usage example

toLower "HOME"
=> "home"

Located at lib/strings.nix:578 in <nixpkgs>.


Type: toUpper :: string -> string

Converts an ASCII string to upper-case.

Example 80. lib.strings.toUpper usage example

toUpper "home"
=> "HOME"

Located at lib/strings.nix:588 in <nixpkgs>.


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.


Function argument


Function argument

Example 81. lib.strings.addContextFrom usage example

pkgs = import <nixpkgs> { };
addContextFrom pkgs.coreutils "bar"
=> "bar"

Located at lib/strings.nix:603 in <nixpkgs>.


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


Function argument


Function argument

Example 82. lib.strings.splitString usage example

splitString "." ""
=> [ "foo" "bar" "baz" ]
splitString "/" "/usr/local/bin"
=> [ "" "usr" "local" "bin" ]

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


Type: string -> string -> string

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


Prefix to remove if it matches


Input string

Example 83. lib.strings.removePrefix usage example

removePrefix "foo." ""
=> "bar.baz"
removePrefix "xxx" ""
=> ""

Located at lib/strings.nix:630 in <nixpkgs>.


Type: string -> string -> string

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


Suffix to remove if it matches


Input string

Example 84. lib.strings.removeSuffix usage example

removeSuffix "front" "homefront"
=> "home"
removeSuffix "xxx" "homefront"
=> "homefront"

Located at lib/strings.nix:663 in <nixpkgs>.


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


Function argument


Function argument

Example 85. lib.strings.versionOlder usage example

versionOlder "1.1" "1.2"
=> true
versionOlder "1.1" "1.1"
=> false

Located at lib/strings.nix:694 in <nixpkgs>.


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


Function argument


Function argument

Example 86. lib.strings.versionAtLeast usage example

versionAtLeast "1.1" "1.0"
=> true
versionAtLeast "1.1" "1.1"
=> true
versionAtLeast "1.1" "1.2"
=> false

Located at lib/strings.nix:706 in <nixpkgs>.


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


Function argument

Example 87. lib.strings.getName usage example

getName "youtube-dl-2016.01.01"
=> "youtube-dl"
=> "youtube-dl"

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


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


Function argument

Example 88. lib.strings.getVersion usage example

getVersion "youtube-dl-2016.01.01"
=> "2016.01.01"
=> "2016.01.01"

Located at lib/strings.nix:735 in <nixpkgs>.


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


Function argument


Function argument

Example 89. lib.strings.nameFromURL usage example

nameFromURL "" "-"
=> "nix"
nameFromURL "" "_"
=> "nix-1.7-x86"

Located at lib/strings.nix:751 in <nixpkgs>.



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
            the possible values (case insensitive) are:
@param value The desired value

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


Function argument


Function argument


Function argument

Example 90. lib.strings.cmakeOptionType usage example

cmakeOptionType "string" "ENGINE" "sdl2"

Located at lib/strings.nix:774 in <nixpkgs>.



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.


Function argument


Function argument

Example 91. lib.strings.cmakeBool usage example

cmakeBool "ENABLE_STATIC_LIBS" false

Located at lib/strings.nix:793 in <nixpkgs>.



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.


Function argument


Function argument

Example 92. lib.strings.cmakeFeature usage example

cmakeFeature "MODULES" "badblock"
=> "-DMODULES:STRING=badblock"

Located at lib/strings.nix:811 in <nixpkgs>.



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.


Function argument


Function argument

Example 93. lib.strings.mesonOption usage example

mesonOption "engine" "opengl"
=> "-Dengine=opengl"

Located at lib/strings.nix:828 in <nixpkgs>.



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.


Function argument


Function argument

Example 94. lib.strings.mesonBool usage example

mesonBool "hardened" true
=> "-Dhardened=true"
mesonBool "static" false
=> "-Dstatic=false"

Located at lib/strings.nix:847 in <nixpkgs>.



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.


Function argument


Function argument

Example 95. lib.strings.mesonEnable usage example

mesonEnable "docs" true
=> "-Ddocs=enabled"
mesonEnable "savage" false
=> "-Dsavage=disabled"

Located at lib/strings.nix:866 in <nixpkgs>.


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


Function argument


Function argument

Example 96. lib.strings.enableFeature usage example

enableFeature true "shared"
=> "--enable-shared"
enableFeature false "shared"
=> "--disable-shared"

Located at lib/strings.nix:880 in <nixpkgs>.


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


Function argument


Function argument


Function argument

Example 97. lib.strings.enableFeatureAs usage example

enableFeatureAs true "shared" "foo"
=> "--enable-shared=foo"
enableFeatureAs false "shared" (throw "ignored")
=> "--disable-shared"

Located at lib/strings.nix:894 in <nixpkgs>.


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


Function argument


Function argument

Example 98. lib.strings.withFeature usage example

withFeature true "shared"
=> "--with-shared"
withFeature false "shared"
=> "--without-shared"

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


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


Function argument


Function argument


Function argument

Example 99. lib.strings.withFeatureAs usage example

withFeatureAs true "shared" "foo"
=> "--with-shared=foo"
withFeatureAs false "shared" (throw "ignored")
=> "--without-shared"

Located at lib/strings.nix:919 in <nixpkgs>.


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.


Function argument


Function argument


Function argument

Example 100. lib.strings.fixedWidthString usage example

fixedWidthString 5 "0" (toString 15)
=> "00015"

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


Format a number adding leading zeroes up to fixed width.


Function argument


Function argument

Example 101. lib.strings.fixedWidthNumber usage example

fixedWidthNumber 5 15
=> "00015"

Located at lib/strings.nix:951 in <nixpkgs>.


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


Function argument

Example 102. lib.strings.floatToString usage example

floatToString 0.000001
=> "0.000001"
floatToString 0.0000001
=> trace: warning: Imprecise conversion from float to string 0.000000

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


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>.


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.


Function argument

Located at lib/strings.nix:980 in <nixpkgs>.


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.


Function argument

Located at lib/strings.nix:991 in <nixpkgs>.


Check whether a value is a store path.


Function argument

Example 103. lib.strings.isStorePath usage example

isStorePath "/nix/store/d945ibfx9x185xf04b890y4f9g3cbb63-python-2.7.11/bin/python"
=> false
isStorePath "/nix/store/d945ibfx9x185xf04b890y4f9g3cbb63-python-2.7.11"
=> true
isStorePath pkgs.python
=> true
isStorePath [] || isStorePath 42 || isStorePath {} || …
=> false

Located at lib/strings.nix:1009 in <nixpkgs>.


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.


Function argument

Example 104. lib.strings.toInt usage example

toInt "1337"
=> 1337

toInt "-4"
=> -4

toInt " 123 "
=> 123

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

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

Located at lib/strings.nix:1039 in <nixpkgs>.


Type: string -> int

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


Function argument

Example 105. lib.strings.toIntBase10 usage example

toIntBase10 "1337"
=> 1337

toIntBase10 "-4"
=> -4

toIntBase10 " 123 "
=> 123

toIntBase10 "00024"
=> 24

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

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


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

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

Example 106. lib.strings.readPathsFromFile usage example

readPathsFromFile /prefix
=> [ "/prefix/dlopen-resolv.patch" "/prefix/tzdir.patch"
     "/prefix/dlopen-libXcursor.patch" "/prefix/dlopen-openssl.patch"
     "/prefix/dlopen-dbus.patch" "/prefix/xdg-config-dirs.patch"
     "/prefix/compose-search-path.patch" ]

Located at lib/strings.nix:1133 in <nixpkgs>.


Type: fileContents :: path -> string

Read the contents of a file removing the trailing \n


Function argument

Example 107. lib.strings.fileContents usage example

$ echo "1.0" > ./version

fileContents ./version
=> "1.0"

Located at lib/strings.nix:1153 in <nixpkgs>.


Type: sanitizeDerivationName :: String -> String

Creates a valid derivation name from a potentially invalid one.

Example 108. lib.strings.sanitizeDerivationName usage example

sanitizeDerivationName "../ # foo"
=> ""
sanitizeDerivationName ""
=> "unknown"
sanitizeDerivationName pkgs.hello
=> "-nix-store-2g75chlbpxlrqn15zlby2dfh8hr9qwbk-hello-2.10"

Located at lib/strings.nix:1168 in <nixpkgs>.


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


Function argument


Function argument

Example 109. lib.strings.levenshtein usage example

levenshtein "foo" "foo"
=> 0
levenshtein "book" "hook"
=> 1
levenshtein "hello" "Heyo"
=> 3

Located at lib/strings.nix:1207 in <nixpkgs>.


Returns the length of the prefix common to both strings.


Function argument


Function argument

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


Returns the length of the suffix common to both strings.


Function argument


Function argument

Located at lib/strings.nix:1236 in <nixpkgs>.


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

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

Example 110. lib.strings.levenshteinAtMost usage example

levenshteinAtMost 0 "foo" "foo"
=> true
levenshteinAtMost 1 "foo" "boa"
=> false
levenshteinAtMost 2 "foo" "boa"
=> true
levenshteinAtMost 2 "This is a sentence" "this is a sentense."
=> false
levenshteinAtMost 3 "This is a sentence" "this is a sentense."
=> true

Located at lib/strings.nix:1260 in <nixpkgs>.

lib.versions: version string functions

Version string functions.


Break a version string into its component parts.

Example 111. lib.versions.splitVersion usage example

splitVersion "1.2.3"
=> ["1" "2" "3"]

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


Get the major version string from a string.


Function argument

Example 112. lib.versions.major usage example

major "1.2.3"
=> "1"

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


Get the minor version string from a string.


Function argument

Example 113. lib.versions.minor usage example

minor "1.2.3"
=> "2"

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


Get the patch version string from a string.


Function argument

Example 114. lib.versions.patch usage example

patch "1.2.3"
=> "3"

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


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


Function argument

Example 115. lib.versions.majorMinor usage example

majorMinor "1.2.3"
=> "1.2"

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


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


Function argument


Function argument

Example 116. lib.versions.pad usage example

pad 3 "1.2"
=> "1.2.0"
pad 3 "1.3-rc1"
=> "1.3.0-rc1"
pad 3 ""
=> "1.2.3"

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

lib.trivial: miscellaneous functions

Type: id :: a -> a

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


The value to return

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


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.


Value to return


Value to ignore

Example 117. lib.trivial.const usage example

let f = const 5; in f 10
=> 5

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


Type: pipe :: a -> [<functions>] -> <return type of last function>

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


Function argument


Function argument

Example 118. lib.trivial.pipe usage example

pipe 2 [
    (x: x + 2)  # 2 + 2 = 4
    (x: x * 2)  # 4 * 2 = 8
  => 8

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

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

    # concatenate all commands into one string

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

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

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

Located at lib/trivial.nix:61 in <nixpkgs>.


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

Concatenate two lists


Function argument


Function argument

Example 119. lib.trivial.concat usage example

concat [ 1 2 ] [ 3 4 ]
=> [ 1 2 3 4 ]

Located at lib/trivial.nix:80 in <nixpkgs>.


boolean “or”


Function argument


Function argument

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


boolean “and”


Function argument


Function argument

Located at lib/trivial.nix:86 in <nixpkgs>.


bitwise “and”

Located at lib/trivial.nix:89 in <nixpkgs>.


bitwise “or”

Located at lib/trivial.nix:94 in <nixpkgs>.


bitwise “xor”

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


bitwise “not”

Located at lib/trivial.nix:104 in <nixpkgs>.


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!).


Function argument

Located at lib/trivial.nix:114 in <nixpkgs>.


Merge two attribute sets shallowly, right side trumps left

mergeAttrs :: attrs -> attrs -> attrs


Left attribute set


Right attribute set (higher precedence for equal keys)

Example 120. lib.trivial.mergeAttrs usage example

mergeAttrs { a = 1; b = 2; } { b = 3; c = 4; }
=> { a = 1; b = 3; c = 4; }

Located at lib/trivial.nix:124 in <nixpkgs>.


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

Flip the order of the arguments of a binary function.


Function argument


Function argument


Function argument

Example 121. lib.trivial.flip usage example

flip concat [1] [2]
=> [ 2 1 ]

Located at lib/trivial.nix:138 in <nixpkgs>.


Apply function if the supplied argument is non-null.


Function to call


Argument to check for null before passing it to f

Example 122. lib.trivial.mapNullable usage example

mapNullable (x: x+1) null
=> null
mapNullable (x: x+1) 22
=> 23

Located at lib/trivial.nix:148 in <nixpkgs>.


Returns the current full nixpkgs version number.

Located at lib/trivial.nix:164 in <nixpkgs>.


Returns the current nixpkgs release number as string.

Located at lib/trivial.nix:167 in <nixpkgs>.


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:180 in <nixpkgs>.


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


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:186 in <nixpkgs>.


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:198 in <nixpkgs>.


Returns the current nixpkgs version suffix as string.

Located at lib/trivial.nix:201 in <nixpkgs>.


Type: revisionWithDefault :: string -> string

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


Default value to return if revision can not be determined

Located at lib/trivial.nix:212 in <nixpkgs>.


Type: inNixShell :: bool

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

Located at lib/trivial.nix:230 in <nixpkgs>.


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:238 in <nixpkgs>.


Return minimum of two numbers.


Function argument


Function argument

Located at lib/trivial.nix:243 in <nixpkgs>.


Return maximum of two numbers.


Function argument


Function argument

Located at lib/trivial.nix:246 in <nixpkgs>.


Integer modulus


Function argument


Function argument

Example 123. lib.trivial.mod usage example

mod 11 10
=> 1
mod 1 10
=> 1

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

C-style comparisons

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


Function argument


Function argument

Located at lib/trivial.nix:267 in <nixpkgs>.


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.




Comparison function if predicate holds for both values


Comparison function if predicate holds for neither value


First value to compare


Second value to compare

Example 124. lib.trivial.splitByAndCompare usage example

let cmp = splitByAndCompare (hasPrefix "foo") compare compare; in

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

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

Located at lib/trivial.nix:292 in <nixpkgs>.


Type: importJSON :: path -> any

Reads a JSON file.


Function argument

Located at lib/trivial.nix:312 in <nixpkgs>.


Type: importTOML :: path -> any

Reads a TOML file.


Function argument

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


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:347 in <nixpkgs>.


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

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


Function argument


Function argument

Located at lib/trivial.nix:357 in <nixpkgs>.


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

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


Function argument


Function argument

Located at lib/trivial.nix:364 in <nixpkgs>.


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.


Function argument


Function argument

Example 125. lib.trivial.throwIfNot usage example

throwIfNot (lib.isList overlays) "The overlays argument to nixpkgs must be a list."
lib.foldr (x: throwIfNot (lib.isFunction x) "All overlays passed to nixpkgs must be functions.") (r: r) overlays

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


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

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


Function argument


Function argument

Located at lib/trivial.nix:393 in <nixpkgs>.


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.


Function argument


Function argument


Function argument

Example 126. lib.trivial.checkListOfEnum usage example

let colorVariants = ["bright" "dark" "black"]
in checkListOfEnum "color variants" [ "standard" "light" "dark" ] colorVariants;
error: color variants: bright, black unexpected; valid ones: standard, light, dark

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


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.


Function argument


Function argument

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


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.


Function argument

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


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


Function argument

Located at lib/trivial.nix:448 in <nixpkgs>.


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).


Function to provide the argument metadata

Example 127. lib.trivial.mirrorFunctionArgs usage example

addab = {a, b}: a + b
addab { a = 2; b = 4; }
=> 6
lib.functionArgs addab
=> { a = false; b = false; }
addab1 = attrs: addab attrs + 1
addab1 { a = 2; b = 4; }
=> 7
lib.functionArgs addab1
=> { }
addab1' = lib.mirrorFunctionArgs addab addab1
addab1' { a = 2; b = 4; }
=> 7
lib.functionArgs addab1'
=> { a = false; b = false; }

Located at lib/trivial.nix:475 in <nixpkgs>.


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


Any value

Example 128. lib.trivial.toFunction usage example

nix-repl> lib.toFunction 1 2

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

Located at lib/trivial.nix:497 in <nixpkgs>.


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

toHexString 0 => “0”

toHexString 16 => “10”

toHexString 250 => “FA”


Function argument

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


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 ]


Function argument


Function argument

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

lib.fixedPoints: explicit recursion functions


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 = +;
}; 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 = +;

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.


Function argument

Example 129. lib.fixedPoints.fix usage example

fix (self: { foo = "foo"; bar = "bar"; foobar = +; })
=> { bar = "bar"; foo = "foo"; foobar = "foobar"; }

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

Located at lib/fixed-points.nix:75 in <nixpkgs>.


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.


Function argument

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


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

Function argument


Function argument

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


Modify the contents of an explicitly recursive attribute set in a way that honors self-references. This is accomplished with a function

g = self: super: { foo = + " + "; }

that has access to the unmodified input (super) as well as the final non-recursive representation of the attribute set (self). extends differs from the native // operator insofar as that it’s applied before references to self are resolved:

nix-repl> fix (extends g f)
{ bar = "bar"; foo = "foo + "; foobar = "foo + bar"; }

The name of the function is inspired by object-oriented inheritance, i.e. think of it as an infix operator g extends f that mimics the syntax from Java. It may seem counter-intuitive to have the “base class” as the second argument, but it’s nice this way if several uses of extends are cascaded.

To get a better understanding how extends turns a function with a fix point (the package set we start with) into a new function with a different fix point (the desired packages set) lets just see, how extends g f unfolds with g and f defined above:

extends g f = self: let super = f self; in super // g self super;
            = self: let super = { foo = "foo"; bar = "bar"; foobar = +; }; in super // g self super
            = self: { foo = "foo"; bar = "bar"; foobar = +; } // g self { foo = "foo"; bar = "bar"; foobar = +; }
            = self: { foo = "foo"; bar = "bar"; foobar = +; } // { foo = "foo" + " + "; }
            = self: { foo = "foo + "; bar = "bar"; foobar = +; }

Function argument


Function argument


Function argument

Located at lib/fixed-points.nix:141 in <nixpkgs>.


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


Function argument


Function argument


Function argument


Function argument

Located at lib/fixed-points.nix:148 in <nixpkgs>.


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:164 in <nixpkgs>.


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 = + " + "; bar = "bar"; foobar = +; })

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

Located at lib/fixed-points.nix:187 in <nixpkgs>.


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


Function argument


Function argument

Located at lib/fixed-points.nix:193 in <nixpkgs>.

lib.lists: list manipulation functions

General list operations.


Type: singleton :: a -> [a]

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.


Function argument

Example 130. lib.lists.singleton usage example

singleton "foo"
=> [ "foo" ]

Located at lib/lists.nix:23 in <nixpkgs>.


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

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


Function argument


Function argument

Example 131. lib.lists.forEach usage example

forEach [ 1 2 ] (x:
  toString x
=> [ "1" "2" ]

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


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

“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)).


Function argument


Function argument


Function argument

Example 132. lib.lists.foldr usage example

concat = foldr (a: b: a + b) "z"
concat [ "a" "b" "c" ]
=> "abcz"
# different types
strange = foldr (int: str: toString (int + 1) + str) "a"
strange [ 1 2 3 4 ]
=> "2345a"

Located at lib/lists.nix:53 in <nixpkgs>.


fold is an alias of foldr for historic reasons

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


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

“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).


Function argument


Function argument


Function argument

Example 133. lib.lists.foldl usage example

lconcat = foldl (a: b: a + b) "z"
lconcat [ "a" "b" "c" ]
=> "zabc"
# different types
lstrange = foldl (str: int: str + toString (int + 1)) "a"
lstrange [ 1 2 3 4 ]
=> "a2345"

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


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

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.

  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  );

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

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


The initial accumulator value


The list to fold

Example 134. lib.lists.foldl' usage example

foldl' (acc: x: acc + x) 0 [1 2 3]
=> 6

Located at lib/lists.nix:129 in <nixpkgs>.


Type: imap0 :: (int -> a -> b) -> [a] -> [b]

Map with index starting from 0


Function argument


Function argument

Example 135. lib.lists.imap0 usage example

imap0 (i: v: "${v}-${toString i}") ["a" "b"]
=> [ "a-0" "b-1" ]

Located at lib/lists.nix:155 in <nixpkgs>.


Type: imap1 :: (int -> a -> b) -> [a] -> [b]

Map with index starting from 1


Function argument


Function argument

Example 136. lib.lists.imap1 usage example

imap1 (i: v: "${v}-${toString i}") ["a" "b"]
=> [ "a-1" "b-2" ]

Located at lib/lists.nix:165 in <nixpkgs>.


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

Map and concatenate the result.

Example 137. lib.lists.concatMap usage example

concatMap (x: [x] ++ ["z"]) ["a" "b"]
=> [ "a" "z" "b" "z" ]

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


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


Function argument

Example 138. lib.lists.flatten usage example

flatten [1 [2 [3] 4] 5]
=> [1 2 3 4 5]
flatten 1
=> [1]

Located at lib/lists.nix:186 in <nixpkgs>.


Type: remove :: a -> [a] -> [a]

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


Element to remove from the list

Example 139. lib.lists.remove usage example

remove 3 [ 1 3 4 3 ]
=> [ 1 4 ]

Located at lib/lists.nix:199 in <nixpkgs>.


Type: findSingle :: (a -> bool) -> a -> a -> [a] -> a

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.




Default value to return if element was not found.


Default value to return if more than one element was found


Input list

Example 140. lib.lists.findSingle usage example

findSingle (x: x == 3) "none" "multiple" [ 1 3 3 ]
=> "multiple"
findSingle (x: x == 3) "none" "multiple" [ 1 3 ]
=> 3
findSingle (x: x == 3) "none" "multiple" [ 1 9 ]
=> "none"

Located at lib/lists.nix:217 in <nixpkgs>.


Type: findFirstIndex :: (a -> Bool) -> b -> [a] -> (Int | b)

Find the first index in the list matching the specified predicate or return default if no such element exists.




Default value to return


Input list

Example 141. lib.lists.findFirstIndex usage example

findFirstIndex (x: x > 3) null [ 0 6 4 ]
=> 1
findFirstIndex (x: x > 9) null [ 0 6 4 ]
=> null

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


Type: findFirst :: (a -> bool) -> a -> [a] -> a

Find the first element in the list matching the specified predicate or return default if no such element exists.




Default value to return


Input list

Example 142. lib.lists.findFirst usage example

findFirst (x: x > 3) 7 [ 1 6 4 ]
=> 6
findFirst (x: x > 9) 7 [ 1 6 4 ]
=> 7

Located at lib/lists.nix:293 in <nixpkgs>.


Type: any :: (a -> bool) -> [a] -> bool

Return true if function pred returns true for at least one element of list.

Example 143. lib.lists.any usage example

any isString [ 1 "a" { } ]
=> true
any isString [ 1 { } ]
=> false

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


Type: all :: (a -> bool) -> [a] -> bool

Return true if function pred returns true for all elements of list.

Example 144. lib.lists.all usage example

all (x: x < 3) [ 1 2 ]
=> true
all (x: x < 3) [ 1 2 3 ]
=> false

Located at lib/lists.nix:332 in <nixpkgs>.


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

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



Example 145. lib.lists.count usage example

count (x: x == 3) [ 3 2 3 4 6 ]
=> 2

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


Type: optional :: bool -> a -> [a]

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).


Function argument


Function argument

Example 146. lib.lists.optional usage example

optional true "foo"
=> [ "foo" ]
optional false "foo"
=> [ ]

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


Type: optionals :: bool -> [a] -> [a]

Return a list or an empty list, depending on a boolean value.




List to return if condition is true

Example 147. lib.lists.optionals usage example

optionals true [ 2 3 ]
=> [ 2 3 ]
optionals false [ 2 3 ]
=> [ ]

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


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.


Function argument

Example 148. lib.lists.toList usage example

toList [ 1 2 ]
=> [ 1 2 ]
toList "hi"
=> [ "hi "]

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


Type: range :: int -> int -> [int]

Return a list of integers from first up to and including last.


First integer in the range


Last integer in the range

Example 149. lib.lists.range usage example

range 2 4
=> [ 2 3 4 ]
range 3 2
=> [ ]

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


Type: replicate :: int -> a -> [a]

Return a list with n copies of an element.


Function argument


Function argument

Example 150. lib.lists.replicate usage example

replicate 3 "a"
=> [ "a" "a" "a" ]
replicate 2 true
=> [ true true ]

Located at lib/lists.nix:420 in <nixpkgs>.


Type: (a -> bool) -> [a] -> { right :: [a]; wrong :: [a]; }

Splits the elements of a list in two lists, right and wrong, depending on the evaluation of a predicate.

Example 151. lib.lists.partition usage example

partition (x: x > 2) [ 5 1 2 3 4 ]
=> { right = [ 5 3 4 ]; wrong = [ 1 2 ]; }

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


Splits the elements of a list into many lists, using the return value of a predicate. Predicate should return a string which becomes keys of attrset groupBy returns.

groupBy' allows to customise the combining function and initial value


Function argument


Function argument


Function argument


Function argument

Example 152. lib.lists.groupBy' usage example

groupBy (x: boolToString (x > 2)) [ 5 1 2 3 4 ]
=> { true = [ 5 3 4 ]; false = [ 1 2 ]; }
groupBy (x: [ {name = "icewm"; script = "icewm &";}
                      {name = "xfce";  script = "xfce4-session &";}
                      {name = "icewm"; script = "icewmbg &";}
                      {name = "mate";  script = "gnome-session &";}
=> { icewm = [ { name = "icewm"; script = "icewm &"; }
               { name = "icewm"; script = "icewmbg &"; } ];
     mate  = [ { name = "mate";  script = "gnome-session &"; } ];
     xfce  = [ { name = "xfce";  script = "xfce4-session &"; } ];

groupBy' builtins.add 0 (x: boolToString (x > 2)) [ 5 1 2 3 4 ]
=> { true = 12; false = 3; }

Located at lib/lists.nix:460 in <nixpkgs>.


Type: zipListsWith :: (a -> b -> c) -> [a] -> [b] -> [c]

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.


Function to zip elements of both lists


First list


Second list

Example 153. lib.lists.zipListsWith usage example

zipListsWith (a: b: a + b) ["h" "l"] ["e" "o"]
=> ["he" "lo"]

Located at lib/lists.nix:480 in <nixpkgs>.


Type: zipLists :: [a] -> [b] -> [{ fst :: a; snd :: b; }]

Merges two lists of the same size together. If the sizes aren’t the same the merging stops at the shortest.

Example 154. lib.lists.zipLists usage example

zipLists [ 1 2 ] [ "a" "b" ]
=> [ { fst = 1; snd = "a"; } { fst = 2; snd = "b"; } ]

Located at lib/lists.nix:499 in <nixpkgs>.


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

Reverse the order of the elements of a list.


Function argument

Example 155. lib.lists.reverseList usage example

reverseList [ "b" "o" "j" ]
=> [ "j" "o" "b" ]

Located at lib/lists.nix:510 in <nixpkgs>.


Depth-First Search (DFS) for lists list != [].

before a b == true means that b depends on a (there’s an edge from b to a).


Function argument


Function argument


Function argument

Example 156. lib.lists.listDfs usage example

listDfs true hasPrefix [ "/home/user" "other" "/" "/home" ]
  == { minimal = "/";                  # minimal element
       visited = [ "/home/user" ];     # seen elements (in reverse order)
       rest    = [ "/home" "other" ];  # everything else

listDfs true hasPrefix [ "/home/user" "other" "/" "/home" "/" ]
  == { cycle   = "/";                  # cycle encountered at this element
       loops   = [ "/" ];              # and continues to these elements
       visited = [ "/" "/home/user" ]; # elements leading to the cycle (in reverse order)
       rest    = [ "/home" "other" ];  # everything else

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


Sort a list based on a partial ordering using DFS. This implementation is O(N^2), if your ordering is linear, use sort instead.

before a b == true means that b should be after a in the result.


Function argument


Function argument

Example 157. lib.lists.toposort usage example

toposort hasPrefix [ "/home/user" "other" "/" "/home" ]
  == { result = [ "/" "/home" "/home/user" "other" ]; }

toposort hasPrefix [ "/home/user" "other" "/" "/home" "/" ]
  == { cycle = [ "/home/user" "/" "/" ]; # path leading to a cycle
       loops = [ "/" ]; }                # loops back to these elements

toposort hasPrefix [ "other" "/home/user" "/home" "/" ]
  == { result = [ "other" "/" "/home" "/home/user" ]; }

toposort (a: b: a < b) [ 3 2 1 ] == { result = [ 1 2 3 ]; }

Located at lib/lists.nix:571 in <nixpkgs>.


Type: sort :: (a -> a -> Bool) -> [a] -> [a]

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.

Example 158. lib.lists.sort usage example

sort (p: q: p < q) [ 5 3 7 ]
=> [ 3 5 7 ]

Located at lib/lists.nix:605 in <nixpkgs>.


Type: sortOn :: (a -> b) -> [a] -> [a], for comparable b

Sort a list based on the default comparison of a derived property b.

The items are returned in b-increasing order.


The passed function f is only evaluated once per item, unlike an unprepared sort using f p < f q.


sortOn f == sort (p: q: f p < f q)

Function argument


Function argument

Example 159. lib.lists.sortOn usage example

sortOn stringLength [ "aa" "b" "cccc" ]
=> [ "b" "aa" "cccc" ]

Located at lib/lists.nix:645 in <nixpkgs>.


Compare two lists element-by-element.


Function argument


Function argument


Function argument

Example 160. lib.lists.compareLists usage example

compareLists compare [] []
=> 0
compareLists compare [] [ "a" ]
=> -1
compareLists compare [ "a" ] []
=> 1
compareLists compare [ "a" "b" ] [ "a" "c" ]
=> -1

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


Sort list using “Natural sorting”. Numeric portions of strings are sorted in numeric order.


Function argument

Example 161. lib.lists.naturalSort usage example

naturalSort ["disk11" "disk8" "disk100" "disk9"]
=> ["disk8" "disk9" "disk11" "disk100"]
naturalSort ["" "" ""]
=> ["" "" ""]
naturalSort ["v0.2" "v0.15" "v0.0.9"]
=> [ "v0.0.9" "v0.2" "v0.15" ]

Located at lib/lists.nix:693 in <nixpkgs>.


Type: take :: int -> [a] -> [a]

Return the first (at most) N elements of a list.


Number of elements to take

Example 162. lib.lists.take usage example

take 2 [ "a" "b" "c" "d" ]
=> [ "a" "b" ]
take 2 [ ]
=> [ ]

Located at lib/lists.nix:711 in <nixpkgs>.


Type: drop :: int -> [a] -> [a]

Remove the first (at most) N elements of a list.


Number of elements to drop


Input list

Example 163. lib.lists.drop usage example

drop 2 [ "a" "b" "c" "d" ]
=> [ "c" "d" ]
drop 2 [ ]
=> [ ]

Located at lib/lists.nix:725 in <nixpkgs>.


Type: hasPrefix :: [a] -> [a] -> bool

Whether the first list is a prefix of the second list.


Function argument


Function argument

Example 164. lib.lists.hasPrefix usage example

hasPrefix [ 1 2 ] [ 1 2 3 4 ]
=> true
hasPrefix [ 0 1 ] [ 1 2 3 4 ]
=> false

Located at lib/lists.nix:741 in <nixpkgs>.


Type: removePrefix :: [a] -> [a] -> [a]

Remove the first list as a prefix from the second list. Error if the first list isn’t a prefix of the second list.


Function argument


Function argument

Example 165. lib.lists.removePrefix usage example

removePrefix [ 1 2 ] [ 1 2 3 4 ]
=> [ 3 4 ]
removePrefix [ 0 1 ] [ 1 2 3 4 ]
=> <error>

Located at lib/lists.nix:757 in <nixpkgs>.


Type: sublist :: int -> int -> [a] -> [a]

Return a list consisting of at most count elements of list, starting at index start.


Index at which to start the sublist


Number of elements to take


Input list

Example 166. lib.lists.sublist usage example

sublist 1 3 [ "a" "b" "c" "d" "e" ]
=> [ "b" "c" "d" ]
sublist 1 3 [ ]
=> [ ]

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


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

The common prefix of two lists.


Function argument


Function argument

Example 167. lib.lists.commonPrefix usage example

commonPrefix [ 1 2 3 4 5 6 ] [ 1 2 4 8 ]
=> [ 1 2 ]
commonPrefix [ 1 2 3 ] [ 1 2 3 4 5 ]
=> [ 1 2 3 ]
commonPrefix [ 1 2 3 ] [ 4 5 6 ]
=> [ ]

Located at lib/lists.nix:802 in <nixpkgs>.


Type: last :: [a] -> a

Return the last element of a list.

This function throws an error if the list is empty.


Function argument

Example 168. lib.lists.last usage example

last [ 1 2 3 ]
=> 3

Located at lib/lists.nix:826 in <nixpkgs>.


Type: init :: [a] -> [a]

Return all elements but the last.

This function throws an error if the list is empty.


Function argument

Example 169. lib.lists.init usage example

init [ 1 2 3 ]
=> [ 1 2 ]

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


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

Example 170. lib.lists.crossLists usage example

crossLists (x:y: "${toString x}${toString y}") [[1 2] [3 4]]
=> [ "13" "14" "23" "24" ]

Located at lib/lists.nix:851 in <nixpkgs>.


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

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

Example 171. lib.lists.unique usage example

unique [ 3 2 3 4 ]
=> [ 3 2 4 ]

Located at lib/lists.nix:864 in <nixpkgs>.


Type: allUnique :: [a] -> bool

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


Function argument

Example 172. lib.lists.allUnique usage example

allUnique [ 3 2 3 4 ]
=> false
allUnique [ 3 2 4 1 ]
=> true

Located at lib/lists.nix:876 in <nixpkgs>.


Intersects list ‘e’ and another list. O(nm) complexity.


Function argument

Example 173. lib.lists.intersectLists usage example

intersectLists [ 1 2 3 ] [ 6 3 2 ]
=> [ 3 2 ]

Located at lib/lists.nix:885 in <nixpkgs>.


Subtracts list ‘e’ from another list. O(nm) complexity.


Function argument

Example 174. lib.lists.subtractLists usage example

subtractLists [ 3 2 ] [ 1 2 3 4 5 3 ]
=> [ 1 4 5 ]

Located at lib/lists.nix:893 in <nixpkgs>.


Test if two lists have no common element. It should be slightly more efficient than (intersectLists a b == [])


Function argument


Function argument

Located at lib/lists.nix:898 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.


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

Conditionally trace the supplied message, based on a predicate.


Predicate to check


Message that should be traced


Value to return

Example 175. lib.debug.traceIf usage example

traceIf true "hello" 3
trace: hello
=> 3

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


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

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


Function to apply


Value to trace and return

Example 176. lib.debug.traceValFn usage example

traceValFn (v: "mystring ${v}") "foo"
trace: mystring foo
=> "foo"

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


Type: traceVal :: a -> a

Trace the supplied value and return it.

Example 177. lib.debug.traceVal usage example

traceVal 42
# trace: 42
=> 42

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


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

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


The value to trace


The value to return

Example 178. lib.debug.traceSeq usage example

trace { a.b.c = 3; } null
trace: { a = <CODE>; }
=> null
traceSeq { a.b.c = 3; } null
trace: { a = { b = { c = 3; }; }; }
=> null

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


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.


Function argument


Function argument


Function argument

Example 179. lib.debug.traceSeqN usage example

traceSeqN 2 { a.b.c = 3; } null
trace: { a = { b = {…}; }; }
=> null

Located at lib/debug.nix:108 in <nixpkgs>.


A combination of traceVal and traceSeq that applies a provided function to the value to be traced after deepSeqing it.


Function to apply


Value to trace

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


A combination of traceVal and traceSeq.

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


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


Function to apply


Function argument


Value to trace

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


A combination of traceVal and traceSeqN.

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


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.


Function argument


Function argument


Function argument


Function argument

Example 180. lib.debug.traceFnSeqN usage example

traceFnSeqN 2 "id" (x: x) { a.b.c = 3; }
trace: { fn = "id"; from = { a.b = {…}; }; to = { a.b = {…}; }; }
=> { a.b.c = 3; }

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



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 to run

Example 181. lib.debug.runTests usage example

runTests {
  testAndOk = {
    expr = lib.and true false;
    expected = false;
  testAndFail = {
    expr = lib.and true false;
    expected = true;
    name = "testAndFail";
    expected = true;
    result = false;

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


Create a test assuming that list elements are true.


Function argument

Example 182. lib.debug.testAllTrue usage example

{ testX = allTrue [ true ]; }

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

lib.options: NixOS / nixpkgs option handling

Nixpkgs/NixOS option handling.


Type: isOption :: a -> bool

Returns true when the given argument is an option

Example 183. lib.options.isOption usage example

isOption 1             // => false
isOption (mkOption {}) // => true

Located at lib/options.nix:56 in <nixpkgs>.


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 value used when no definition is given in the configuration.


Textual representation of the default, for the manual.


Example value used in the manual.


String describing the option.


Related packages used in the manual (see genRelatedPackages in …/nixos/lib/make-options-doc/default.nix).


Option type, providing type-checking and value merging.


Function that converts the option value to something else.


Whether the option is for NixOS developers only.


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.


Whether the option can be set only once

Example 184. lib.options.mkOption usage example

mkOption { }  // => { _type = "option"; }
mkOption { default = "foo"; } // => { _type = "option"; default = "foo"; }

Located at lib/options.nix:66 in <nixpkgs>.


Creates an Option attribute set for a boolean value option i.e an option to be toggled on or off:


Name for the created option

Example 185. lib.options.mkEnableOption usage example

mkEnableOption "foo"
=> { _type = "option"; default = false; description = "Whether to enable foo."; example = true; type = { ... }; }

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


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.


Package set (an instantiation of nixpkgs such as pkgs in modules or another package set)


Name for the package, shown in option description

structured function argument

Whether the package can be null, for example to disable installing a package altogether (defaults to false)


The attribute path where the default package is located (may be omitted, in which case it is copied from name)


A string or an attribute path to use as an example (may be omitted)


Additional text to include in the option description (may be omitted)


Representation of the package set passed as pkgs (defaults to "pkgs")

Example 186. 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>.


Alias of mkPackageOption. Previously used to create options with markdown documentation, which is no longer required.

Located at lib/options.nix:226 in <nixpkgs>.


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.


Function argument

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


“Merge” option definitions by checking that they all have the same value.


Function argument


Function argument

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


Type: getValues :: [ { value :: a; } ] -> [a]

Extracts values of all “value” keys of the given list.

Example 187. lib.options.getValues usage example

getValues [ { value = 1; } { value = 2; } ] // => [ 1 2 ]
getValues [ ]                               // => [ ]

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


Type: getFiles :: [ { file :: a; } ] -> [a]

Extracts values of all “file” keys of the given list

Example 188. lib.options.getFiles usage example

getFiles [ { file = "file1"; } { file = "file2"; } ] // => [ "file1" "file2" ]
getFiles [ ]                                         // => [ ]

Located at lib/options.nix:296 in <nixpkgs>.


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.


Function argument

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


Ensures that the given option value (default or example) is a _typed string by rendering Nix values to literalExpressions.


Function argument

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


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.


Function argument

Located at lib/options.nix:378 in <nixpkgs>.


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:387 in <nixpkgs>.


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.


Function argument

Located at lib/options.nix:393 in <nixpkgs>.


Convert an option, described as a list of the option parts to a human-readable version.


Function argument

Example 189. lib.options.showOption usage example

(showOption ["foo" "bar" "baz"]) == ""
  (showOption ["foo" "bar.baz" "tux"]) == "foo.\"bar.baz\".tux"
  (showOption ["windowManager" "2bwm" "enable"]) == "windowManager.\"2bwm\".enable"

Placeholders will not be quoted as they are not actual values:
  (showOption ["foo" "*" "bar"]) == "foo.*.bar"
  (showOption ["foo" "<name>" "bar"]) == "foo.<name>.bar"

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

lib.path: path functions

Functions for working with path values.


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.


  • Not influenced by subpath normalisation:

    append p s == append p (subpath.normalise s)

The absolute path to append to


The subpath string to append

Example 190. lib.path.append usage example

append /foo "bar/baz"
=> /foo/bar/baz

# subpaths don't need to be normalised
append /foo "./bar//baz/./"
=> /foo/bar/baz

# can append to root directory
append /. "foo/bar"
=> /foo/bar

# first argument needs to be a path value type
append "/foo" "bar"
=> <error>

# second argument needs to be a valid subpath string
append /foo /bar
=> <error>
append /foo ""
=> <error>
append /foo "/bar"
=> <error>
append /foo "../bar"
=> <error>

Located at lib/path/default.nix:192 in <nixpkgs>.


Type: hasPrefix :: Path -> Path -> Bool

Whether the first path is a component-wise prefix of the second path.



Function argument

Example 191. lib.path.hasPrefix usage example

hasPrefix /foo /foo/bar
=> true
hasPrefix /foo /foo
=> true
hasPrefix /foo/bar /foo
=> false
hasPrefix /. /foo
=> true

Located at lib/path/default.nix:226 in <nixpkgs>.


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.



Function argument

Example 192. lib.path.removePrefix usage example

removePrefix /foo /foo/bar/baz
=> "./bar/baz"
removePrefix /foo /foo
=> "./."
removePrefix /foo/bar /foo
=> <error>
removePrefix /. /foo
=> "./foo"

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


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.


  • Appending the root and subpath gives the original path:

    p ==
        (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

The path to split the root off of

Example 193. lib.path.splitRoot usage example

splitRoot /foo/bar
=> { root = /.; subpath = "./foo/bar"; }

splitRoot /.
=> { root = /.; subpath = "./."; }

# Nix neutralises `..` path components for all path values automatically
splitRoot /foo/../bar
=> { root = /.; subpath = "./bar"; }

splitRoot "/foo/bar"
=> <error>

Located at lib/path/default.nix:336 in <nixpkgs>.


Type: hasStorePathPrefix :: Path -> Bool

Whether a path has a store path as a prefix.


As with all functions of this lib.path library, it does not work on paths in strings, which is how you’d typically get store paths.

Instead, this function only handles path values themselves, which occur when Nix files in the store use relative path expressions.


Function argument

Example 194. lib.path.hasStorePathPrefix usage example

# Subpaths of derivation outputs have a store path as a prefix
hasStorePathPrefix /nix/store/nvl9ic0pj1fpyln3zaqrf4cclbqdfn1j-foo/bar/baz
=> true

# The store directory itself is not a store path
hasStorePathPrefix /nix/store
=> false

# Derivation outputs are store paths themselves
hasStorePathPrefix /nix/store/nvl9ic0pj1fpyln3zaqrf4cclbqdfn1j-foo
=> true

# Paths outside the Nix store don't have a store path prefix
hasStorePathPrefix /home/user
=> false

# Not all paths under the Nix store are store paths
hasStorePathPrefix /nix/store/.links/10gg8k3rmbw8p7gszarbk7qyd9jwxhcfq9i6s5i0qikx8alkk4hq
=> false

# Store derivations are also store paths themselves
hasStorePathPrefix /nix/store/nvl9ic0pj1fpyln3zaqrf4cclbqdfn1j-foo.drv
=> true

Located at lib/path/default.nix:390 in <nixpkgs>.


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.


The value to check

Example 195. lib.path.subpath.isValid usage example

# Not a string
subpath.isValid null
=> false

# Empty string
subpath.isValid ""
=> false

# Absolute path
subpath.isValid "/foo"
=> false

# Contains a `..` path component
subpath.isValid "../foo"
=> false

# Valid subpath
subpath.isValid "foo/bar"
=> true

# Doesn't need to be normalised
subpath.isValid "./foo//bar/"
=> true

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


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 "./.".


  • 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)

The list of subpaths to join together

Example 196. lib.path.subpath.join usage example

subpath.join [ "foo" "bar/baz" ]
=> "./foo/bar/baz"

# normalise the result
subpath.join [ "./foo" "." "bar//./baz/" ]
=> "./foo/bar/baz"

# passing an empty list results in the current directory
subpath.join [ ]
=> "./."

# elements must be valid subpath strings
subpath.join [ /foo ]
=> <error>
subpath.join [ "" ]
=> <error>
subpath.join [ "/foo" ]
=> <error>
subpath.join [ "../foo" ]
=> <error>

Located at lib/path/default.nix:510 in <nixpkgs>.


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.


  • Splitting a subpath into components and joining the components gives the same subpath but normalised:

    subpath.join (subpath.components s) == subpath.normalise s

The subpath string to split into components

Example 197. lib.path.subpath.components usage example

subpath.components "."
=> [ ]

subpath.components "./foo//bar/./baz/"
=> [ "foo" "bar" "baz" ]

subpath.components "/foo"
=> <error>

Located at lib/path/default.nix:552 in <nixpkgs>.


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 ./.


  • 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

The subpath string to normalise

Example 198. lib.path.subpath.normalise usage example

# limit repeating `/` to a single one
subpath.normalise "foo//bar"
=> "./foo/bar"

# remove redundant `.` components
subpath.normalise "foo/./bar"
=> "./foo/bar"

# add leading `./`
subpath.normalise "foo/bar"
=> "./foo/bar"

# remove trailing `/`
subpath.normalise "foo/bar/"
=> "./foo/bar"

# remove trailing `/.`
subpath.normalise "foo/bar/."
=> "./foo/bar"

# Return the current directory as `./.`
subpath.normalise "."
=> "./."

# error on `..` path components
subpath.normalise "foo/../bar"
=> <error>

# error on empty string
subpath.normalise ""
=> <error>

# error on absolute path
subpath.normalise "/foo"
=> <error>

Located at lib/path/default.nix:633 in <nixpkgs>.

lib.filesystem: filesystem functions

Functions for querying information about the filesystem without copying any files to the Nix store.


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.

Example 199. lib.filesystem.pathType usage example

pathType /.
=> "directory"

pathType /some/file.nix
=> "regular"

Located at lib/filesystem.nix:35 in <nixpkgs>.


Type: pathIsDirectory :: Path -> Bool

Whether a path exists and is a directory.


Function argument

Example 200. lib.filesystem.pathIsDirectory usage example

pathIsDirectory /.
=> true

pathIsDirectory /this/does/not/exist
=> false

pathIsDirectory /some/file.nix
=> false

Located at lib/filesystem.nix:67 in <nixpkgs>.


Type: pathIsRegularFile :: Path -> Bool

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


Function argument

Example 201. lib.filesystem.pathIsRegularFile usage example

pathIsRegularFile /.
=> false

pathIsRegularFile /this/does/not/exist
=> false

pathIsRegularFile /some/file.nix
=> true

Located at lib/filesystem.nix:86 in <nixpkgs>.


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.


The directory within to search

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


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’.


The pattern to search for


The file to start searching upward from

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


Type: Path -> [ Path ]

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


The path to recursively list

Located at lib/filesystem.nix:147 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.






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.


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.


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.


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.


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.


Function argument

Example 202. lib.fileset.maybeMissing usage example

# All files in the current directory, but excluding main.o if it exists
difference ./. (maybeMissing ./main.o)

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


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.


The file set to trace.

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

Example 203. lib.fileset.trace usage example

trace (unions [ ./Makefile ./src ./tests/ ]) null
trace: /home/user/src/myProject
trace: - Makefile (regular)
trace: - src (all files in directory)
trace: - tests
trace:   - (regular)

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


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.


The file set to trace and return.

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

Example 204. lib.fileset.traceVal usage example

toSource {
  root = ./.;
  fileset = traceVal (unions [
trace: /home/user/src/myProject
trace: - Makefile (regular)
trace: - src (all files in directory)
trace: - tests
trace:   - (regular)

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



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

(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.


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.


(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.


If a directory does not recursively contain any file, it is omitted from the store path contents.

Example 205. lib.fileset.toSource usage example

# Import the current directory into the store
# but only include files under ./src
toSource {
  root = ./.;
  fileset = ./src;
=> "/nix/store/...-source"

# Import the current directory into the store
# but only include ./Makefile and all files under ./src
toSource {
  root = ./.;
  fileset = union
=> "/nix/store/...-source"

# Trying to include a file outside the root will fail
toSource {
  root = ./.;
  fileset = unions [
=> <error>

# The root needs to point to a directory that contains all the files
toSource {
  root = ../.;
  fileset = unions [
=> "/nix/store/...-source"

# The root has to be a local filesystem path
toSource {
  root = "/nix/store/...-source";
  fileset = ./.;
=> <error>

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


Type: toList :: FileSet -> [ Path ]

The list of file paths contained in the given file set.


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.


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

Example 206. lib.fileset.toList usage example

toList ./.
[ ./ ./Makefile ./src/main.c ./src/main.h ]

toList (difference ./. ./src)
[ ./ ./Makefile ]

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


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.


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


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

Example 207. lib.fileset.union usage example

# Create a file set containing the file `Makefile`
# and all files recursively in the `src` directory
union ./Makefile ./src

# Create a file set containing the file `Makefile`
# and the LICENSE file from the parent directory
union ./Makefile ../LICENSE

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


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.


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

Example 208. lib.fileset.unions usage example

# Create a file set containing selected files
unions [
  # Include the single file `Makefile` in the current directory
  # This errors if the file doesn't exist

  # Recursively include all files in the `src/code` directory
  # If this directory is empty this has no effect

  # Include the files `` and `unit.c` from the `tests` directory

  # Include the `LICENSE` file from the parent directory

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


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.


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


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

Example 209. lib.fileset.intersection usage example

# Limit the selected files to the ones in ./., so only ./src and ./Makefile
intersection ./. (unions [ ../LICENSE ./src ./Makefile ])

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


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.


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.


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.

Example 210. lib.fileset.difference usage example

# Create a file set containing all files from the current directory,
# except ones under ./tests
difference ./. ./tests

  # A set of Nix-related files
  nixFiles = unions [ ./default.nix ./nix ./tests/default.nix ];
# Create a file set containing all files under ./tests, except ones in `nixFiles`,
# meaning only without ./tests/default.nix
difference ./tests nixFiles

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



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

Filter a file set to only contain files matching some 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.


The path whose files to filter

Example 211. lib.fileset.fileFilter usage example

# Include all regular `default.nix` files in the current directory
fileFilter (file: == "default.nix") ./.

# Include all non-Nix files from the current directory
fileFilter (file: ! file.hasExt "nix") ./.

# Include all files that start with a "." in the current directory
fileFilter (file: hasPrefix "." ./.

# Include all regular files (not symlinks or others) in the current directory
fileFilter (file: file.type == "regular") ./.

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


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.


File sets cannot represent empty directories. Turning the result of this function back into a source using toSource will therefore not preserve empty directories.


Function argument

Example 212. lib.fileset.fromSource usage example

# There's no cleanSource-like function for file sets yet,
# but we can just convert cleanSource to a file set and use it that way
toSource {
  root = ./.;
  fileset = fromSource (lib.sources.cleanSource ./.);

# Keeping a previous sourceByRegex (which could be migrated to `lib.fileset.unions`),
# but removing a subdirectory using file set functions
  (fromSource (lib.sources.sourceByRegex ./. [
    # This regex includes everything in ./doc

# Use cleanSource, but limit it to only include ./Makefile and files under ./src
  (fromSource (lib.sources.cleanSource ./.))
  (unions [

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


Type: gitTracked :: Path -> FileSet

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

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


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

Example 213. lib.fileset.gitTracked usage example

# Include all files tracked by the Git repository in the current directory
gitTracked ./.

# Include only files tracked by the Git repository in the parent directory
# that are also in the current directory
intersection ./. (gitTracked ../.)

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


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).


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

(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.


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

Example 214. lib.fileset.gitTrackedWith usage example

# Include all files tracked by the Git repository in the current directory
# and any submodules under it
gitTracked { recurseSubmodules = true; } ./.

Located at lib/fileset/default.nix:831 in <nixpkgs>.

lib.sources: source filtering functions

Functions for copying sources to the Nix store.


Get the commit id of a git repo.


Function argument

Example 215. lib.sources.commitIdFromGitRepo usage example

commitIdFromGitRepo <nixpkgs/.git>

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


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


Function argument

Example 216. lib.sources.cleanSource usage example

cleanSource ./.

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


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

A path or cleanSourceWith result to filter and/or rename.


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 Type: A function (path -> type -> bool)


Optional name to use as part of the store path. This defaults to or otherwise "source".

Example 217. lib.sources.cleanSourceWith usage example

lib.cleanSourceWith {
  filter = f;
  src = lib.cleanSourceWith {
    filter = g;
    src = ./.;
# Succeeds!

builtins.filterSource f (builtins.filterSource g ./.)
# Fails!

Located at lib/sources.nix:276 in <nixpkgs>.


A basic filter for cleanSourceWith that removes directories of version control system, backup files (*~) and some generated files.


Function argument


Function argument

Located at lib/sources.nix:277 in <nixpkgs>.


Filter sources by a list of regular expressions.


Function argument


Function argument

Example 218. lib.sources.sourceByRegex usage example

src = sourceByRegex ./my-subproject [".*\.py$" "^database.sql$"]

Located at lib/sources.nix:281 in <nixpkgs>.


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.


Path or source containing the files to be returned


A list of file suffix strings

Example 219. lib.sources.sourceFilesBySuffices usage example

sourceFilesBySuffices ./. [ ".xml" ".c" ]

Located at lib/sources.nix:282 in <nixpkgs>.


Type: sources.trace :: sourceLike -> Source

Add logging to a source, for troubleshooting the filtering behavior.


Source to debug. The returned source will behave like this source, but also log its filter invocations.

Located at lib/sources.nix:284 in <nixpkgs>.

lib.cli: command-line serialization functions


Automatically convert an attribute set to command-line options.

This helps protect against malformed command lines and also to reduce boilerplate related to command-line construction for simple use cases.

toGNUCommandLine returns a list of nix strings. toGNUCommandLineShell returns an escaped shell string.


Function argument


Function argument

Example 220. lib.cli.toGNUCommandLineShell usage example

cli.toGNUCommandLine {} {
  data = builtins.toJSON { id = 0; };
  X = "PUT";
  retry = 3;
  retry-delay = null;
  url = [ "" "" ];
  silent = false;
  verbose = true;
=> [
  "-X" "PUT"
  "--data" "{\"id\":0}"
  "--retry" "3"
  "--url" ""
  "--url" ""

cli.toGNUCommandLineShell {} {
  data = builtins.toJSON { id = 0; };
  X = "PUT";
  retry = 3;
  retry-delay = null;
  url = [ "" "" ];
  silent = false;
  verbose = true;
=> "'-X' 'PUT' '--data' '{\"id\":0}' '--retry' '3' '--url' '' '--url' '' '--verbose'";

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

lib.gvariant: GVariant formatted string serialization functions

A partial and basic implementation of GVariant formatted strings. See GVariant Format Strings for details.


This API is not considered fully stable and it might therefore change in backwards incompatible ways without prior notice.


Type: isGVariant :: Any -> Bool

Check if a value is a GVariant value


Function argument

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


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.


Function argument

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


Type: mkArray :: [Any] -> gvariant

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


Function argument

Example 221. lib.gvariant.mkArray usage example

# Creating a string array
lib.gvariant.mkArray [ "a" "b" "c" ]

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


Type: mkEmptyArray :: gvariant.type -> gvariant

Returns the GVariant array from the given empty Nix list.


Function argument

Example 222. lib.gvariant.mkEmptyArray usage example

# Creating an empty string array
lib.gvariant.mkEmptyArray (lib.gvariant.type.string)

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


Type: mkVariant :: Any -> gvariant

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


Function argument

Example 223. lib.gvariant.mkVariant usage example

lib.gvariant.mkArray [
  (lib.gvariant.mkVariant "a string")
  (lib.gvariant.mkVariant (lib.gvariant.mkInt32 1))

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


Type: mkDictionaryEntry :: String -> Any -> gvariant

Returns the GVariant dictionary entry from the given key and value.


The key of the entry


The value of the entry

Example 224. lib.gvariant.mkDictionaryEntry usage example

# A dictionary describing an Epiphany’s search provider
  (lib.gvariant.mkDictionaryEntry "url" (lib.gvariant.mkVariant ""))
  (lib.gvariant.mkDictionaryEntry "bang" (lib.gvariant.mkVariant "!d"))
  (lib.gvariant.mkDictionaryEntry "name" (lib.gvariant.mkVariant "DuckDuckGo"))

Located at lib/gvariant.nix:142 in <nixpkgs>.


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

Returns the GVariant maybe from the given element type.


Function argument


Function argument

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


Type: mkNothing :: gvariant.type -> gvariant

Returns the GVariant nothing from the given element type.


Function argument

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


Type: mkJust :: Any -> gvariant

Returns the GVariant just from the given Nix value.


Function argument

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


Type: mkTuple :: [Any] -> gvariant

Returns the GVariant tuple from the given Nix list.


Function argument

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


Type: mkBoolean :: Bool -> gvariant

Returns the GVariant boolean from the given Nix bool value.


Function argument

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


Type: mkString :: String -> gvariant

Returns the GVariant string from the given Nix string value.


Function argument

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


Type: mkObjectpath :: String -> gvariant

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


Function argument

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


Type: mkUchar :: Int -> gvariant

Returns the GVariant uchar from the given Nix int value.

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


Type: mkInt16 :: Int -> gvariant

Returns the GVariant int16 from the given Nix int value.

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


Type: mkUint16 :: Int -> gvariant

Returns the GVariant uint16 from the given Nix int value.

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


Type: mkInt32 :: Int -> gvariant

Returns the GVariant int32 from the given Nix int value.


Function argument

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


Type: mkUint32 :: Int -> gvariant

Returns the GVariant uint32 from the given Nix int value.

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


Type: mkInt64 :: Int -> gvariant

Returns the GVariant int64 from the given Nix int value.

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


Type: mkUint64 :: Int -> gvariant

Returns the GVariant uint64 from the given Nix int value.

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


Type: mkDouble :: Float -> gvariant

Returns the GVariant double from the given Nix float value.


Function argument

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

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


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.


Function argument


Function argument

Example 225. lib.customisation.overrideDerivation usage example

mySed = overrideDerivation pkgs.gnused (oldAttrs: {
  name = "sed-4.2.2-pre";
  src = fetchurl {
    url =;
    hash = "sha256-MxBJRcM2rYzQYwJ5XKxhXTQByvSg5jZc5cSHEZoB2IY=";
  patches = [];

Located at lib/customisation.nix:43 in <nixpkgs>.


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.


Function argument

Example 226. lib.customisation.makeOverridable usage example

nix-repl> x = {a, b}: { result = a + b; }

nix-repl> y = lib.makeOverridable x { a = 1; b = 2; }

nix-repl> y
{ override = «lambda»; overrideDerivation = «lambda»; result = 3; }

nix-repl> y.override { a = 10; }
{ override = «lambda»; overrideDerivation = «lambda»; result = 12; }

Located at lib/customisation.nix:79 in <nixpkgs>.


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;

Function argument


Function argument


Function argument

Located at lib/customisation.nix:141 in <nixpkgs>.


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.


Function argument


Function argument


Function argument

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


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.


Function argument


Function argument


Function argument

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


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.


Function argument

Located at lib/customisation.nix:261 in <nixpkgs>.


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

Make a set of packages with a common scope. All packages called with the provided callPackage will be evaluated with the same arguments. Any package in the set may depend on any other. The overrideScope' function allows subsequent modification of the package set in a consistent way, i.e. all packages in the set will be called with the overridden packages. The package sets may be hierarchical: the packages in the set are called with the scope provided by newScope and the set provides a newScope attribute which can form the parent scope for later package sets.


Function argument


Function argument

Located at lib/customisation.nix:303 in <nixpkgs>.


backward compatibility with old uncurried form; deprecated


Function argument


Function argument


Function argument


Function argument


Function argument


Function argument

Located at lib/customisation.nix:317 in <nixpkgs>.



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

Function argument


Function argument

structured function argument

Function argument


Function argument


Function argument


Function argument

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

lib.meta: functions for derivation metadata

Some functions for manipulating meta attributes, as well as the name attribute.


Add to or override the meta attributes of the given derivation.


Function argument


Function argument

Example 227. lib.meta.addMetaAttrs usage example

addMetaAttrs {description = "Bla blah";} somePkg

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


Disable Hydra builds of given derivation.


Function argument

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


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


Function argument


Function argument

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


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


Function argument


Function argument

Example 228. lib.meta.updateName usage example

updateName (oldName: oldName + "-experimental") somePkg

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


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


Function argument

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


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


Function argument


Function argument

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


Set the nix-env priority of the package.


Function argument

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


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>.


Apply lowPrio to an attrset with derivations


Function argument

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


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>.


Apply hiPrio to an attrset with derivations


Function argument

Located at lib/meta.nix:74 in <nixpkgs>.


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

  3. (modern) a pattern for the platform parsed field (see

We can inject these into a pattern for the whole of a structured platform, and then match that.


Function argument


Function argument

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


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.


Function argument


Function argument

Located at lib/meta.nix:116 in <nixpkgs>.


Type: getLicenseFromSpdxId :: str -> AttrSet

Get the corresponding attribute in lib.licenses from the SPDX ID. For SPDX IDs, see

Example 229. lib.meta.getLicenseFromSpdxId usage example

lib.getLicenseFromSpdxId "MIT" ==
=> true
lib.getLicenseFromSpdxId "mIt" ==
=> true
lib.getLicenseFromSpdxId "MY LICENSE"
=> trace: warning: getLicenseFromSpdxId: No license matches the given SPDX ID: MY LICENSE
=> { shortName = "MY LICENSE"; }

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


Type: getExe :: package -> string

Get the path to the main program of a package based on meta.mainProgram


Function argument

Example 230. lib.meta.getExe usage example

getExe pkgs.hello
=> "/nix/store/g124820p9hlv4lj8qplzxw1c44dxaw1k-hello-2.12/bin/hello"
getExe pkgs.mustache-go
=> "/nix/store/am9ml4f4ywvivxnkiaqwr0hyxka1xjsf-mustache-go-1.3.0/bin/mustache"

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


Type: getExe' :: derivation -> string -> string

Get the path of a program of a derivation.


Function argument


Function argument

Example 231. lib.meta.getExe' usage example

getExe' pkgs.hello "hello"
=> "/nix/store/g124820p9hlv4lj8qplzxw1c44dxaw1k-hello-2.12/bin/hello"
getExe' pkgs.imagemagick "convert"
=> "/nix/store/5rs48jamq7k6sal98ymj9l4k2bnwq515-imagemagick-7.1.1-15/bin/convert"

Located at lib/meta.nix:177 in <nixpkgs>.


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:

with lib;
  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:




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


pkgs.nix-gitignore is a function that acts similarly to builtins.filterSource but also allows filtering with the help of the gitignore format.


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



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.


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.


Evaluate a set of modules. This function is typically only used once per application (e.g. once in NixOS, once in Home Manager, …).



A list of modules. These are merged together to form the final configuration.


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.


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


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:


The nested attribute set of all option declarations.


The nested attribute set of all option values.


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.


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.


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.


A portion of the configuration tree which is elided from config.


A nominal type marker, always "configuration".


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 = "";
    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 = "${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 = ./;

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


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.


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.


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


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.


Consider for example this simplified derivation for solo5, a sandboxing tool:

stdenv.mkDerivation rec {
  pname = "solo5";
  version = "0.7.5";

  src = fetchurl {
    url = "${version}/solo5-v${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”.


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 three axes: their host and target platforms relative to the new derivation’s, and whether they are propagated. 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 or not the dependency is needed at run-time or build-time, a concept that makes perfect sense outside of cross compilation. By default, the run-time/build-time distinction is just a hint for mental clarity, but with strictDeps set it is mostly enforced even in the native case.

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.

A dependency is said to be propagated when some of its other-transitive (non-immediate) downstream dependencies also need it as an immediate dependency. [3]

It is important to note that dependencies are not necessarily propagated as the same sort of dependency that they were before, but rather as the corresponding sort so that the platform rules still line up. 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! [4] 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


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, 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.


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.


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.


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.


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.


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.


The propagated equivalent of depsBuildBuild. This perhaps never ought to be used, but it is included for consistency [see below for the others].


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.


The propagated equivalent of depsBuildTarget. This is prefixed for the same reason of alerting potential users.


The propagated equivalent of depsHostHost.


The propagated equivalent of buildInputs. This would be called depsHostTargetPropagated but for historical continuity.


The propagated equivalent of depsTargetTarget. This is prefixed for the same reason of alerting potential users.


Variables affecting stdenv initialisation


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


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


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.


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

  • an executable file, either on the file system:

    passthru.updateScript = ./;

    or inside the expression itself:

    passthru.updateScript = writeScript "update-zoom-us" ''
      #!/usr/bin/env nix-shell
      #!nix-shell -i bash -p curl pcre common-updater-scripts
      set -eu -o pipefail
      version="$(curl -sI | grep -Fi 'Location:' | pcregrep -o1 '/(([0-9]\.?)+)/')"
      update-source-version zoom-us "$version"
  • a list, a script followed by arguments to be passed to it:

    passthru.updateScript = [ ../../ pname "--requested-release=unstable" ];
  • an attribute set containing:

    • command – a string or list in the format expected by passthru.updateScript.

    • attrPath (optional) – a string containing the canonical attribute path for the package. If present, it will be passed to the update script instead of the attribute path on which the package was discovered during Nixpkgs traversal.

    • supportedFeatures (optional) – a list of the extra features the script supports.

    passthru.updateScript = {
      command = [ ../../ pname ];
      attrPath = pname;
      supportedFeatures = [ … ];


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:

  • UPDATE_NIX_NAME – content of the name attribute of the updated package.

  • UPDATE_NIX_PNAME – content of the pname attribute of the updated package.

  • UPDATE_NIX_OLD_VERSION – content of the version attribute of the updated package.

  • UPDATE_NIX_ATTR_PATH – attribute path the update.nix discovered the package on (or the canonical attrPath when available). Example: pantheon.elementary-terminal


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.


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

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:

  • attrPath – a string containing attribute path.

  • oldVersion – a string containing old version.

  • newVersion – a string containing new version.

  • files – a non-empty list of file paths (as strings) to add to the commit.

  • commitBody (optional) – a string with extra content to be appended to the default commit message (useful for adding changelog links).

  • commitMessage (optional) – a string to use instead of the default commit message.

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

Example 232. Standard output of an update script using commit feature

    "attrPath": "volume_key",
    "oldVersion": "0.3.11",
    "newVersion": "0.3.12",
    "files": [

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.


stdenv.mkDerivation sets the Nix derivation’s builder to a script that loads the stdenv bash library and calls genericBuild. Most packaging functions rely on this default builder.

This generic command invokes 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/

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


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).


Additional phases executed before any of the default phases.


Additional phases executed just before the configure phase.


Additional phases executed just before the build phase.


Additional phases executed just before the install phase.


Additional phases executed just before the fixup phase.


Additional phases executed just before the distribution phase.


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.


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 = "${}/my-sub-directory".


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)";.


Hook executed at the start of the unpack phase.


Hook executed at the end of the unpack phase.


Set to true to skip the unpack phase.


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.


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


Set to true to skip the patch phase.


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).


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.


Hook executed at the start of the patch phase.


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


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 ./


A list of strings passed as additional arguments to the configure script.


Set to true to skip the configure phase.


A shell array containing additional arguments passed to the configure script. You must use this instead of configureFlags if the arguments contain spaces.


By default, the flag --prefix=$prefix is added to the configure flags. If this is undesirable, set this variable to true.


The prefix under which the package must be installed, passed via the --prefix option to the configure script. It defaults to $out.


The key to use when specifying the prefix. By default, this is set to --prefix= as that is used by the majority of packages.


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.


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.


By default, the configure phase applies some special hackery to all files called before running the configure script in order to improve the purity of Libtool-based packages [5] . If this is undesirable, set this variable to true.


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.


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. [6]


Hook executed at the start of the configure phase.


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


Set to true to skip the build phase.


The file name of the Makefile.


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)" ];


The flags are quoted in bash, but environment variables can be specified by using the make syntax.


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.


Hook executed at the start of the build phase.


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


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.


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.


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.


A list of native dependencies used by the phase, notably tools needed on $PATH. This gets included in nativeBuildInputs when doCheck is set.


Hook executed at the start of the check phase.


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


Set to true to skip the install phase.

makeFlags / makeFlagsArray / makefile

See the build phase for details.


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.


Hook executed at the start of the install phase.


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 “ for details.

Variables controlling the fixup phase


Set to true to skip the fixup phase.


If set, libraries and executables are not stripped. By default, they are.


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.


Like dontStrip, but only affects the strip command targeting the packages’ target platform. Useful when supporting cross compilation, but otherwise feel free to ignore.


If set, files in $out/sbin are not moved to $out/bin. By default, they are.


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.


Like stripAllList, but only applies to packages’ target platform. By default, it’s empty. Useful when supporting cross compilation.


Flags passed to the strip command applied to the files in the directories listed in stripAllList. Defaults to -s (i.e. --strip-all).


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.


Like stripDebugList, but only applies to packages’ target platform. By default, it’s empty. Useful when supporting cross compilation.


Flags passed to the strip command applied to the files in the directories listed in stripDebugList. Defaults to -S (i.e. --strip-debug).


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/* ]

If set, the patchelf command is not used to remove unnecessary RPATH entries. Only applies to Linux.


If set, scripts starting with #! do not have their interpreter paths rewritten to paths in the Nix store. See the section called “ on how patching shebangs works.


If set, libtool .la files associated with shared libraries won’t have their dependency_libs field cleared.


The list of directories that must be moved from $out to $out/share. Defaults to man doc info.


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.


Hook executed at the start of the fixup phase.


Hook executed at the end of the fixup phase.


If set to true, the standard environment will enable debug information in C/C++ builds. After installation, the debug information will be separated from the executables and stored in the output named debug. (This output is enabled automatically; you don’t need to set the outputs attribute explicitly.) To be precise, the debug information is stored in debug/lib/debug/.build-id/XX/YYYY…, where <XXYYYY…> is the <build ID> of the binary — a SHA-1 hash of the contents of the binary. Debuggers like GDB use the build ID to look up the separated debug information.

Example 233. Enable debug symbols for use with GDB

To make GDB find debug information for the socat package and its dependencies, you can use the following shell.nix:

  pkgs = import ./. {
    config = {};
    overlays = [
      (final: prev: {
        ncurses = prev.ncurses.overrideAttrs { separateDebugInfo = true; };
        readline = prev.readline.overrideAttrs { separateDebugInfo = true; };

  myDebugInfoDirs = pkgs.symlinkJoin {
    name = "myDebugInfoDirs";
    paths = with pkgs; [
  pkgs.mkShell {

    NIX_DEBUG_INFO_DIRS = "${pkgs.lib.getLib myDebugInfoDirs}/lib/debug";

    packages = [

    shellHook = ''
      ${pkgs.lib.getBin pkgs.gdb}/bin/gdb ${pkgs.lib.getBin pkgs.socat}/bin/socat

This setup works as follows:

  • Add overlays to the package set, since debug symbols are disabled for ncurses and readline by default.

  • Create a derivation to combine all required debug symbols under one path with symlinkJoin.

  • Set the environment variable NIX_DEBUG_INFO_DIRS in the shell. Nixpkgs patches gdb to use it for looking up debug symbols.

  • Run gdb on the socat binary on shell startup in the shellHook. Here we use lib.getBin to ensure that the correct derivation output is selected rather than the default one.

The installCheck phase

The installCheck phase checks whether the package was installed correctly by running its test suite against the installed directories. The default installCheck calls make installcheck.

It is often better to add tests that are not part of the source distribution to passthru.tests (see the section called “tests). This avoids adding overhead to every build and enables us to run them independently.

Variables controlling the installCheck phase


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.


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.


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.


A list of native dependencies used by the phase, notably tools needed on $PATH. This gets included in nativeBuildInputs when doInstallCheck is set.


Hook executed at the start of the installCheck phase.


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


If set, the distribution phase is executed.


The make target that produces the distribution. Defaults to dist.

distFlags / distFlagsArray

Additional flags passed to make.


The names of the source distribution files to be copied to $out/tarballs/. It can contain shell wildcards. The default is *.tar.gz.


If set, no files are copied to $out/tarballs/.


Hook executed at the start of the distribution phase.


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.


--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/ for the makeWrapper implementation and in nixpkgs/pkgs/build-support/setup-hooks/make-binary-wrapper/ 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 ${} '{}' +

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>.


substitute ./ ./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
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
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:

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.


$ 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.


$ 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.

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.

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.

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.

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.


The generic builder populates PATH from inputs of the derivation.


Multiple paths can be specified.

patchShebangs [--build | --host] PATH...

Look up commands available at build time


Look up commands available at run time

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.


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 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.

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.

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.

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.

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.

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.

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. [7] 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.


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]
cc1plus: some warnings being treated as errors


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'


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


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'


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.


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.


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: 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.


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]

Nix itself already takes a package’s transitive dependencies into account, but this propagation ensures nixpkgs-specific infrastructure like setup hooks also are run as if it were a propagated dependency.[3]

The findInputs function, currently residing in pkgs/stdenv/generic/, implements the propagation logic.[4]

It clears the sys_lib_*search_path variables in the Libtool script to prevent Libtool from using libraries in /usr/lib and such.[5]

Eventually these will be passed building natively as well, to improve determinism: build-time guessing, as is done today, is a risk of impurity.[6]

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.[7]


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 = with lib; {
  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 = "";
  license = licenses.gpl3Plus;
  maintainers = with maintainers; [ eelco ];
  platforms = 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:


A short (one-line) description of the package. This is displayed on

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"


An arbitrarily long description of the package in CommonMark Markdown.


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.


The package’s homepage. Example:


The page where a link to the current version can be found. Example:


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: "${version}"


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.


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 ].


The name of the main binary for the package. This affects the binary nix run executes. Example: "rg"


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).


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.


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 = [ ];

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.



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.actual
  diff -U3 --color=auto example.expected example.actual
  mkdir $out


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.


The list of Nix platform types for which the Hydra instance at 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 to build the package on a subset of meta.platforms, or not at all, e.g.

meta.platforms = lib.platforms.linux;
meta.hydraPlatforms = [];


If set to true, the package is marked as “broken”, meaning that it won’t show up in, 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.


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:, "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.


Package elements which are produced by a nixpkgs derivation which builds them from source code.


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.


Code to be executed on a peripheral device or embedded controller, built by a third party.


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.


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., 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/>; 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.


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/ The executables provided by glibc can be accessed via its bin attribute (e.g. ${lib.getBin}/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.


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.


is meant for user-facing binaries, typically residing in bin/. They go to bin or out by default.


is meant for libraries, typically residing in lib/ and libexec/. They go to lib or out by default.


is for user documentation, typically residing in share/doc/. It goes to doc or out by default.


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.


is for man pages (except for section 3), typically residing in share/man/man[0-9]/. They go to man or $outputBin by default.


is for section 3 man pages, typically residing in share/man/man[0-9]/. They go to devman or $outputMan by default.


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” 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...

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.


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.


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; note how they are not all very consistent. For now, here are few fields can count on them containing:


This is a two-component shorthand for the platform. Examples of this would be “x86_64-darwin” and “i686-linux”; see 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.


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!


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 for the exact representation.


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.


These predicates are defined in, 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.


This is, quite frankly, a dumping ground of ad-hoc settings (it’s an attribute set). See 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


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, 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 (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 ${}cc.

makeFlags = [ "CC=${}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 = [ ];

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.


nativeBuildInputs = [
] ++ lib.optionals (!stdenv.buildPlatform.canExecute stdenv.hostPlatform) [

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, 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


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.


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.


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.


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”.


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.


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/' 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.


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.


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


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 = "";
  hash = "sha256-lTeyxzJNQeMdu1IVdovNMtgn77jRIhSybLdMbTkf2Ww=";

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

fetchurl {
  url = "";
  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 = "";
  hash = "";

Use the resulting error message to determine the correct hash.

error: hash mismatch in fixed-output derivation '/path/to/my.drv':
            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 = "";

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.


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.


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, 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 to archive the patch if it needs to be available longer-term.


Used with Subversion. Expects url to a Subversion directory, rev, and hash.


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 = [


Used with Fossil. Expects url to a Fossil archive, rev, and hash.


Used with CVS. Expects cvsRoot, tag, and hash.


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 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 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 "").

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.


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 "").


This is used with Gitiles repositories. The arguments expected are similar to fetchgit.


This is used with BitBucket repositories. The arguments expected are very similar to fetchFromGitHub above.


This is used with Savannah repositories. The arguments expected are very similar to fetchFromGitHub above.


This is used with repositories. The arguments expected are very similar to fetchFromGitHub above.


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 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 = "";
  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 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
  nix-prefetch-url --type sha256 file:///path/to/jdk-11.0.10_linux-x64_bin.tar.gz



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 = "";


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


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

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

name :: String

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

drvAttr :: AttrSet

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

buildCommand :: String

Shell commands to run in the derivation builder.


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

Example 234. Invocation of runCommand

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

  mkdir $out

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

  echo I can also run basic commands like:

  echo ls

  echo whoami

  echo date


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.


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.


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

writeTextFile, writeText, writeTextDir, writeScript, writeScriptBin

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

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

Here are a few examples:

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

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

concatTextFile, concatText, concatScript

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

Here are a few examples:

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

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

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


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

For example, look at the following code:

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

  runtimeInputs = [ curl w3m ];

  text = ''
    curl -s '' | w3m -dump -T text/html

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


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:

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


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


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.


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


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


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


Checks whether a package exposes a given list of pkg-config modules. If the moduleNames argument is omitted, hasPkgConfigModules will use meta.pkgConfigModules.


passthru.tests.pkg-config = testers.hasPkgConfigModules {
  package = finalAttrs.finalPackage;
  moduleNames = [ "libfoo" ];

If the package in question has meta.pkgConfigModules set, it is even simpler:

passthru.tests.pkg-config = testers.hasPkgConfigModules {
  package = finalAttrs.finalPackage;

meta.pkgConfigModules = [ "libfoo" ];


Checks the command output contains the specified version

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.


passthru.tests.version = testers.testVersion { package = hello; };

passthru.tests.version = testers.testVersion {
  package = seaweedfs;
  command = "weed version";

passthru.tests.version = testers.testVersion {
  package = key;
  command = "KeY --help";
  # Wrong '2.5' version in the code. Drop on next version.
  version = "2.5";

passthru.tests.version = testers.testVersion {
  package = ghr;
  # The output needs to contain the 'version' string without any prefix or suffix.
  version = "v${version}";


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)


runCommand "example" {
  failed = testers.testBuildFailure (runCommand "fail" {} ''
    echo ok-ish >$out
    echo failing though
    exit 3
} ''
  grep -F 'ok-ish' $failed/result
  grep -F 'failing though' $failed/testBuildFailure.log
  [[ 3 = $(cat $failed/testBuildFailure.exit) ]]
  touch $out

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, 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.


Check that two paths have the same contents.


testers.testEqualContents {
  assertion = "sed -e performs replacement";
  expected = writeText "expected" ''
    foo baz baz
  actual = runCommand "actual" {
    # not really necessary for a package that's in stdenv
    nativeBuildInputs = [ gnused ];
    base = writeText "base" ''
      foo bar baz
  } ''
    sed -e 's/bar/baz/g' $base >$out


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.


  "The hello package must stay the same when enabling checks."
  (hello.overrideAttrs(o: { doCheck = true; }))


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.


tests.fetchgit = testers.invalidateFetcherByDrvHash fetchgit {
  name = "nix-source";
  url = "";
  rev = "9d9dbe6ed05854e03811c361a3380e09183f4f4a";
  hash = "sha256-7DszvbCNTjpzGRmpIVAWXk20P0/XTrWZ79KSOGLrUWY=";


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.


pkgs.testers.runNixOSTest ({ lib, ... }: {
  name = "hello";
  nodes.machine = { pkgs, ... }: {
    environment.systemPackages = [ pkgs.hello ];
  testScript = ''


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.


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

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


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.


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; [
  ]) ++ (with pkgs.xorg; [
  multiPkgs = pkgs: (with pkgs; [
  runScript = "bash";

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/ 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 is a build helper that produces hooks that go in to nativeBuildInputs


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

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 "" ''


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


Here is a common usage example:

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

  inputsFrom = [ pkgs.hello pkgs.gnutar ];

  shellHook = ''
    export DEBUG=1


  • 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.

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.


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


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


  • 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.


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.


  • 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.


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";


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


  • 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.


Extract the contents of an ISO file:

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


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


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.


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


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


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


  • 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.


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

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


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


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


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


{ pkgs ? import <nixpkgs> {} }:
  inherit (pkgs.checkpointBuildTools)
  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


This chapter describes tools for creating various types of images.


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 it would take too long. To quickly run an AppImage file, pkgs.appimage-run can be used as well.


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

AppImage formats

There are different formats for AppImages, see the specification for details.

  • Type 1 images are ISO 9660 files that are also ELF executables.

  • Type 2 images are ELF executables with an appended filesystem.

They can be told apart with file -k:

$ file -k type1.AppImage
type1.AppImage: ELF 64-bit LSB executable, x86-64, version 1 (SYSV) ISO 9660 CD-ROM filesystem data 'AppImage' (Lepton 3.x), scale 0-0,
spot sensor temperature 0.000000, unit celsius, color scheme 0, calibration: offset 0.000000, slope 0.000000, dynamically linked, interpreter /lib64/, for GNU/Linux 2.6.18, BuildID[sha1]=d629f6099d2344ad82818172add1d38c5e11bc6d, stripped\012- data

$ file -k type2.AppImage
type2.AppImage: ELF 64-bit LSB executable, x86-64, version 1 (SYSV) (Lepton 3.x), scale 232-60668, spot sensor temperature -4.187500, color scheme 15, show scale bar, calibration: offset -0.000000, slope 0.000000 (Lepton 2.x), scale 4111-45000, spot sensor temperature 412442.250000, color scheme 3, minimum point enabled, calibration: offset -75402534979642766821519867692934234112.000000, slope 5815371847733706829839455140374904832.000000, dynamically linked, interpreter /lib64/, for GNU/Linux 2.6.18, BuildID[sha1]=79dcc4e55a61c293c5e19edbd8d65b202842579f, stripped\012- data

Note how the type 1 AppImage is described as an ISO 9660 CD-ROM filesystem, and the type 2 AppImage is not.


Depending on the type of AppImage you’re wrapping, you’ll have to use wrapType1 or wrapType2.

appimageTools.wrapType2 { # or wrapType1
  name = "patchwork";
  src = fetchurl {
    url = "";
    hash = "sha256-OqTitCeZ6xmWbqYTXp8sDrmVgTNjPZNW0hzUPW++mq4=";
  extraPkgs = pkgs: with pkgs; [ ];
  • name specifies the name of the resulting image.

  • src specifies the AppImage file to extract.

  • extraPkgs allows you to pass a function to include additional packages inside the FHS environment your AppImage is going to run in. 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.


pkgs.dockerTools is a set of functions for creating and manipulating Docker images according to the Docker Image Specification v1.2.0. Docker itself is not used to perform any of the operations done by these functions.


This function is analogous to the docker build command, in that it can be used to build a Docker-compatible repository tarball containing a single image with one or multiple layers. As such, the result is suitable for being loaded in Docker with docker load.

The parameters of buildImage with relative example values are described below:

buildImage {
  name = "redis";
  tag = "latest";

  fromImage = someBaseImage;
  fromImageName = null;
  fromImageTag = "latest";

  copyToRoot = pkgs.buildEnv {
    name = "image-root";
    paths = [ pkgs.redis ];
    pathsToLink = [ "/bin" ];

  runAsRoot = ''
    mkdir -p /data

  config = {
    Cmd = [ "/bin/redis-server" ];
    WorkingDir = "/data";
    Volumes = { "/data" = { }; };

  diskSize = 1024;
  buildVMMemorySize = 512;

The above example will build a Docker image redis/latest from the given base image. Loading and running this image in Docker results in redis-server being started automatically.

  • name specifies the name of the resulting image. This is the only required argument for buildImage.

  • tag specifies the tag of the resulting image. By default it’s null, which indicates that the nix output hash will be used as tag.

  • fromImage is the repository tarball containing the base image. It must be a valid Docker image, such as exported by docker save. By default it’s null, which can be seen as equivalent to FROM scratch of a Dockerfile.

  • fromImageName can be used to further specify the base image within the repository, in case it contains multiple images. By default it’s null, in which case buildImage will peek the first image available in the repository.

  • fromImageTag can be used to further specify the tag of the base image within the repository, in case an image contains multiple tags. By default it’s null, in which case buildImage will peek the first tag available for the base image.

  • copyToRoot is a derivation that will be copied in the new layer of the resulting image. This can be similarly seen as ADD contents/ / in a Dockerfile. By default it’s null.

  • runAsRoot is a bash script that will run as root in an environment that overlays the existing layers of the base image with the new resulting layer, including the previously copied contents derivation. This can be similarly seen as RUN ... in a Dockerfile.

NOTE: Using this parameter requires the kvm device to be available.

  • config is used to specify the configuration of the containers that will be started off the built image in Docker. The available options are listed in the Docker Image Specification v1.2.0.

  • architecture is optional and used to specify the image architecture, this is useful for multi-architecture builds that don’t need cross compiling. If not specified it will default to hostPlatform.

  • diskSize is used to specify the disk size of the VM used to build the image in megabytes. By default it’s 1024 MiB.

  • buildVMMemorySize is used to specify the memory size of the VM to build the image in megabytes. By default it’s 512 MiB.

After the new layer has been created, its closure (to which contents, config and runAsRoot contribute) will be copied in the layer itself. Only new dependencies that are not already in the existing layers will be copied.

At the end of the process, only one new single layer will be produced and added to the resulting image.

The resulting repository will only list the single image image/tag. In the case of the buildImage example, it would be redis/latest.

It is possible to inspect the arguments with which an image was built using its buildArgs attribute.

NOTE: If you see errors similar to getProtocolByName: does not exist (no such protocol name: tcp) you may need to add pkgs.iana-etc to contents.

NOTE: If you see errors similar to Error_Protocol ("certificate has unknown CA",True,UnknownCa) you may need to add pkgs.cacert to contents.

By default buildImage will use a static date of one second past the UNIX Epoch. This allows buildImage to produce binary reproducible images. When listing images with docker images, the newly created images will be listed like this:

$ docker images
hello        latest   08c791c7846e   48 years ago   25.2MB

You can break binary reproducibility but have a sorted, meaningful CREATED column by setting created to now.

pkgs.dockerTools.buildImage {
  name = "hello";
  tag = "latest";
  created = "now";
  copyToRoot = pkgs.buildEnv {
    name = "image-root";
    paths = [ pkgs.hello ];
    pathsToLink = [ "/bin" ];

  config.Cmd = [ "/bin/hello" ];

Now the Docker CLI will display a reasonable date and sort the images as expected:

$ docker images
REPOSITORY   TAG      IMAGE ID       CREATED              SIZE
hello        latest   de2bf4786de6   About a minute ago   25.2MB

However, the produced images will not be binary reproducible.


Create a Docker image with many of the store paths being on their own layer to improve sharing between images. The image is realized into the Nix store as a gzipped tarball. Depending on the intended usage, many users might prefer to use streamLayeredImage instead, which this function uses internally.


The name of the resulting image.

tag optional

Tag of the generated image.

Default: the output path’s hash

fromImage optional

The repository tarball containing the base image. It must be a valid Docker image, such as one exported by docker save.

Default: null, which can be seen as equivalent to FROM scratch of a Dockerfile.

contents optional

Top-level paths in the container. Either a single derivation, or a list of derivations.

Default: []

config optional

architecture is optional and used to specify the image architecture, this is useful for multi-architecture builds that don’t need cross compiling. If not specified it will default to hostPlatform.

Run-time configuration of the container. A full list of the options available is in the Docker Image Specification v1.2.0.

Default: {}

created optional

Date and time the layers were created. Follows the same now exception supported by buildImage.

Default: 1970-01-01T00:00:01Z

uid optional gid optional uname optional gname optional

: Credentials for Nix store ownership. Can be overridden to e.g. 1000 / 1000 / "user" / "user" to enable building a container where Nix can be used as an unprivileged user in single-user mode.

Default: 0 / 0 / "root" / "root"

maxLayers optional

Maximum number of layers to create.

Default: 100

Maximum: 125

extraCommands optional

Shell commands to run while building the final layer, without access to most of the layer contents. Changes to this layer are “on top” of all the other layers, so can create additional directories and files.

fakeRootCommands optional

Shell commands to run while creating the archive for the final layer in a fakeroot environment. Unlike extraCommands, you can run chown to change the owners of the files in the archive, changing fakeroot’s state instead of the real filesystem. The latter would require privileges that the build user does not have. Static binaries do not interact with the fakeroot environment. By default all files in the archive will be owned by root.

enableFakechroot optional

Whether to run in fakeRootCommands in fakechroot, making programs behave as though / is the root of the image being created, while files in the Nix store are available as usual. This allows scripts that perform installation in / to work as expected. Considering that fakechroot is implemented via the same mechanism as fakeroot, the same caveats apply.

Default: false

Behavior of contents in the final image

Each path directly listed in contents will have a symlink in the root of the image.

For example:

pkgs.dockerTools.buildLayeredImage {
  name = "hello";
  contents = [ pkgs.hello ];

will create symlinks for all the paths in the hello package:

/bin/hello -> /nix/store/h1zb1padqbbb7jicsvkmrym3r6snphxg-hello-2.10/bin/hello
/share/info/ -> /nix/store/h1zb1padqbbb7jicsvkmrym3r6snphxg-hello-2.10/share/info/
/share/locale/bg/LC_MESSAGES/ -> /nix/store/h1zb1padqbbb7jicsvkmrym3r6snphxg-hello-2.10/share/locale/bg/LC_MESSAGES/

Automatic inclusion of config references

The closure of config is automatically included in the closure of the final image.

This allows you to make very simple Docker images with very little code. This container will start up and run hello:

pkgs.dockerTools.buildLayeredImage {
  name = "hello";
  config.Cmd = [ "${pkgs.hello}/bin/hello" ];

Adjusting maxLayers

Increasing the maxLayers increases the number of layers which have a chance to be shared between different images.

Modern Docker installations support up to 128 layers, but older versions support as few as 42.

If the produced image will not be extended by other Docker builds, it is safe to set maxLayers to 128. However, it will be impossible to extend the image further.

The first (maxLayers-2) most “popular” paths will have their own individual layers, then layer #maxLayers-1 will contain all the remaining “unpopular” paths, and finally layer #maxLayers will contain the Image configuration.

Docker’s Layers are not inherently ordered, they are content-addressable and are not explicitly layered until they are composed in to an Image.


Builds a script which, when run, will stream an uncompressed tarball of a Docker image to stdout. The arguments to this function are as for buildLayeredImage. This method of constructing an image does not realize the image into the Nix store, so it saves on IO and disk/cache space, particularly with large images.

The image produced by running the output script can be piped directly into docker load, to load it into the local docker daemon:

$(nix-build) | docker load

Alternatively, the image be piped via gzip into skopeo, e.g., to copy it into a registry:

$(nix-build) | gzip --fast | skopeo copy docker-archive:/dev/stdin docker://some_docker_registry/myimage:tag


This function is analogous to the docker pull command, in that it can be used to pull a Docker image from a Docker registry. By default Docker Hub is used to pull images.

Its parameters are described in the example below:

pullImage {
  imageName = "nixos/nix";
  imageDigest =
  finalImageName = "nix";
  finalImageTag = "2.11.1";
  sha256 = "sha256-qvhj+Hlmviz+KEBVmsyPIzTB3QlVAFzwAY1zDPIBGxc=";
  os = "linux";
  arch = "x86_64";
  • imageName specifies the name of the image to be downloaded, which can also include the registry namespace (e.g. nixos). This argument is required.

  • imageDigest specifies the digest of the image to be downloaded. This argument is required.

  • finalImageName, if specified, this is the name of the image to be created. Note it is never used to fetch the image since we prefer to rely on the immutable digest ID. By default it’s equal to imageName.

  • finalImageTag, if specified, this is the tag of the image to be created. Note it is never used to fetch the image since we prefer to rely on the immutable digest ID. By default it’s latest.

  • sha256 is the checksum of the whole fetched image. This argument is required.

  • os, if specified, is the operating system of the fetched image. By default it’s linux.

  • arch, if specified, is the cpu architecture of the fetched image. By default it’s x86_64.

nix-prefetch-docker command can be used to get required image parameters:

$ nix run nixpkgs#nix-prefetch-docker -- --image-name mysql --image-tag 5

Since a given imageName may transparently refer to a manifest list of images which support multiple architectures and/or operating systems, you can supply the --os and --arch arguments to specify exactly which image you want. By default it will match the OS and architecture of the host the command is run on.

$ nix-prefetch-docker --image-name mysql --image-tag 5 --arch x86_64 --os linux

Desired image name and tag can be set using --final-image-name and --final-image-tag arguments:

$ nix-prefetch-docker --image-name mysql --image-tag 5 --final-image-name --final-image-tag prod


This function is analogous to the docker export command, in that it can be used to flatten a Docker image that contains multiple layers. It is in fact the result of the merge of all the layers of the image. As such, the result is suitable for being imported in Docker with docker import.

NOTE: Using this function requires the kvm device to be available.

The parameters of exportImage are the following:

exportImage {
  fromImage = someLayeredImage;
  fromImageName = null;
  fromImageTag = null;

  name =;

The parameters relative to the base image have the same synopsis as described in buildImage, except that fromImage is the only required argument in this case.

The name argument is the name of the derivation output, which defaults to

Environment Helpers

Some packages expect certain files to be available globally. When building an image from scratch (i.e. without fromImage), these files are missing. pkgs.dockerTools provides some helpers to set up an environment with the necessary files. You can include them in copyToRoot like this:

buildImage {
  name = "environment-example";
  copyToRoot = with pkgs.dockerTools; [


This provides the env utility at /usr/bin/env.


This provides bashInteractive at /bin/sh.


This sets up /etc/ssl/certs/ca-certificates.crt.


Provides /etc/passwd and /etc/group that contain root and nobody. Useful when packaging binaries that insist on using nss to look up username/groups (like nginx).


This constant string is a helper for setting up the base files for managing users and groups, only if such files don’t exist already. It is suitable for being used in a buildImage runAsRoot script for cases like in the example below:

buildImage {
  name = "shadow-basic";

  runAsRoot = ''
    groupadd -r redis
    useradd -r -g redis redis
    mkdir /data
    chown redis:redis /data

Creating base files like /etc/passwd or /etc/login.defs is necessary for shadow-utils to manipulate users and groups.


If your primary goal is providing a basic skeleton for user lookups to work, and/or a lesser privileged user, adding pkgs.fakeNss to the container image root might be the better choice than a custom script running useradd and friends.

It provides a /etc/passwd and /etc/group, containing root and nobody users and groups.

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

You can pair it with binSh, which provides bin/sh as a symlink to bashInteractive (as /bin/sh is configured as a shell).

buildImage {
  name = "shadow-basic";

  copyToRoot = pkgs.buildEnv {
    name = "image-root";
    paths = [ binSh pkgs.fakeNss ];
    pathsToLink = [ "/bin" "/etc" "/var" ];


Create a Docker image that sets up an environment similar to that of running nix-shell on a derivation. When run in Docker, this environment somewhat resembles the Nix sandbox typically used by nix-build, with a major difference being that access to the internet is allowed. It additionally also behaves like an interactive nix-shell, running things like shellHook and setting an interactive prompt. If the derivation is fully buildable (i.e. nix-build can be used on it), running buildDerivation inside such a Docker image will build the derivation, with all its outputs being available in the correct /nix/store paths, pointed to by the respective environment variables like $out, etc.


The behavior doesn’t match nix-shell or nix-build exactly and this function is known not to work correctly for e.g. fixed-output derivations, content-addressed derivations, impure derivations and other special types of derivations.



The derivation on which to base the Docker image.

Adding packages to the Docker image is possible by e.g. extending the list of nativeBuildInputs of this derivation like

buildNixShellImage {
  drv = someDrv.overrideAttrs (old: {
    nativeBuildInputs = old.nativeBuildInputs or [] ++ [
  # ...

Similarly, you can extend the image initialization script by extending shellHook

name optional

The name of the resulting image.

Default: + "-env"

tag optional

Tag of the generated image.

Default: the resulting image derivation output path’s hash

uid/gid optional

The user/group ID to run the container as. This is like a nixbld build user.

Default: 1000/1000

homeDirectory optional

The home directory of the user the container is running as

Default: /build

shell optional

The path to the bash binary to use as the shell. This shell is started when running the image.

Default: pkgs.bashInteractive + "/bin/bash"

command optional

Run this command in the environment of the derivation, in an interactive shell. See the --command option in the nix-shell documentation.

Default: (none)

run optional

Same as command, but runs the command in a non-interactive shell instead. See the --run option in the nix-shell documentation.

Default: (none)


The following shows how to build the pkgs.hello package inside a Docker container built with buildNixShellImage.

with import <nixpkgs> {};
dockerTools.buildNixShellImage {
  drv = hello;

Build the derivation:

nix-build hello.nix
these 8 derivations will be built:
Creating layer 56 from paths: ['/nix/store/crpnj8ssz0va2q0p5ibv9i6k6n52gcya-stdenv-linux']
Creating layer 57 with customisation...
Adding manifests...

Load the image:

docker load -i result
0d9f4c4cd109: Loading layer [==================================================>]   2.56MB/2.56MB
ab1d897c0697: Loading layer [==================================================>]  10.24kB/10.24kB
Loaded image: hello-2.12-env:pgj9h98nal555415faa43vsydg161bdz

Run the container:

docker run -it hello-2.12-env:pgj9h98nal555415faa43vsydg161bdz

In the running container, run the build:

unpacking sources
unpacking source archive /nix/store/8nqv6kshb3vs5q5bs2k600xpj5bkavkc-hello-2.12.tar.gz
patching script interpreter paths in /nix/store/z5wwy5nagzy15gag42vv61c2agdpz2f2-hello-2.12
checking for references to /build/ in /nix/store/z5wwy5nagzy15gag42vv61c2agdpz2f2-hello-2.12...

Check the build result:

Hello, world!


pkgs.ociTools is a set of functions for creating containers according to the OCI container specification v1.0.0. Beyond that, it makes no assumptions about the container runner you choose to use to run the created container.


This function creates a simple OCI container that runs a single command inside of it. An OCI container consists of a config.json and a rootfs directory. The nix store of the container will contain all referenced dependencies of the given command.

The parameters of buildContainer with an example value are described below:

buildContainer {
  args = [
    (with pkgs;
      writeScript "" ''
        exec ${bash}/bin/bash

  mounts = {
    "/data" = {
      type = "none";
      source = "/var/lib/mydata";
      options = [ "bind" ];

  readonly = false;
  • args specifies a set of arguments to run inside the container. This is the only required argument for buildContainer. All referenced packages inside the derivation will be made available inside the container.

  • mounts specifies additional mount points chosen by the user. By default only a minimal set of necessary filesystems are mounted into the container (e.g procfs, cgroupfs)

  • readonly makes the container’s rootfs read-only if it is set to true. The default value is false false.


pkgs.snapTools is a set of functions for creating Snapcraft images. Snap and Snapcraft is not used to perform these operations.

The makeSnap Function

makeSnap takes a single named argument, meta. This argument mirrors the upstream snap.yaml format exactly.

The base should not be specified, as makeSnap will force set it.

Currently, makeSnap does not support creating GUI stubs.

Build a Hello World Snap

The following expression packages GNU Hello as a Snapcraft snap.

  inherit (import <nixpkgs> { }) snapTools hello;
in snapTools.makeSnap {
  meta = {
    name = "hello";
    summary = hello.meta.description;
    description = hello.meta.longDescription;
    architectures = [ "amd64" ];
    confinement = "strict";
    apps.hello.command = "${hello}/bin/hello";

nix-build this expression and install it with snap install ./result --dangerous. hello will now be the Snapcraft version of the package.

Build a Graphical Snap

Graphical programs require many more integrations with the host. This example uses Firefox as an example because it is one of the most complicated programs we could package.

  inherit (import <nixpkgs> { }) snapTools firefox;
in snapTools.makeSnap {
  meta = {
    name = "nix-example-firefox";
    summary = firefox.meta.description;
    architectures = [ "amd64" ];
    apps.nix-example-firefox = {
      command = "${firefox}/bin/firefox";
      plugs = [
    confinement = "strict";

nix-build this expression and install it with snap install ./result --dangerous. nix-example-firefox will now be the Snapcraft version of the Firefox package.

The specific meaning behind plugs can be looked up in the Snapcraft interface documentation.


pkgs.portableService is a function to create portable service images, as read-only, immutable, squashfs archives.

systemd supports a concept of Portable Services. Portable Services are a delivery method for system services that uses two specific features of container management:

  • Applications are bundled. I.e. multiple services, their binaries and all their dependencies are packaged in an image, and are run directly from it.

  • Stricter default security policies, i.e. sandboxing of applications.

This allows using Nix to build images which can be run on many recent Linux distributions.

The primary tool for interacting with Portable Services is portablectl, and they are managed by the systemd-portabled system service.


Portable services are supported starting with systemd 239 (released on 2018-06-22).

A very simple example of using portableService is described below:

pkgs.portableService {
  pname = "demo";
  version = "1.0";
  units = [ demo-service demo-socket ];

The above example will build an squashfs archive image in result/$pname_$version.raw. The image will contain the file system structure as required by the portable service specification, and a subset of the Nix store with all the dependencies of the two derivations in the units list. units must be a list of derivations, and their names must be prefixed with the service name ("demo" in this case). Otherwise systemd-portabled will ignore them.


The .raw file extension of the image is required by the portable services specification.

Some other options available are:

  • description, homepage

    Are added to the /etc/os-release in the image and are shown by the portable services tooling. Default to empty values, not added to os-release.

  • symlinks

    A list of attribute sets {object, symlink}. Symlinks will be created in the root filesystem of the image to objects in the Nix store. Defaults to an empty list.

  • contents

    A list of additional derivations to be included in the image Nix store, as-is. Defaults to an empty list.

  • squashfsTools

    Defaults to pkgs.squashfsTools, allows you to override the package that provides mksquashfs.

  • squash-compression, squash-block-size

    Options to mksquashfs. Default to "xz -Xdict-size 100%" and "1M" respectively.

A typical usage of symlinks would be:

  symlinks = [
    { object = "${pkgs.cacert}/etc/ssl"; symlink = "/etc/ssl"; }
    { object = "${pkgs.bash}/bin/bash"; symlink = "/bin/sh"; }
    { object = "${pkgs.php}/bin/php"; symlink = "/usr/bin/php"; }

to create these symlinks for legacy applications that assume them existing globally.

Once the image is created, and deployed on a host in /var/lib/portables/, you can attach the image and run the service. As root run:

portablectl attach demo_1.0.raw
systemctl enable --now demo.socket
systemctl enable --now demo.service


See the man page of portablectl for more info on its usage.


<nixpkgs/nixos/lib/make-disk-image.nix> is a function to create disk images in multiple formats: raw, QCOW2 (QEMU), QCOW2-Compressed (compressed version), VDI (VirtualBox), VPC (VirtualPC).

This function can create images in two ways:

  • using cptofs without any virtual machine to create a Nix store disk image,

  • using a virtual machine to create a full NixOS installation.

When testing early-boot or lifecycle parts of NixOS such as a bootloader or multiple generations, it is necessary to opt for a full NixOS system installation. Whereas for many web servers, applications, it is possible to work with a Nix store only disk image and is faster to build.

NixOS tests also use this function when preparing the VM. The cptofs method is used when virtualisation.useBootLoader is false (the default). Otherwise the second method is used.


For reference, read the function signature source code for documentation on arguments: Features are separated in various sections depending on if you opt for a Nix-store only image or a full NixOS image.


  • arbitrary NixOS configuration

  • automatic or bound disk size: diskSize parameter, additionalSpace can be set when diskSize is auto to add a constant of disk space

  • multiple partition table layouts: EFI, legacy, legacy + GPT, hybrid, none through partitionTableType parameter

  • OVMF or EFI firmwares and variables templates can be customized

  • root filesystem fsType can be customized to whatever mkfs.${fsType} exist during operations

  • root filesystem label can be customized, defaults to nix-store if it’s a Nix store image, otherwise nixpkgs/nixos

  • arbitrary code can be executed after disk image was produced with postVM

  • the current nixpkgs can be realized as a channel in the disk image, which will change the hash of the image when the sources are updated

  • additional store paths can be provided through additionalPaths

Full NixOS image

  • arbitrary contents with permissions can be placed in the target filesystem using contents

  • a /etc/nixpkgs/nixos/configuration.nix can be provided through configFile

  • bootloaders are supported

  • EFI variables can be mutated during image production and the result is exposed in $out

  • boot partition size when partition table is efi or hybrid

On bit-to-bit reproducibility

Images are NOT deterministic, please do not hesitate to try to fix this, source of determinisms are (not exhaustive) :

  • bootloader installation have timestamps

  • SQLite Nix store database contain registration times

  • /etc/shadow is in a non-deterministic order

A deterministic flag is available for best efforts determinism.


To produce a Nix-store only image:

  pkgs = import <nixpkgs> {};
  lib = pkgs.lib;
  make-disk-image = import <nixpkgs/nixos/lib/make-disk-image.nix>;
  make-disk-image {
    inherit pkgs lib;
    config = {};
    additionalPaths = [ ];
    format = "qcow2";
    onlyNixStore = true;
    partitionTableType = "none";
    installBootLoader = false;
    touchEFIVars = false;
    diskSize = "auto";
    additionalSpace = "0M"; # Defaults to 512M.
    copyChannel = false;

Some arguments can be left out, they are shown explicitly for the sake of the example.

Building this derivation will provide a QCOW2 disk image containing only the Nix store and its registration information.

To produce a NixOS installation image disk with UEFI and bootloader installed:

  pkgs = import <nixpkgs> {};
  lib = pkgs.lib;
  make-disk-image = import <nixpkgs/nixos/lib/make-disk-image.nix>;
  evalConfig = import <nixpkgs/nixos/lib/eval-config.nix>;
  make-disk-image {
    inherit pkgs lib;
    config = evalConfig {
      modules = [
          fileSystems."/" = { device = "/dev/vda"; fsType = "ext4"; autoFormat = true; };
          boot.grub.device = "/dev/vda";
    format = "qcow2";
    onlyNixStore = false;
    partitionTableType = "legacy+gpt";
    installBootLoader = true;
    touchEFIVars = true;
    diskSize = "auto";
    additionalSpace = "0M"; # Defaults to 512M.
    copyChannel = false;
    memSize = 2048; # Qemu VM memory size in megabytes. Defaults to 1024M.


pkgs.mkBinaryCache is a function for creating Nix flat-file binary caches. Such a cache exists as a directory on disk, and can be used as a Nix substituter by passing --substituter file:///path/to/cache to Nix commands.

Nix packages are most commonly shared between machines using HTTP, SSH, or S3, but a flat-file binary cache can still be useful in some situations. For example, you can copy it directly to another machine, or make it available on a network file system. It can also be a convenient way to make some Nix packages available inside a container via bind-mounting.

Note that this function is meant for advanced use-cases. The more idiomatic way to work with flat-file binary caches is via the nix-copy-closure command. You may also want to consider dockerTools for your containerization needs.


The following derivation will construct a flat-file binary cache containing the closure of hello.

mkBinaryCache {
  rootPaths = [hello];
  • rootPaths specifies a list of root derivations. The transitive closure of these derivations’ outputs will be copied into the cache.

Here’s an example of building and using the cache.

Build the cache on one machine, host1:

nix-build -E 'with import <nixpkgs> {}; mkBinaryCache { rootPaths = [hello]; }'

Copy the resulting directory to the other machine, host2:

scp result host2:/tmp/hello-cache

Substitute the derivation using the flat-file binary cache on the other machine, host2:

nix-build -A hello '<nixpkgs>' \
  --option require-sigs false \
  --option trusted-substituters file:///tmp/hello-cache \
  --option substituters file:///tmp/hello-cache

Hooks reference

Nixpkgs has several hook packages that augment the stdenv phases.

The stdenv built-in hooks are documented in the section called “Package setup hooks”.


The autoreconfHook derivation adds autoreconfPhase, which runs autoreconf, libtoolize and automake, essentially preparing the configure script in autotools-based builds. Most autotools-based packages come with the configure script pre-generated, but this hook is necessary for a few packages and when you need to patch the package’s configure scripts.


Adds the share/aclocal subdirectory of each build input to the ACLOCAL_PATH environment variable.


This is a special setup hook which helps in packaging proprietary software in that it automatically tries to find missing shared library dependencies of ELF files based on the given buildInputs and nativeBuildInputs.

You can also specify a runtimeDependencies variable which lists dependencies to be unconditionally added to rpath of all executables. This is useful for programs that use dlopen 3 to load libraries at runtime.

In certain situations you may want to run the main command (autoPatchelf) of the setup hook on a file or a set of directories instead of unconditionally patching all outputs. This can be done by setting the dontAutoPatchelf environment variable to a non-empty value.

By default autoPatchelf will fail as soon as any ELF file requires a dependency which cannot be resolved via the given build inputs. In some situations you might prefer to just leave missing dependencies unpatched and continue to patch the rest. This can be achieved by setting the autoPatchelfIgnoreMissingDeps environment variable to a non-empty value. autoPatchelfIgnoreMissingDeps can be set to a list like autoPatchelfIgnoreMissingDeps = [ "" "" ]; or to [ "*" ] to ignore all missing dependencies.

The autoPatchelf command also recognizes a --no-recurse command line flag, which prevents it from recursing into subdirectories.


bmake is the portable variant of NetBSD make utility.

In Nixpkgs, bmake comes with a hook that overrides the default build, check, install and dist phases.


This hook will make a build pause instead of stopping when a failure happens. It prevents nix from cleaning up the build environment immediately and allows the user to attach to a build environment using the cntr command. Upon build error it will print instructions on how to use cntr, which can be used to enter the environment for debugging. Installing cntr and running the command will provide shell access to the build sandbox of failed build. At /var/lib/cntr the sandboxed filesystem is mounted. All commands and files of the system are still accessible within the shell. To execute commands from the sandbox use the cntr exec subcommand. cntr is only supported on Linux-based platforms. To use it first add cntr to your environment.systemPackages on NixOS or alternatively to the root user on non-NixOS systems. Then in the package that is supposed to be inspected, add breakpointHook to nativeBuildInputs.

nativeBuildInputs = [ breakpointHook ];

When a build failure happens there will be an instruction printed that shows how to attach with cntr to the build sandbox.


Caution with remote builds

This won’t work with remote builds as the build environment is on a different machine and can’t be accessed by cntr. Remote builds can be turned off by setting --option builders '' for nix-build or --builders '' for nix build.


Overrides the default configure phase to run the CMake command. By default, we use the Make generator of CMake. In addition, dependencies are added automatically to CMAKE_PREFIX_PATH so that packages are correctly detected by CMake. Some additional flags are passed in to give similar behavior to configure-based packages. You can disable this hook’s behavior by setting configurePhase to a custom value, or by setting dontUseCmakeConfigure. cmakeFlags controls flags passed only to CMake. By default, parallel building is enabled as CMake supports parallel building almost everywhere. When Ninja is also in use, CMake will detect that and use the ninja generator.


Exports GDK_PIXBUF_MODULE_FILE environment variable to the builder. Add librsvg package to buildInputs to get svg support. See also the setup hook description in GNOME platform docs.


Creates a temporary package database and registers every Haskell build input in it (TODO: how?).

GNOME platform

Hooks related to GNOME platform and related libraries like GLib, GTK and GStreamer are described in the section called “GNOME”.


This hook helps with installing manpages and shell completion files. It exposes 2 shell functions installManPage and installShellCompletion that can be used from your postInstall hook.

The installManPage function takes one or more paths to manpages to install. The manpages must have a section suffix, and may optionally be compressed (with .gz suffix). This function will place them into the correct directory.

The installShellCompletion function takes one or more paths to shell completion files. By default it will autodetect the shell type from the completion file extension, but you may also specify it by passing one of --bash, --fish, or --zsh. These flags apply to all paths listed after them (up until another shell flag is given). Each path may also have a custom installation name provided by providing a flag --name NAME before the path. If this flag is not provided, zsh completions will be renamed automatically such that foobar.zsh becomes _foobar. A root name may be provided for all paths using the flag --cmd NAME; this synthesizes the appropriate name depending on the shell (e.g. --cmd foo will synthesize the name foo.bash for bash and _foo for zsh). The path may also be a fifo or named fd (such as produced by <(cmd)), in which case the shell and name must be provided.

nativeBuildInputs = [ installShellFiles ];
postInstall = ''
  installManPage doc/foobar.1 doc/barfoo.3
  # explicit behavior
  installShellCompletion --bash --name foobar.bash share/completions.bash
  installShellCompletion --fish --name share/
  installShellCompletion --zsh --name _foobar share/completions.zsh
  # implicit behavior
  installShellCompletion share/completions/foobar.{bash,fish,zsh}
  # using named fd
  installShellCompletion --cmd foobar \
    --bash <($out/bin/foobar --bash-completion) \
    --fish <($out/bin/foobar --fish-completion) \
    --zsh <($out/bin/foobar --zsh-completion)

libiconv, libintl

A few libraries automatically add to NIX_LDFLAGS their library, making their symbols automatically available to the linker. This includes libiconv and libintl (gettext). This is done to provide compatibility between GNU Linux, where libiconv and libintl are bundled in, and other systems where that might not be the case. Sometimes, this behavior is not desired. To disable this behavior, set dontAddExtraLibs.


Adds every file named catalog.xml found under the xml/dtd and xml/xsl subdirectories of each build input to the XML_CATALOG_FILES environment variable.


Meson is an open source meta build system meant to be fast and user-friendly.

In Nixpkgs, meson comes with a setup hook that overrides the configure, check, and install phases.

Being a meta build system, meson needs an accompanying backend. In the context of Nixpkgs, the typical companion backend is Ninja, that provides a setup hook registering ninja-based build and install phases.

Variables controlling Meson

Meson Exclusive Variables


Controls the flags passed to meson setup during configure phase.


Which value is passed as -Dwrap_mode= to. In Nixpkgs the default value is nodownload, so that no subproject will be downloaded (since network access is already disabled during deployment in Nixpkgs).

Note: Meson allows pre-population of subprojects that would otherwise be downloaded.


Which value is passed as --buildtype to meson setup during configure phase. In Nixpkgs the default value is plain.


Which value is passed as -Dauto_features= to meson setup during configure phase. In Nixpkgs the default value is enabled, meaning that every feature declared as “auto” by the meson scripts will be enabled.


Controls the flags passed to meson test during check phase.


Controls the flags passed to meson install during install phase.


A list of installation tags passed to Meson’s commandline option --tags during install phase.

Note: mesonInstallTags should be a list of strings, that will be converted to a comma-separated string that is recognized to --tags. Example: mesonInstallTags = [ "emulator" "assembler" ]; will be converted to --tags emulator,assembler.


When set to true, don’t use the predefined mesonConfigurePhase.


When set to true, don’t use the predefined mesonCheckPhase.


When set to true, don’t use the predefined mesonInstallPhase.

Honored variables

The following variables commonly used by stdenv.mkDerivation are honored by Meson setup hook.

  • prefixKey

  • enableParallelBuilding


This hook can be used to setup a check phase that requires running a MPI application. It detects the used present MPI implementation type and exports the neceesary environment variables to use mpirun and mpiexec in a Nix sandbox.


  { mpiCheckPhaseHook, mpi, ... }:


  nativeCheckInputs = [


Overrides the build, install, and check phase to run ninja instead of make. You can disable this behavior with the dontUseNinjaBuild, dontUseNinjaInstall, and dontUseNinjaCheck, respectively. Parallel building is enabled by default in Ninja.

Note that if the Meson setup hook is also active, Ninja’s install and check phases will be disabled in favor of Meson’s.

patchRcPath hooks

These hooks provide shell-specific utilities (with the same name as the hook) to patch shell scripts meant to be sourced by software users.

The typical usage is to patch initialisation or rc scripts inside $out/bin or $out/etc. Such scripts, when being sourced, would insert the binary locations of certain commands into PATH, modify other environment variables or run a series of start-up commands. When shipped from the upstream, they sometimes use commands that might not be available in the environment they are getting sourced in.

The compatible shells for each hook are:

  • patchRcPathBash: Bash, ksh, zsh and other shells supporting the Bash-like parameter expansions.

  • patchRcPathCsh: Csh scripts, such as those targeting tcsh.

  • patchRcPathFish: Fish scripts.

  • patchRcPathPosix: POSIX-conformant shells supporting the limited parameter expansions specified by the POSIX standard. Current implementation uses the parameter expansion ${foo-} only.

For each supported shell, it modifies the script with a PATH prefix that is later removed when the script ends. It allows nested patching, which guarantees that a patched script may source another patched script.

Syntax to apply the utility to a script:

patchRcPath<shell> <file> <PATH-prefix>

Example usage:

Given a package foo containing an init script that depends on coreutils, man and which, patch the init script for users to source without having the above dependencies in their PATH:

{ lib, stdenv, patchRcPathFish}:
stdenv.mkDerivation {

  # ...

  nativeBuildInputs = [

  postFixup = ''
    patchRcPathFish $out/bin/ ${lib.makeBinPath [ coreutils man which ]}


patchRcPathCsh and patchRcPathPosix implementation depends on sed to do the string processing. The others are in vanilla shell and have no third-party dependencies.


Adds the lib/site_perl subdirectory of each build input to the PERL5LIB environment variable. For instance, if buildInputs contains Perl, then the lib/site_perl subdirectory of each input is added to the PERL5LIB environment variable.


Adds the lib/pkgconfig and share/pkgconfig subdirectories of each build input to the PKG_CONFIG_PATH environment variable.


This hook starts a PostgreSQL server during the checkPhase. Example:

{ stdenv, postgresql, postgresqlTestHook }:
stdenv.mkDerivation {

  # ...

  nativeCheckInputs = [

If you use a custom checkPhase, remember to add the runHook calls:

  checkPhase ''
    runHook preCheck

    # ... your tests

    runHook postCheck


The hook logic will read a number of variables and set them to a default value if unset or empty.

Exported variables:

  • PGDATA: location of server files.

  • PGHOST: location of UNIX domain socket directory; the default host in a connection string.

  • PGUSER: user to create / log in with, default: test_user.

  • PGDATABASE: database name, default: test_db.

Bash-only variables:

  • postgresqlTestUserOptions: SQL options to use when creating the $PGUSER role, default: "LOGIN". Example: "LOGIN SUPERUSER"

  • postgresqlTestSetupSQL: SQL commands to run as database administrator after startup, default: statements that create $PGUSER and $PGDATABASE.

  • postgresqlTestSetupCommands: bash commands to run after database start, defaults to running $postgresqlTestSetupSQL as database administrator.

  • postgresqlEnableTCP: set to 1 to enable TCP listening. Flaky; not recommended.

  • postgresqlStartCommands: defaults to pg_ctl start.


A number of additional hooks are ran in postgresqlTestHook

  • postgresqlTestSetupPost: ran after postgresql has been set up.

TCP and the Nix sandbox

postgresqlEnableTCP relies on network sandboxing, which is not available on macOS and some custom Nix installations, resulting in flaky tests. For this reason, it is disabled by default.

The preferred solution is to make the test suite use a UNIX domain socket connection. This is the default behavior when no host connection parameter is provided. Some test suites hardcode a value for host though, so a patch may be required. If you can upstream the patch, you can make host default to the PGHOST environment variable when set. Otherwise, you can patch it locally to omit the host connection string parameter altogether.


The error libpq: failed (could not receive data from server: Connection refused is generally an indication that the test suite is trying to connect through TCP.


Adds the lib/${python.libPrefix}/site-packages subdirectory of each build input to the PYTHONPATH environment variable.


Overrides the build, install, and check phases. This uses the scons build system as a replacement for make. scons does not provide a configure phase, so everything is managed at build and install time.

teTeX / TeX Live

Adds the share/texmf-nix subdirectory of each build input to the TEXINPUTS environment variable.


This setup hook will allow you to unzip .zip files specified in $src. There are many similar packages like unrar, undmg, etc.


The validatePkgConfig hook validates all pkg-config (.pc) files in a package. This helps catching some common errors in pkg-config files, such as undefined variables.


Waf is a Python-based software building system.

In Nixpkgs, wafHook overrides the default configure, build, and install phases.

Variables controlling wafHook

wafHook Exclusive Variables

The variables below are exclusive of wafHook.


Location of the waf tool. It defaults to ./waf, to honor software projects that include it directly inside their source trees.

If wafPath doesn’t exist, then wafHook will copy the waf provided from Nixpkgs to it.


Controls the flags passed to waf tool during build and install phases. For settings specific to build or install phases, use wafBuildFlags or wafInstallFlags respectively.


When set to true, don’t add cross compilation flags during configure phase.


When set to true, don’t use the predefined wafConfigurePhase.


When set to true, don’t use the predefined wafBuildPhase.


When set to true, don’t use the predefined wafInstallPhase.

Similar variables

The following variables are similar to their stdenv.mkDerivation counterparts.

wafHook Variablestdenv.mkDerivation Counterpart

Honored variables

The following variables commonly used by stdenv.mkDerivation are honored by wafHook.

  • prefixKey

  • enableParallelBuilding

  • enableParallelInstalling


Zig is a general-purpose programming language and toolchain for maintaining robust, optimal and reusable software.

In Nixpkgs, zig.hook overrides the default build, check and install phases.

Example code snippet

{ lib
, stdenv
, zig_0_11

stdenv.mkDerivation {
  # . . .

  nativeBuildInputs = [

  zigBuildFlags = [ "-Dman-pages=true" ];

  dontUseZigCheck = true;

  # . . .

Variables controlling zig.hook

zig.hook Exclusive Variables

The variables below are exclusive to zig.hook.


Disables using zigBuildPhase.


Disables using zigCheckPhase.


Disables using zigInstallPhase.

Similar variables

The following variables are similar to their stdenv.mkDerivation counterparts.

zig.hook Variablestdenv.mkDerivation Counterpart

Variables honored by zig.hook

The following variables commonly used by stdenv.mkDerivation are honored by zig.hook.

  • prefixKey

  • dontAddPrefix


Overrides the build and install phases to run the “xcbuild” command. This hook is needed when a project only comes with build files for the XCode build system. You can disable this behavior by setting buildPhase and configurePhase to a custom value. xcbuildFlags controls flags passed only to xcbuild.

Languages and frameworks

The standard build environment makes it easy to build typical Autotools-based packages with very little code. Any other kind of package can be accommodated by overriding the appropriate phases of stdenv. However, there are specialised functions in Nixpkgs to easily build packages for other programming languages, such as Perl or Haskell. These are described in this chapter.


How to use Agda

Agda is available as the agda package.

The agda package installs an Agda-wrapper, which calls agda with --library-file set to a generated library-file within the nix store, this means your library-file in $HOME/.agda/libraries will be ignored. By default the agda package installs Agda with no libraries, i.e. the generated library-file is empty. To use Agda with libraries, the agda.withPackages function can be used. This function either takes:

  • A list of packages,

  • or a function which returns a list of packages when given the agdaPackages attribute set,

  • or an attribute set containing a list of packages and a GHC derivation for compilation (see below).

  • or an attribute set containing a function which returns a list of packages when given the agdaPackages attribute set and a GHC derivation for compilation (see below).

For example, suppose we wanted a version of Agda which has access to the standard library. This can be obtained with the expressions:

agda.withPackages [ agdaPackages.standard-library ]


agda.withPackages (p: [ p.standard-library ])

or can be called as in the Compiling Agda section.

If you want to use a different version of a library (for instance a development version) override the src attribute of the package to point to your local repository

agda.withPackages (p: [
  (p.standard-library.overrideAttrs (oldAttrs: {
    version = "local version";
    src = /path/to/local/repo/agda-stdlib;

You can also reference a GitHub repository

agda.withPackages (p: [
  (p.standard-library.overrideAttrs (oldAttrs: {
    version = "1.5";
    src =  fetchFromGitHub {
      repo = "agda-stdlib";
      owner = "agda";
      rev = "v1.5";
      hash = "sha256-nEyxYGSWIDNJqBfGpRDLiOAnlHJKEKAOMnIaqfVZzJk=";

If you want to use a library not added to Nixpkgs, you can add a dependency to a local library by calling agdaPackages.mkDerivation.

agda.withPackages (p: [
  (p.mkDerivation {
    pname = "your-agda-lib";
    version = "1.0.0";
    src = /path/to/your-agda-lib;

Again you can reference GitHub

agda.withPackages (p: [
  (p.mkDerivation {
    pname = "your-agda-lib";
    version = "1.0.0";
    src = fetchFromGitHub {
      repo = "repo";
      owner = "owner";
      version = "...";
      rev = "...";
      hash = "...";

See Building Agda Packages for more information on mkDerivation.

Agda will not by default use these libraries. To tell Agda to use a library we have some options:

  • Call agda with the library flag:

    $ agda -l standard-library -i . MyFile.agda
  • Write a my-library.agda-lib file for the project you are working on which may look like:

    name: my-library
    include: .
    depend: standard-library
  • Create the file ~/.agda/defaults and add any libraries you want to use by default.

More information can be found in the official Agda documentation on library management.

Compiling Agda

Agda modules can be compiled using the GHC backend with the --compile flag. A version of ghc with ieee754 is made available to the Agda program via the --with-compiler flag. This can be overridden by a different version of ghc as follows:

agda.withPackages {
  pkgs = [ ... ];
  ghc = haskell.compiler.ghcHEAD;

Writing Agda packages

To write a nix derivation for an Agda library, first check that the library has a *.agda-lib file.

A derivation can then be written using agdaPackages.mkDerivation. This has similar arguments to stdenv.mkDerivation with the following additions:

  • everythingFile can be used to specify the location of the Everything.agda file, defaulting to ./Everything.agda. If this file does not exist then either it should be patched in or the buildPhase should be overridden (see below).

  • libraryName should be the name that appears in the *.agda-lib file, defaulting to pname.

  • libraryFile should be the file name of the *.agda-lib file, defaulting to ${libraryName}.agda-lib.

Here is an example default.nix

{ nixpkgs ?  <nixpkgs> }:
with (import nixpkgs {});
agdaPackages.mkDerivation {
  version = "1.0";
  pname = "my-agda-lib";
  src = ./.;
  buildInputs = [

Building Agda packages

The default build phase for agdaPackages.mkDerivation runs agda on the Everything.agda file. If something else is needed to build the package (e.g. make) then the buildPhase should be overridden. Additionally, a preBuild or configurePhase can be used if there are steps that need to be done prior to checking the Everything.agda file. agda and the Agda libraries contained in buildInputs are made available during the build phase.

Installing Agda packages

The default install phase copies Agda source files, Agda interface files (*.agdai) and *.agda-lib files to the output directory. This can be overridden.

By default, Agda sources are files ending on .agda, or literate Agda files ending on .lagda, .lagda.tex,,, .lagda.rst. The list of recognised Agda source extensions can be extended by setting the extraExtensions config variable.

Maintaining the Agda package set on Nixpkgs

We are aiming at providing all common Agda libraries as packages on nixpkgs, and keeping them up to date. Contributions and maintenance help is always appreciated, but the maintenance effort is typically low since the Agda ecosystem is quite small.

The nixpkgs Agda package set tries to take up a role similar to that of Stackage in the Haskell world. It is a curated set of libraries that:

  1. Always work together.

  2. Are as up-to-date as possible.

While the Haskell ecosystem is huge, and Stackage is highly automatised, the Agda package set is small and can (still) be maintained by hand.

Adding Agda packages to Nixpkgs

To add an Agda package to nixpkgs, the derivation should be written to pkgs/development/libraries/agda/${library-name}/ and an entry should be added to pkgs/top-level/agda-packages.nix. Here it is called in a scope with access to all other Agda libraries, so the top line of the default.nix can look like:

{ mkDerivation, standard-library, fetchFromGitHub }:

Note that the derivation function is called with mkDerivation set to agdaPackages.mkDerivation, therefore you could use a similar set as in your default.nix from Writing Agda Packages with agdaPackages.mkDerivation replaced with mkDerivation.

Here is an example skeleton derivation for iowa-stdlib:

mkDerivation {
  version = "1.5.0";
  pname = "iowa-stdlib";

  src = ...

  libraryFile = "";
  libraryName = "IAL-1.3";

  buildPhase = ''

This library has a file called .agda-lib, and so we give an empty string to libraryFile as nothing precedes .agda-lib in the filename. This file contains name: IAL-1.3, and so we let libraryName = "IAL-1.3". This library does not use an Everything.agda file and instead has a Makefile, so there is no need to set everythingFile and we set a custom