List of Tables

9.1. Possible dependency types

Table of Contents

1.1. 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/X11 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.

1.1. 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-19.09, which includes all packages and modules for the stable NixOS 19.09. 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-19.09-small).

Chapter 2. 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.

2.1. 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;

2.2. 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.

2.3. Installing unfree packages

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 flash player and visual studio code:

      allowUnfreePredicate = pkg: builtins.elem (lib.getName pkg) [

  • It is also possible to whitelist and blacklist licenses that are specifically acceptable or not acceptable, using whitelistedLicenses and blacklistedLicenses, respectively.

    The following example configuration whitelists the licenses amd and wtfpl:

      whitelistedLicenses = with stdenv.lib.licenses; [ amd wtfpl ];

    The following example configuration blacklists the gpl3Only and agpl3Only licenses:

      blacklistedLicenses = with stdenv.lib.licenses; [ agpl3Only gpl3Only ];

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

2.4. 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.

2.5. 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 = { ... };

2.6. Declarative Package Management

2.6.1. 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.

2.6.2. 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 source $HOME/.profile and you can starting loading man pages from your environment.

2.6.3. 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.

Chapter 3. Overlays

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

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

3.1. Installing overlays

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

3.1.1. 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 ]; }.

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.

3.1.2. 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.

3.2. 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 Section 2.5, “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.

3.3. 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)

    The Nixpkgs attribute is lapack-reference.

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

    The Nixpkgs attribute is mkl.

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

    The AMD BLIS library, with attribute amd-blis, provides a BLAS implementation. 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 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.

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 versions of BLAS/LAPACK currently in the wild, LP64 (integer size = 32 bits) and ILP64 (integer size = 64 bits). 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 {

Chapter 4. Overriding

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

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

4.1. <pkg>.override

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

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

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

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.

4.2. <pkg>.overrideAttrs

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

Example usage:

helloWithDebug = pkgs.hello.overrideAttrs (oldAttrs: rec {
  separateDebugInfo = true;

In the above example, the separateDebugInfo attribute is overridden to be true, thus building debug info for helloWithDebug, while all other attributes will be retained from the original hello package.

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

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

4.3. <pkg>.overrideDerivation

Warning: You should prefer overrideAttrs in almost all cases, see its documentation for the reasons why. overrideDerivation is not deprecated and will continue to work, but is less nice to use and does not have as many abilities as overrideAttrs.
Warning: Do not use this function in Nixpkgs as it evaluates a Derivation before modifying it, which breaks package abstraction and removes error-checking of function arguments. 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 =;
    sha256 = "11nq06d131y4wmf3drm0yk502d2xc6n5qy82cg88rb9nqd2lj41k";
  patches = [];

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

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

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

4.4. lib.makeOverridable

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

Example usage:

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

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

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

Chapter 5. Functions reference

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

5.1. Nixpkgs Library Functions

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

5.1.1. Assert functions lib.asserts.assertMsg

assertMsg :: Bool -> String -> Bool

Located at lib/asserts.nix:21 in <nixpkgs>.

Print a trace message if pred is false.

Intended to be used to augment asserts with helpful error messages.


Condition under which the msg should not be printed.


Message to print.

Example 5.1. Printing when the predicate is false

assert lib.asserts.assertMsg ("foo" == "bar") "foo is not bar, silly"
stderr> trace: foo is not bar, silly
stderr> assert failed lib.asserts.assertOneOf

assertOneOf :: String -> String -> StringList -> Bool

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

Specialized asserts.assertMsg for checking if val is one of the elements of 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 5.2. Ensuring a user provided a possible value

let sslLibrary = "bearssl";
in lib.asserts.assertOneOf "sslLibrary" sslLibrary [ "openssl" "bearssl" ];
=> false
stderr> trace: sslLibrary must be one of "openssl", "libressl", but is: "bearssl"

5.1.2. Attribute-Set Functions lib.attrset.attrByPath

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

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

Return an attribute from within nested attribute sets.


A list of strings representing the path through the nested attribute set set.


Default value if attrPath does not resolve to an existing value.


The nested attributeset to select values from.

Example 5.3. Extracting a value from a nested attribute set

let set = { a = { b = 3; }; };
in lib.attrsets.attrByPath [ "a" "b" ] 0 set
=> 3

Example 5.4. No value at the path, instead using the default

lib.attrsets.attrByPath [ "a" "b" ] 0 {}
=> 0 lib.attrsets.hasAttrByPath

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

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

Determine if an attribute exists within a nested attribute set.


A list of strings representing the path through the nested attribute set set.


The nested attributeset to check.

Example 5.5. A nested value does exist inside a set

  [ "a" "b" "c" "d" ]
  { a = { b = { c = { d = 123; }; }; }; }
=> true lib.attrsets.setAttrByPath

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

Located at lib/attrsets.nix:57 in <nixpkgs>.

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


A list of strings representing the path through the nested attribute set.


The value to set at the location described by attrPath.

Example 5.6. Creating a new nested attribute set

lib.attrsets.setAttrByPath [ "a" "b" ] 3
=> { a = { b = 3; }; } lib.attrsets.getAttrFromPath

getAttrFromPath :: [String] -> AttrSet -> Value

Located at lib/attrsets.nix:73 in <nixpkgs>.

Like Section, “lib.attrset.attrByPath except without a default, and it will throw if the value doesn't exist.


A list of strings representing the path through the nested attribute set set.


The nested attribute set to find the value in.

Example 5.7. Succesfully getting a value from an attribute set

lib.attrsets.getAttrFromPath [ "a" "b" ] { a = { b = 3; }; }
=> 3

Example 5.8. Throwing after failing to get a value from an attribute set

lib.attrsets.getAttrFromPath [ "x" "y" ] { }
=> error: cannot find attribute `x.y' lib.attrsets.attrVals

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

Located at lib/attrsets.nix:84 in <nixpkgs>.

Return the specified attributes from a set. All values must exist.


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 5.9. Getting several values from an attribute set

lib.attrsets.attrVals [ "a" "b" "c" ] { a = 1; b = 2; c = 3; }
=> [ 1 2 3 ]

Example 5.10. Getting missing values from an attribute set

lib.attrsets.attrVals [ "d" ] { }
error: attribute 'd' missing lib.attrsets.attrValues

attrValues :: AttrSet -> [Any]

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

Get all the attribute values from an attribute set.

Provides a backwards-compatible interface of builtins.attrValues for Nix version older than 1.8.


The attribute set.

Example 5.11. 

lib.attrsets.attrValues { a = 1; b = 2; c = 3; }
=> [ 1 2 3 ] lib.attrsets.catAttrs

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

Located at lib/attrsets.nix:113 in <nixpkgs>.

Collect each attribute named `attr' from the list of attribute sets, sets. Sets that don't contain the named attribute are ignored.

Provides a backwards-compatible interface of builtins.catAttrs for Nix version older than 1.9.


Attribute name to select from each attribute set in sets.


The list of attribute sets to select attr from.

Example 5.12. Collect an attribute from a list of attribute sets.

Attribute sets which don't have the attribute are ignored.

catAttrs "a" [{a = 1;} {b = 0;} {a = 2;}]
=> [ 1 2 ] lib.attrsets.filterAttrs

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

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

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


String -> Any -> Bool

Predicate which returns true to include an attribute, or returns false to exclude it.


The attribute's name


The attribute's value

Returns true to include the attribute, false to exclude the attribute.


The attribute set to filter

Example 5.13. Filtering an attributeset

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

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

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

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


String -> Any -> Bool

Predicate which returns true to include an attribute, or returns false to exclude it.


The attribute's name


The attribute's value

Returns true to include the attribute, false to exclude the attribute.


The attribute set to filter

Example 5.14. Recursively filtering an attribute set

  (n: v: v != null)
    levelA = {
      example = "hi";
      levelB = {
        hello = "there";
        this-one-is-present = {
          this-is-excluded = null;
      this-one-is-also-excluded = null;
    also-excluded = null;
=> {
     levelA = {
       example = "hi";
       levelB = {
         hello = "there";
         this-one-is-present = { };
   } lib.attrsets.foldAttrs

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

Located at lib/attrsets.nix:154 in <nixpkgs>.

Apply fold function to values grouped by key.


Any -> Any -> Any

Given a value val and a collector col, combine the two.


An attribute's value


The result of previous op calls with other values and nul.


The null-value, the starting value.


A list of attribute sets to fold together by key.

Example 5.15. Combining an attribute of lists in to one attribute set

  (n: a: [n] ++ a) []
    { a = 2; b = 7; }
    { a = 3; }
    { b = 6; }
=> { a = [ 2 3 ]; b = [ 7 6 ]; } lib.attrsets.collect

collect :: (Any -> Bool) -> AttrSet -> [Any]

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

Recursively collect sets that verify a given predicate named pred from the set attrs. The recursion stops when pred returns true.


Any -> Bool

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


The attribute set value.


The attribute set to recursively collect.

Example 5.16. Collecting all lists from an attribute set

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

Example 5.17. Collecting all attribute-sets which contain the outPath attribute name.

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

nameValuePair :: String -> Any -> AttrSet

Located at lib/attrsets.nix:194 in <nixpkgs>.

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


The attribute name.


The attribute value.

Example 5.18. Creating a name value pair

nameValuePair "some" 6
=> { name = "some"; value = 6; } lib.attrsets.mapAttrs

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

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

Provides a backwards-compatible interface of builtins.mapAttrs for Nix version older than 2.1.


String -> Any -> Any

Given an attribute's name and value, return a new value.


The name of the attribute.


The attribute's value.

Example 5.19. Modifying each value of an attribute set

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

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

Located at lib/attrsets.nix:221 in <nixpkgs>.

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.


String -> Any -> { name = String; value = Any }

Given an attribute's name and value, return a new name value pair.


The name of the attribute.


The attribute's value.


The attribute set to map over.

Example 5.20. Change the name and value of each attribute of an attribute set

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

mapAttrsToList :: (String -> Any -> Any) -> AttrSet -> Any

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

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


String -> Any -> Any

Given an attribute's name and value, return a new value.


The name of the attribute.


The attribute's value.


The attribute set to map over.

Example 5.21. Combine attribute values and names in to a list

lib.attrsets.mapAttrsToList (name: value: "${name}=${value}")
   { x = "a"; y = "b"; }
=> [ "x=a" "y=b" ] lib.attrsets.mapAttrsRecursive

mapAttrsRecursive :: ([String] > Any -> Any) -> AttrSet -> AttrSet

Located at lib/attrsets.nix:250 in <nixpkgs>.

Like mapAttrs, except that it recursively applies itself to attribute sets. Also, the first argument of the argument function is a list of the names of the containing attributes.


[ String ] -> Any -> Any

Given a list of attribute names and value, return a new value.


The list of attribute names to this value.

For example, the name_path for the example string in the attribute set { foo = { bar = "example"; }; } is [ "foo" "bar" ].


The attribute's value.


The attribute set to recursively map over.

Example 5.22. A contrived example of using lib.attrsets.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";
   } lib.attrsets.mapAttrsRecursiveCond

mapAttrsRecursiveCond :: (AttrSet -> Bool) -> ([ String ] -> Any -> Any) -> AttrSet -> AttrSet

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

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


(AttrSet -> Bool)

Determine if mapAttrsRecursive should recurse deeper in to the attribute set.


An attribute set.


[ String ] -> Any -> Any

Given a list of attribute names and value, return a new value.


The list of attribute names to this value.

For example, the name_path for the example string in the attribute set { foo = { bar = "example"; }; } is [ "foo" "bar" ].


The attribute's value.


The attribute set to recursively map over.

Example 5.23. Only convert attribute values to JSON if the containing attribute set is marked for recursion

  ({ recurse ? false, ... }: recurse)
  (name: value: builtins.toJSON value)
    dorecur = {
      recurse = true;
      hello = "there";
    dontrecur = {
      converted-to- = "json";
=> {
     dorecur = {
       hello = "\"there\"";
       recurse = "true";
     dontrecur = "{\"converted-to\":\"json\"}";
   } lib.attrsets.genAttrs

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

Located at lib/attrsets.nix:291 in <nixpkgs>.

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


Names of values in the resulting attribute set.


String -> Any

Takes the name of the attribute and return the attribute's value.


The name of the attribute to generate a value for.

Example 5.24. Generate an attrset based on names only

lib.attrsets.genAttrs [ "foo" "bar" ] (name: "x_${name}")
=> { foo = "x_foo"; bar = "x_bar"; } lib.attrsets.isDerivation

isDerivation :: Any -> Bool

Located at lib/attrsets.nix:305 in <nixpkgs>.

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


The value which is possibly a derivation.

Example 5.25. A package is a derivation

lib.attrsets.isDerivation (import <nixpkgs> {}).ruby
=> true

Example 5.26. Anything else is not a derivation

lib.attrsets.isDerivation "foobar"
=> false lib.attrsets.toDerivation

toDerivation :: Path -> Derivation

Located at lib/attrsets.nix:308 in <nixpkgs>.

Converts a store path to a fake derivation.


A store path to convert to a derivation. lib.attrsets.optionalAttrs

optionalAttrs :: Bool -> AttrSet

Located at lib/attrsets.nix:331 in <nixpkgs>.

Conditionally return an attribute set or an empty attribute set.


Condition under which the as attribute set is returned.


The attribute set to return if cond is true.

Example 5.27. Return the provided attribute set when cond is true

lib.attrsets.optionalAttrs true { my = "set"; }
=> { my = "set"; }

Example 5.28. Return an empty attribute set when cond is false

lib.attrsets.optionalAttrs false { my = "set"; }
=> { } lib.attrsets.zipAttrsWithNames

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

Located at lib/attrsets.nix:341 in <nixpkgs>.

Merge sets of attributes and use the function f to merge attribute values where the attribute name is in names.


A list of attribute names to zip.


(String -> [ Any ] -> Any

Accepts an attribute name, all the values, and returns a combined value.


The name of the attribute each value came from.


A list of values collected from the list of attribute sets.


A list of attribute sets to zip together.

Example 5.29. Summing a list of attribute sets of numbers

  [ "a" "b" ]
  (name: vals: "${name} ${toString (builtins.foldl' (a: b: a + b) 0 vals)}")
    { a = 1; b = 1; c = 1; }
    { a = 10; }
    { b = 100; }
    { c = 1000; }
=> { a = "a 11"; b = "b 101"; } lib.attrsets.zipAttrsWith

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

Located at lib/attrsets.nix:356 in <nixpkgs>.

Merge sets of attributes and use the function f to merge attribute values. Similar to Section, “lib.attrsets.zipAttrsWithNames where all key names are passed for names.


(String -> [ Any ] -> Any

Accepts an attribute name, all the values, and returns a combined value.


The name of the attribute each value came from.


A list of values collected from the list of attribute sets.


A list of attribute sets to zip together.

Example 5.30. Summing a list of attribute sets of numbers

  (name: vals: "${name} ${toString (builtins.foldl' (a: b: a + b) 0 vals)}")
    { a = 1; b = 1; c = 1; }
    { a = 10; }
    { b = 100; }
    { c = 1000; }
=> { a = "a 11"; b = "b 101"; c = "c 1001"; } lib.attrsets.zipAttrs

zipAttrsWith :: [ AttrSet ] -> AttrSet

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

Merge sets of attributes and combine each attribute value in to a list. Similar to Section, “lib.attrsets.zipAttrsWith where the merge function returns a list of all values.


A list of attribute sets to zip together.

Example 5.31. Combining a list of attribute sets

    { a = 1; b = 1; c = 1; }
    { a = 10; }
    { b = 100; }
    { c = 1000; }
=> { a = [ 1 10 ]; b = [ 1 100 ]; c = [ 1 1000 ]; } lib.attrsets.recursiveUpdateUntil

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

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

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 verified, the value of the first attribute set is replaced by the value of the second attribute set.


[ String ] -> AttrSet -> AttrSet -> Bool


The path to the values in the left and right hand sides.


The left hand side value.


The right hand side value.


The left hand attribute set of the merge.


The right hand attribute set of the merge.

Example 5.32. Recursively merging two attribute sets

lib.attrsets.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
} lib.attrsets.recursiveUpdate

recursiveUpdate :: AttrSet -> AttrSet -> AttrSet

Located at lib/attrsets.nix:424 in <nixpkgs>.

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.


The left hand attribute set of the merge.


The right hand attribute set of the merge.

Example 5.33. Recursively merging two attribute sets

    boot.loader.grub.enable = true;
    boot.loader.grub.device = "/dev/hda";
    boot.loader.grub.device = "";
=> {
  boot.loader.grub.enable = true;
  boot.loader.grub.device = "";
} lib.attrsets.recurseIntoAttrs

recurseIntoAttrs :: AttrSet -> AttrSet

Located at lib/attrsets.nix:483 in <nixpkgs>.

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 5.34. Making Nix look inside an attribute set

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

5.1.3. String manipulation functions lib.strings.concatStrings

concatStrings :: [string] -> string

Concatenate a list of strings.

Example 5.35. lib.strings.concatStrings usage example

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

Located at lib/strings.nix:21 in <nixpkgs>. lib.strings.concatMapStrings

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

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


Function argument


Function argument

Example 5.36. lib.strings.concatMapStrings usage example

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

Located at lib/strings.nix:31 in <nixpkgs>. lib.strings.concatImapStrings

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 5.37. lib.strings.concatImapStrings usage example

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

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

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

Place an element between each element of a list


Separator to add between elements


Input list

Example 5.38. lib.strings.intersperse usage example

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

Located at lib/strings.nix:52 in <nixpkgs>. lib.strings.concatStringsSep

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

Concatenate a list of strings with a separator between each element

Example 5.39. lib.strings.concatStringsSep usage example

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

Located at lib/strings.nix:69 in <nixpkgs>. lib.strings.concatMapStringsSep

concatMapStringsSep :: string -> (string -> string) -> [string] -> 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 5.40. lib.strings.concatMapStringsSep usage example

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

Located at lib/strings.nix:82 in <nixpkgs>. lib.strings.concatImapStringsSep

concatIMapStringsSep :: string -> (int -> string -> string) -> [string] -> 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 5.41. lib.strings.concatImapStringsSep usage example

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

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

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 5.42. 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:118 in <nixpkgs>. lib.strings.makeSearchPathOutput

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 5.43. 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:136 in <nixpkgs>. lib.strings.makeLibraryPath

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

Example 5.44. 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:154 in <nixpkgs>. lib.strings.makeBinPath

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

Example 5.45. lib.strings.makeBinPath usage example

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

Located at lib/strings.nix:163 in <nixpkgs>. lib.strings.optionalString

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 5.46. lib.strings.optionalString usage example

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

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

hasPrefix :: string -> string -> bool

Determine whether a string has given prefix.


Prefix to check for


Input string

Example 5.47. lib.strings.hasPrefix usage example

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

Located at lib/strings.nix:192 in <nixpkgs>. lib.strings.hasSuffix

hasSuffix :: string -> string -> bool

Determine whether a string has given suffix.


Suffix to check for


Input string

Example 5.48. lib.strings.hasSuffix usage example

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

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

hasInfix :: string -> string -> bool

Determine whether a string contains the given infix


Function argument


Function argument

Example 5.49. 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:233 in <nixpkgs>. lib.strings.stringToCharacters

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 5.50. lib.strings.stringToCharacters usage example

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

Located at lib/strings.nix:257 in <nixpkgs>. lib.strings.stringAsChars

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 5.51. lib.strings.stringAsChars usage example

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

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

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

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


Function argument

Example 5.52. lib.strings.escape usage example

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

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

escapeShellArg :: string -> string

Quote string to be used safely within the Bourne shell.


Function argument

Example 5.53. lib.strings.escapeShellArg usage example

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

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

escapeShellArgs :: [string] -> string

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

Example 5.54. lib.strings.escapeShellArgs usage example

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

Located at lib/strings.nix:306 in <nixpkgs>. lib.strings.escapeNixString

string -> string

Turn a string into a Nix expression representing that string


Function argument

Example 5.55. lib.strings.escapeNixString usage example

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

Located at lib/strings.nix:316 in <nixpkgs>. lib.strings.escapeNixIdentifier

string -> string

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


Function argument

Example 5.56. lib.strings.escapeNixIdentifier usage example

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

Located at lib/strings.nix:328 in <nixpkgs>. lib.strings.toLower

toLower :: string -> string

Converts an ASCII string to lower-case.

Example 5.57. lib.strings.toLower usage example

toLower "HOME"
=> "home"

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

toUpper :: string -> string

Converts an ASCII string to upper-case.

Example 5.58. lib.strings.toUpper usage example

toUpper "home"
=> "HOME"

Located at lib/strings.nix:369 in <nixpkgs>. lib.strings.addContextFrom

Appends string context from another string. This is an implementation detail of Nix.

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 5.59. lib.strings.addContextFrom usage example

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

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

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

NOTE: this function is not performant and should never be used.


Function argument


Function argument

Example 5.60. lib.strings.splitString usage example

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

Located at lib/strings.nix:397 in <nixpkgs>. lib.strings.removePrefix

string -> string -> string

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


Prefix to remove if it matches


Input string

Example 5.61. lib.strings.removePrefix usage example

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

Located at lib/strings.nix:430 in <nixpkgs>. lib.strings.removeSuffix

string -> string -> string

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


Suffix to remove if it matches


Input string

Example 5.62. lib.strings.removeSuffix usage example

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

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

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


Function argument


Function argument

Example 5.63. lib.strings.versionOlder usage example

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

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

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


Function argument


Function argument

Example 5.64. 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:488 in <nixpkgs>. lib.strings.getName

This function takes an argument that's either a derivation or a derivation's "name" attribute and extracts the name part from that argument.


Function argument

Example 5.65. lib.strings.getName usage example

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

Located at lib/strings.nix:500 in <nixpkgs>. lib.strings.getVersion

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


Function argument

Example 5.66. lib.strings.getVersion usage example

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

Located at lib/strings.nix:517 in <nixpkgs>. lib.strings.nameFromURL

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


Function argument


Function argument

Example 5.67. lib.strings.nameFromURL usage example

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

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

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


Function argument


Function argument

Example 5.68. lib.strings.enableFeature usage example

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

Located at lib/strings.nix:549 in <nixpkgs>. lib.strings.enableFeatureAs

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


Function argument


Function argument


Function argument

Example 5.69. lib.strings.enableFeatureAs usage example

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

Located at lib/strings.nix:560 in <nixpkgs>. lib.strings.withFeature

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


Function argument


Function argument

Example 5.70. lib.strings.withFeature usage example

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

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

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


Function argument


Function argument


Function argument

Example 5.71. lib.strings.withFeatureAs usage example

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

Located at lib/strings.nix:582 in <nixpkgs>. lib.strings.fixedWidthString

fixedWidthString :: int -> 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 5.72. lib.strings.fixedWidthString usage example

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

Located at lib/strings.nix:596 in <nixpkgs>. lib.strings.fixedWidthNumber

Format a number adding leading zeroes up to fixed width.


Function argument


Function argument

Example 5.73. lib.strings.fixedWidthNumber usage example

fixedWidthNumber 5 15
=> "00015"

Located at lib/strings.nix:613 in <nixpkgs>. lib.strings.floatToString

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


Function argument

Example 5.74. 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:625 in <nixpkgs>. lib.strings.isCoercibleToString

Check whether a value can be coerced to a string


Function argument

Located at lib/strings.nix:632 in <nixpkgs>. lib.strings.isStorePath

Check whether a value is a store path.


Function argument

Example 5.75. 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:650 in <nixpkgs>. lib.strings.toInt

string -> int

Parse a string string as an int.


Function argument

Example 5.76. lib.strings.toInt usage example

toInt "1337"
=> 1337
toInt "-4"
=> -4
toInt "3.14"
=> error: floating point JSON numbers are not supported

Located at lib/strings.nix:671 in <nixpkgs>. lib.strings.readPathsFromFile

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

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

Example 5.77. 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:692 in <nixpkgs>. lib.strings.fileContents

fileContents :: path -> string

Read the contents of a file removing the trailing \n


Function argument

Example 5.78. lib.strings.fileContents usage example

$ echo "1.0" > ./version

fileContents ./version
=> "1.0"

Located at lib/strings.nix:712 in <nixpkgs>. lib.strings.sanitizeDerivationName

sanitizeDerivationName :: String -> String

Creates a valid derivation name from a potentially invalid one.


Function argument

Example 5.79. lib.strings.sanitizeDerivationName usage example

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

Located at lib/strings.nix:727 in <nixpkgs>.

5.1.4. Miscellaneous functions

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>. lib.trivial.const

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 5.80. lib.trivial.const usage example

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

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

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

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


Function argument


Function argument

Example 5.81. 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>. lib.trivial.concat

note please don’t add a function like `compose = flip pipe`. This would confuse users, because the order of the functions in the list is not clear. With pipe, it’s obvious that it goes first-to-last. With `compose`, not so much.


Function argument


Function argument

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

boolean “or”


Function argument


Function argument

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

boolean “and”


Function argument


Function argument

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

bitwise “and”

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

bitwise “or”

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

bitwise “xor”

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

bitwise “not”

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

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>. lib.trivial.mergeAttrs

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 5.82. 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>. lib.trivial.flip

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

Flip the order of the arguments of a binary function.


Function argument


Function argument


Function argument

Example 5.83. lib.trivial.flip usage example

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

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

Apply function if the supplied argument is non-null.


Function to call


Argument to check for null before passing it to `f`

Example 5.84. 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>. lib.trivial.version

Returns the current full nixpkgs version number.

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

Returns the current nixpkgs release number as string.

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

Returns the current nixpkgs release code name.

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

Located at lib/trivial.nix:174 in <nixpkgs>. lib.trivial.versionSuffix

Returns the current nixpkgs version suffix as string.

Located at lib/trivial.nix:177 in <nixpkgs>. lib.trivial.revisionWithDefault

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:188 in <nixpkgs>. lib.trivial.inNixShell

inNixShell :: bool

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

Located at lib/trivial.nix:206 in <nixpkgs>. lib.trivial.min

Return minimum of two numbers.


Function argument


Function argument

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

Return maximum of two numbers.


Function argument


Function argument

Located at lib/trivial.nix:215 in <nixpkgs>. lib.trivial.mod

Integer modulus


Function argument


Function argument

Example 5.85. lib.trivial.mod usage example

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

Located at lib/trivial.nix:225 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:236 in <nixpkgs>. lib.trivial.splitByAndCompare

(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 5.86. 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:261 in <nixpkgs>. lib.trivial.importJSON

Reads a JSON file.

Type :: path -> any


Function argument

Located at lib/trivial.nix:281 in <nixpkgs>. lib.trivial.setFunctionArgs

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

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


Function argument


Function argument

Located at lib/trivial.nix:316 in <nixpkgs>. lib.trivial.functionArgs

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


Function argument

Located at lib/trivial.nix:328 in <nixpkgs>. lib.trivial.isFunction

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


Function argument

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

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

toHexString 0 => "0"

toHexString 16 => "10"

toHexString 250 => "FA"


Function argument

Located at lib/trivial.nix:345 in <nixpkgs>. lib.trivial.toBaseDigits

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

toBaseDigits 10 123 => [ 1 2 3 ]

toBaseDigits 2 6 => [ 1 1 0 ]

toBaseDigits 16 250 => [ 15 10 ]


Function argument


Function argument

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

5.1.5. List manipulation functions lib.lists.singleton

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 5.87. lib.lists.singleton usage example

singleton "foo"
=> [ "foo" ]

Located at lib/lists.nix:22 in <nixpkgs>. lib.lists.forEach

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 5.88. lib.lists.forEach usage example

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

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

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 5.89. 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:52 in <nixpkgs>. lib.lists.fold

`fold` is an alias of `foldr` for historic reasons

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

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 5.90. 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:80 in <nixpkgs>. lib.lists.foldl'

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

Strict version of `foldl`.

The difference is that evaluation is forced upon access. Usually used with small whole results (in contrast with lazily-generated list or large lists where only a part is consumed.)

Located at lib/lists.nix:96 in <nixpkgs>. lib.lists.imap0

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

Map with index starting from 0


Function argument


Function argument

Example 5.91. lib.lists.imap0 usage example

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

Located at lib/lists.nix:106 in <nixpkgs>. lib.lists.imap1

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

Map with index starting from 1


Function argument


Function argument

Example 5.92. lib.lists.imap1 usage example

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

Located at lib/lists.nix:116 in <nixpkgs>. lib.lists.concatMap

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

Map and concatenate the result.

Example 5.93. lib.lists.concatMap usage example

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

Located at lib/lists.nix:126 in <nixpkgs>. lib.lists.flatten

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


Function argument

Example 5.94. lib.lists.flatten usage example

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

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

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

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


Element to remove from the list

Example 5.95. lib.lists.remove usage example

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

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

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 5.96. 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:168 in <nixpkgs>. lib.lists.findFirst

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 5.97. 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:193 in <nixpkgs>. lib.lists.any

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

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

Example 5.98. lib.lists.any usage example

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

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

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

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

Example 5.99. 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:227 in <nixpkgs>. lib.lists.count

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

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



Example 5.100. lib.lists.count usage example

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

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

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") flashplayer').


Function argument


Function argument

Example 5.101. lib.lists.optional usage example

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

Located at lib/lists.nix:254 in <nixpkgs>. lib.lists.optionals

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

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




List to return if condition is true

Example 5.102. lib.lists.optionals usage example

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

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

If argument is a list, return it; else, wrap it in a singleton list. If you're using this, you should almost certainly reconsider if there isn't a more "well-typed" approach.


Function argument

Example 5.103. lib.lists.toList usage example

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

Located at lib/lists.nix:283 in <nixpkgs>. lib.lists.range

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 5.104. lib.lists.range usage example

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

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

(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 5.105. 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:314 in <nixpkgs>. lib.lists.groupBy'

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

`groupBy'` allows to customise the combining function and initial value


Function argument


Function argument


Function argument


Function argument

Example 5.106. 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:343 in <nixpkgs>. lib.lists.zipListsWith

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 5.107. lib.lists.zipListsWith usage example

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

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

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 5.108. lib.lists.zipLists usage example

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

Located at lib/lists.nix:382 in <nixpkgs>. lib.lists.reverseList

reverseList :: [a] -> [a]

Reverse the order of the elements of a list.


Function argument

Example 5.109. lib.lists.reverseList usage example

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

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

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

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


Function argument


Function argument


Function argument

Example 5.110. 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:415 in <nixpkgs>. lib.lists.toposort

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

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


Function argument


Function argument

Example 5.111. 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:454 in <nixpkgs>. lib.lists.sort

Sort a list based on a comparator function which compares two elements and returns true if the first argument is strictly below the second argument. The returned list is sorted in an increasing order. The implementation does a quick-sort.

Example 5.112. lib.lists.sort usage example

sort (a: b: a < b) [ 5 3 7 ]
=> [ 3 5 7 ]

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

Compare two lists element-by-element.


Function argument


Function argument


Function argument

Example 5.113. 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:511 in <nixpkgs>. lib.lists.naturalSort

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


Function argument

Example 5.114. 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:534 in <nixpkgs>. lib.lists.take

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

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


Number of elements to take

Example 5.115. lib.lists.take usage example

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

Located at lib/lists.nix:552 in <nixpkgs>. lib.lists.drop

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

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


Number of elements to drop


Input list

Example 5.116. lib.lists.drop usage example

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

Located at lib/lists.nix:566 in <nixpkgs>. lib.lists.sublist

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 5.117. lib.lists.sublist usage example

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

Located at lib/lists.nix:583 in <nixpkgs>. lib.lists.last

last :: [a] -> a

Return the last element of a list.

This function throws an error if the list is empty.


Function argument

Example 5.118. lib.lists.last usage example

last [ 1 2 3 ]
=> 3

Located at lib/lists.nix:607 in <nixpkgs>. lib.lists.init

init :: [a] -> [a]

Return all elements but the last.

This function throws an error if the list is empty.


Function argument

Example 5.119. lib.lists.init usage example

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

Located at lib/lists.nix:621 in <nixpkgs>. lib.lists.crossLists

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


Function argument

Example 5.120. 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:632 in <nixpkgs>. lib.lists.unique

unique :: [a] -> [a]

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

Example 5.121. lib.lists.unique usage example

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

Located at lib/lists.nix:643 in <nixpkgs>. lib.lists.intersectLists

Intersects list 'e' and another list. O(nm) complexity.


Function argument

Example 5.122. lib.lists.intersectLists usage example

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

Located at lib/lists.nix:651 in <nixpkgs>. lib.lists.subtractLists

Subtracts list 'e' from another list. O(nm) complexity.


Function argument

Example 5.123. lib.lists.subtractLists usage example

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

Located at lib/lists.nix:659 in <nixpkgs>. lib.lists.mutuallyExclusive

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:664 in <nixpkgs>.

5.1.6. Debugging functions lib.debug.traceIf

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 5.124. lib.debug.traceIf usage example

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

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

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 5.125. lib.debug.traceValFn usage example

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

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

traceVal :: a -> a

Trace the supplied value and return it.

Example 5.126. lib.debug.traceVal usage example

traceVal 42
# trace: 42
=> 42

Located at lib/debug.nix:68 in <nixpkgs>. lib.debug.traceSeq

traceSeq :: a -> b -> b

`builtins.trace`, but the value is `builtins.deepSeq`ed first.


The value to trace


The value to return

Example 5.127. 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:82 in <nixpkgs>. lib.debug.traceSeqN

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


Function argument


Function argument


Function argument

Example 5.128. lib.debug.traceSeqN usage example

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

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

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


Function to apply


Value to trace

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

A combination of `traceVal` and `traceSeq`.

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

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:125 in <nixpkgs>. lib.debug.traceValSeqN

A combination of `traceVal` and `traceSeqN`.

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

Evaluate 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, actual}`, denoting the attribute name of the failing test and its expected and actual results.

Used for regression testing of the functions in lib; see tests.nix for an example. Only tests having names starting with "test" are run.

Add attr { tests = ["testName"]; } to run these tests only.


Tests to run

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

Create a test assuming that list elements are `true`.


Function argument

Example 5.129. lib.debug.testAllTrue usage example

{ testX = allTrue [ true ]; }

Located at lib/debug.nix:166 in <nixpkgs>.

5.1.7. NixOS / nixpkgs option handling lib.options.isOption

isOption :: a -> bool

Returns true when the given argument is an option

Example 5.130. lib.options.isOption usage example

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

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

Creates an Option attribute set. mkOption accepts an attribute set with the following keys:

All keys default to `null` when not given.


Structured function argument


Default 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.


Whether the option can be set only once


Deprecated, used by types.optionSet.

Example 5.131. lib.options.mkOption usage example

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

Located at lib/options.nix:29 in <nixpkgs>. lib.options.mkEnableOption

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


Name for the created option

Example 5.132. lib.options.mkEnableOption usage example

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

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

This option accepts anything, but it does not produce any result.

This is useful for sharing a module across different module sets without having to implement similar features as long as the values of the options are not accessed.


Function argument

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

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


Function argument


Function argument

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

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

Extracts values of all "value" keys of the given list.

Example 5.133. lib.options.getValues usage example

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

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

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

Extracts values of all "file" keys of the given list

Example 5.134. lib.options.getFiles usage example

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

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

This function recursively removes all derivation attributes from `x` except for the `name` attribute.

This is to make the generation of `options.xml` much more efficient: the XML representation of derivations is very large (on the order of megabytes) and is not actually used by the manual generator.


Function argument

Located at lib/options.nix:173 in <nixpkgs>. lib.options.literalExample

For use in the `example` option attribute. It causes the given text to be included verbatim in documentation. This is necessary for example values that are not simple values, e.g., functions.


Function argument

Located at lib/options.nix:185 in <nixpkgs>. lib.options.showOption

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


Function argument

Example 5.135. lib.options.showOption usage example

(showOption ["foo" "bar" "baz"]) == ""
(showOption ["foo" "bar.baz" "tux"]) == ""

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

Unlike attributes, options can also start with numbers:
(showOption ["windowManager" "2bwm" "enable"]) == "windowManager.2bwm.enable"

Located at lib/options.nix:203 in <nixpkgs>.

5.2. Generators

Generators are functions that create file formats from nix data structures, e. g. for configuration files. There are generators available for: INI, JSON and YAML

All generators follow a similar call interface: generatorName configFunctions data, where configFunctions is an attrset of user-defined functions that format nested parts of the content. They each have common defaults, so often they do not need to be set manually. An example is mkSectionName ? (name: libStr.escape [ "[" "]" ] name) from the INI generator. It receives the name of a section and sanitizes it. The default mkSectionName escapes [ and ] with a backslash.

Generators can be fine-tuned to produce exactly the file format required by your application/service. One example is an INI-file format which uses : as separator, the strings "yes"/"no" as boolean values and requires all string values to be quoted:

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 delegats 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:


Note: Nix store paths can be converted to strings by enclosing a derivation attribute like so: "${drv}".

Detailed documentation for each generator can be found in lib/generators.nix.

5.3. 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 runnnig. 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.

5.4. 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

5.5. pkgs.nix-gitignore

pkgs.nix-gitignore is a function that acts similarly to builtins.filterSource but also allows filtering with the help of the gitignore format.

5.5.1. Usage

pkgs.nix-gitignore exports a number of functions, but you'll most likely need either gitignoreSource or gitignoreSourcePure. As their first argument, they both accept either 1. a file with gitignore lines or 2. a string with gitignore lines, or 3. a list of either of the two. They will be concatenated into a single big string.

{ pkgs ? import <nixpkgs> {} }:

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

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

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

 nix-gitignore.gitignoreSourcePure ["ignore-this\nignore-that\n", ~/.gitignore] ./source
     # It also accepts a list (of strings and paths) that will be concatenated
     # once the paths are turned to strings via readFile.

These functions are derived from the Filter functions by setting the first filter argument to (_: _: true):

gitignoreSourcePure = gitignoreFilterSourcePure (_: _: true);
gitignoreSource = gitignoreFilterSource (_: _: true);

Those filter functions accept the same arguments the builtins.filterSource function would pass to its filters, thus fn: gitignoreFilterSourcePure fn "" should be extensionally equivalent to filterSource. The file is blacklisted iff 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;

5.5.2. 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);

Chapter 6. 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.

6.1. 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 = "";
    sha256 = "0x2g1jqygyr5wiwg4ma1nd7w4ydpy82z9gkcv8vh2v8dn3y58v5m";

(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";
    sha256 = "0x2g1jqygyr5wiwg4ma1nd7w4ydpy82z9gkcv8vh2v8dn3y58v5m";

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 {
  name = "libfoo-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 Section 6.7, “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 Section 6.5, “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 {
  name = "fnord-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 Section 6.4, “Attributes”.

While the standard environment provides a generic builder, you can still supply your own build script:

stdenv.mkDerivation {
  name = "libfoo-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., process the buildInputs). 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


6.2. 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.

6.3. Specifying dependencies

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, between them, 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 Section 6.7, “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 Chapter 9, 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.

The dependency is propagated when it forces some of its other-transitive (non-immediate) downstream dependencies to also take it on as an immediate dependency. Nix itself already takes a package's transitive dependencies into account, but this propagation ensures nixpkgs-specific infrastructure like setup hooks (mentioned above) also are run as if the propagated dependency.

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. The exact rules for dependency propagation can be given by assigning to each dependency two integers based one how its host and target platforms are offset from the depending derivation's platforms. Those offsets are given below in the descriptions of each dependency list attribute. 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 in sort a transitive closure of the dependency relation, with the offsets being approximately summed when two dependency links are combined. We also prune transitive dependencies whose combined offsets go out-of-bounds, which can be viewed as a filter over that transitive closure removing dependencies that are blatantly absurd.

We can define the process precisely with Natural Deduction using the inference rules. This probably seems a bit obtuse, but so is the bash code that actually implements it! [3] They're confusing in very different ways so... hopefully if something doesn't make sense in one presentation, it will in the other!

let mapOffset(h, t, i) = i + (if i <= 0 then h else t - 1)

propagated-dep(h0, t0, A, B)
propagated-dep(h1, t1, B, C)
h0 + h1 in {-1, 0, 1}
h0 + t1 in {-1, 0, 1}
-------------------------------------- Transitive property
propagated-dep(mapOffset(h0, t0, h1),
               mapOffset(h0, t0, t1),
               A, C)

let mapOffset(h, t, i) = i + (if i <= 0 then h else t - 1)

dep(h0, _, 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 simply 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 dependending 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 unforagable 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. This means a -1 host and -1 target offset from the new derivation's platforms. 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. This means a -1 host offset and 0 target offset from the new derivation's platforms. 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. This means a -1 host offset and 1 target offset from the new derivation's platforms. 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 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. This means a 0 host offset and 0 target offset from the new derivation's host platform. These are packages used at run-time to generate code also used at run-time. 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 means a 0 host offset and a 1 target offset from the new derivation's host platform. 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. This means a 1 offset from the new derivation's platforms. 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.

6.4. Attributes

Variables affecting stdenv initialisation


A natural number 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. It needs to be 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"

The attribute can also contain a list, a script followed by arguments to be passed to it:

passthru.updateScript = [ ../../ pname "--requested-release=unstable" ];

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

For information about how to run the updates, execute nix-shell maintainers/scripts/update.nix.

6.5. Phases

The generic builder has 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). Furthermore, it allows a nicer presentation of build logs in the Nix build farm.

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/

6.5.1. 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.

Usually, if you just want to add a few phases, it’s more convenient to set one of the variables below (such as preInstallPhases), as you then don’t specify all the normal phases.


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.

6.5.2. 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 simply 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.


After running unpackPhase, the generic builder changes the current directory to the directory created by unpacking the sources. If there are multiple source directories, you should set sourceRoot to the name of the intended 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.


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.

6.5.3. 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.

6.5.4. 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, 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 [4] . 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.


By default, when cross compiling, the configure script has --build=... and --host=... passed. Packages can instead pass [ "build" "host" "target" ] or a subset to control exactly which platform flags are passed. Compilers and other tools can use this to also pass the target platform. [5]


Hook executed at the start of the configure phase.


Hook executed at the end of the configure phase.

6.5.5. The build phase

The build phase is responsible for actually building the package (e.g. compiling it). The default buildPhase simply 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)" ];

Note: 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.

6.5.6. The check phase

The check phase checks whether the package was built correctly by running its test suite. The default checkPhase calls make check, 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. Defaults to check.

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 dependencies used by the phase. 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.

6.5.7. 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.

6.5.8. The fixup phase

The fixup phase performs some (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.

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 targetting the package's host platform. Useful when supporting cross compilation, but otherwise feel free to ignore.


Like dontStrip, but only affects the strip command targetting 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.


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.


Flags passed to the strip command applied to the files in the directories listed in stripDebugList. Defaults to -S (i.e. --strip-debug).


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.


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.

For example, with GDB, you can add

set debug-file-directory ~/.nix-profile/lib/debug

to ~/.gdbinit. GDB will then be able to find debug information installed via nix-env -i.

6.5.9. 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.

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 dependencies used by the phase. 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.

6.5.10. 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


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.

6.6. Shell functions

The standard environment provides a number of useful functions.

makeWrapper executable wrapperfile args

Constructs a wrapper for a program with various possible arguments. For example:

# 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`
# 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 ]}

There’s many more kinds of arguments, they are documented in nixpkgs/pkgs/build-support/setup-hooks/

wrapProgram is a convenience function you probably want to use most of the time.

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 s1 s2

Replace every occurrence of the string s1 by s2.

--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 /usr/bin/bar $bar/bin/bar \
    --replace "a string containing spaces" "some other text" \
    --subst-var someVar

substitute is implemented using the replace command. Unlike with the sed command, you don’t have to worry about escaping special characters. It supports performing substitutions on binary files (such as executables), though there you’ll probably want to make sure that the replacement string is as long as the replaced string.

substituteInPlace file subs

Like substitute, but performs the substitutions in place on the file file.

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 automatically creates a sane wrapper file. It takes all the same arguments as makeWrapper, except for --argv0.

It cannot be applied multiple times, since it will overwrite the wrapper file.

6.7. 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 letter isn't. For example, if a derivation path is mentioned more than once, Nix itself doesn't care and simply 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. 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 use the full path to the shebang interpreter. A shebang interpreter is the first commented line of a script telling the operating system which program will run the script (e.g #!/bin/bash). In Nix, we want an exact path to that interpreter to be used. This often replaces /bin/sh with a path in the Nix store.

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 Chapter 8, 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 sets SOURCE_DATE_EPOCH to the modification time of the most recent file.

Bintools Wrapper

The Bintools Wrapper wraps the binary utilities for a bunch of miscellaneous purposes. These are GNU Binutils when targetting Linux, and a mix of cctools and GNU binutils for Darwin. [The "Bintools" name is supposed to be a compromise between "Binutils" and "cctools" not denoting any specific implementation.] Specifically, the underlying bintools package, and a C standard library (glibc or Darwin's libSystem, just for the dynamic loader) are all fed in, and dependency finding, hardening (see below), and purity checks for each are handled by the Bintools Wrapper. Packages typically depend on CC Wrapper, which in turn (at run time) depends on the Bintools Wrapper.

The Bintools Wrapper was only just recently split off from CC Wrapper, so the division of labor is still being worked out. For example, it shouldn't care about the C standard library, but just take a derivation with the dynamic loader (which happens to be the glibc on linux). Dependency finding however is a task both wrappers will continue to need to share, and probably the most important to understand. It is currently accomplished by collecting directories of host-platform dependencies (i.e. buildInputs and nativeBuildInputs) in environment variables. The Bintools Wrapper's setup hook causes any lib and lib64 subdirectories to be added to NIX_LDFLAGS. Since the CC Wrapper and the Bintools Wrapper use the same strategy, most of the Bintools Wrapper code is sparsely commented and refers to the CC Wrapper. But the CC Wrapper's code, by contrast, has quite lengthy comments. The Bintools Wrapper merely cites those, rather than repeating them, to avoid falling out of sync.

A final task of the setup hook is defining a number of standard environment variables to tell build systems which executables fulfill which purpose. They are defined to just be the base name of the tools, under the assumption that the Bintools Wrapper's binaries will be on the path. Firstly, this helps poorly-written packages, e.g. ones that look for just gcc when CC isn't defined yet clang is to be used. Secondly, this helps packages not get confused when cross-compiling, in which case multiple Bintools Wrappers may simultaneously be in use. [6] BUILD_- and TARGET_-prefixed versions of the normal environment variable are defined for additional Bintools Wrappers, properly disambiguating them.

A problem with this final task is that the Bintools Wrapper is honest and defines LD as ld. Most packages, however, firstly use the C compiler for linking, secondly use LD anyways, defining it as the C compiler, and thirdly, only so define LD when it is undefined as a fallback. This triple-threat means Bintools Wrapper will break those packages, as LD is already defined as the actual linker which the package won't override yet doesn't want to use. The workaround is to define, just for the problematic package, LD as the C compiler. A good way to do this would be preConfigure = "LD=$CC".

CC Wrapper

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 some lengthy comments describing the exact convoluted 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.


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/${python.libPrefix}/site-packages subdirectory of each build input to the PYTHONPATH environment variable.


Adds the lib/pkgconfig and share/pkgconfig subdirectories of each build input to the PKG_CONFIG_PATH environment variable.


Adds the share/aclocal subdirectory of each build input to the ACLOCAL_PATH environment variable.


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 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.

teTeX / TeX Live

Adds the share/texmf-nix subdirectory of each build input to the TEXINPUTS environment variable.

Qt 4

Sets the QTDIR environment variable to Qt’s path.


Exports GDK_PIXBUF_MODULE_FILE environment variable to the builder. Add librsvg package to buildInputs to get svg support.


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 Section 15.8, “GNOME”.


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.

The autoPatchelf command also recognizes a --no-recurse command line flag, which prevents it from recursing into subdirectories.


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.

Note: 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.

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.

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}

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.


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.


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.


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.


Overrides the configure phase to run meson to generate Ninja files. To run these files, you should accompany Meson with ninja. By default, enableParallelBuilding is enabled as Meson supports parallel building almost everywhere.

Variables controlling Meson


Controls the flags passed to meson.


Which --buildtype to pass to Meson. We default to plain.


What value to set -Dauto_features= to. We default to enabled.


What value to set -Dwrap_mode= to. We default to nodownload as we disallow network access.


Disables using Meson's configurePhase.


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.


This setup hook will allow you to unzip .zip files specified in $src. There are many similar packages like unrar, undmg, etc.


Overrides the configure, build, and install phases. This will run the "waf" script used by many projects. If wafPath (default ./waf) doesn’t exist, it will copy the version of waf available in Nixpkgs. wafFlags can be used to pass flags to the waf script.


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.

6.8. 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.

6.9. 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.

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 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 bindnow 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

The following flags are disabled by default and should be enabled with hardeningEnable for packages that take untrusted input like network services.


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.

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.

[1] 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.

[2] 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.

[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.

Chapter 7. Meta-attributes

Nix packages can declare meta-attributes that contain information about a package such as a description, its homepage, its license, and so on. For instance, the GNU Hello package has a meta declaration like this:

meta = with stdenv.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 = [ 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. The value of a meta-attribute must be a string.

The meta-attributes of a package can be queried from the command-line using nix-env:

$ nix-env -qa hello --json
    "hello": {
        "meta": {
            "description": "A program that produces a familiar, friendly greeting",
            "homepage": "",
            "license": {
                "fullName": "GNU General Public License version 3 or later",
                "shortName": "GPLv3+",
                "url": ""
            "longDescription": "GNU Hello is a program that prints \"Hello, world!\" when you run it.\nIt is fully customizable.\n",
            "maintainers": [
                "Ludovic Court\u00e8s <>"
            "platforms": [
            "position": "/home/user/dev/nixpkgs/pkgs/applications/misc/hello/default.nix:14"
        "name": "hello-2.9",
        "system": "x86_64-linux"

nix-env knows about the description field specifically:

$ nix-env -qa hello --description
hello-2.3  A program that produces a familiar, friendly greeting

7.1. 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 shown by nix-env -q --description and also on the Nixpkgs release pages.

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.


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) stdenv.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 stdenv.lib.licenses; [ asl20 free ofl ].

  • Multiple licenses referenced as a space delimited string of attribute shortNames (frowned upon) "asl20 free ofl".

For details, see Section 7.2, “Licenses”.


A list of names and e-mail addresses of the maintainers of this Nix expression. If you would like to be a maintainer of a package, you may want to add yourself to nixpkgs/maintainers/maintainer-list.nix and write something like [ stdenv.lib.maintainers.alice stdenv.lib.maintainers.bob ].


The priority of the package, used by nix-env to resolve file name conflicts between packages. See the Nix 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 = stdenv.lib.platforms.linux;

Attribute Set stdenv.lib.platforms defines various common lists of platforms types.

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

An attribute set with as values tests. A test is a derivation, which builds successfully when the test passes, and fails to build otherwise. A derivation that is a test needs to have meta.timeout defined.

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;


A timeout (in seconds) for building the derivation. If the derivation takes longer than this time to build, it can fail 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.


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 = stdenv.lib.platforms.linux;
meta.hydraPlatforms = [];


If set to true, the package is marked as “broken”, meaning that it won’t show up in nix-env -qa, and cannot be built or installed. Such packages should be removed from Nixpkgs eventually unless they are fixed.


If set to true, the package is tested to be updated correctly by the script without additional settings. Such packages have meta.version set and their homepage (or the page specified by meta.downloadPage) contains a direct link to the package tarball.

7.2. Licenses

The meta.license attribute should preferrably contain a value from stdenv.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.

stdenv.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.

stdenv.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.

stdenv.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.

Chapter 8. Multiple-output packages

8.1. Introduction

The Nix language allows a derivation to produce multiple outputs, which is similar to what is utilized by other Linux distribution packaging systems. The outputs reside in separate Nix store paths, so they can be mostly handled independently of each other, including passing to build inputs, garbage collection or binary substitution. The exception is that building from source always produces all the outputs.

The main motivation is to save disk space by reducing runtime closure sizes; consequently also sizes of substituted binaries get reduced. Splitting can be used to have more granular runtime dependencies, for example the typical reduction is to split away development-only files, as those are typically not needed during runtime. As a result, closure sizes of many packages can get reduced to a half or even much less.

Note: The reduction effects could be instead achieved by building the parts in completely separate derivations. That would often additionally reduce build-time closures, but it tends to be much harder to write such derivations, as build systems typically assume all parts are being built at once. This compromise approach of single source package producing multiple binary packages is also utilized often by rpm and deb.

A number of attributes can be used to work with a derivation with multiple outputs. The attribute outputs is a list of strings, which are the names of the outputs. For each of these names, an identically named attribute is created, corresponding to that output. The attribute meta.outputsToInstall is used to determine the default set of outputs to install when using the derivation name unqualified.

8.2. Installing a split package

When installing a package with multiple outputs, the package's meta.outputsToInstall attribute determines which outputs are actually installed. meta.outputsToInstall is a list whose default installs binaries and the associated man pages. The following sections describe ways to install different outputs.

8.2.1. Selecting outputs to install via NixOS

NixOS provides two ways to select the outputs to install for packages listed in environment.systemPackages:

  • The configuration option environment.extraOutputsToInstall is appended to each package's meta.outputsToInstall attribute to determine the outputs to install. It can for example be used to install info documentation or debug symbols for all packages.

  • The outputs can be listed as packages in environment.systemPackages. For example, the "out" and "info" outputs for the coreutils package can be installed by including coreutils and in environment.systemPackages.

8.2.2. Selecting outputs to install via nix-env

nix-env lacks an easy way to select the outputs to install. When installing a package, nix-env always installs the outputs listed in meta.outputsToInstall, even when the user explicitly selects an output.

nix-env silenty disregards the outputs selected by the user, and instead installs the outputs from meta.outputsToInstall. For example,

$ nix-env -iA

installs the "out" output (coreutils.meta.outputsToInstall is [ "out" ]) instead of the requested "info".

The only recourse to select an output with nix-env is to override the package's meta.outputsToInstall, using the functions described in Chapter 4, Overriding. For example, the following overlay adds the "info" output for the coreutils package:

self: super:
  coreutils = super.coreutils.overrideAttrs (oldAttrs: {
    meta = oldAttrs.meta // { outputsToInstall = oldAttrs.meta.outputsToInstall or [ "out" ] ++ [ "info" ]; };

8.3. 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 Section 8.4.2, “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.)

8.4. 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.

Note: There is a special handling of the debug output, described at separateDebugInfo .

8.4.1. 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. ${stdenv.glibc}/lib/ The executables provided by glibc can be accessed via its bin attribute (e.g. ${stdenv.glibc.bin}/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).

8.4.2. 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, pkg-config, cmake and aclocal files. 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 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). They go to man or $outputBin by default.


is for section 3 man pages. They go to devman or $outputMan by default.


is for info pages. They go to info or $outputBin by default.

8.4.3. 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.

Chapter 9. Cross-compilation

9.1. Introduction

"Cross-compilation" means compiling a program on one machine for another type of machine. For example, a typical use of cross-compilation is to compile programs for embedded devices. These devices often don't have the computing power and memory to compile their own programs. One might think that cross-compilation is a fairly niche concern. However, there are significant advantages to rigorously distinguishing between build-time and run-time environments! Significant, because the benefits apply even when one is developing and deploying on the same machine. Nixpkgs is increasingly adopting the opinion that packages should be written with cross-compilation in mind, and nixpkgs should evaluate in a similar way (by minimizing cross-compilation-specific special cases) whether or not one is cross-compiling.

This chapter will be organized in three parts. First, it will describe the basics of how to package software in a way that supports cross-compilation. Second, it will describe how to use Nixpkgs when cross-compiling. Third, it will describe the internal infrastructure supporting cross-compilation.

9.2. Packaging in a cross-friendly manner

9.2.1. 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 platfom" 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!

9.2.2. Theory of dependency categorization

Note: This is a rather philosophical description that isn't very Nixpkgs-specific. For an overview of all the relevant attributes given to mkDerivation, see Section 6.3, “Specifying dependencies”. For a description of how everything is implemented, see Section 9.4.1, “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 build-time and run-time (respectively) changes 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 langauges' 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 we have dependencies in the form "host → target", in at most the following six combinations:

Table 9.1. Possible dependency types

Dependency's host platform Dependency's target platform
build build
build host
build target
host host
host target
target target

Some examples will make this table clearer. Suppose there's some package that is being built with a (build, host, target) platform triple of (foo, bar, baz). If it has a build-time library dependency, that would be a "host → build" dependency with a triple of (foo, foo, *) (the target platform is irrelevant). If it needs a compiler to be built, that would be a "build → host" dependency with a triple of (foo, foo, *) (the target platform is irrelevant). That compiler, would be built with another compiler, also "build → host" dependency, with a triple of (foo, foo, foo).

9.2.3. 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! What if my package's build system needs to build a C program to be run under the build environment? My package fails to find ar. My package's testsuite needs to run host platform code.

What if my package's build system needs to build a C program to be run under the build environment?

depsBuildBuild = [ ];

Add it to your mkDerivation invocation.

My package fails to find ar.

Many packages assume that an unprefixed ar 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 prefixed ar.

My package's testsuite needs to run host platform code.

doCheck = stdenv.hostPlatform == stdenv.buildPlatfrom;

Add it to your mkDerivation invocation.

9.3. 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.

Note: Many sources (manual, wiki, etc) probably mention passing system, platform, along with the optional crossSystem to nixpkgs: import <nixpkgs> { system = ..; platform = ..; crossSystem = ..; }. Passing those two instead of localSystem is still supported for compatibility, but is discouraged. Indeed, much of the inference we do for these parameters is motivated by compatibility as much as convenience.

One would think that localSystem and crossSystem overlap horribly with the three *Platforms (buildPlatform, hostPlatform, and targetPlatform; see stage.nix or the manual). Actually, those identifiers are purposefully not used here to draw a subtle but important distinction: While the granularity of having 3 platforms is necessary to properly *build* packages, it is overkill for specifying the user's *intent* when making a build plan or package set. A simple "build vs deploy" dichotomy is adequate: the sliding window principle described in the previous section shows how to interpolate between the these two "end points" to get the 3 platform triple for each bootstrapping stage. That means for any package a given package set, even those not bound on the top level but only reachable via dependencies or buildPackages, the three platforms will be defined as one of localSystem or crossSystem, with the former replacing the latter as one traverses build-time dependencies. A last simple difference is that crossSystem should be null when one doesn't want to cross-compile, while the *Platforms are always non-null. localSystem is always non-null.

9.4. Cross-compilation infrastructure

9.4.1. Implementation of dependencies

The categorizes of dependencies developed in Section 9.2.2, “Theory of dependency categorization” are specified as lists of derivations given to mkDerivation, as documented in Section 6.3, “Specifying dependencies”. In short, each list of dependencies for "host → target" of "foo → bar" is called depsFooBar, with exceptions for backwards compatibility that depsBuildHost is instead called nativeBuildInputs and depsHostTarget is instead called buildInputs. Nixpkgs is now structured so that each depsFooBar is automatically taken from pkgsFooBar. (These pkgsFooBars 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 pkgsFooBar 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.

9.4.2. Bootstrapping

Each of the package sets described above come from a single bootstrapping stage. While pkgs/top-level/default.nix, coordinates the composition of stages at a high level, pkgs/top-level/stage.nix "ties the knot" (creates the fixed point) of each stage. The package sets are defined per-stage however, so they can be thought of as edges between stages (the nodes) in a graph. Compositions like pkgsBuildTarget.targetPackages can be thought of as paths to this graph.

While there are many package sets, and thus many edges, the stages can also be arranged in a linear chain. In other words, many of the edges are redundant as far as connectivity is concerned. This hinges on the type of bootstrapping we do. Currently for cross it is:

  1. (native, native, native)

  2. (native, native, foreign)

  3. (native, foreign, foreign)

In each stage, pkgsBuildHost refers to the previous stage, pkgsBuildBuild refers to the one before that, and pkgsHostTarget refers to the current one, and pkgsTargetTarget refers to the next one. When there is no previous or next stage, they instead refer to the current stage. Note how all the invariants regarding the mapping between dependency and depending packages' build host and target platforms are preserved. pkgsBuildTarget and pkgsHostHost are more complex in that the stage fitting the requirements isn't always a fixed chain of "prevs" and "nexts" away (modulo the "saturating" self-references at the ends). We just special case each instead. All the primary edges are implemented is in pkgs/stdenv/booter.nix, and secondarily aliases in pkgs/top-level/stage.nix.

Note: Note the native stages are bootstrapped in legacy ways that predate the current cross implementation. This is why the bootstrapping stages leading up to the final stages are ignored inthe 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...)

On 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" bootstraping stage where build != host != target, more bootstrapping stages are needed since no sliding window providess 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.

Note: If one explores Nixpkgs, they will see derivations with names like gccCross. Such *Cross derivations is a holdover from before we properly distinguished between the host and target platforms—the derivation with "Cross" in the name covered the build = host != target case, while the other covered the host = target, with build platform the same or not based on whether one was using its .nativeDrv or .crossDrv. This ugliness will disappear soon.

Chapter 10. Platform Notes

Table of Contents

10.1. Darwin (macOS)

10.1. 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 = stdenv.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 '/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.

Chapter 11. Fetchers

When using Nix, you will frequently need to download source code and other files from the internet. Nixpkgs comes with a few helper functions that allow you to fetch fixed-output derivations in a structured way.

The two fetcher primitives are fetchurl and fetchzip. Both of these have two required arguments, a URL and a hash. The hash is typically sha256, although many more hash algorithms are supported. Nixpkgs contributors are currently recommended to use sha256. 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 = "";
    sha256 = "1111111111111111111111111111111111111111111111111111";

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 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.

Other fetcher functions allow you to add source code directly from a VCS such as subversion or git. These are mostly straightforward names based on the name of the command used with the VCS system. Because they give you a working repository, they act most like fetchzip.


Used with Subversion. Expects url to a Subversion directory, rev, and sha256.


Used with Git. Expects url to a Git repo, rev, and sha256. rev in this case can be full the git commit id (SHA1 hash) or a tag name like refs/tags/v1.0.


Used with Fossil. Expects url to a Fossil archive, rev, and sha256.


Used with CVS. Expects cvsRoot, tag, and sha256.


Used with Mercurial. Expects url, rev, and sha256.

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.


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, sha256 corresponds to the hash of the extracted directory. Again, other hash algorithms are also available but sha256 is currently preferred.


This is used with GitLab repositories. The arguments expected are very similar to fetchFromGitHub above.


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.

Chapter 12. Trivial builders

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.


This takes three arguments, name, env, and buildCommand. name is just the name that Nix will append to the store path in the same way that stdenv.mkDerivation uses its name attribute. env is an attribute set specifying environment variables that will be set for this derivation. These attributes are then passed to the wrapped stdenv.mkDerivation. buildCommand specifies the commands that will be run to create this derivation. Note that you will need to create $out for Nix to register the command as successful.

An example of using runCommand is provided below.

(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 roundrip and can speed up a build.

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

These functions write text to the Nix store. This is useful for creating scripts from Nix expressions. writeTextFile takes an attribute set and expects two arguments, name and text. name corresponds to the name used in the Nix store path. text will be the contents of the file. You can also set executable to true to make this file have the executable bit set.

Many more commands wrap writeTextFile including writeText, writeTextDir, writeScript, and writeScriptBin. These are convenience functions over writeTextFile.


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.

Chapter 13. Special builders

This chapter describes several special builders.

13.1. buildFHSUserEnv

buildFHSUserEnv provides a way to build and run FHS-compatible lightweight sandboxes. It creates an isolated root with bound /nix/store, so its footprint in terms of disk space needed is quite small. This allows one 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. It uses Linux namespaces feature to create temporary lightweight environments which are destroyed after all child processes exit, without root user rights requirement. Accepted arguments are:


Environment name.


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.


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.


Additional commands to be executed for finalizing the directory structure.


Like extraBuildCommands, but executed only on multilib architectures.


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


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


A command that would be executed inside the sandbox and passed all the command line arguments. It defaults to bash.

One can create a simple environment using a shell.nix like that:

{ pkgs ? import <nixpkgs> {} }:

(pkgs.buildFHSUserEnv {
  name = "simple-x11-env";
  targetPkgs = pkgs: (with pkgs;
    [ udev
    ]) ++ (with pkgs.xorg;
    [ libX11
  multiPkgs = pkgs: (with pkgs;
    [ udev
  runScript = "bash";

Running nix-shell would then drop you into a shell with these libraries and binaries available. You can use this to run closed-source applications which expect FHS structure without hassles: simply change runScript to the application path, e.g. ./bin/ -- relative paths are supported.

13.2. pkgs.mkShell

pkgs.mkShell is a special kind of derivation that is only useful when using it combined with nix-shell. It will in fact fail to instantiate when invoked with nix-build.

13.2.1. Usage

{ pkgs ? import <nixpkgs> {} }:
pkgs.mkShell {
  # this will make all the build inputs from hello and gnutar
  # available to the shell environment
  inputsFrom = with pkgs; [ hello gnutar ];
  buildInputs = [ pkgs.gnumake ];

Chapter 14. Images

This chapter describes tools for creating various types of images.

14.1. pkgs.appimageTools

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

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

14.1.1. 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.

14.1.2. Wrapping

Depending on the type of AppImage you're wrapping, you'll have to use wrapType1 or wrapType2.

appimageTools.wrapType2 { # or wrapType1
  name = "patchwork"; 1
  src = fetchurl { 2
    url = "";
    sha256 =  "1blsprpkvm0ws9b96gb36f0rbf8f5jgmw4x6dsb1kswr4ysf591s";
  extraPkgs = pkgs: with pkgs; [ ]; 3


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.

14.2. pkgs.dockerTools

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.

14.2.1. buildImage

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:

Example 14.1. Docker build

buildImage {
  name = "redis"; 1
  tag = "latest"; 2

  fromImage = someBaseImage; 3
  fromImageName = null; 4
  fromImageTag = "latest"; 5

  contents = pkgs.redis; 6
  runAsRoot = '' 7
    mkdir -p /data

  config = { 8
    Cmd = [ "/bin/redis-server" ];
    WorkingDir = "/data";
    Volumes = {
      "/data" = {};

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.


contents 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 .

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 Example 14.1, “Docker build” 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.

Example 14.2. Impurely Defining a Docker Layer's Creation Date

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";
  contents = pkgs.hello;

  config.Cmd = [ "/bin/hello" ];

and 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.

14.2.2. buildLayeredImage

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

contents optional

Top level paths in the container. Either a single derivation, or a list of derivations.

Default: []

config optional

Run-time configuration of the container. A full list of the options are available at 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

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. 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, however 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.

14.2.3. streamLayeredImage

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

14.2.4. pullImage

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:

Example 14.3. Docker pull

pullImage {
  imageName = "nixos/nix"; 1
  imageDigest = "sha256:20d9485b25ecfd89204e843a962c1bd70e9cc6858d65d7f5fadc340246e2116b"; 2
  finalImageName = "nix"; 3
  finalImageTag = "1.11";  4
  sha256 = "0mqjy3zq2v6rrhizgb9nvhczl87lcfphq9601wcprdika2jz7qh8"; 5
  os = "linux"; 6
  arch = "x86_64"; 7


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 -c 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

14.2.5. exportImage

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:

Example 14.4. Docker export

exportImage {
  fromImage = someLayeredImage;
  fromImageName = null;
  fromImageTag = null;

  name =;

The parameters relative to the base image have the same synopsis as described in Section 14.2.1, “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

14.2.6. shadowSetup

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 runAsRoot 7 script for cases like in the example below:

Example 14.5. Shadow base files

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.

14.3. pkgs.ociTools

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.

14.3.1. buildContainer

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:

Example 14.6. Build Container

buildContainer {
  args = [ (with pkgs; writeScript "" ''
    exec ${bash}/bin/bash
  '').outPath ]; 1

  mounts = {
    "/data" = {
      type = "none";
      source = "/var/lib/mydata";
      options = [ "bind" ];

  readonly = false; 3


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.

14.4. pkgs.snapTools

pkgs.snapTools is a set of functions for creating Snapcraft images. Snap and Snapcraft is not used to perform these operations.

14.4.1. The makeSnap Function

makeSnap takes a single named argument, meta. This argument mirrors the upstream snap.yaml format exactly.

The base should not be be specified, as makeSnap will force set it.

Currently, makeSnap does not support creating GUI stubs.

14.4.2. Build a Hello World Snap

Example 14.7. Making 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.

14.4.3. Build a Hello World Snap

Example 14.8. Making 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.

Chapter 15. 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 accomodated 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.

15.1. Agda

15.1.1. How to use Agda

Agda can be installed from agda:

$ nix-env -iA agda

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).

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 library in your home directory (for instance if it is a development version) then typecheck it manually (using agda.withPackages if necessary) and then override the src attribute of the package to point to your local repository.

Agda will not by default use these libraries. To tell agda to use the 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.

15.1.2. Compiling Agda

Agda modules can be compiled with the --compile flag. A version of ghc with ieee 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;

15.1.3. 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. Building Agda packages

The default build phase for agdaPackages.mkDerivation simply 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.

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 }:

and mkDerivation should be called instead of agdaPackages.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 buildPhase.

When writing an agda package it is essential to make sure that no .agda-lib file gets added to the store as a single file (for example by using writeText). This causes agda to think that the nix store is a agda library and it will attempt to write to it whenever it typechecks something. See

15.2. Android

The Android build environment provides three major features and a number of supporting features.

15.2.1. Deploying an Android SDK installation with plugins

The first use case is deploying the SDK with a desired set of plugins or subsets of an SDK.

with import <nixpkgs> {};

  androidComposition = androidenv.composeAndroidPackages {
    toolsVersion = "25.2.5";
    platformToolsVersion = "27.0.1";
    buildToolsVersions = [ "27.0.3" ];
    includeEmulator = false;
    emulatorVersion = "27.2.0";
    platformVersions = [ "24" ];
    includeSources = false;
    includeDocs = false;
    includeSystemImages = false;
    systemImageTypes = [ "default" ];
    abiVersions = [ "armeabi-v7a" ];
    lldbVersions = [ "2.0.2558144" ];
    cmakeVersions = [ "3.6.4111459" ];
    includeNDK = false;
    ndkVersion = "16.1.4479499";
    useGoogleAPIs = false;
    useGoogleTVAddOns = false;
    includeExtras = [

The above function invocation states that we want an Android SDK with the above specified plugin versions. By default, most plugins are disabled. Notable exceptions are the tools, platform-tools and build-tools sub packages.

The following parameters are supported:

  • toolsVersion, specifies the version of the tools package to use

  • platformsToolsVersion specifies the version of the platform-tools plugin

  • buildToolsVersion specifies the versions of the build-tools plugins to use.

  • includeEmulator specifies whether to deploy the emulator package (false by default). When enabled, the version of the emulator to deploy can be specified by setting the emulatorVersion parameter.

  • includeDocs specifies whether the documentation catalog should be included.

  • lldbVersions specifies what LLDB versions should be deployed.

  • cmakeVersions specifies which CMake versions should be deployed.

  • includeNDK specifies that the Android NDK bundle should be included. Defaults to: false.

  • ndkVersion specifies the NDK version that we want to use.

  • includeExtras is an array of identifier strings referring to arbitrary add-on packages that should be installed.

  • platformVersions specifies which platform SDK versions should be included.

For each platform version that has been specified, we can apply the following options:

  • includeSystemImages specifies whether a system image for each platform SDK should be included.

  • includeSources specifies whether the sources for each SDK version should be included.

  • useGoogleAPIs specifies that for each selected platform version the Google API should be included.

  • useGoogleTVAddOns specifies that for each selected platform version the Google TV add-on should be included.

For each requested system image we can specify the following options:

  • systemImageTypes specifies what kind of system images should be included. Defaults to: default.

  • abiVersions specifies what kind of ABI version of each system image should be included. Defaults to: armeabi-v7a.

Most of the function arguments have reasonable default settings.

When building the above expression with:

$ nix-build

The Android SDK gets deployed with all desired plugin versions.

We can also deploy subsets of the Android SDK. For example, to only the platform-tools package, you can evaluate the following expression:

with import <nixpkgs> {};

  androidComposition = androidenv.composeAndroidPackages {
    # ...

15.2.2. Using predefine Android package compositions

In addition to composing an Android package set manually, it is also possible to use a predefined composition that contains all basic packages for a specific Android version, such as version 9.0 (API-level 28).

The following Nix expression can be used to deploy the entire SDK with all basic plugins:

with import <nixpkgs> {};


It is also possible to use one plugin only:

with import <nixpkgs> {};


15.2.3. Building an Android application

In addition to the SDK, it is also possible to build an Ant-based Android project and automatically deploy all the Android plugins that a project requires.

with import <nixpkgs> {};

androidenv.buildApp {
  name = "MyAndroidApp";
  src = ./myappsources;
  release = true;

  # If release is set to true, you need to specify the following parameters
  keyStore = ./keystore;
  keyAlias = "myfirstapp";
  keyStorePassword = "mykeystore";
  keyAliasPassword = "myfirstapp";

  # Any Android SDK parameters that install all the relevant plugins that a
  # build requires
  platformVersions = [ "24" ];

  # When we include the NDK, then ndk-build is invoked before Ant gets invoked
  includeNDK = true;

Aside from the app-specific build parameters (name, src, release and keystore parameters), the buildApp {} function supports all the function parameters that the SDK composition function (the function shown in the previous section) supports.

This build function is particularly useful when it is desired to use Hydra: the Nix-based continuous integration solution to build Android apps. An Android APK gets exposed as a build product and can be installed on any Android device with a web browser by navigating to the build result page.

15.2.4. Spawning emulator instances

For testing purposes, it can also be quite convenient to automatically generate scripts that spawn emulator instances with all desired configuration settings.

An emulator spawn script can be configured by invoking the emulateApp {} function:

with import <nixpkgs> {};

androidenv.emulateApp {
  name = "emulate-MyAndroidApp";
  platformVersion = "28";
  abiVersion = "x86"; # armeabi-v7a, mips, x86_64
  systemImageType = "google_apis_playstore";

Additional flags may be applied to the Android SDK’s emulator through the runtime environment variable $NIX_ANDROID_EMULATOR_FLAGS.

It is also possible to specify an APK to deploy inside the emulator and the package and activity names to launch it:

with import <nixpkgs> {};

androidenv.emulateApp {
  name = "emulate-MyAndroidApp";
  platformVersion = "24";
  abiVersion = "armeabi-v7a"; # mips, x86, x86_64
  systemImageType = "default";
  useGoogleAPIs = false;
  app = ./MyApp.apk;
  package = "MyApp";
  activity = "MainActivity";

In addition to prebuilt APKs, you can also bind the APK parameter to a buildApp {} function invocation shown in the previous example.

15.2.5. Querying the available versions of each plugin

When using any of the previously shown functions, it may be a bit inconvenient to find out what options are supported, since the Android SDK provides many plugins.

A shell script in the pkgs/development/mobile/androidenv/ sub directory can be used to retrieve all possible options:

sh ./ packages build-tools

The above command-line instruction queries all build-tools versions in the generated packages.nix expression.

15.2.6. Updating the generated expressions

Most of the Nix expressions are generated from XML files that the Android package manager uses. To update the expressions run the script that is stored in the pkgs/development/mobile/androidenv/ sub directory:


15.3. BEAM Languages (Erlang, Elixir & LFE)

15.3.1. Introduction

In this document and related Nix expressions, we use the term, BEAM, to describe the environment. BEAM is the name of the Erlang Virtual Machine and, as far as we're concerned, from a packaging perspective, all languages that run on the BEAM are interchangeable. That which varies, like the build system, is transparent to users of any given BEAM package, so we make no distinction.

15.3.2. Structure

All BEAM-related expressions are available via the top-level beam attribute, which includes:

  • interpreters: a set of compilers running on the BEAM, including multiple Erlang/OTP versions (beam.interpreters.erlangR19, etc), Elixir (beam.interpreters.elixir) and LFE (beam.interpreters.lfe).

  • packages: a set of package builders (Mix and rebar3), each compiled with a specific Erlang/OTP version, e.g. beam.packages.erlangR19.

The default Erlang compiler, defined by beam.interpreters.erlang, is aliased as erlang. The default BEAM package set is defined by beam.packages.erlang and aliased at the top level as beamPackages.

To create a package builder built with a custom Erlang version, use the lambda, beam.packagesWith, which accepts an Erlang/OTP derivation and produces a package builder similar to beam.packages.erlang.

Many Erlang/OTP distributions available in beam.interpreters have versions with ODBC and/or Java enabled or without wx (no observer support). For example, there's beam.interpreters.erlangR22_odbc_javac, which corresponds to beam.interpreters.erlangR22 and beam.interpreters.erlangR22_nox, which corresponds to beam.interpreters.erlangR22.

15.3.3. Build Tools Rebar3

We provide a version of Rebar3, under rebar3. We also provide a helper to fetch Rebar3 dependencies from a lockfile under fetchRebar3Deps. Mix &

Both Mix and work exactly as expected. There is a bootstrap process that needs to be run for both, however, which is supported by the buildMix and buildErlangMk derivations, respectively.

15.3.4. How to Install BEAM Packages

BEAM builders are not registered at the top level, simply because they are not relevant to the vast majority of Nix users. To install any of those builders into your profile, refer to them by their attribute path beamPackages.rebar3:

$ nix-env -f "<nixpkgs>" -iA beamPackages.rebar3

15.3.5. Packaging BEAM Applications Erlang Applications Rebar3 Packages

The Nix function, buildRebar3, defined in beam.packages.erlang.buildRebar3 and aliased at the top level, can be used to build a derivation that understands how to build a Rebar3 project.

If a package needs to compile native code via Rebar3's port compilation mechanism, add compilePort = true; to the derivation. Packages functions similarly to Rebar3, except we use buildErlangMk instead of buildRebar3. Mix Packages

Mix functions similarly to Rebar3, except we use buildMix instead of buildRebar3.

Alternatively, we can use buildHex as a shortcut:

15.3.6. How to Develop Creating a Shell

Usually, we need to create a shell.nix file and do our development inside of the environment specified therein. Just install your version of erlang and other interpreter, and then user your normal build tools. As an example with elixir:

{ pkgs ? import "<nixpkgs"> {} }:

with pkgs;


  elixir = beam.packages.erlangR22.elixir_1_9;

mkShell {
  buildInputs = [ elixir ];

} Building in a Shell (for Mix Projects)

Using a shell.nix as described (see Section, “Creating a Shell”) should just work.

15.4. Bower

Bower is a package manager for web site front-end components. Bower packages (comprising of build artefacts and sometimes sources) are stored in git repositories, typically on Github. The package registry is run by the Bower team with package metadata coming from the bower.json file within each package.

The end result of running Bower is a bower_components directory which can be included in the web app's build process.

Bower can be run interactively, by installing nodePackages.bower. More interestingly, the Bower components can be declared in a Nix derivation, with the help of nodePackages.bower2nix.

15.4.1. bower2nix usage

Suppose you have a bower.json with the following contents:

Example 15.1. bower.json

  "name": "my-web-app",
  "dependencies": {
    "angular": "~1.5.0",
    "bootstrap": "~3.3.6"

Running bower2nix will produce something like the following output:

{ fetchbower, buildEnv }:
buildEnv { name = "bower-env"; ignoreCollisions = true; paths = [
  (fetchbower "angular" "1.5.3" "~1.5.0" "1749xb0firxdra4rzadm4q9x90v6pzkbd7xmcyjk6qfza09ykk9y")
  (fetchbower "bootstrap" "3.3.6" "~3.3.6" "1vvqlpbfcy0k5pncfjaiskj3y6scwifxygfqnw393sjfxiviwmbv")
  (fetchbower "jquery" "2.2.2" "1.9.1 - 2" "10sp5h98sqwk90y4k6hbdviwqzvzwqf47r3r51pakch5ii2y7js1")
]; }

Using the bower2nix command line arguments, the output can be redirected to a file. A name like bower-packages.nix would be fine.

The resulting derivation is a union of all the downloaded Bower packages (and their dependencies). To use it, they still need to be linked together by Bower, which is where buildBowerComponents is useful.

15.4.2. buildBowerComponents function

The function is implemented in pkgs/development/bower-modules/generic/default.nix. Example usage:

Example 15.2. buildBowerComponents

bowerComponents = buildBowerComponents {
  name = "my-web-app";
  generated = ./bower-packages.nix; 1
  src = myWebApp; 2

In Example 15.2, “buildBowerComponents”, the following arguments are of special significance to the function:


generated specifies the file which was created by bower2nix.


src is your project's sources. It needs to contain a bower.json file.

buildBowerComponents will run Bower to link together the output of bower2nix, resulting in a bower_components directory which can be used.

Here is an example of a web frontend build process using gulp. You might use grunt, or anything else.

Example 15.3. Example build script (gulpfile.js)

var gulp = require('gulp');

gulp.task('default', [], function () {

gulp.task('build', [], function () {
  console.log("Just a dummy gulp build");

Example 15.4. Full example — default.nix

{ myWebApp ? { outPath = ./.; name = "myWebApp"; }
, pkgs ? import <nixpkgs> {}

pkgs.stdenv.mkDerivation {
  name = "my-web-app-frontend";
  src = myWebApp;

  buildInputs = [ pkgs.nodePackages.gulp ];

  bowerComponents = pkgs.buildBowerComponents { 1
    name = "my-web-app";
    generated = ./bower-packages.nix;
    src = myWebApp;

  buildPhase = ''
    cp --reflink=auto --no-preserve=mode -R $bowerComponents/bower_components . 2
    export HOME=$PWD 3
    ${pkgs.nodePackages.gulp}/bin/gulp build 4

  installPhase = "mv gulpdist $out";

A few notes about Example 15.4, “Full example — default.nix:


The result of buildBowerComponents is an input to the frontend build.


Whether to symlink or copy the bower_components directory depends on the build tool in use. In this case a copy is used to avoid gulp silliness with permissions.


gulp requires HOME to refer to a writeable directory.


The actual build command. Other tools could be used.

15.4.3. Troubleshooting

ENOCACHE errors from buildBowerComponents

This means that Bower was looking for a package version which doesn't exist in the generated bower-packages.nix.

If bower.json has been updated, then run bower2nix again.

It could also be a bug in bower2nix or fetchbower. If possible, try reformulating the version specification in bower.json.

15.5. Coq

Coq libraries should be installed in $(out)/lib/coq/${coq.coq-version}/user-contrib/. Such directories are automatically added to the $COQPATH environment variable by the hook defined in the Coq derivation.

Some extensions (plugins) might require OCaml and sometimes other OCaml packages. The coq.ocamlPackages attribute can be used to depend on the same package set Coq was built against.

Coq libraries may be compatible with some specific versions of Coq only. The compatibleCoqVersions attribute is used to precisely select those versions of Coq that are compatible with this derivation.

Here is a simple package example. It is a pure Coq library, thus it depends on Coq. It builds on the Mathematical Components library, thus it also takes mathcomp as buildInputs. Its Makefile has been generated using coq_makefile so we only have to set the $COQLIB variable at install time.

{ stdenv, fetchFromGitHub, coq, mathcomp }:

stdenv.mkDerivation rec {
  name = "coq${coq.coq-version}-multinomials-${version}";
  version = "1.0";
  src = fetchFromGitHub {
    owner = "math-comp";
    repo = "multinomials";
    rev = version;
    sha256 = "1qmbxp1h81cy3imh627pznmng0kvv37k4hrwi2faa101s6bcx55m";

  buildInputs = [ coq ];
  propagatedBuildInputs = [ mathcomp ];

  installFlags = "COQLIB=$(out)/lib/coq/${coq.coq-version}/";

  meta = {
    description = "A Coq/SSReflect Library for Monoidal Rings and Multinomials";
    inherit (src.meta) homepage;
    license = stdenv.lib.licenses.cecill-b;
    inherit (coq.meta) platforms;

  passthru = {
    compatibleCoqVersions = v: builtins.elem v [ "8.5" "8.6" "8.7" ];

15.6. Crystal

15.6.1. Building a Crystal package

This section uses Mint as an example for how to build a Crystal package.

If the Crystal project has any dependencies, the first step is to get a shards.nix file encoding those. Get a copy of the project and go to its root directory such that its shard.lock file is in the current directory, then run crystal2nix in it

$ git clone
$ cd mint
$ git checkout 0.5.0
$ nix-shell -p crystal2nix --run crystal2nix

This should have generated a shards.nix file.

Next create a Nix file for your derivation and use pkgs.crystal.buildCrystalPackage as follows:

with import <nixpkgs> {};
crystal.buildCrystalPackage rec {
  pname = "mint";
  version = "0.5.0";

  src = fetchFromGitHub {
    owner = "mint-lang";
    repo = "mint";
    rev = version;
    sha256 = "0vxbx38c390rd2ysvbwgh89v2232sh5rbsp3nk9wzb70jybpslvl";

  # Insert the path to your shards.nix file here
  shardsFile = ./shards.nix;


This won’t build anything yet, because we haven’t told it what files build. We can specify a mapping from binary names to source files with the crystalBinaries attribute. The project’s compilation instructions should show this. For Mint, the binary is called mint, which is compiled from the source file src/, so we’ll specify this as follows: = "src/";

  # ...

Additionally you can override the default crystal build options (which are currently --release --progress --no-debug --verbose) with = [ "--release" "--verbose" ];

Depending on the project, you might need additional steps to get it to compile successfully. In Mint’s case, we need to link against openssl, so in the end the Nix file looks as follows:

with import <nixpkgs> {};
crystal.buildCrystalPackage rec {
  version = "0.5.0";
  pname = "mint";
  src = fetchFromGitHub {
    owner = "mint-lang";
    repo = "mint";
    rev = version;
    sha256 = "0vxbx38c390rd2ysvbwgh89v2232sh5rbsp3nk9wzb70jybpslvl";

  shardsFile = ./shards.nix; = "src/";

  buildInputs = [ openssl ];

15.7. Emscripten

Emscripten: An LLVM-to-JavaScript Compiler

This section of the manual covers how to use emscripten in nixpkgs.

Minimal requirements:

  • nix

  • nixpkgs

Modes of use of emscripten:

  • Imperative usage (on the command line):

    If you want to work with emcc, emconfigure and emmake as you are used to from Ubuntu and similar distributions you can use these commands:

    • nix-env -i emscripten

    • nix-shell -p emscripten

  • Declarative usage:

    This mode is far more power full since this makes use of nix for dependency management of emscripten libraries and targets by using the mkDerivation which is implemented by pkgs.emscriptenStdenv and pkgs.buildEmscriptenPackage. The source for the packages is in pkgs/top-level/emscripten-packages.nix and the abstraction behind it in pkgs/development/em-modules/generic/default.nix.

    • build and install all packages:

      • nix-env -iA emscriptenPackages

    • dev-shell for zlib implementation hacking:

      • nix-shell -A emscriptenPackages.zlib

15.7.1. Imperative usage

A few things to note:

  • export EMCC_DEBUG=2 is nice for debugging

  • ~/.emscripten, the build artifact cache sometimes creates issues and needs to be removed from time to time

15.7.2. Declarative usage

Let’s see two different examples from pkgs/top-level/emscripten-packages.nix:

  • pkgs.zlib.override

  • pkgs.buildEmscriptenPackage

Both are interesting concepts.

A special requirement of the pkgs.buildEmscriptenPackage is the doCheck = true is a default meaning that each emscriptenPackage requires a checkPhase implemented.

  • Use export EMCC_DEBUG=2 from within a emscriptenPackage’s phase to get more detailed debug output what is going wrong.

  • ~/.emscripten cache is requiring us to set HOME=$TMPDIR in individual phases. This makes compilation slower but also makes it more deterministic. Usage 1: pkgs.zlib.override

This example uses zlib from nixpkgs but instead of compiling C to ELF it compiles C to JS since we were using pkgs.zlib.override and changed stdenv to pkgs.emscriptenStdenv. A few adaptions and hacks were set in place to make it working. One advantage is that when pkgs.zlib is updated, it will automatically update this package as well. However, this can also be the downside…

See the zlib example:

zlib = (pkgs.zlib.override {
  stdenv = pkgs.emscriptenStdenv;
(old: rec {
  buildInputs = old.buildInputs ++ [ pkgconfig ];
  # we need to reset this setting!
  configurePhase = ''
    # FIXME: Some tests require writing at $HOME
    runHook preConfigure

    #export EMCC_DEBUG=2
    emconfigure ./configure --prefix=$out --shared

    runHook postConfigure
  dontStrip = true;
  outputs = [ "out" ];
  buildPhase = ''
    emmake make
  installPhase = ''
    emmake make install
  checkPhase = ''
    echo "================= testing zlib using node ================="

    echo "Compiling a custom test"
    set -x
    emcc -O2 -s EMULATE_FUNCTION_POINTER_CASTS=1 test/example.c -DZ_SOLO \${old.version} -I . -o example.js

    echo "Using node to execute the test"
    ${pkgs.nodejs}/bin/node ./example.js

    set +x
    if [ $? -ne 0 ]; then
      echo "test failed for some reason"
      exit 1;
      echo "it seems to work! very good."
    echo "================= /testing zlib using node ================="

  postPatch = pkgs.stdenv.lib.optionalString pkgs.stdenv.isDarwin ''
    substituteInPlace configure \
      --replace '/usr/bin/libtool' 'ar' \
      --replace 'AR="libtool"' 'AR="ar"' \
      --replace 'ARFLAGS="-o"' 'ARFLAGS="-r"'
}); Usage 2: pkgs.buildEmscriptenPackage

This xmlmirror example features a emscriptenPackage which is defined completely from this context and no pkgs.zlib.override is used.

xmlmirror = pkgs.buildEmscriptenPackage rec {
  name = "xmlmirror";

  buildInputs = [ pkgconfig autoconf automake libtool gnumake libxml2 nodejs openjdk json_c ];
  nativeBuildInputs = [ pkgconfig zlib ];

  src = pkgs.fetchgit {
    url = "";
    rev = "4fd7e86f7c9526b8f4c1733e5c8b45175860a8fd";
    sha256 = "1jasdqnbdnb83wbcnyrp32f36w3xwhwp0wq8lwwmhqagxrij1r4b";

  configurePhase = ''
    rm -f fastXmlLint.js*
    # a fix for ERROR:root:For asm.js, TOTAL_MEMORY must be a multiple of 16MB, was 234217728
    sed -e "s/TOTAL_MEMORY=234217728/TOTAL_MEMORY=268435456/g" -i Makefile.emEnv
    sed -e "s/-o fastXmlLint.js/-s EXTRA_EXPORTED_RUNTIME_METHODS='[\"ccall\", \"cwrap\"]' -o fastXmlLint.js/g" -i Makefile.emEnv

  buildPhase = ''
    make -f Makefile.emEnv

  outputs = [ "out" "doc" ];

  installPhase = ''
    mkdir -p $out/share
    mkdir -p $doc/share/${name}

    cp Demo* $out/share
    cp -R codemirror-5.12 $out/share
    cp fastXmlLint.js* $out/share
    cp *.xsd $out/share
    cp *.js $out/share
    cp *.xhtml $out/share
    cp *.html $out/share
    cp *.json $out/share
    cp *.rng $out/share
    cp $doc/share/${name}
  checkPhase = ''

}; Declarative debugging

Use nix-shell -I nixpkgs=/some/dir/nixpkgs -A emscriptenPackages.libz and from there you can go trough the individual steps. This makes it easy to build a good unit test or list the files of the project.

  1. nix-shell -I nixpkgs=/some/dir/nixpkgs -A emscriptenPackages.libz

  2. cd /tmp/

  3. unpackPhase

  4. cd libz-1.2.3

  5. configurePhase

  6. buildPhase

  7. … happy hacking…

15.7.3. Summary

Using this toolchain makes it easy to leverage nix from NixOS, MacOSX or even Windows (WSL+ubuntu+nix). This toolchain is reproducible, behaves like the rest of the packages from nixpkgs and contains a set of well working examples to learn and adapt from.

If in trouble, ask the maintainers.

15.8. GNOME

15.8.1. Packaging GNOME applications

Programs in the GNOME universe are written in various languages but they all use GObject-based libraries like GLib, GTK or GStreamer. These libraries are often modular, relying on looking into certain directories to find their modules. However, due to Nix’s specific file system organization, this will fail without our intervention. Fortunately, the libraries usually allow overriding the directories through environment variables, either natively or thanks to a patch in nixpkgs. Wrapping the executables to ensure correct paths are available to the application constitutes a significant part of packaging a modern desktop application. In this section, we will describe various modules needed by such applications, environment variables needed to make the modules load, and finally a script that will do the work for us. Settings

GSettings API is often used for storing settings. GSettings schemas are required, to know the type and other metadata of the stored values. GLib looks for glib-2.0/schemas/gschemas.compiled files inside the directories of XDG_DATA_DIRS.

On Linux, GSettings API is implemented using dconf backend. You will need to add dconf GIO module to GIO_EXTRA_MODULES variable, otherwise the memory backend will be used and the saved settings will not be persistent.

Last you will need the dconf database D-Bus service itself. You can enable it using programs.dconf.enable.

Some applications will also require gsettings-desktop-schemas for things like reading proxy configuration or user interface customization. This dependency is often not mentioned by upstream, you should grep for org.gnome.desktop and org.gnome.system to see if the schemas are needed. Icons

When an application uses icons, an icon theme should be available in XDG_DATA_DIRS during runtime. The package for the default, icon-less hicolor-icon-theme (should be propagated by every icon theme) contains a setup hook that will pick up icon themes from buildInputs and pass it to our wrapper. Unfortunately, relying on that would mean every user has to download the theme included in the package expression no matter their preference. For that reason, we leave the installation of icon theme on the user. If you use one of the desktop environments, you probably already have an icon theme installed.

To avoid costly file system access when locating icons, GTK, as well as Qt, can rely on icon-theme.cache files from the themes’ top-level directories. These files are generated using gtk-update-icon-cache, which is expected to be run whenever an icon is added or removed to an icon theme (typically an application icon into hicolor theme) and some programs do indeed run this after icon installation. However, since packages are installed into their own prefix by Nix, this would lead to conflicts. For that reason, gtk3 provides a setup hook that will clean the file from installation. Since most applications only ship their own icon that will be loaded on start-up, it should not affect them too much. On the other hand, icon themes are much larger and more widely used so we need to cache them. Because we recommend installing icon themes globally, we will generate the cache files from all packages in a profile using a NixOS module. You can enable the cache generation using gtk.iconCache.enable option if your desktop environment does not already do that. Packaging icon themes

Icon themes may inherit from other icon themes. The inheritance is specified using the Inherits key in the index.theme file distributed with the icon theme. According to the icon theme specification, icons not provided by the theme are looked for in its parent icon themes. Therefore the parent themes should be installed as dependencies for a more complete experience regarding the icon sets used.

The package hicolor-icon-theme provides a setup hook which makes symbolic links for the parent themes into the directory share/icons of the current theme directory in the nix store, making sure they can be found at runtime. For that to work the packages providing parent icon themes should be listed as propagated build dependencies, together with hicolor-icon-theme.

Also make sure that icon-theme.cache is installed for each theme provided by the package, and set dontDropIconThemeCache to true so that the cache file is not removed by the gtk3 setup hook. GTK Themes

Previously, a GTK theme needed to be in XDG_DATA_DIRS. This is no longer necessary for most programs since GTK incorporated Adwaita theme. Some programs (for example, those designed for elementary HIG) might require a special theme like pantheon.elementary-gtk-theme. GObject introspection typelibs

GObject introspection allows applications to use C libraries in other languages easily. It does this through typelib files searched in GI_TYPELIB_PATH. Various plug-ins

If your application uses GStreamer or Grilo, you should set GST_PLUGIN_SYSTEM_PATH_1_0 and GRL_PLUGIN_PATH, respectively.

15.8.2. Onto wrapGAppsHook

Given the requirements above, the package expression would become messy quickly:

preFixup = ''
  for f in $(find $out/bin/ $out/libexec/ -type f -executable); do
    wrapProgram "$f" \
      --prefix GIO_EXTRA_MODULES : "${getLib dconf}/lib/gio/modules" \
      --prefix XDG_DATA_DIRS : "$out/share" \
      --prefix XDG_DATA_DIRS : "$out/share/gsettings-schemas/${name}" \
      --prefix XDG_DATA_DIRS : "${gsettings-desktop-schemas}/share/gsettings-schemas/${}" \
      --prefix XDG_DATA_DIRS : "${hicolor-icon-theme}/share" \
      --prefix GI_TYPELIB_PATH : "${lib.makeSearchPath "lib/girepository-1.0" [ pango json-glib ]}"

Fortunately, there is wrapGAppsHook, that does the wrapping for us. In particular, it works in conjunction with other setup hooks that will populate the variable:

  • wrapGAppsHook itself will add the package’s share directory to XDG_DATA_DIRS.

  • glib setup hook will populate GSETTINGS_SCHEMAS_PATH and then wrapGAppsHook will prepend it to XDG_DATA_DIRS.

  • One of gtk3’s setup hooks will remove icon-theme.cache files from package’s icon theme directories to avoid conflicts. Icon theme packages should prevent this with dontDropIconThemeCache = true;.

  • dconf.lib is a dependency of wrapGAppsHook, which then also adds it to the GIO_EXTRA_MODULES variable.

  • hicolor-icon-theme’s setup hook will add icon themes to XDG_ICON_DIRS which is prepended to XDG_DATA_DIRS by wrapGAppsHook.

  • gobject-introspection setup hook populates GI_TYPELIB_PATH variable with lib/girepository-1.0 directories of dependencies, which is then added to wrapper by wrapGAppsHook. It also adds share directories of dependencies to XDG_DATA_DIRS, which is intended to promote GIR files but it also pollutes the closures of packages using wrapGAppsHook.

    Warning: The setup hook currently does not work in expressions with strictDeps enabled, like Python packages. In those cases, you will need to disable it with strictDeps = false;.
  • Setup hooks of gst_all_1.gstreamer and gnome3.grilo will populate the GST_PLUGIN_SYSTEM_PATH_1_0 and GRL_PLUGIN_PATH variables, respectively, which will then be added to the wrapper by wrapGAppsHook.

You can also pass additional arguments to makeWrapper using gappsWrapperArgs in preFixup hook:

preFixup = ''
    # Thumbnailers
    --prefix XDG_DATA_DIRS : "${gdk-pixbuf}/share"
    --prefix XDG_DATA_DIRS : "${librsvg}/share"
    --prefix XDG_DATA_DIRS : "${shared-mime-info}/share"

15.8.3. Updating GNOME packages

Most GNOME package offer updateScript, it is therefore possible to update to latest source tarball by running nix-shell maintainers/scripts/update.nix --argstr package gnome3.nautilus or even en masse with nix-shell maintainers/scripts/update.nix --argstr path gnome3. Read the package’s NEWS file to see what changed.

15.8.4. Frequently encountered issues

GLib-GIO-ERROR **: 06:04:50.903: No GSettings schemas are installed on the system

There are no schemas avalable in XDG_DATA_DIRS. Temporarily add a random package containing schemas like gsettings-desktop-schemas to buildInputs. glib and wrapGAppsHook setup hooks will take care of making the schemas available to application and you will see the actual missing schemas with the next error. Or you can try looking through the source code for the actual schemas used.

GLib-GIO-ERROR **: 06:04:50.903: Settings schema ‘’ is not installed

Package is missing some GSettings schemas. You can find out the package containing the schema with nix-locate and let the hooks handle the wrapping as above.

When using wrapGAppsHook with special derivers you can end up with double wrapped binaries.

This is because derivers like python.pkgs.buildPythonApplication or qt5.mkDerivation have setup-hooks automatically added that produce wrappers with makeWrapper. The simplest way to workaround that is to disable the wrapGAppsHook automatic wrapping with dontWrapGApps = true; and pass the arguments it intended to pass to makeWrapper to another.

In the case of a Python application it could look like:

python3.pkgs.buildPythonApplication {
  pname = "gnome-music";
  version = "3.32.2";

  nativeBuildInputs = [

  dontWrapGApps = true;

  # Arguments to be passed to `makeWrapper`, only used by buildPython*
  preFixup = ''

And for a QT app like:

mkDerivation {
  pname = "calibre";
  version = "3.47.0";

  nativeBuildInputs = [

  dontWrapGApps = true;

  # Arguments to be passed to `makeWrapper`, only used by qt5’s mkDerivation
  preFixup = ''

I am packaging a project that cannot be wrapped, like a library or GNOME Shell extension.

You can rely on applications depending on the library setting the necessary environment variables but that is often easy to miss. Instead we recommend to patch the paths in the source code whenever possible. Here are some examples:

I need to wrap a binary outside bin and libexec directories.

You can manually trigger the wrapping with wrapGApp in preFixup phase. It takes a path to a program as a first argument; the remaining arguments are passed directly to wrapProgram function.

15.9. Go

15.9.1. Go modules

The function buildGoModule builds Go programs managed with Go modules. It builds a Go modules through a two phase build:

  • An intermediate fetcher derivation. This derivation will be used to fetch all of the dependencies of the Go module.

  • A final derivation will use the output of the intermediate derivation to build the binaries and produce the final output.

Example 15.5. buildGoModule

pet = buildGoModule rec {
  pname = "pet";
  version = "0.3.4";

  src = fetchFromGitHub {
    owner = "knqyf263";
    repo = "pet";
    rev = "v${version}";
    sha256 = "0m2fzpqxk7hrbxsgqplkg7h2p7gv6s1miymv3gvw0cz039skag0s";

  vendorSha256 = "1879j77k96684wi554rkjxydrj8g3hpp0kvxz03sd8dmwr3lh83j"; 1

  subPackages = [ "." ]; 2

  deleteVendor = true; 3

  runVend = true; 4

  meta = with lib; {
    description = "Simple command-line snippet manager, written in Go";
    homepage = "";
    license =;
    maintainers = with maintainers; [ kalbasit ];
    platforms = platforms.linux ++ platforms.darwin;

Example 15.5, “buildGoModule” is an example expression using buildGoModule, the following arguments are of special significance to the function:


vendorSha256 is the hash of the output of the intermediate fetcher derivation.


subPackages limits the builder from building child packages that have not been listed. If subPackages is not specified, all child packages will be built.


deleteVendor removes the pre-existing vendor directory and fetches the dependencies. This should only be used if the dependencies included in the vendor folder are broken or incomplete.


runVend runs the vend command to generate the vendor directory. This is useful if your code depends on c code and go mod tidy does not include the needed sources to build.

vendorSha256 can also take null as an input. When `null` is used as a value, rather than fetching the dependencies and vendoring them, we use the vendoring included within the source repo. If you'd like to not have to update this field on dependency changes, run `go mod vendor` in your source repo and set 'vendorSha256 = null;'

15.9.2. Go legacy

The function buildGoPackage builds legacy Go programs, not supporting Go modules.

Example 15.6. buildGoPackage

deis = buildGoPackage rec {
  pname = "deis";
  version = "1.13.0";

  goPackagePath = ""; 1
  subPackages = [ "client" ]; 2

  src = fetchFromGitHub {
    owner = "deis";
    repo = "deis";
    rev = "v${version}";
    sha256 = "1qv9lxqx7m18029lj8cw3k7jngvxs4iciwrypdy0gd2nnghc68sw";

  goDeps = ./deps.nix; 3

  buildFlags = [ "--tags" "release" ]; 4

Example 15.6, “buildGoPackage” is an example expression using buildGoPackage, the following arguments are of special significance to the function:


goPackagePath specifies the package's canonical Go import path.


subPackages limits the builder from building child packages that have not been listed. If subPackages is not specified, all child packages will be built.

In this example only will be built.


goDeps is where the Go dependencies of a Go program are listed as a list of package source identified by Go import path. It could be imported as a separate deps.nix file for readability. The dependency data structure is described below.


buildFlags is a list of flags passed to the go build command.

The goDeps attribute can be imported from a separate nix file that defines which Go libraries are needed and should be included in GOPATH for buildPhase.

Example 15.7. deps.nix

[ 1
    goPackagePath = ""; 2
    fetch = {
      type = "git"; 3
      url = "";
      rev = "a83829b6f1293c91addabc89d0571c246397bbf4";
      sha256 = "1m4dsmk90sbi17571h6pld44zxz7jc4lrnl4f27dpd1l8g5xvjhh";
    goPackagePath = "";
    fetch = {
      type = "git";
      url = "";
      rev = "784ddc588536785e7299f7272f39101f7faccc3f";
      sha256 = "0wwz48jl9fvl1iknvn9dqr4gfy1qs03gxaikrxxp9gry6773v3sj";


goDeps is a list of Go dependencies.


goPackagePath specifies Go package import path.


fetch type that needs to be used to get package source. If git is used there should be url, rev and sha256 defined next to it.

To extract dependency information from a Go package in automated way use go2nix. It can produce complete derivation and goDeps file for Go programs.

You may use Go packages installed into the active Nix profiles by adding the following to your ~/.bashrc:

for p in $NIX_PROFILES; do

15.10. Haskell

The documentation for the Haskell infrastructure is published at The source code for that site lives in the doc/ sub-directory of the cabal2nix Git repository and changes can be submitted there.

15.11. Idris

15.11.1. Installing Idris

The easiest way to get a working idris version is to install the idris attribute:

$ # On NixOS
$ nix-env -i nixos.idris
$ # On non-NixOS
$ nix-env -i nixpkgs.idris

This however only provides the prelude and base libraries. To install idris with additional libraries, you can use the idrisPackages.with-packages function, e.g. in an overlay in ~/.config/nixpkgs/overlays/my-idris.nix:

self: super: {
  myIdris = with self.idrisPackages; with-packages [ contrib pruviloj ];

And then:

$ # On NixOS
$ nix-env -iA nixos.myIdris
$ # On non-NixOS
$ nix-env -iA nixpkgs.myIdris

To see all available Idris packages:

$ # On NixOS
$ nix-env -qaPA nixos.idrisPackages
$ # On non-NixOS
$ nix-env -qaPA nixpkgs.idrisPackages

Similarly, entering a nix-shell:

$ nix-shell -p 'idrisPackages.with-packages (with idrisPackages; [ contrib pruviloj ])'

15.11.2. Starting Idris with library support

To have access to these libraries in idris, call it with an argument -p <library name> for each library:

$ nix-shell -p 'idrisPackages.with-packages (with idrisPackages; [ contrib pruviloj ])'
[nix-shell:~]$ idris -p contrib -p pruviloj

A listing of all available packages the Idris binary has access to is available via --listlibs:

$ idris --listlibs

15.11.3. Building an Idris project with Nix

As an example of how a Nix expression for an Idris package can be created, here is the one for idrisPackages.yaml:

{ build-idris-package
, fetchFromGitHub
, contrib
, lightyear
, lib
build-idris-package  {
  name = "yaml";
  version = "2018-01-25";

  # This is the .ipkg file that should be built, defaults to the package name
  # In this case it should build `Yaml.ipkg` instead of `yaml.ipkg`
  # This is only necessary because the yaml packages ipkg file is
  # different from its package name here.
  ipkgName = "Yaml";
  # Idris dependencies to provide for the build
  idrisDeps = [ contrib lightyear ];

  src = fetchFromGitHub {
    owner = "Heather";
    repo = "Idris.Yaml";
    rev = "5afa51ffc839844862b8316faba3bafa15656db4";
    sha256 = "1g4pi0swmg214kndj85hj50ccmckni7piprsxfdzdfhg87s0avw7";

  meta = {
    description = "Idris YAML lib";
    homepage = "";
    license =;
    maintainers = [ lib.maintainers.brainrape ];

Assuming this file is saved as yaml.nix, it’s buildable using

$ nix-build -E '(import <nixpkgs> {}).idrisPackages.callPackage ./yaml.nix {}'

Or it’s possible to use

with import <nixpkgs> {};

  yaml = idrisPackages.callPackage ./yaml.nix {};

in another file (say default.nix) to be able to build it with

$ nix-build -A yaml

15.11.4. Passing options to idris commands

The build-idris-package function provides also optional input values to set additional options for the used idris commands.

Specifically, you can set idrisBuildOptions, idrisTestOptions, idrisInstallOptions and idrisDocOptions to provide additional options to the idris command respectively when building, testing, installing and generating docs for your package.

For example you could set

build-idris-package {
  idrisBuildOptions = [ "--log" "1" "--verbose" ]


to require verbose output during idris build phase.

15.12. iOS

This component is basically a wrapper/workaround that makes it possible to expose an Xcode installation as a Nix package by means of symlinking to the relevant executables on the host system.

Since Xcode can’t be packaged with Nix, nor we can publish it as a Nix package (because of its license) this is basically the only integration strategy making it possible to do iOS application builds that integrate with other components of the Nix ecosystem

The primary objective of this project is to use the Nix expression language to specify how iOS apps can be built from source code, and to automatically spawn iOS simulator instances for testing.

This component also makes it possible to use Hydra, the Nix-based continuous integration server to regularly build iOS apps and to do wireless ad-hoc installations of enterprise IPAs on iOS devices through Hydra.

The Xcode build environment implements a number of features.

15.12.1. Deploying a proxy component wrapper exposing Xcode

The first use case is deploying a Nix package that provides symlinks to the Xcode installation on the host system. This package can be used as a build input to any build function implemented in the Nix expression language that requires Xcode.

  pkgs = import <nixpkgs> {};

  xcodeenv = import ./xcodeenv {
    inherit (pkgs) stdenv;
xcodeenv.composeXcodeWrapper {
  version = "9.2";
  xcodeBaseDir = "/Applications/";

By deploying the above expression with nix-build and inspecting its content you will notice that several Xcode-related executables are exposed as a Nix package:

$ ls result/bin
lrwxr-xr-x  1 sander  staff  94  1 jan  1970 Simulator -> /Applications/
lrwxr-xr-x  1 sander  staff  17  1 jan  1970 codesign -> /usr/bin/codesign
lrwxr-xr-x  1 sander  staff  17  1 jan  1970 security -> /usr/bin/security
lrwxr-xr-x  1 sander  staff  21  1 jan  1970 xcode-select -> /usr/bin/xcode-select
lrwxr-xr-x  1 sander  staff  61  1 jan  1970 xcodebuild -> /Applications/
lrwxr-xr-x  1 sander  staff  14  1 jan  1970 xcrun -> /usr/bin/xcrun

15.12.2. Building an iOS application

We can build an iOS app executable for the simulator, or an IPA/xcarchive file for release purposes, e.g. ad-hoc, enterprise or store installations, by executing the xcodeenv.buildApp {} function:

  pkgs = import <nixpkgs> {};

  xcodeenv = import ./xcodeenv {
    inherit (pkgs) stdenv;
xcodeenv.buildApp {
  name = "MyApp";
  src = ./myappsources;
  sdkVersion = "11.2";

  target = null; # Corresponds to the name of the app by default
  configuration = null; # Release for release builds, Debug for debug builds
  scheme = null; # -scheme will correspond to the app name by default
  sdk = null; # null will set it to 'iphonesimulator` for simulator builds or `iphoneos` to real builds
  xcodeFlags = "";

  release = true;
  certificateFile = ./mycertificate.p12;
  certificatePassword = "secret";
  provisioningProfile = ./myprovisioning.profile;
  signMethod = "ad-hoc"; # 'enterprise' or 'store'
  generateIPA = true;
  generateXCArchive = false;

  enableWirelessDistribution = true;
  installURL = "/installipa.php";
  bundleId = "mycompany.myapp";
  appVersion = "1.0";

  # Supports all xcodewrapper parameters as well
  xcodeBaseDir = "/Applications/";

The above function takes a variety of parameters: * The name and src parameters are mandatory and specify the name of the app and the location where the source code resides * sdkVersion specifies which version of the iOS SDK to use.

It also possile to adjust the xcodebuild parameters. This is only needed in rare circumstances. In most cases the default values should suffice:

  • Specifies which xcodebuild target to build. By default it takes the target that has the same name as the app.

  • The configuration parameter can be overridden if desired. By default, it will do a debug build for the simulator and a release build for real devices.

  • The scheme parameter specifies which -scheme parameter to propagate to xcodebuild. By default, it corresponds to the app name.

  • The sdk parameter specifies which SDK to use. By default, it picks iphonesimulator for simulator builds and iphoneos for release builds.

  • The xcodeFlags parameter specifies arbitrary command line parameters that should be propagated to xcodebuild.

By default, builds are carried out for the iOS simulator. To do release builds (builds for real iOS devices), you must set the release parameter to true. In addition, you need to set the following parameters:

  • certificateFile refers to a P12 certificate file.

  • certificatePassword specifies the password of the P12 certificate.

  • provisioningProfile refers to the provision profile needed to sign the app

  • signMethod should refer to ad-hoc for signing the app with an ad-hoc certificate, enterprise for enterprise certificates and app-store for App store certificates.

  • generateIPA specifies that we want to produce an IPA file (this is probably what you want)

  • generateXCArchive specifies thet we want to produce an xcarchive file.

When building IPA files on Hydra and when it is desired to allow iOS devices to install IPAs by browsing to the Hydra build products page, you can enable the enableWirelessDistribution parameter.

When enabled, you need to configure the following options:

  • The installURL parameter refers to the URL of a PHP script that composes the itms-services:// URL allowing iOS devices to install the IPA file.

  • bundleId refers to the bundle ID value of the app

  • appVersion refers to the app’s version number

To use wireless adhoc distributions, you must also install the corresponding PHP script on a web server (see section: Installing the PHP script for wireless ad hoc installations from Hydra for more information).

In addition to the build parameters, you can also specify any parameters that the xcodeenv.composeXcodeWrapper {} function takes. For example, the xcodeBaseDir parameter can be overridden to refer to a different Xcode version.

15.12.3. Spawning simulator instances

In addition to building iOS apps, we can also automatically spawn simulator instances:

  pkgs = import <nixpkgs> {};

  xcodeenv = import ./xcodeenv {
    inherit (pkgs) stdenv;
xcode.simulateApp {
  name = "simulate";

  # Supports all xcodewrapper parameters as well
  xcodeBaseDir = "/Applications/";

The above expression produces a script that starts the simulator from the provided Xcode installation. The script can be started as follows:


By default, the script will show an overview of UDID for all available simulator instances and asks you to pick one. You can also provide a UDID as a command-line parameter to launch an instance automatically:

./result/bin/run-test-simulator 5C93129D-CF39-4B1A-955F-15180C3BD4B8

You can also extend the simulator script to automatically deploy and launch an app in the requested simulator instance:

  pkgs = import <nixpkgs> {};

  xcodeenv = import ./xcodeenv {
    inherit (pkgs) stdenv;
xcode.simulateApp {
  name = "simulate";
  bundleId = "mycompany.myapp";
  app = xcode.buildApp {
    # ...

  # Supports all xcodewrapper parameters as well
  xcodeBaseDir = "/Applications/";

By providing the result of an xcode.buildApp {} function and configuring the app bundle id, the app gets deployed automatically and started.

15.12.4. Troubleshooting

In some rare cases, it may happen that after a failure, changes are not picked up. Most likely, this is caused by a derived data cache that Xcode maintains. To wipe it you can run:

$ rm -rf ~/Library/Developer/Xcode/DerivedData

15.13. Java

Ant-based Java packages are typically built from source as follows:

stdenv.mkDerivation {
  name = "...";
  src = fetchurl { ... };

  nativeBuildInputs = [ jdk ant ];

  buildPhase = "ant";

Note that jdk is an alias for the OpenJDK (self-built where available, or pre-built via Zulu). Platforms with OpenJDK not (yet) in Nixpkgs (Aarch32, Aarch64) point to the (unfree) oraclejdk.

JAR files that are intended to be used by other packages should be installed in $out/share/java. JDKs have a stdenv setup hook that add any JARs in the share/java directories of the build inputs to the CLASSPATH environment variable. For instance, if the package libfoo installs a JAR named foo.jar in its share/java directory, and another package declares the attribute

buildInputs = [ libfoo ];
nativeBuildInputs = [ jdk ];

then CLASSPATH will be set to /nix/store/...-libfoo/share/java/foo.jar.

Private JARs should be installed in a location like $out/share/package-name.

If your Java package provides a program, you need to generate a wrapper script to run it using the OpenJRE. You can use makeWrapper for this:

nativeBuildInputs = [ makeWrapper ];

installPhase =
    mkdir -p $out/bin
    makeWrapper ${jre}/bin/java $out/bin/foo \
      --add-flags "-cp $out/share/java/foo.jar"

Note the use of jre, which is the part of the OpenJDK package that contains the Java Runtime Environment. By using ${jre}/bin/java instead of ${jdk}/bin/java, you prevent your package from depending on the JDK at runtime.

Note all JDKs passthru home, so if your application requires environment variables like JAVA_HOME being set, that can be done in a generic fashion with the --set argument of makeWrapper:

--set JAVA_HOME ${jdk.home}

It is possible to use a different Java compiler than javac from the OpenJDK. For instance, to use the GNU Java Compiler:

nativeBuildInputs = [ gcj ant ];

Here, Ant will automatically use gij (the GNU Java Runtime) instead of the OpenJRE.

15.14. User’s Guide to Lua Infrastructure

15.14.1. Using Lua Overview of Lua

Several versions of the Lua interpreter are available: luajit, lua 5.1, 5.2, 5.3. The attribute lua refers to the default interpreter, it is also possible to refer to specific versions, e.g. lua5_2 refers to Lua 5.2.

Lua libraries are in separate sets, with one set per interpreter version.

The interpreters have several common attributes. One of these attributes is pkgs, which is a package set of Lua libraries for this specific interpreter. E.g., the busted package corresponding to the default interpreter is lua.pkgs.busted, and the lua 5.2 version is lua5_2.pkgs.busted. The main package set contains aliases to these package sets, e.g. luaPackages refers to lua5_1.pkgs and lua52Packages to lua5_2.pkgs. Installing Lua and packages Lua environment defined in separate .nix file

Create a file, e.g. build.nix, with the following expression

with import <nixpkgs> {};

lua5_2.withPackages (ps: with ps; [ busted luafilesystem ])

and install it in your profile with

nix-env -if build.nix

Now you can use the Lua interpreter, as well as the extra packages (busted, luafilesystem) that you added to the environment. Lua environment defined in ~/.config/nixpkgs/config.nix

If you prefer to, you could also add the environment as a package override to the Nixpkgs set, e.g. using config.nix,

{ # ...

  packageOverrides = pkgs: with pkgs; {
    myLuaEnv = lua5_2.withPackages (ps: with ps; [ busted luafilesystem ]);

and install it in your profile with

nix-env -iA nixpkgs.myLuaEnv

The environment is is installed by referring to the attribute, and considering the nixpkgs channel was used. Lua environment defined in /etc/nixos/configuration.nix

For the sake of completeness, here’s another example how to install the environment system-wide.

{ # ...

  environment.systemPackages = with pkgs; [
    (lua.withPackages(ps: with ps; [ busted luafilesystem ]))
} How to override a Lua package using overlays?

Use the following overlay template:

final: prev:

  lua = prev.lua.override {
    packageOverrides = luaself: luaprev: {

      luarocks-nix = luaprev.luarocks-nix.overrideAttrs(oa: {
        pname = "luarocks-nix";
        src = /home/my_luarocks/repository;

  luaPackages = lua.pkgs;
} Temporary Lua environment with nix-shell

There are two methods for loading a shell with Lua packages. The first and recommended method is to create an environment with lua.buildEnv or lua.withPackages and load that. E.g.

$ nix-shell -p 'lua.withPackages(ps: with ps; [ busted luafilesystem ])'

opens a shell from which you can launch the interpreter

[nix-shell:~] lua

The other method, which is not recommended, does not create an environment and requires you to list the packages directly,

$ nix-shell -p lua.pkgs.busted lua.pkgs.luafilesystem

Again, it is possible to launch the interpreter from the shell. The Lua interpreter has the attribute pkgs which contains all Lua libraries for that specific interpreter.

15.14.2. Developing with Lua

Now that you know how to get a working Lua environment with Nix, it is time to go forward and start actually developing with Lua. There are two ways to package lua software, either it is on luarocks and most of it can be taken care of by the luarocks2nix converter or the packaging has to be done manually. Let’s present the luarocks way first and the manual one in a second time. Packaging a library on luarocks is the main repository of lua packages. The site proposes two types of packages, the rockspec and the src.rock (equivalent of a rockspec but with the source). These packages can have different build types such as cmake, builtin etc .

Luarocks-based packages are generated in pkgs/development/lua-modules/generated-packages.nix from the whitelist maintainers/scripts/luarocks-packages.csv and updated by running maintainers/scripts/update-luarocks-packages.

luarocks2nix is a tool capable of generating nix derivations from both rockspec and src.rock (and favors the src.rock). The automation only goes so far though and some packages need to be customized. These customizations go in pkgs/development/lua-modules/overrides.nix. For instance if the rockspec defines external_dependencies, these need to be manually added in in its rockspec file then it won’t work.

You can try converting luarocks packages to nix packages with the command nix-shell -p luarocks-nix and then luarocks nix PKG_NAME. Nix rely on luarocks to install lua packages, basically it runs: luarocks make --deps-mode=none --tree $out Packaging a library manually

You can develop your package as you usually would, just don’t forget to wrap it within a toLuaModule call, for instance

mynewlib = toLuaModule ( stdenv.mkDerivation { ... });

There is also the buildLuaPackage function that can be used when lua modules are not packaged for luarocks. You can see a few examples at pkgs/top-level/lua-packages.nix.

15.14.3. Lua Reference Lua interpreters

Versions 5.1, 5.2 and 5.3 of the lua interpreter are available as respectively lua5_1, lua5_2 and lua5_3. Luajit is available too. The Nix expressions for the interpreters can be found in pkgs/development/interpreters/lua-5. Attributes on lua interpreters packages

Each interpreter has the following attributes:

  • interpreter. Alias for ${pkgs.lua}/bin/lua.

  • buildEnv. Function to build lua interpreter environments with extra packages bundled together. See section lua.buildEnv function for usage and documentation.

  • withPackages. Simpler interface to buildEnv.

  • pkgs. Set of Lua packages for that specific interpreter. The package set can be modified by overriding the interpreter and passing packageOverrides. buildLuarocksPackage function

The buildLuarocksPackage function is implemented in pkgs/development/interpreters/lua-5/build-lua-package.nix The following is an example:

luaposix = buildLuarocksPackage {
  pname = "luaposix";
  version = "34.0.4-1";

  src = fetchurl {
    url    = "";
    sha256 = "0yrm5cn2iyd0zjd4liyj27srphvy0gjrjx572swar6zqr4dwjqp2";
  disabled = (luaOlder "5.1") || (luaAtLeast "5.4");
  propagatedBuildInputs = [ bit32 lua std_normalize ];

  meta = with stdenv.lib; {
    homepage = "";
    description = "Lua bindings for POSIX";
    maintainers = with maintainers; [ vyp lblasc ];
    license.fullName = "MIT/X11";

The buildLuarocksPackage delegates most tasks to luarocks:

  • it adds luarocks as an unpacker for src.rock files (zip files really).

  • configurePhasewrites a temporary luarocks configuration file which location is exported via the environment variableLUAROCKS_CONFIG`.

  • the buildPhase does nothing.

  • installPhase calls luarocks make --deps-mode=none --tree $out to build and install the package

  • In the postFixup phase, the wrapLuaPrograms bash function is called to wrap all programs in the $out/bin/* directory to include $PATH environment variable and add dependent libraries to script’s LUA_PATH and LUA_CPATH.

By default meta.platforms is set to the same value as the interpreter unless overridden otherwise. buildLuaApplication function

The buildLuaApplication function is practically the same as buildLuaPackage. The difference is that buildLuaPackage by default prefixes the names of the packages with the version of the interpreter. Because with an application we’re not interested in multiple version the prefix is dropped. lua.withPackages function

The lua.withPackages takes a function as an argument that is passed the set of lua packages and returns the list of packages to be included in the environment. Using the withPackages function, the previous example for the luafilesystem environment can be written like this:

with import <nixpkgs> {};

lua.withPackages (ps: [ps.luafilesystem])

withPackages passes the correct package set for the specific interpreter version as an argument to the function. In the above example, ps equals luaPackages. But you can also easily switch to using lua5_2:

with import <nixpkgs> {};

lua5_2.withPackages (ps: [ps.lua])

Now, ps is set to lua52Packages, matching the version of the interpreter. Possible Todos

  • export/use version specific variables such as LUA_PATH_5_2/LUAROCKS_CONFIG_5_2

  • let luarocks check for dependencies via exporting the different rocktrees in temporary config Lua Contributing guidelines

Following rules should be respected:

  • Make sure libraries build for all Lua interpreters.

  • Commit names of Lua libraries should reflect that they are Lua libraries, so write for example luaPackages.luafilesystem: 1.11 -> 1.12.

15.15. Node.js

The pkgs/development/node-packages folder contains a generated collection of NPM packages that can be installed with the Nix package manager.

As a rule of thumb, the package set should only provide end user software packages, such as command-line utilities. Libraries should only be added to the package set if there is a non-NPM package that requires it.

When it is desired to use NPM libraries in a development project, use the node2nix generator directly on the package.json configuration file of the project.

The package set provides support for the official stable Node.js versions. The latest stable LTS release in nodePackages, as well as the latest stable Current release in nodePackages_latest.

If your package uses native addons, you need to examine what kind of native build system it uses. Here are some examples:

  • node-gyp

  • node-gyp-builder

  • node-pre-gyp

After you have identified the correct system, you need to override your package expression while adding in build system as a build input. For example, dat requires node-gyp-build, so we override its expression in default.nix:

    dat = super.dat.override {
      buildInputs = [ self.node-gyp-build pkgs.libtool pkgs.autoconf pkgs.automake ];
      meta.broken = since "12";

To add a package from NPM to nixpkgs:

  1. Modify pkgs/development/node-packages/node-packages.json to add, update or remove package entries to have it included in nodePackages and nodePackages_latest.

  2. Run the script: (cd pkgs/development/node-packages && ./

  3. Build your new package to test your changes: cd /path/to/nixpkgs && nix-build -A nodePackages.<new-or-updated-package>. To build against the latest stable Current Node.js version (e.g. 14.x): nix-build -A nodePackages_latest.<new-or-updated-package>

  4. Add and commit all modified and generated files.

For more information about the generation process, consult the file of the node2nix tool.

15.16. OCaml

OCaml libraries should be installed in $(out)/lib/ocaml/${ocaml.version}/site-lib/. Such directories are automatically added to the $OCAMLPATH environment variable when building another package that depends on them or when opening a nix-shell.

Given that most of the OCaml ecosystem is now built with dune, nixpkgs includes a convenience build support function called buildDunePackage that will build an OCaml package using dune, OCaml and findlib and any additional dependencies provided as buildInputs or propagatedBuildInputs.

Here is a simple package example. It defines an (optional) attribute minimumOCamlVersion that will be used to throw a descriptive evaluation error if building with an older OCaml is attempted. It uses the fetchFromGitHub fetcher to get its source. It sets the doCheck (optional) attribute to true which means that tests will be run with dune runtest -p angstrom after the build (dune build -p angstrom) is complete. It uses alcotest as a build input (because it is needed to run the tests) and bigstringaf and result as propagated build inputs (thus they will also be available to libraries depending on this library). The library will be installed using the angstrom.install file that dune generates.

{ stdenv, fetchFromGitHub, buildDunePackage, alcotest, result, bigstringaf }:

buildDunePackage rec {
  pname = "angstrom";
  version = "0.10.0";

  minimumOCamlVersion = "4.03";

  src = fetchFromGitHub {
    owner  = "inhabitedtype";
    repo   = pname;
    rev    = version;
    sha256 = "0lh6024yf9ds0nh9i93r9m6p5psi8nvrqxl5x7jwl13zb0r9xfpw";

  buildInputs = [ alcotest ];
  propagatedBuildInputs = [ bigstringaf result ];
  doCheck = true;

  meta = {
    homepage = "";
    description = "OCaml parser combinators built for speed and memory efficiency";
    license = stdenv.lib.licenses.bsd3;
    maintainers = with stdenv.lib.maintainers; [ sternenseemann ];

Here is a second example, this time using a source archive generated with dune-release. It is a good idea to use this archive when it is available as it will usually contain substituted variables such as a %%VERSION%% field. This library does not depend on any other OCaml library and no tests are run after building it.

{ stdenv, fetchurl, buildDunePackage }:

buildDunePackage rec {
  pname = "wtf8";
  version = "1.0.1";

  minimumOCamlVersion = "4.01";

  src = fetchurl {
    url = "${pname}/releases/download/v${version}/${pname}-${version}.tbz";
    sha256 = "1msg3vycd3k8qqj61sc23qks541cxpb97vrnrvrhjnqxsqnh6ygq";

  meta = with stdenv.lib; {
    homepage = "";
    description = "WTF-8 is a superset of UTF-8 that allows unpaired surrogates.";
    license =;
    maintainers = [ maintainers.eqyiel ];

15.17. Perl

15.17.1. Running perl programs on the shell

When executing a Perl script, it is possible you get an error such as ./ bad interpreter: /usr/bin/perl: no such file or directory. This happens when the script expects Perl to be installed at /usr/bin/perl, which is not the case when using Perl from nixpkgs. You can fix the script by changing the first line to:

#!/usr/bin/env perl

to take the Perl installation from the PATH environment variable, or invoke Perl directly with:

$ perl ./

When the script is using a Perl library that is not installed globally, you might get an error such as Can't locate in @INC (you may need to install the DB_File module). In that case, you can use nix-shell to start an ad-hoc shell with that library installed, for instance:

$ nix-shell -p perl perlPackages.DBFile --run ./

If you are always using the script in places where nix-shell is available, you can embed the nix-shell invocation in the shebang like this:

#!/usr/bin/env nix-shell
#! nix-shell -i perl -p perl perlPackages.DBFile

15.17.2. Packaging Perl programs

Nixpkgs provides a function buildPerlPackage, a generic package builder function for any Perl package that has a standard Makefile.PL. It’s implemented in pkgs/development/perl-modules/generic.

Perl packages from CPAN are defined in pkgs/top-level/perl-packages.nix, rather than pkgs/all-packages.nix. Most Perl packages are so straight-forward to build that they are defined here directly, rather than having a separate function for each package called from perl-packages.nix. However, more complicated packages should be put in a separate file, typically in pkgs/development/perl-modules. Here is an example of the former:

ClassC3 = buildPerlPackage rec {
  name = "Class-C3-0.21";
  src = fetchurl {
    url = "mirror://cpan/authors/id/F/FL/FLORA/${name}.tar.gz";
    sha256 = "1bl8z095y4js66pwxnm7s853pi9czala4sqc743fdlnk27kq94gz";

Note the use of mirror://cpan/, and the ${name} in the URL definition to ensure that the name attribute is consistent with the source that we’re actually downloading. Perl packages are made available in all-packages.nix through the variable perlPackages. For instance, if you have a package that needs ClassC3, you would typically write

foo = import ../path/to/foo.nix {
  inherit stdenv fetchurl ...;
  inherit (perlPackages) ClassC3;

in all-packages.nix. You can test building a Perl package as follows:

$ nix-build -A perlPackages.ClassC3

buildPerlPackage adds perl- to the start of the name attribute, so the package above is actually called perl-Class-C3-0.21. So to install it, you can say:

$ nix-env -i perl-Class-C3

(Of course you can also install using the attribute name: nix-env -i -A perlPackages.ClassC3.)

So what does buildPerlPackage do? It does the following:

  1. In the configure phase, it calls perl Makefile.PL to generate a Makefile. You can set the variable makeMakerFlags to pass flags to Makefile.PL

  2. It adds the contents of the PERL5LIB environment variable to #! .../bin/perl line of Perl scripts as -Idir flags. This ensures that a script can find its dependencies. (This can cause this shebang line to become too long for Darwin to handle; see the note below.)

  3. In the fixup phase, it writes the propagated build inputs (propagatedBuildInputs) to the file $out/nix-support/propagated-user-env-packages. nix-env recursively installs all packages listed in this file when you install a package that has it. This ensures that a Perl package can find its dependencies.

buildPerlPackage is built on top of stdenv, so everything can be customised in the usual way. For instance, the BerkeleyDB module has a preConfigure hook to generate a configuration file used by Makefile.PL:

{ buildPerlPackage, fetchurl, db }:

buildPerlPackage rec {
  name = "BerkeleyDB-0.36";

  src = fetchurl {
    url = "mirror://cpan/authors/id/P/PM/PMQS/${name}.tar.gz";
    sha256 = "07xf50riarb60l1h6m2dqmql8q5dij619712fsgw7ach04d8g3z1";

  preConfigure = ''
    echo "LIB = ${db.out}/lib" >
    echo "INCLUDE = ${}/include" >>

Dependencies on other Perl packages can be specified in the buildInputs and propagatedBuildInputs attributes. If something is exclusively a build-time dependency, use buildInputs; if it’s (also) a runtime dependency, use propagatedBuildInputs. For instance, this builds a Perl module that has runtime dependencies on a bunch of other modules:

ClassC3Componentised = buildPerlPackage rec {
  name = "Class-C3-Componentised-1.0004";
  src = fetchurl {
    url = "mirror://cpan/authors/id/A/AS/ASH/${name}.tar.gz";
    sha256 = "0xql73jkcdbq4q9m0b0rnca6nrlvf5hyzy8is0crdk65bynvs8q1";
  propagatedBuildInputs = [
    ClassC3 ClassInspector TestException MROCompat

On Darwin, if a script has too many -Idir flags in its first line (its “shebang line”), it will not run. This can be worked around by calling the shortenPerlShebang function from the postInstall phase:

{ stdenv, buildPerlPackage, fetchurl, shortenPerlShebang }:

ImageExifTool = buildPerlPackage {
  pname = "Image-ExifTool";
  version = "11.50";

  src = fetchurl {
    url = "";
    sha256 = "0d8v48y94z8maxkmw1rv7v9m0jg2dc8xbp581njb6yhr7abwqdv3";

  buildInputs = stdenv.lib.optional stdenv.isDarwin shortenPerlShebang;
  postInstall = stdenv.lib.optional stdenv.isDarwin ''
    shortenPerlShebang $out/bin/exiftool

This will remove the -I flags from the shebang line, rewrite them in the use lib form, and put them on the next line instead. This function can be given any number of Perl scripts as arguments; it will modify them in-place. Generation from CPAN

Nix expressions for Perl packages can be generated (almost) automatically from CPAN. This is done by the program nix-generate-from-cpan, which can be installed as follows:

$ nix-env -i nix-generate-from-cpan

This program takes a Perl module name, looks it up on CPAN, fetches and unpacks the corresponding package, and prints a Nix expression on standard output. For example:

$ nix-generate-from-cpan XML::Simple
  XMLSimple = buildPerlPackage rec {
    name = "XML-Simple-2.22";
    src = fetchurl {
      url = "mirror://cpan/authors/id/G/GR/GRANTM/${name}.tar.gz";
      sha256 = "b9450ef22ea9644ae5d6ada086dc4300fa105be050a2030ebd4efd28c198eb49";
    propagatedBuildInputs = [ XMLNamespaceSupport XMLSAX XMLSAXExpat ];
    meta = {
      description = "An API for simple XML files";
      license = with stdenv.lib.licenses; [ artistic1 gpl1Plus ];

The output can be pasted into pkgs/top-level/perl-packages.nix or wherever else you need it. Cross-compiling modules

Nixpkgs has experimental support for cross-compiling Perl modules. In many cases, it will just work out of the box, even for modules with native extensions. Sometimes, however, the Makefile.PL for a module may (indirectly) import a native module. In that case, you will need to make a stub for that module that will satisfy the Makefile.PL and install it into lib/perl5/site_perl/cross_perl/${perl.version}. See the postInstall for DBI for an example.

15.18. PHP

15.18.1. User Guide Overview

Several versions of PHP are available on Nix, each of which having a wide variety of extensions and libraries available.

The different versions of PHP that nixpkgs provides are located under attributes named based on major and minor version number; e.g., php74 is PHP 7.4.

Only versions of PHP that are supported by upstream for the entirety of a given NixOS release will be included in that release of NixOS. See PHP Supported Versions.

The attribute php refers to the version of PHP considered most stable and thoroughly tested in nixpkgs for any given release of NixOS - not necessarily the latest major release from upstream.

All available PHP attributes are wrappers around their respective binary PHP package and provide commonly used extensions this way. The real PHP 7.4 package, i.e. the unwrapped one, is available as php74.unwrapped; see the next section for more details.

Interactive tools built on PHP are put in php.packages; composer is for example available at php.packages.composer.

Most extensions that come with PHP, as well as some popular third-party ones, are available in php.extensions; for example, the opcache extension shipped with PHP is available at php.extensions.opcache and the third-party ImageMagick extension at php.extensions.imagick. Installing PHP with extensions

A PHP package with specific extensions enabled can be built using php.withExtensions. This is a function which accepts an anonymous function as its only argument; the function should accept two named parameters: enabled - a list of currently enabled extensions and all - the set of all extensions, and return a list of wanted extensions. For example, a PHP package with all default extensions and ImageMagick enabled:

php.withExtensions ({ enabled, all }:
  enabled ++ [ all.imagick ])

To exclude some, but not all, of the default extensions, you can filter the enabled list like this:

php.withExtensions ({ enabled, all }:
  (lib.filter (e: e != php.extensions.opcache) enabled)
  ++ [ all.imagick ])

To build your list of extensions from the ground up, you can simply ignore enabled:

php.withExtensions ({ all, ... }: with all; [ imagick opcache ])

php.withExtensions provides extensions by wrapping a minimal php base package, providing a php.ini file listing all extensions to be loaded. You can access this package through the php.unwrapped attribute; useful if you, for example, need access to the dev output. The generated php.ini file can be accessed through the php.phpIni attribute.

If you want a PHP build with extra configuration in the php.ini file, you can use php.buildEnv. This function takes two named and optional parameters: extensions and extraConfig. extensions takes an extension specification equivalent to that of php.withExtensions, extraConfig a string of additional php.ini configuration parameters. For example, a PHP package with the opcache and ImageMagick extensions enabled, and memory_limit set to 256M:

php.buildEnv {
  extensions = { all, ... }: with all; [ imagick opcache ];
  extraConfig = "memory_limit=256M";
} Example setup for phpfpm

You can use the previous examples in a phpfpm pool called foo as follows:

  myPhp = php.withExtensions ({ all, ... }: with all; [ imagick opcache ]);
in {
  services.phpfpm.pools."foo".phpPackage = myPhp;
  myPhp = php.buildEnv {
    extensions = { all, ... }: with all; [ imagick opcache ];
    extraConfig = "memory_limit=256M";
in {
  services.phpfpm.pools."foo".phpPackage = myPhp;
}; Example usage with nix-shell

This brings up a temporary environment that contains a PHP interpreter with the extensions imagick and opcache enabled:

nix-shell -p 'php.withExtensions ({ all, ... }: with all; [ imagick opcache ])' Installing PHP packages with extensions

All interactive tools use the PHP package you get them from, so all packages at php.packages.* use the php package with its default extensions. Sometimes this default set of extensions isn’t enough and you may want to extend it. A common case of this is the composer package: a project may depend on certain extensions and composer won’t work with that project unless those extensions are loaded.

Example of building composer with additional extensions:

(php.withExtensions ({ all, enabled }:
  enabled ++ (with all; [ imagick redis ]))

15.19. Python

15.19.1. User Guide Using Python Overview

Several versions of the Python interpreter are available on Nix, as well as a high amount of packages. The attribute python refers to the default interpreter, which is currently CPython 2.7. It is also possible to refer to specific versions, e.g. python38 refers to CPython 3.8, and pypy refers to the default PyPy interpreter.

Python is used a lot, and in different ways. This affects also how it is packaged. In the case of Python on Nix, an important distinction is made between whether the package is considered primarily an application, or whether it should be used as a library, i.e., of primary interest are the modules in site-packages that should be importable.

In the Nixpkgs tree Python applications can be found throughout, depending on what they do, and are called from the main package set. Python libraries, however, are in separate sets, with one set per interpreter version.

The interpreters have several common attributes. One of these attributes is pkgs, which is a package set of Python libraries for this specific interpreter. E.g., the toolz package corresponding to the default interpreter is python.pkgs.toolz, and the CPython 3.8 version is python38.pkgs.toolz. The main package set contains aliases to these package sets, e.g. pythonPackages refers to python.pkgs and python38Packages to python38.pkgs. Installing Python and packages

The Nix and NixOS manuals explain how packages are generally installed. In the case of Python and Nix, it is important to make a distinction between whether the package is considered an application or a library.

Applications on Nix are typically installed into your user profile imperatively using nix-env -i, and on NixOS declaratively by adding the package name to environment.systemPackages in /etc/nixos/configuration.nix. Dependencies such as libraries are automatically installed and should not be installed explicitly.

The same goes for Python applications. Python applications can be installed in your profile, and will be wrapped to find their exact library dependencies, without impacting other applications or polluting your user environment.

But Python libraries you would like to use for development cannot be installed, at least not individually, because they won’t be able to find each other resulting in import errors. Instead, it is possible to create an environment with python.buildEnv or python.withPackages where the interpreter and other executables are wrapped to be able to find each other and all of the modules.

In the following examples we will start by creating a simple, ad-hoc environment with a nix-shell that has numpy and toolz in Python 3.8; then we will create a re-usable environment in a single-file Python script; then we will create a full Python environment for development with this same environment.

Philosphically, this should be familiar to users who are used to a venv style of development: individual projects create their own Python environments without impacting the global environment or each other. Ad-hoc temporary Python environment with nix-shell

The simplest way to start playing with the way nix wraps and sets up Python environments is with nix-shell at the cmdline. These environments create a temporary shell session with a Python and a precise list of packages (plus their runtime dependencies), with no other Python packages in the Python interpreter’s scope.

To create a Python 3.8 session with numpy and toolz available, run:

$ nix-shell -p 'python38.withPackages(ps: with ps; [ numpy toolz ])'

By default nix-shell will start a bash session with this interpreter in our PATH, so if we then run:

[nix-shell:~/src/nixpkgs]$ python3
Python 3.8.1 (default, Dec 18 2019, 19:06:26)
[GCC 9.2.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import numpy; import toolz

Note that no other modules are in scope, even if they were imperatively installed into our user environment as a dependency of a Python application:

>>> import requests
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ModuleNotFoundError: No module named 'requests'

We can add as many additional modules onto the nix-shell as we need, and we will still get 1 wrapped Python interpreter. We can start the interpreter directly like so:

$ nix-shell -p 'python38.withPackages(ps: with ps; [ numpy toolz requests ])' --run python3
these derivations will be built:
building '/nix/store/xbdsrqrsfa1yva5s7pzsra8k08gxlbz1-python3-3.8.1-env.drv'...
created 277 symlinks in user environment
Python 3.8.1 (default, Dec 18 2019, 19:06:26)
[GCC 9.2.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import requests

Notice that this time it built a new Python environment, which now includes requests. Building an environment just creates wrapper scripts that expose the selected dependencies to the interpreter while re-using the actual modules. This means if any other env has installed requests or numpy in a different context, we don’t need to recompile them – we just recompile the wrapper script that sets up an interpreter pointing to them. This matters much more for big modules like pytorch or tensorflow.

Module names usually match their names on, but you can use the Nixpkgs search website to find them as well (along with non-python packages).

At this point we can create throwaway experimental Python environments with arbitrary dependencies. This is a good way to get a feel for how the Python interpreter and dependencies work in Nix and NixOS, but to do some actual development, we’ll want to make it a bit more persistent. Running Python scripts and using nix-shell as shebang

Sometimes, we have a script whose header looks like this:

#!/usr/bin/env python3
import numpy as np
a = np.array([1,2])
b = np.array([3,4])
print(f"The dot product of {a} and {b} is: {, b)}")

Executing this script requires a python3 that has numpy. Using what we learned in the previous section, we could startup a shell and just run it like so:

nix-shell -p 'python38.withPackages(ps: with ps; [ numpy ])' --run 'python3'
The dot product of [1 2] and [3 4] is: 11

But if we maintain the script ourselves, and if there are more dependencies, it may be nice to encode those dependencies in source to make the script re-usable without that bit of knowledge. That can be done by using nix-shell as a [shebang](, like so:

#!/usr/bin/env nix-shell
#!nix-shell -i python3 -p "python3.withPackages(ps: [ ps.numpy ])"
import numpy as np
a = np.array([1,2])
b = np.array([3,4])
print(f"The dot product of {a} and {b} is: {, b)}")

Then we simply execute it, without requiring any environment setup at all!

$ ./
The dot product of [1 2] and [3 4] is: 11

If the dependencies are not available on the host where is executed, it will build or download them from a Nix binary cache prior to starting up, prior that it is executed on a machine with a multi-user nix installation.

This provides a way to ship a self bootstrapping Python script, akin to a statically linked binary, where it can be run on any machine (provided nix is installed) without having to assume that numpy is installed globally on the system.

By default it is pulling the import checkout of Nixpkgs itself from our nix channel, which is nice as it cache aligns with our other package builds, but we can make it fully reproducible by pinning the nixpkgs import:

#!/usr/bin/env nix-shell
#!nix-shell -i python3 -p "python3.withPackages(ps: [ ps.numpy ])"
#!nix-shell -I nixpkgs=
import numpy as np
a = np.array([1,2])
b = np.array([3,4])
print(f"The dot product of {a} and {b} is: {, b)}")

This will execute with the exact same versions of Python 3.8, numpy, and system dependencies a year from now as it does today, because it will always use exactly git commit d373d80b1207d52621961b16aa4a3438e4f98167 of Nixpkgs for all of the package versions.

This is also a great way to ensure the script executes identically on different servers. Load environment from .nix expression

We’ve now seen how to create an ad-hoc temporary shell session, and how to create a single script with Python dependencies, but in the course of normal development we’re usually working in an entire package repository.

As explained in the Nix manual, nix-shell can also load an expression from a .nix file. Say we want to have Python 3.8, numpy and toolz, like before, in an environment. We can add a shell.nix file describing our dependencies:

with import <nixpkgs> {};
(python38.withPackages (ps: [ps.numpy ps.toolz])).env

And then at the command line, just typing nix-shell produces the same environment as before. In a normal project, we’ll likely have many more dependencies; this can provide a way for developers to share the environments with each other and with CI builders.

What’s happening here?

  1. We begin with importing the Nix Packages collections. import <nixpkgs> imports the <nixpkgs> function, {} calls it and the with statement brings all attributes of nixpkgs in the local scope. These attributes form the main package set.

  2. Then we create a Python 3.8 environment with the withPackages function, as before.

  3. The withPackages function expects us to provide a function as an argument that takes the set of all Python packages and returns a list of packages to include in the environment. Here, we select the packages numpy and toolz from the package set.

To combine this with mkShell you can:

with import <nixpkgs> {};
  pythonEnv = python38.withPackages (ps: [
in mkShell {
  buildInputs = [



This will create a unified environment that has not just our Python interpreter and its Python dependencies, but also tools like black or mypy and libraries like libffi the openssl in scope. This is generic and can span any number of tools or languages across the Nixpkgs ecosystem. Installing environments globally on the system

Up to now, we’ve been creating environments scoped to an ad-hoc shell session, or a single script, or a single project. This is generally advisable, as it avoids pollution across contexts.

However, sometimes we know we will often want a Python with some basic packages, and want this available without having to enter into a shell or build context. This can be useful to have things like vim/emacs editors and plugins or shell tools just work without having to set them up, or when running other software that expects packages to be installed globally.

To create your own custom environment, create a file in ~/.config/nixpkgs/overlays/ that looks like this:

# ~/.config/nixpkgs/overlays/myEnv.nix
self: super: {
  myEnv = super.buildEnv {
    name = "myEnv";
    paths = [
      # A Python 3 interpreter with some packages
      (self.python3.withPackages (
        ps: with ps; [

      # Some other packages we'd like as part of this env

You can then build and install this to your profile with:

nix-env -iA myEnv

One limitation of this is that you can only have 1 Python env installed globally, since they conflict on the python to load out of your PATH.

If you get a conflict or prefer to keep the setup clean, you can have nix-env atomically uninstall all other imperatively installed packages and replace your profile with just myEnv by using the --replace flag. Environment defined in /etc/nixos/configuration.nix

For the sake of completeness, here’s how to install the environment system-wide on NixOS.

{ # ...

  environment.systemPackages = with pkgs; [
    (python38.withPackages(ps: with ps; [ numpy toolz ]))
} Developing with Python

Above, we were mostly just focused on use cases and what to do to get started creating working Python environments in nix.

Now that you know the basics to be up and running, it is time to take a step back and take a deeper look at at how Python packages are packaged on Nix. Then, we will look at how you can use development mode with your code. Python library packages in Nixpkgs

With Nix all packages are built by functions. The main function in Nix for building Python libraries is buildPythonPackage. Let’s see how we can build the toolz package.

{ lib, buildPythonPackage, fetchPypi }:

buildPythonPackage rec {
  pname = "toolz";
  version = "0.10.0";

  src = fetchPypi {
    inherit pname version;
    sha256 = "08fdd5ef7c96480ad11c12d472de21acd32359996f69a5259299b540feba4560";

  doCheck = false;

  meta = with lib; {
    homepage = "";
    description = "List processing tools and functional utilities";
    license = licenses.bsd3;
    maintainers = with maintainers; [ fridh ];

What happens here? The function buildPythonPackage is called and as argument it accepts a set. In this case the set is a recursive set, rec. One of the arguments is the name of the package, which consists of a basename (generally following the name on PyPi) and a version. Another argument, src specifies the source, which in this case is fetched from PyPI using the helper function fetchPypi. The argument doCheck is used to set whether tests should be run when building the package. Furthermore, we specify some (optional) meta information. The output of the function is a derivation.

An expression for toolz can be found in the Nixpkgs repository. As explained in the introduction of this Python section, a derivation of toolz is available for each interpreter version, e.g. python38.pkgs.toolz refers to the toolz derivation corresponding to the CPython 3.8 interpreter.

The above example works when you’re directly working on pkgs/top-level/python-packages.nix in the Nixpkgs repository. Often though, you will want to test a Nix expression outside of the Nixpkgs tree.

The following expression creates a derivation for the toolz package, and adds it along with a numpy package to a Python environment.

with import <nixpkgs> {};

( let
    my_toolz = python38.pkgs.buildPythonPackage rec {
      pname = "toolz";
      version = "0.10.0";

      src = python38.pkgs.fetchPypi {
        inherit pname version;
        sha256 = "08fdd5ef7c96480ad11c12d472de21acd32359996f69a5259299b540feba4560";

      doCheck = false;

      meta = {
        homepage = "";
        description = "List processing tools and functional utilities";

  in python38.withPackages (ps: [ps.numpy my_toolz])

Executing nix-shell will result in an environment in which you can use Python 3.8 and the toolz package. As you can see we had to explicitly mention for which Python version we want to build a package.

So, what did we do here? Well, we took the Nix expression that we used earlier to build a Python environment, and said that we wanted to include our own version of toolz, named my_toolz. To introduce our own package in the scope of withPackages we used a let expression. You can see that we used ps.numpy to select numpy from the nixpkgs package set (ps). We did not take toolz from the Nixpkgs package set this time, but instead took our own version that we introduced with the let expression. Handling dependencies

Our example, toolz, does not have any dependencies on other Python packages or system libraries. According to the manual, buildPythonPackage uses the arguments buildInputs and propagatedBuildInputs to specify dependencies. If something is exclusively a build-time dependency, then the dependency should be included in buildInputs, but if it is (also) a runtime dependency, then it should be added to propagatedBuildInputs. Test dependencies are considered build-time dependencies and passed to checkInputs.

The following example shows which arguments are given to buildPythonPackage in order to build datashape.

{ lib, buildPythonPackage, fetchPypi, numpy, multipledispatch, dateutil, pytest }:

buildPythonPackage rec {
  pname = "datashape";
  version = "0.4.7";

  src = fetchPypi {
    inherit pname version;
    sha256 = "14b2ef766d4c9652ab813182e866f493475e65e558bed0822e38bf07bba1a278";

  checkInputs = [ pytest ];
  propagatedBuildInputs = [ numpy multipledispatch dateutil ];

  meta = with lib; {
    homepage = "";
    description = "A data description language";
    license = licenses.bsd2;
    maintainers = with maintainers; [ fridh ];

We can see several runtime dependencies, numpy, multipledispatch, and dateutil. Furthermore, we have one checkInputs, i.e. pytest. pytest is a test runner and is only used during the checkPhase and is therefore not added to propagatedBuildInputs.

In the previous case we had only dependencies on other Python packages to consider. Occasionally you have also system libraries to consider. E.g., lxml provides Python bindings to libxml2 and libxslt. These libraries are only required when building the bindings and are therefore added as buildInputs.

{ lib, pkgs, buildPythonPackage, fetchPypi }:

buildPythonPackage rec {
  pname = "lxml";
  version = "3.4.4";

  src = fetchPypi {
    inherit pname version;
    sha256 = "16a0fa97hym9ysdk3rmqz32xdjqmy4w34ld3rm3jf5viqjx65lxk";

  buildInputs = [ pkgs.libxml2 pkgs.libxslt ];

  meta = with lib; {
    description = "Pythonic binding for the libxml2 and libxslt libraries";
    homepage = "";
    license = licenses.bsd3;
    maintainers = with maintainers; [ sjourdois ];

In this example lxml and Nix are able to work out exactly where the relevant files of the dependencies are. This is not always the case.

The example below shows bindings to The Fastest Fourier Transform in the West, commonly known as FFTW. On Nix we have separate packages of FFTW for the different types of floats ("single", "double", "long-double"). The bindings need all three types, and therefore we add all three as buildInputs. The bindings don’t expect to find each of them in a different folder, and therefore we have to set LDFLAGS and CFLAGS.

{ lib, pkgs, buildPythonPackage, fetchPypi, numpy, scipy }:

buildPythonPackage rec {
  pname = "pyFFTW";
  version = "0.9.2";

  src = fetchPypi {
    inherit pname version;
    sha256 = "f6bbb6afa93085409ab24885a1a3cdb8909f095a142f4d49e346f2bd1b789074";

  buildInputs = [ pkgs.fftw pkgs.fftwFloat pkgs.fftwLongDouble];

  propagatedBuildInputs = [ numpy scipy ];

  # Tests cannot import pyfftw. pyfftw works fine though.
  doCheck = false;

  preConfigure = ''
    export LDFLAGS="-L${}/lib -L${pkgs.fftwFloat.out}/lib -L${pkgs.fftwLongDouble.out}/lib"
    export CFLAGS="-I${}/include -I${}/include -I${}/include"

  meta = with lib; {
    description = "A pythonic wrapper around FFTW, the FFT library, presenting a unified interface for all the supported transforms";
    homepage = "";
    license = with licenses; [ bsd2 bsd3 ];
    maintainers = with maintainers; [ fridh ];

Note also the line doCheck = false;, we explicitly disabled running the test-suite. Testing Python Packages

It is highly encouraged to have testing as part of the package build. This helps to avoid situations where the package was able to build and install, but is not usable at runtime. Currently, all packages will use the test command provided by the (i.e. python test). However, this is currently deprecated and your package should provide its own checkPhase.

NOTE: The checkPhase for python maps to the installCheckPhase on a normal derivation. This is due to many python packages not behaving well to the pre-installed version of the package. Version info, and natively compiled extensions generally only exist in the install directory, and thus can cause issues when a test suite asserts on that behavior.

NOTE: Tests should only be disabled if they don’t agree with nix (e.g. external dependencies, network access, flakey tests), however, as many tests should be enabled as possible. Failing tests can still be a good indication that the package is not in a valid state. Using pytest

Pytest is the most common test runner for python repositories. A trivial test run would be:

  checkInputs = [ pytest ];
  checkPhase = "pytest";

However, many repositories’ test suites do not translate well to nix’s build sandbox, and will generally need many tests to be disabled.

To filter tests using pytest, one can do the following:

  checkInputs = [ pytest ];
  # avoid tests which need additional data or touch network
  checkPhase = ''
    pytest tests/ --ignore=tests/integration -k 'not download and not update'

--ignore will tell pytest to ignore that file or directory from being collected as part of a test run. This is useful is a file uses a package which is not available in nixpkgs, thus skipping that test file is much easier than having to create a new package.

-k is used to define a predicate for test names. In this example, we are filtering out tests which contain download or update in their test case name. Only one -k argument is allows, and thus a long predicate should be concatenated with "" and wrapped to the next line.

NOTE: In pytest==6.0.1, the use of "" to continue a line (e.g. -k 'not download \') has been removed, in this case, it’s recommended to use pytestCheckHook. Using pytestCheckHook

pytestCheckHook is a convenient hook which will substitute the setuptools test command for a checkPhase which runs pytest. This is also beneficial when a package may need many items disabled to run the test suite.

Using the example above, the analagous pytestCheckHook usage would be:

  checkInputs = [ pytestCheckHook ];

  # requires additional data
  pytestFlagsArray = [ "tests/" "--ignore=tests/integration" ];

  disabledTests = [
    # touches network

This is expecially useful when tests need to be conditionallydisabled, for example:

  disabledTests = [
    # touches network
  ] ++ lib.optionals (pythonAtLeast "3.8") [
    # broken due to python3.8 async changes
  ] ++ lib.optionals stdenv.isDarwin [
    # can fail when building with other packages

Trying to concatenate the related strings to disable tests in a regular checkPhase would be much harder to read. This also enables us to comment on why specific tests are disabled. Using pythonImportsCheck

Although unit tests are highly prefered to valid correctness of a package. Not all packages have test suites that can be ran easily, and some have none at all. To help ensure the package still works, pythonImportsCheck can attempt to import the listed modules.

  pythonImportsCheck = [ "requests" "urllib" ];

roughly translates to:

  postCheck = ''
    python -c "import requests; import urllib"

However, this is done in it’s own phase, and not dependent on whether doCheck = true;

This can also be useful in verifying that the package doesn’t assume commonly present packages (e.g. setuptools) Develop local package

As a Python developer you’re likely aware of development mode (python develop); instead of installing the package this command creates a special link to the project code. That way, you can run updated code without having to reinstall after each and every change you make. Development mode is also available. Let’s see how you can use it.

In the previous Nix expression the source was fetched from an url. We can also refer to a local source instead using src = ./path/to/source/tree;

If we create a shell.nix file which calls buildPythonPackage, and if src is a local source, and if the local source has a, then development mode is activated.

In the following example we create a simple environment that has a Python 3.8 version of our package in it, as well as its dependencies and other packages we like to have in the environment, all specified with propagatedBuildInputs. Indeed, we can just add any package we like to have in our environment to propagatedBuildInputs.

with import <nixpkgs> {};
with python38Packages;

buildPythonPackage rec {
  name = "mypackage";
  src = ./path/to/package/source;
  propagatedBuildInputs = [ pytest numpy pkgs.libsndfile ];

It is important to note that due to how development mode is implemented on Nix it is not possible to have multiple packages simultaneously in development mode. Organising your packages

So far we discussed how you can use Python on Nix, and how you can develop with it. We’ve looked at how you write expressions to package Python packages, and we looked at how you can create environments in which specified packages are available.

At some point you’ll likely have multiple packages which you would like to be able to use in different projects. In order to minimise unnecessary duplication we now look at how you can maintain a repository with your own packages. The important functions here are import and callPackage. Including a derivation using callPackage

Earlier we created a Python environment using withPackages, and included the toolz package via a let expression. Let’s split the package definition from the environment definition.

We first create a function that builds toolz in ~/path/to/toolz/release.nix

{ lib, buildPythonPackage }:

buildPythonPackage rec {
  pname = "toolz";
  version = "0.10.0";

  src = fetchPypi {
    inherit pname version;
    sha256 = "08fdd5ef7c96480ad11c12d472de21acd32359996f69a5259299b540feba4560";

  meta = with lib; {
    homepage = "";
    description = "List processing tools and functional utilities";
    license = licenses.bsd3;
    maintainers = with maintainers; [ fridh ];

It takes an argument buildPythonPackage. We now call this function using callPackage in the definition of our environment

with import <nixpkgs> {};

( let
    toolz = callPackage /path/to/toolz/release.nix {
      buildPythonPackage = python38Packages.buildPythonPackage;
  in python38.withPackages (ps: [ ps.numpy toolz ])

Important to remember is that the Python version for which the package is made depends on the python derivation that is passed to buildPythonPackage. Nix tries to automatically pass arguments when possible, which is why generally you don’t explicitly define which python derivation should be used. In the above example we use buildPythonPackage that is part of the set python38Packages, and in this case the python38 interpreter is automatically used.

15.19.2. Reference Interpreters

Versions 2.7, 3.6, 3.7, 3.8 and 3.9 of the CPython interpreter are available as respectively python27, python36, python37, python38 and python39. The aliases python2 and python3 correspond to respectively python27 and python38. The default interpreter, python, maps to python2. The PyPy interpreters compatible with Python 2.7 and 3 are available as pypy27 and pypy3, with aliases pypy2 mapping to pypy27 and pypy mapping to pypy2. The Nix expressions for the interpreters can be found in pkgs/development/interpreters/python.

All packages depending on any Python interpreter get appended out/{python.sitePackages} to $PYTHONPATH if such directory exists. Missing tkinter module standard library

To reduce closure size the Tkinter/tkinter is available as a separate package, pythonPackages.tkinter. Attributes on interpreters packages

Each interpreter has the following attributes:

  • libPrefix. Name of the folder in ${python}/lib/ for corresponding interpreter.

  • interpreter. Alias for ${python}/bin/${executable}.

  • buildEnv. Function to build python interpreter environments with extra packages bundled together. See section python.buildEnv function for usage and documentation.

  • withPackages. Simpler interface to buildEnv. See section python.withPackages function for usage and documentation.

  • sitePackages. Alias for lib/${libPrefix}/site-packages.

  • executable. Name of the interpreter executable, e.g. python3.8.

  • pkgs. Set of Python packages for that specific interpreter. The package set can be modified by overriding the interpreter and passing packageOverrides. Building packages and applications

Python libraries and applications that use setuptools or distutils are typically built with respectively the buildPythonPackage and buildPythonApplication functions. These two functions also support installing a wheel.

All Python packages reside in pkgs/top-level/python-packages.nix and all applications elsewhere. In case a package is used as both a library and an application, then the package should be in pkgs/top-level/python-packages.nix since only those packages are made available for all interpreter versions. The preferred location for library expressions is in pkgs/development/python-modules. It is important that these packages are called from pkgs/top-level/python-packages.nix and not elsewhere, to guarantee the right version of the package is built.

Based on the packages defined in pkgs/top-level/python-packages.nix an attribute set is created for each available Python interpreter. The available sets are

  • pkgs.python27Packages

  • pkgs.python36Packages

  • pkgs.python37Packages

  • pkgs.python38Packages

  • pkgs.python39Packages

  • pkgs.pypyPackages

and the aliases

  • pkgs.python2Packages pointing to pkgs.python27Packages

  • pkgs.python3Packages pointing to pkgs.python38Packages

  • pkgs.pythonPackages pointing to pkgs.python2Packages buildPythonPackage function

The buildPythonPackage function is implemented in pkgs/development/interpreters/python/mk-python-derivation using setup hooks.

The following is an example:

{ lib, buildPythonPackage, fetchPypi, hypothesis, setuptools_scm, attrs, py, setuptools, six, pluggy }:

buildPythonPackage rec {
  pname = "pytest";
  version = "3.3.1";

  src = fetchPypi {
    inherit pname version;
    sha256 = "cf8436dc59d8695346fcd3ab296de46425ecab00d64096cebe79fb51ecb2eb93";

  postPatch = ''
    # don't test bash builtins
    rm testing/

  checkInputs = [ hypothesis ];
  nativeBuildInputs = [ setuptools_scm ];
  propagatedBuildInputs = [ attrs py setuptools six pluggy ];

  meta = with lib; {
    maintainers = with maintainers; [ domenkozar lovek323 madjar lsix ];
    description = "Framework for writing tests";

The buildPythonPackage mainly does four things:

  • In the buildPhase, it calls ${python.interpreter} bdist_wheel to build a wheel binary zipfile.

  • In the installPhase, it installs the wheel file using pip install *.whl.

  • In the postFixup phase, the wrapPythonPrograms bash function is called to wrap all programs in the $out/bin/* directory to include $PATH environment variable and add dependent libraries to script’s sys.path.

  • In the installCheck phase, ${python.interpreter} test is ran.

By default tests are run because doCheck = true. Test dependencies, like e.g. the test runner, should be added to checkInputs.

By default meta.platforms is set to the same value as the interpreter unless overridden otherwise. buildPythonPackage parameters

All parameters from stdenv.mkDerivation function are still supported. The following are specific to buildPythonPackage:

  • catchConflicts ? true: If true, abort package build if a package name appears more than once in dependency tree. Default is true.

  • disabled ? false: If true, package is not built for the particular Python interpreter version.

  • dontWrapPythonPrograms ? false: Skip wrapping of Python programs.

  • permitUserSite ? false: Skip setting the PYTHONNOUSERSITE environment variable in wrapped programs.

  • format ? "setuptools": Format of the source. Valid options are "setuptools", "pyproject", "flit", "wheel", and "other". "setuptools" is for when the source has a and setuptools is used to build a wheel, flit, in case flit should be used to build a wheel, and wheel in case a wheel is provided. Use other when a custom buildPhase and/or installPhase is needed.

  • makeWrapperArgs ? []: A list of strings. Arguments to be passed to makeWrapper, which wraps generated binaries. By default, the arguments to makeWrapper set PATH and PYTHONPATH environment variables before calling the binary. Additional arguments here can allow a developer to set environment variables which will be available when the binary is run. For example, makeWrapperArgs = ["--set FOO BAR" "--set BAZ QUX"].

  • namePrefix: Prepends text to ${name} parameter. In case of libraries, this defaults to "python3.8-" for Python 3.8, etc., and in case of applications to "".

  • pipInstallFlags ? []: A list of strings. Arguments to be passed to pip install. To pass options to python install, use --install-option. E.g., pipInstallFlags=["--install-option='--cpp_implementation'"].

  • pythonPath ? []: List of packages to be added into $PYTHONPATH. Packages in pythonPath are not propagated (contrary to propagatedBuildInputs).

  • preShellHook: Hook to execute commands before shellHook.

  • postShellHook: Hook to execute commands after shellHook.

  • removeBinByteCode ? true: Remove bytecode from /bin. Bytecode is only created when the filenames end with .py.

  • setupPyGlobalFlags ? []: List of flags passed to command.

  • setupPyBuildFlags ? []: List of flags passed to build_ext command.

The stdenv.mkDerivation function accepts various parameters for describing build inputs (see Specifying dependencies). The following are of special interest for Python packages, either because these are primarily used, or because their behaviour is different:

  • nativeBuildInputs ? []: Build-time only dependencies. Typically executables as well as the items listed in setup_requires.

  • buildInputs ? []: Build and/or run-time dependencies that need to be be compiled for the host machine. Typically non-Python libraries which are being linked.

  • checkInputs ? []: Dependencies needed for running the checkPhase. These are added to nativeBuildInputs when doCheck = true. Items listed in tests_require go here.

  • propagatedBuildInputs ? []: Aside from propagating dependencies, buildPythonPackage also injects code into and wraps executables with the paths included in this list. Items listed in install_requires go here. Overriding Python packages

The buildPythonPackage function has a overridePythonAttrs method that can be used to override the package. In the following example we create an environment where we have the blaze package using an older version of pandas. We override first the Python interpreter and pass packageOverrides which contains the overrides for packages in the package set.

with import <nixpkgs> {};

  python = let
    packageOverrides = self: super: {
      pandas = super.pandas.overridePythonAttrs(old: rec {
        version = "0.19.1";
        src =  super.fetchPypi {
          pname = "pandas";
          inherit version;
          sha256 = "08blshqj9zj1wyjhhw3kl2vas75vhhicvv72flvf1z3jvapgw295";
  in pkgs.python3.override {inherit packageOverrides; self = python;};

in python.withPackages(ps: [ps.blaze])).env buildPythonApplication function

The buildPythonApplication function is practically the same as buildPythonPackage. The main purpose of this function is to build a Python package where one is interested only in the executables, and not importable modules. For that reason, when adding this package to a python.buildEnv, the modules won’t be made available.

Another difference is that buildPythonPackage by default prefixes the names of the packages with the version of the interpreter. Because this is irrelevant for applications, the prefix is omitted.

When packaging a Python application with buildPythonApplication, it should be called with callPackage and passed python or pythonPackages (possibly specifying an interpreter version), like this:

{ lib, python3Packages }:

python3Packages.buildPythonApplication rec {
  pname = "luigi";
  version = "2.7.9";

  src = python3Packages.fetchPypi {
    inherit pname version;
    sha256 = "035w8gqql36zlan0xjrzz9j4lh9hs0qrsgnbyw07qs7lnkvbdv9x";

  propagatedBuildInputs = with python3Packages; [ tornado_4 python-daemon ];

  meta = with lib; {

This is then added to all-packages.nix just as any other application would be.

luigi = callPackage ../applications/networking/cluster/luigi { };

Since the package is an application, a consumer doesn’t need to care about Python versions or modules, which is why they don’t go in pythonPackages. toPythonApplication function

A distinction is made between applications and libraries, however, sometimes a package is used as both. In this case the package is added as a library to python-packages.nix and as an application to all-packages.nix. To reduce duplication the toPythonApplication can be used to convert a library to an application.

The Nix expression shall use buildPythonPackage and be called from python-packages.nix. A reference shall be created from all-packages.nix to the attribute in python-packages.nix, and the toPythonApplication shall be applied to the reference:

youtube-dl = with pythonPackages; toPythonApplication youtube-dl; toPythonModule function

In some cases, such as bindings, a package is created using stdenv.mkDerivation and added as attribute in all-packages.nix. The Python bindings should be made available from python-packages.nix. The toPythonModule function takes a derivation and makes certain Python-specific modifications.

opencv = toPythonModule (pkgs.opencv.override {
  enablePython = true;
  pythonPackages = self;

Do pay attention to passing in the right Python version! python.buildEnv function

Python environments can be created using the low-level pkgs.buildEnv function. This example shows how to create an environment that has the Pyramid Web Framework. Saving the following as default.nix

with import <nixpkgs> {};

python.buildEnv.override {
  extraLibs = [ pythonPackages.pyramid ];