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 unsuported for the given system.

  • 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 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 gpl3 and agpl3 licenses:

      blacklistedLicenses = with stdenv.lib.licenses; [ agpl3 gpl3 ];

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 environent.

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.

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 = "";

5.1.3. String manipulation functions lib.strings.concatStrings

concatStrings :: [string] -> string

Concatenate a list of strings.

Example 5.34. 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 5.35. 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 5.36. 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 5.37. 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 5.38. 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 5.39. 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 5.40. 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 5.41. 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 5.42. 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 5.43. 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 5.44. 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 5.45. 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 5.46. 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 5.47. 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 5.48. 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 5.49. 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 5.50. 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 5.51. 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 5.52. 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 5.53. 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 5.54. lib.strings.escapeNixString usage example

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

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

toLower :: string -> string

Converts an ASCII string to lower-case.

Example 5.55. lib.strings.toLower usage example

toLower "HOME"
=> "home"

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

toUpper :: string -> string

Converts an ASCII string to upper-case.

Example 5.56. lib.strings.toUpper usage example

toUpper "home"
=> "HOME"

Located at lib/strings.nix:354 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 5.57. lib.strings.addContextFrom usage example

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

Located at lib/strings.nix:369 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 5.58. lib.strings.splitString usage example

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

Located at lib/strings.nix:382 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 5.59. lib.strings.removePrefix usage example

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

Located at lib/strings.nix:415 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 5.60. lib.strings.removeSuffix usage example

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

Located at lib/strings.nix:439 in <nixpkgs>. lib.strings.versionOlder

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


Function argument


Function argument

Example 5.61. lib.strings.versionOlder usage example

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

Located at lib/strings.nix:461 in <nixpkgs>. lib.strings.versionAtLeast

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


Function argument


Function argument

Example 5.62. 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:473 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 5.63. lib.strings.getName usage example

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

Located at lib/strings.nix:485 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 5.64. lib.strings.getVersion usage example

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

Located at lib/strings.nix:502 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 5.65. lib.strings.nameFromURL usage example

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

Located at lib/strings.nix:518 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 5.66. lib.strings.enableFeature usage example

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

Located at lib/strings.nix:534 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 5.67. lib.strings.enableFeatureAs usage example

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

Located at lib/strings.nix:545 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 5.68. lib.strings.withFeature usage example

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

Located at lib/strings.nix:556 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 5.69. 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:567 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 5.70. lib.strings.fixedWidthString usage example

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

Located at lib/strings.nix:581 in <nixpkgs>. lib.strings.fixedWidthNumber

Format a number adding leading zeroes up to fixed width.


Function argument


Function argument

Example 5.71. lib.strings.fixedWidthNumber usage example

fixedWidthNumber 5 15
=> "00015"

Located at lib/strings.nix:598 in <nixpkgs>. lib.strings.isCoercibleToString

Check whether a value can be coerced to a string


Function argument

Located at lib/strings.nix:601 in <nixpkgs>. lib.strings.isStorePath

Check whether a value is a store path.


Function argument

Example 5.72. 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:619 in <nixpkgs>. lib.strings.toInt

string -> int

Parse a string string as an int.


Function argument

Example 5.73. 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:640 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.


Function argument


Function argument

Example 5.74. 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:661 in <nixpkgs>. lib.strings.fileContents

fileContents :: path -> string

Read the contents of a file removing the trailing \n


Function argument

Example 5.75. lib.strings.fileContents usage example

$ echo "1.0" > ./version

fileContents ./version
=> "1.0"

Located at lib/strings.nix:680 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 5.76. 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 5.77. 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 5.78. 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 5.79. 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 5.80. 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 5.81. 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 5.82. 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>.

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 5.83. 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 5.84. 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 5.85. 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 5.86. 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)) ""
strange [ 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 5.87. 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 5.88. 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 5.89. 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 5.90. 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 5.91. 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 5.92. 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 5.93. 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 5.94. 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 5.95. 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 5.96. 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 5.97. 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 5.98. 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 5.99. 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 5.100. 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 5.101. 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 5.102. 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 5.103. 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 5.104. 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 5.105. 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 5.106. 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 5.107. 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 5.108. 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 5.109. 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 5.110. 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 5.111. 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 5.112. 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 5.113. 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 5.114. 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 5.115. 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 5.116. 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.


Function argument

Example 5.117. 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 5.118. lib.lists.intersectLists usage example

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

Located at lib/lists.nix:657 in <nixpkgs>. lib.lists.subtractLists

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


Function argument

Example 5.119. lib.lists.subtractLists usage example

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

Located at lib/lists.nix:665 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:670 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 5.120. 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 5.121. 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 5.122. 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 5.123. 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 5.124. 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 5.125. 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 5.126. 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 5.127. 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 5.128. 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 5.129. 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 5.130. 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 5.131. lib.options.showOption usage example

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

Located at lib/options.nix:196 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. It has been patched to provide nested output that can be fed into the nix-log2xml command and log2html stylesheet to create a structured, readable output of the build steps performed by 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


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 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 15.7, “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 environment variable which lists dependencies that are unconditionally added to 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.

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.


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.gpl3.

  • Single license referenced by its attribute shortName (frowned upon) "gpl3".

  • Single license referenced by its attribute spdxId (frowned upon) "GPL-3.0".

  • 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.

8.2. Installing a split package

When installing a package via systemPackages or nix-env you have several options:

  • You can install particular outputs explicitly, as each is available in the Nix language as an attribute of the package. The outputs attribute contains a list of output names.

  • You can let it use the default outputs. These are handled by meta.outputsToInstall attribute that contains a list of output names.

    TODO: more about tweaking the attribute, etc.

  • NixOS provides configuration option environment.extraOutputsToInstall that allows adding extra outputs of environment.systemPackages atop the default ones. It's mainly meant for documentation and debug symbols, and it's also modified by specific options.

    Note: At this moment there is no similar configurability for packages installed by nix-env. You can still use approach from Section 2.5, “Modify packages via packageOverrides to override meta.outputsToInstall attributes, but that's a rather inconvenient way.

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 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. 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

14.2.4. 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

14.2.5. 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 "" ''
    ${coreutils}/bin/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. Android

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

15.1.1. 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 {
    # ...

15.1.2. 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> {};


15.1.3. 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.

15.1.4. 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_64"; # armeabi-v7a, mips, x86
  systemImageType = "google_apis_playstore";

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.

15.1.5. 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.

15.1.6. 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:

sh ./

15.2. BEAM Languages (Erlang, Elixir & LFE)

15.2.1. 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.

15.2.2. 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.

15.2.3. 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.

15.2.4. 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

15.2.5. 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:

15.2.6. 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.

15.3. 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.

15.3.1. 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.

15.3.2. 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.

15.3.3. 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.

15.4. 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" ];

15.5. Crystal

15.5.1. 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 ];

15.6. 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

15.6.1. 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

15.6.2. 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…

15.6.3. 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.

15.7. GNOME

15.7.1. 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. 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.

15.7.2. 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"

15.7.3. 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.

15.7.4. 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.

15.8. Go

15.8.1. 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";

  modSha256 = "1879j77k96684wi554rkjxydrj8g3hpp0kvxz03sd8dmwr3lh83j"; 1

  subPackages = [ "." ]; 2

  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:


modSha256 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.

modSha256 can also take null as an input. When `null` is used as a value, the derivation won't be a fixed-output derivation but disable the build sandbox instead. This can be useful outside of nixpkgs where re-generating the modSha256 on each mod.sum changes is cumbersome, but will fail to build by Hydra, as builds with a disabled sandbox are discouraged.

15.8.2. 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.

buildGoPackage produces Multiple-output packages where bin includes program binaries. You can test build a Go binary as follows:

$ nix-build -A deis.bin

or build all outputs with:

$ nix-build -A deis.all

bin output will be installed by default with nix-env -i or systemPackages.

You may use Go packages installed into the active Nix profiles by adding the following to your ~/.bashrc:

for p in $NIX_PROFILES; do

15.9. Haskell

15.9.1. How to install Haskell packages

Nixpkgs distributes build instructions for all Haskell packages registered on Hackage, but strangely enough normal Nix package lookups don’t seem to discover any of them, except for the default version of ghc, cabal-install, and stack:

$ nix-env -i alex
error: selector ‘alex’ matches no derivations
$ nix-env -qa ghc

The Haskell package set is not registered in the top-level namespace because it is huge. If all Haskell packages were visible to these commands, then name-based search/install operations would be much slower than they are now. We avoided that by keeping all Haskell-related packages in a separate attribute set called haskellPackages, which the following command will list:

$ nix-env -f "<nixpkgs>" -qaP -A haskellPackages
haskellPackages.a50                                             a50-0.5
haskellPackages.AAI                                             AAI-
haskellPackages.abacate                                         abacate-                                      abc-puzzle-0.2.1
haskellPackages.abcBridge                                       abcBridge-0.15
haskellPackages.abcnotation                                     abcnotation-1.9.0
haskellPackages.abeson                                          abeson-
[... some 14000 entries omitted  ...]

To install any of those packages into your profile, refer to them by their attribute path (first column):

nix-env -f "<nixpkgs>" -iA haskellPackages.Allure ...

The attribute path of any Haskell packages corresponds to the name of that particular package on Hackage: the package cabal-install has the attribute haskellPackages.cabal-install, and so on. (Actually, this convention causes trouble with packages like 3dmodels and 4Blocks, because these names are invalid identifiers in the Nix language. The issue of how to deal with these rare corner cases is currently unresolved.)

Haskell packages whose Nix name (second column) begins with a haskell- prefix are packages that provide a library whereas packages without that prefix provide just executables. Libraries may provide executables too, though: the package haskell-pandoc, for example, installs both a library and an application. You can install and use Haskell executables just like any other program in Nixpkgs, but using Haskell libraries for development is a bit trickier and we’ll address that subject in great detail in section How to create a development environment.

Attribute paths are deterministic inside of Nixpkgs, but the path necessary to reach Nixpkgs varies from system to system. We dodged that problem by giving nix-env an explicit -f "<nixpkgs>" parameter, but if you call nix-env without that flag, then chances are the invocation fails:

$ nix-env -iA haskellPackages.cabal-install
error: attribute ‘haskellPackages’ in selection path
       ‘haskellPackages.cabal-install’ not found

On NixOS, for example, Nixpkgs does not exist in the top-level namespace by default. To figure out the proper attribute path, it’s easiest to query for the path of a well-known Nixpkgs package, i.e.:

$ nix-env -qaP coreutils
nixos.coreutils  coreutils-8.23

If your system responds like that (most NixOS installations will), then the attribute path to haskellPackages is nixos.haskellPackages. Thus, if you want to use nix-env without giving an explicit -f flag, then that’s the way to do it:

nix-env -qaP -A nixos.haskellPackages
nix-env -iA nixos.haskellPackages.cabal-install

Our current default compiler is GHC 8.6.x and the haskellPackages set contains packages built with that particular version. Nixpkgs contains the last three major releases of GHC and there is a whole family of package sets available that defines Hackage packages built with each of those compilers, too:

nix-env -f "<nixpkgs>" -qaP -A haskell.packages.ghc844
nix-env -f "<nixpkgs>" -qaP -A haskell.packages.ghc882

The name haskellPackages is really just a synonym for haskell.packages.ghc865, because we prefer that package set internally and recommend it to our users as their default choice, but ultimately you are free to compile your Haskell packages with any GHC version you please. The following command displays the complete list of available compilers:

$ nix-env -f "<nixpkgs>" -qaP -A haskell.compiler
haskell.compiler.ghc8101                 ghc-
haskell.compiler.integer-simple.ghc8101  ghc-
haskell.compiler.ghcHEAD                 ghc-8.10.20191119
haskell.compiler.integer-simple.ghcHEAD  ghc-8.10.20191119
haskell.compiler.ghc822Binary            ghc-8.2.2-binary
haskell.compiler.ghc844                  ghc-8.4.4
haskell.compiler.ghc863Binary            ghc-8.6.3-binary
haskell.compiler.ghc865                  ghc-8.6.5
haskell.compiler.integer-simple.ghc865   ghc-8.6.5
haskell.compiler.ghc881                  ghc-8.8.1
haskell.compiler.integer-simple.ghc881   ghc-8.8.1
haskell.compiler.ghc882                  ghc-8.8.2
haskell.compiler.integer-simple.ghc882   ghc-8.8.2
haskell.compiler.ghc883                  ghc-8.8.3
haskell.compiler.integer-simple.ghc883   ghc-8.8.3
haskell.compiler.ghcjs                   ghcjs-

We have no package sets for jhc or uhc yet, unfortunately, but for every version of GHC listed above, there exists a package set based on that compiler. Also, the attributes haskell.compiler.ghcXYC and haskell.packages.ghcXYC.ghc are synonymous for the sake of convenience.

15.9.2. How to create a development environment How to install a compiler

A simple development environment consists of a Haskell compiler and one or both of the tools cabal-install and stack. We saw in section How to install Haskell packages how you can install those programs into your user profile:

nix-env -f "<nixpkgs>" -iA haskellPackages.ghc haskellPackages.cabal-install

Instead of the default package set haskellPackages, you can also use the more precise name haskell.compiler.ghc7102, which has the advantage that it refers to the same GHC version regardless of what Nixpkgs considers default at any given time.

Once you’ve made those tools available in $PATH, it’s possible to build Hackage packages the same way people without access to Nix do it all the time:

cabal get lens-4.11 && cd lens-4.11
cabal install -j --dependencies-only
cabal configure
cabal build

If you enjoy working with Cabal sandboxes, then that’s entirely possible too: just execute the command

cabal sandbox init

before installing the required dependencies.

The nix-shell utility makes it easy to switch to a different compiler version; just enter the Nix shell environment with the command

nix-shell -p haskell.compiler.ghc784

to bring GHC 7.8.4 into $PATH. Alternatively, you can use Stack instead of nix-shell directly to select compiler versions and other build tools per-project. It uses nix-shell under the hood when Nix support is turned on. See How to build a Haskell project using Stack.

If you’re using cabal-install, re-running cabal configure inside the spawned shell switches your build to use that compiler instead. If you’re working on a project that doesn’t depend on any additional system libraries outside of GHC, then it’s even sufficient to just run the cabal configure command inside of the shell:

nix-shell -p haskell.compiler.ghc784 --command "cabal configure"

Afterwards, all other commands like cabal build work just fine in any shell environment, because the configure phase recorded the absolute paths to all required tools like GHC in its build configuration inside of the dist/ directory. Please note, however, that nix-collect-garbage can break such an environment because the Nix store paths created by nix-shell aren’t alive anymore once nix-shell has terminated. If you find that your Haskell builds no longer work after garbage collection, then you’ll have to re-run cabal configure inside of a new nix-shell environment. How to install a compiler with libraries

GHC expects to find all installed libraries inside of its own lib directory. This approach works fine on traditional Unix systems, but it doesn’t work for Nix, because GHC’s store path is immutable once it’s built. We cannot install additional libraries into that location. As a consequence, our copies of GHC don’t know any packages except their own core libraries, like base, containers, Cabal, etc.

We can register additional libraries to GHC, however, using a special build function called ghcWithPackages. That function expects one argument: a function that maps from an attribute set of Haskell packages to a list of packages, which determines the libraries known to that particular version of GHC. For example, the Nix expression ghcWithPackages (pkgs: [pkgs.mtl]) generates a copy of GHC that has the mtl library registered in addition to its normal core packages:

$ nix-shell -p "haskellPackages.ghcWithPackages (pkgs: [pkgs.mtl])"

[nix-shell:~]$ ghc-pkg list mtl

This function allows users to define their own development environment by means of an override. After adding the following snippet to ~/.config/nixpkgs/config.nix,

  packageOverrides = super: let self = super.pkgs; in
    myHaskellEnv = self.haskell.packages.ghc7102.ghcWithPackages
                     (haskellPackages: with haskellPackages; [
                       # libraries
                       arrows async cgi criterion
                       # tools
                       cabal-install haskintex

it’s possible to install that compiler with nix-env -f "<nixpkgs>" -iA myHaskellEnv. If you’d like to switch that development environment to a different version of GHC, just replace the ghc7102 bit in the previous definition with the appropriate name. Of course, it’s also possible to define any number of these development environments! (You can’t install two of them into the same profile at the same time, though, because that would result in file conflicts.)

The generated ghc program is a wrapper script that re-directs the real GHC executable to use a new lib directory — one that we specifically constructed to contain all those packages the user requested:

$ cat $(type -p ghc)
#! /nix/store/xlxj...-bash-4.3-p33/bin/bash -e
export NIX_GHC=/nix/store/19sm...-ghc-7.10.2/bin/ghc
export NIX_GHCPKG=/nix/store/19sm...-ghc-7.10.2/bin/ghc-pkg
export NIX_GHC_DOCDIR=/nix/store/19sm...-ghc-7.10.2/share/doc/ghc/html
export NIX_GHC_LIBDIR=/nix/store/19sm...-ghc-7.10.2/lib/ghc-7.10.2
exec /nix/store/j50p...-ghc-7.10.2/bin/ghc "-B$NIX_GHC_LIBDIR" "$@"

The variables $NIX_GHC, $NIX_GHCPKG, etc. point to the new store path ghcWithPackages constructed specifically for this environment. The last line of the wrapper script then executes the real ghc, but passes the path to the new lib directory using GHC’s -B flag.

The purpose of those environment variables is to work around an impurity in the popular ghc-paths library. That library promises to give its users access to GHC’s installation paths. Only, the library can’t possible know that path when it’s compiled, because the path GHC considers its own is determined only much later, when the user configures it through ghcWithPackages. So we patched ghc-paths to return the paths found in those environment variables at run-time rather than trying to guess them at compile-time.

To make sure that mechanism works properly all the time, we recommend that you set those variables to meaningful values in your shell environment, too, i.e. by adding the following code to your ~/.bashrc:

if type >/dev/null 2>&1 -p ghc; then
  eval "$(egrep ^export "$(type -p ghc)")"

If you are certain that you’ll use only one GHC environment which is located in your user profile, then you can use the following code, too, which has the advantage that it doesn’t contain any paths from the Nix store, i.e. those settings always remain valid even if a nix-env -u operation updates the GHC environment in your profile:

if [ -e ~/.nix-profile/bin/ghc ]; then
  export NIX_GHC="$HOME/.nix-profile/bin/ghc"
  export NIX_GHCPKG="$HOME/.nix-profile/bin/ghc-pkg"
  export NIX_GHC_DOCDIR="$HOME/.nix-profile/share/doc/ghc/html"
  export NIX_GHC_LIBDIR="$HOME/.nix-profile/lib/ghc-$($NIX_GHC --numeric-version)"
fi How to install a compiler with libraries, hoogle and documentation indexes

If you plan to use your environment for interactive programming, not just compiling random Haskell code, you might want to replace ghcWithPackages in all the listings above with ghcWithHoogle.

This environment generator not only produces an environment with GHC and all the specified libraries, but also generates a hoogle and haddock indexes for all the packages, and provides a wrapper script around hoogle binary that uses all those things. A precise name for this thing would be ghcWithPackagesAndHoogleAndDocumentationIndexes, which is, regrettably, too long and scary.

For example, installing the following environment

  packageOverrides = super: let self = super.pkgs; in
    myHaskellEnv = self.haskellPackages.ghcWithHoogle
                     (haskellPackages: with haskellPackages; [
                       # libraries
                       arrows async cgi criterion
                       # tools
                       cabal-install haskintex

allows one to browse module documentation index not too dissimilar to this for all the specified packages and their dependencies by directing a browser of choice to ~/.nix-profile/share/doc/hoogle/index.html (or /run/current-system/sw/share/doc/hoogle/index.html in case you put it in environment.systemPackages in NixOS).

After you’ve marveled enough at that try adding the following to your ~/.ghc/ghci.conf

:def hoogle \s -> return $ ":! hoogle search -cl --count=15 \"" ++ s ++ "\""
:def doc \s -> return $ ":! hoogle search -cl --info \"" ++ s ++ "\""

and test it by typing into ghci:

:hoogle a -> a
:doc a -> a

Be sure to note the links to haddock files in the output. With any modern and properly configured terminal emulator you can just click those links to navigate there.

Finally, you can run

hoogle server --local -p 8080

and navigate to http://localhost:8080/ for your own local Hoogle. The --local flag makes the hoogle server serve files from your nix store over http, without the flag it will use file:// URIs. Note, however, that Firefox and possibly other browsers disallow navigation from http:// to file:// URIs for security reasons, which might be quite an inconvenience. Versions before v5 did not have this flag. See this page for workarounds.

For NixOS users there’s a service which runs this exact command for you. Specify the packages you want documentation for and the haskellPackages set you want them to come from. Add the following to configuration.nix.

services.hoogle = {
  enable = true;
  packages = (hpkgs: with hpkgs; [text cryptonite]);
  haskellPackages = pkgs.haskellPackages;
}; How to build a Haskell project using Stack

Stack is a popular build tool for Haskell projects. It has first-class support for Nix. Stack can optionally use Nix to automatically select the right version of GHC and other build tools to build, test and execute apps in an existing project downloaded from somewhere on the Internet. Pass the --nix flag to any stack command to do so, e.g.

git clone --recurse-submodules
cd wai
stack --nix build

If you want stack to use Nix by default, you can add a nix section to the stack.yaml file, as explained in the Stack documentation. For example:

  enable: true
  packages: [pkgconfig zeromq zlib]

The example configuration snippet above tells Stack to create an ad hoc environment for nix-shell as in the below section, in which the pkgconfig, zeromq and zlib packages from Nixpkgs are available. All stack commands will implicitly be executed inside this ad hoc environment.

Some projects have more sophisticated needs. For examples, some ad hoc environments might need to expose Nixpkgs packages compiled in a certain way, or with extra environment variables. In these cases, you’ll need a shell field instead of packages:

  enable: true
  shell-file: shell.nix

For more on how to write a shell.nix file see the below section. You’ll need to express a derivation. Note that Nixpkgs ships with a convenience wrapper function around mkDerivation called haskell.lib.buildStackProject to help you create this derivation in exactly the way Stack expects. However for this to work you need to disable the sandbox, which you can do by using --option sandbox relaxed or --option sandbox false to the Nix command. All of the same inputs as mkDerivation can be provided. For example, to build a Stack project that including packages that link against a version of the R library compiled with special options turned on:

with (import <nixpkgs> { });

let R = pkgs.R.override { enableStrictBarrier = true; };
haskell.lib.buildStackProject {
  name = "HaskellR";
  buildInputs = [ R zeromq zlib ];

You can select a particular GHC version to compile with by setting the ghc attribute as an argument to buildStackProject. Better yet, let Stack choose what GHC version it wants based on the snapshot specified in stack.yaml (only works with Stack >= 1.1.3):

{nixpkgs ? import <nixpkgs> { }, ghc ? nixpkgs.ghc}:

with nixpkgs;

let R = pkgs.R.override { enableStrictBarrier = true; };
haskell.lib.buildStackProject {
  name = "HaskellR";
  buildInputs = [ R zeromq zlib ];
  inherit ghc;
} How to create ad hoc environments for nix-shell

The easiest way to create an ad hoc development environment is to run nix-shell with the appropriate GHC environment given on the command-line:

nix-shell -p "haskellPackages.ghcWithPackages (pkgs: with pkgs; [mtl pandoc])"

For more sophisticated use-cases, however, it’s more convenient to save the desired configuration in a file called shell.nix that looks like this:

{ nixpkgs ? import <nixpkgs> {}, compiler ? "ghc7102" }:
  inherit (nixpkgs) pkgs;
  ghc = pkgs.haskell.packages.${compiler}.ghcWithPackages (ps: with ps; [
          monad-par mtl
pkgs.stdenv.mkDerivation {
  name = "my-haskell-env-0";
  buildInputs = [ ghc ];
  shellHook = "eval $(egrep ^export ${ghc}/bin/ghc)";

Now run nix-shell — or even nix-shell --pure — to enter a shell environment that has the appropriate compiler in $PATH. If you use --pure, then add all other packages that your development environment needs into the buildInputs attribute. If you’d like to switch to a different compiler version, then pass an appropriate compiler argument to the expression, i.e. nix-shell --argstr compiler ghc784.

If you need such an environment because you’d like to compile a Hackage package outside of Nix — i.e. because you’re hacking on the latest version from Git —, then the package set provides suitable nix-shell environments for you already! Every Haskell package has an env attribute that provides a shell environment suitable for compiling that particular package. If you’d like to hack the lens library, for example, then you just have to check out the source code and enter the appropriate environment:

$ cabal get lens-4.11 && cd lens-4.11
Downloading lens-4.11...
Unpacking to lens-4.11/

$ nix-shell "<nixpkgs>" -A haskellPackages.lens.env

At point, you can run cabal configure, cabal build, and all the other development commands. Note that you need cabal-install installed in your $PATH already to use it here — the nix-shell environment does not provide it.

15.9.3. How to create Nix builds for your own private Haskell packages

If your own Haskell packages have build instructions for Cabal, then you can convert those automatically into build instructions for Nix using the cabal2nix utility, which you can install into your profile by running nix-env -i cabal2nix. How to build a stand-alone project

For example, let’s assume that you’re working on a private project called foo. To generate a Nix build expression for it, change into the project’s top-level directory and run the command:

cabal2nix . > foo.nix

Then write the following snippet into a file called default.nix:

{ nixpkgs ? import <nixpkgs> {}, compiler ? "ghc7102" }:
nixpkgs.pkgs.haskell.packages.${compiler}.callPackage ./foo.nix { }

Finally, store the following code in a file called shell.nix:

{ nixpkgs ? import <nixpkgs> {}, compiler ? "ghc7102" }:
(import ./default.nix { inherit nixpkgs compiler; }).env

At this point, you can run nix-build to have Nix compile your project and install it into a Nix store path. The local directory will contain a symlink called result after nix-build returns that points into that location. Of course, passing the flag --argstr compiler ghc763 allows switching the build to any version of GHC currently supported.

Furthermore, you can call nix-shell to enter an interactive development environment in which you can use cabal configure and cabal build to develop your code. That environment will automatically contain a proper GHC derivation with all the required libraries registered as well as all the system-level libraries your package might need.

If your package does not depend on any system-level libraries, then it’s sufficient to run

nix-shell --command "cabal configure"

once to set up your build. cabal-install determines the absolute paths to all resources required for the build and writes them into a config file in the dist/ directory. Once that’s done, you can run cabal build and any other command for that project even outside of the nix-shell environment. This feature is particularly nice for those of us who like to edit their code with an IDE, like Emacs’ haskell-mode, because it’s not necessary to start Emacs inside of nix-shell just to make it find out the necessary settings for building the project; cabal-install has already done that for us.

If you want to do some quick-and-dirty hacking and don’t want to bother setting up a default.nix and shell.nix file manually, then you can use the --shell flag offered by cabal2nix to have it generate a stand-alone nix-shell environment for you. With that feature, running

cabal2nix --shell . > shell.nix
nix-shell --command "cabal configure"

is usually enough to set up a build environment for any given Haskell package. You can even use that generated file to run nix-build, too:

nix-build shell.nix How to build projects that depend on each other

If you have multiple private Haskell packages that depend on each other, then you’ll have to register those packages in the Nixpkgs set to make them visible for the dependency resolution performed by callPackage. First of all, change into each of your projects top-level directories and generate a default.nix file with cabal2nix:

cd ~/src/foo && cabal2nix . > default.nix
cd ~/src/bar && cabal2nix . > default.nix

Then edit your ~/.config/nixpkgs/config.nix file to register those builds in the default Haskell package set:

  packageOverrides = super: let self = super.pkgs; in
    haskellPackages = super.haskellPackages.override {
      overrides = self: super: {
        foo = self.callPackage ../src/foo {};
        bar = self.callPackage ../src/bar {};

Once that’s accomplished, nix-env -f "<nixpkgs>" -qA haskellPackages will show your packages like any other package from Hackage, and you can build them

nix-build "<nixpkgs>" -A

or enter an interactive shell environment suitable for building them:

nix-shell "<nixpkgs>" -A

15.9.4. Miscellaneous Topics How to build with profiling enabled

Every Haskell package set takes a function called overrides that you can use to manipulate the package as much as you please. One useful application of this feature is to replace the default mkDerivation function with one that enables library profiling for all packages. To accomplish that add the following snippet to your ~/.config/nixpkgs/config.nix file:

  packageOverrides = super: let self = super.pkgs; in
    profiledHaskellPackages = self.haskellPackages.override {
      overrides = self: super: {
        mkDerivation = args: super.mkDerivation (args // {
          enableLibraryProfiling = true;

Then, replace instances of haskellPackages in the cabal2nix-generated default.nix or shell.nix files with profiledHaskellPackages. How to override package versions in a compiler-specific package set

Nixpkgs provides the latest version of ghc-events, which is at the time of this writing. This is fine for users of GHC 7.10.x, but GHC 7.8.4 cannot compile that binary. Now, one way to solve that problem is to register an older version of ghc-events in the 7.8.x-specific package set. The first step is to generate Nix build instructions with cabal2nix:

cabal2nix cabal://ghc-events- > ~/.nixpkgs/ghc-events-

Then add the override in ~/.config/nixpkgs/config.nix:

  packageOverrides = super: let self = super.pkgs; in
    haskell = super.haskell // {
      packages = super.haskell.packages // {
        ghc784 = super.haskell.packages.ghc784.override {
          overrides = self: super: {
            ghc-events = self.callPackage ./ghc-events- {};

This code is a little crazy, no doubt, but it’s necessary because the intuitive version

{ # ...

  haskell.packages.ghc784 = super.haskell.packages.ghc784.override {
    overrides = self: super: {
      ghc-events = self.callPackage ./ghc-events- {};

doesn’t do what we want it to: that code replaces the haskell package set in Nixpkgs with one that contains only one entry,packages, which contains only one entry ghc784. This override loses the haskell.compiler set, and it loses the haskell.packages.ghcXYZ sets for all compilers but GHC 7.8.4. To avoid that problem, we have to perform the convoluted little dance from above, iterating over each step in hierarchy.

Once it’s accomplished, however, we can install a variant of ghc-events that’s compiled with GHC 7.8.4:

nix-env -f "<nixpkgs>" -iA haskell.packages.ghc784.ghc-events

Unfortunately, it turns out that this build fails again while executing the test suite! Apparently, the release archive on Hackage is missing some data files that the test suite requires, so we cannot run it. We accomplish that by re-generating the Nix expression with the --no-check flag:

cabal2nix --no-check cabal://ghc-events- > ~/.nixpkgs/ghc-events-

Now the builds succeeds.

Of course, in the concrete example of ghc-events this whole exercise is not an ideal solution, because ghc-events can analyze the output emitted by any version of GHC later than 6.12 regardless of the compiler version that was used to build the ghc-events executable, so strictly speaking there’s no reason to prefer one built with GHC 7.8.x in the first place. However, for users who cannot use GHC 7.10.x at all for some reason, the approach of downgrading to an older version might be useful. How to override packages in all compiler-specific package sets

In the previous section we learned how to override a package in a single compiler-specific package set. You may have some overrides defined that you want to use across multiple package sets. To accomplish this you could use the technique that we learned in the previous section by repeating the overrides for all the compiler-specific package sets. For example:

  packageOverrides = super: let self = super.pkgs; in
    haskell = super.haskell // {
      packages = super.haskell.packages // {
        ghc784 = super.haskell.packages.ghc784.override {
          overrides = self: super: {
            my-package = ...;
            my-other-package = ...;
        ghc822 = super.haskell.packages.ghc784.override {
          overrides = self: super: {
            my-package = ...;
            my-other-package = ...;

However there’s a more convenient way to override all compiler-specific package sets at once:

  packageOverrides = super: let self = super.pkgs; in
    haskell = super.haskell // {
      packageOverrides = self: super: {
        my-package = ...;
        my-other-package = ...;
} How to specify source overrides for your Haskell package

When starting a Haskell project you can use developPackage to define a derivation for your package at the root path as well as source override versions for Hackage packages, like so:

# default.nix
{ compilerVersion ? "ghc842" }:
  # pinning nixpkgs using new Nix 2.0 builtin `fetchGit`
  pkgs = import (fetchGit (import ./version.nix)) { };
  compiler = pkgs.haskell.packages."${compilerVersion}";
  pkg = compiler.developPackage {
    root = ./.;
    source-overrides = {
      # Let's say the GHC 8.4.2 haskellPackages uses and your test suite is incompatible with >=
      HUnit = "";
in pkg

This could be used in place of a simplified stack.yaml defining a Nix derivation for your Haskell package.

As you can see this allows you to specify only the source version found on Hackage and nixpkgs will take care of the rest.

You can also specify buildInputs for your Haskell derivation for packages that directly depend on external libraries like so:

# default.nix
{ compilerVersion ? "ghc842" }:
  # pinning nixpkgs using new Nix 2.0 builtin `fetchGit`
  pkgs = import (fetchGit (import ./version.nix)) { };
  compiler = pkgs.haskell.packages."${compilerVersion}";
  pkg = compiler.developPackage {
    root = ./.;
    source-overrides = {
      HUnit = ""; # Let's say the GHC 8.4.2 haskellPackages uses and your test suite is incompatible with >=
  # in case your package source depends on any libraries directly, not just transitively.
  buildInputs = [ zlib ];
in pkg.overrideAttrs(attrs: {
  buildInputs = attrs.buildInputs ++ buildInputs;

Notice that you will need to override (via overrideAttrs or similar) the derivation returned by the developPackage Nix lambda as there is no buildInputs named argument you can pass directly into the developPackage lambda. How to recover from GHC’s infamous non-deterministic library ID bug

GHC and distributed build farms don’t get along well:


When you see an error like this one

package foo- is broken due to missing package

then you have to download and re-install foo and all its dependents from scratch:

nix-store -q --referrers /nix/store/*-haskell-text- \
  | xargs -L 1 nix-store --repair-path

If you’re using additional Hydra servers other than, then it might be necessary to purge the local caches that store data from those machines to disable these binary channels for the duration of the previous command, i.e. by running:

rm ~/.cache/nix/binary-cache*.sqlite Builds on Darwin fail with math.h not found

Users of GHC on Darwin have occasionally reported that builds fail, because the compiler complains about a missing include file:

fatal error: 'math.h' file not found

The issue has been discussed at length in ticket 6390, and so far no good solution has been proposed. As a work-around, users who run into this problem can configure the environment variables

export NIX_CFLAGS_COMPILE="-idirafter /usr/include"
export NIX_CFLAGS_LINK="-L/usr/lib"

in their ~/.bashrc file to avoid the compiler error. Builds using Stack complain about missing system libraries

--  While building package zlib- using:
  runhaskell -package=Cabal- -clear-package-db [... lots of flags ...]
Process exited with code: ExitFailure 1
Logs have been written to: /home/foo/src/stack-ide/.stack-work/logs/zlib-

Configuring zlib-
Setup.hs: Missing dependency on a foreign library:
* Missing (or bad) header file: zlib.h
This problem can usually be solved by installing the system package that
provides this library (you may need the "-dev" version). If the library is
already installed but in a non-standard location then you can use the flags
--extra-include-dirs= and --extra-lib-dirs= to specify where it is.
If the header file does exist, it may contain errors that are caught by the C
compiler at the preprocessing stage. In this case you can re-run configure
with the verbosity flag -v3 to see the error messages.

When you run the build inside of the nix-shell environment, the system is configured to find without any special flags – the compiler and linker just know how to find it. Consequently, Cabal won’t record any search paths for in the package description, which means that the package works fine inside of nix-shell, but once you leave the shell the shared object can no longer be found. That issue is by no means specific to Stack: you’ll have that problem with any other Haskell package that’s built inside of nix-shell but run outside of that environment.

You can remedy this issue in several ways. The easiest is to add a nix section to the stack.yaml like the following:

  enable: true
  packages: [ zlib ]

Stack’s Nix support knows to add ${zlib.out}/lib and ${}/include as an --extra-lib-dirs and extra-include-dirs, respectively. Alternatively, you can achieve the same effect by hand. First of all, run

$ nix-build --no-out-link "<nixpkgs>" -A zlib

to find out the store path of the system’s zlib library. Now, you can

  1. add that path (plus a /lib suffix) to your $LD_LIBRARY_PATH environment variable to make sure your system linker finds automatically. It’s no pretty solution, but it will work.

  2. As a variant of (1), you can also install any number of system libraries into your user’s profile (or some other profile) and point $LD_LIBRARY_PATH to that profile instead, so that you don’t have to list dozens of those store paths all over the place.

  3. The solution I prefer is to call stack with an appropriate –extra-lib-dirs flag like so: shell stack --extra-lib-dirs=/nix/store/alsvwzkiw4b7ip38l4nlfjijdvg3fvzn-zlib-1.2.8/lib build

Typically, you’ll need --extra-include-dirs as well. It’s possible to add those flag to the project’s stack.yaml or your user’s global ~/.stack/global/stack.yaml file so that you don’t have to specify them manually every time. But again, you’re likely better off using Stack’s Nix support instead.

The same thing applies to cabal configure, of course, if you’re building with cabal-install instead of Stack. Creating statically linked binaries

There are two levels of static linking. The first option is to configure the build with the Cabal flag --disable-executable-dynamic. In Nix expressions, this can be achieved by setting the attribute:

enableSharedExecutables = false;

That gives you a binary with statically linked Haskell libraries and dynamically linked system libraries.

To link both Haskell libraries and system libraries statically, the additional flags --ghc-option=-optl=-static --ghc-option=-optl=-pthread need to be used. In Nix, this is accomplished with:

configureFlags = [ "--ghc-option=-optl=-static" "--ghc-option=-optl=-pthread" ];

It’s important to realize, however, that most system libraries in Nix are built as shared libraries only, i.e. there is just no static library available that Cabal could link! Building GHC with integer-simple

By default GHC implements the Integer type using the GNU Multiple Precision Arithmetic (GMP) library. The implementation can be found in the integer-gmp package.

A potential problem with this is that GMP is licensed under the GNU Lesser General Public License (LGPL), a kind of copyleft license. According to the terms of the LGPL, paragraph 5, you may distribute a program that is designed to be compiled and dynamically linked with the library under the terms of your choice (i.e., commercially) but if your program incorporates portions of the library, if it is linked statically, then your program is a derivative–a work based on the library–and according to paragraph 2, section c, you must cause the whole of the work to be licensed under the terms of the LGPL (including for free).

The LGPL licensing for GMP is a problem for the overall licensing of binary programs compiled with GHC because most distributions (and builds) of GHC use static libraries. (Dynamic libraries are currently distributed only for macOS.) The LGPL licensing situation may be worse: even though The Glasgow Haskell Compiler License is essentially a free software license (BSD3), according to paragraph 2 of the LGPL, GHC must be distributed under the terms of the LGPL!

To work around these problems GHC can be build with a slower but LGPL-free alternative implementation for Integer called integer-simple.

To get a GHC compiler build with integer-simple instead of integer-gmp use the attribute: haskell.compiler.integer-simple."${ghcVersion}". For example:

$ nix-build -E '(import <nixpkgs> {}).haskell.compiler.integer-simple.ghc802'
$ result/bin/ghc-pkg list | grep integer

The following command displays the complete list of GHC compilers build with integer-simple:

$ nix-env -f "<nixpkgs>" -qaP -A haskell.compiler.integer-simple
haskell.compiler.integer-simple.ghc7102  ghc-7.10.2
haskell.compiler.integer-simple.ghc7103  ghc-7.10.3
haskell.compiler.integer-simple.ghc722   ghc-7.2.2
haskell.compiler.integer-simple.ghc742   ghc-7.4.2
haskell.compiler.integer-simple.ghc783   ghc-7.8.3
haskell.compiler.integer-simple.ghc784   ghc-7.8.4
haskell.compiler.integer-simple.ghc801   ghc-8.0.1
haskell.compiler.integer-simple.ghc802   ghc-8.0.2
haskell.compiler.integer-simple.ghcHEAD  ghc-8.1.20170106

To get a package set supporting integer-simple use the attribute: haskell.packages.integer-simple."${ghcVersion}". For example use the following to get the scientific package build with integer-simple:

nix-build -A haskell.packages.integer-simple.ghc802.scientific Quality assurance

The haskell.lib library includes a number of functions for checking for various imperfections in Haskell packages. It’s useful to apply these functions to your own Haskell packages and integrate that in a Continuous Integration server like hydra to assure your packages maintain a minimum level of quality. This section discusses some of these functions. failOnAllWarnings

Applying haskell.lib.failOnAllWarnings to a Haskell package enables the -Wall and -Werror GHC options to turn all warnings into build failures. buildStrictly

Applying haskell.lib.buildStrictly to a Haskell package calls failOnAllWarnings on the given package to turn all warnings into build failures. Additionally the source of your package is gotten from first invoking cabal sdist to ensure all needed files are listed in the Cabal file. checkUnusedPackages

Applying haskell.lib.checkUnusedPackages to a Haskell package invokes the packunused tool on the package. packunused complains when it finds packages listed as build-depends in the Cabal file which are redundant. For example:

$ nix-build -E 'let pkgs = import <nixpkgs> {}; in pkgs.haskell.lib.checkUnusedPackages {} pkgs.haskellPackages.scientific'
these derivations will be built:
detected package components

 - library
 - testsuite(s): test-scientific
 - benchmark(s): bench-scientific*

(component names suffixed with '*' are not configured to be built)


The following package dependencies seem redundant:

 - ghc-prim-


no redundant packages dependencies found

builder for ‘/nix/store/3lc51cxj2j57y3zfpq5i69qbzjpvyci1-scientific-’ failed with exit code 1
error: build of ‘/nix/store/3lc51cxj2j57y3zfpq5i69qbzjpvyci1-scientific-’ failed

As you can see, packunused finds out that although the testsuite component has no redundant dependencies the library component of scientific- depends on ghc-prim which is unused in the library. Using hackage2nix with nixpkgs

Hackage package derivations are found in the hackage-packages.nix file within nixpkgs and are used as the initial package set for haskellPackages. The hackage-packages.nix file is not meant to be edited by hand, but rather autogenerated by hackage2nix, which by default uses the configuration-hackage2nix.yaml file to generate all the derivations.

To modify the contents configuration-hackage2nix.yaml, follow the instructions on hackage2nix.

15.9.5. Other resources

  • The Youtube video Nix Loves Haskell provides an introduction into Haskell NG aimed at beginners. The slides are available at and also – in a form ready for cut & paste – at

  • Another Youtube video is Escaping Cabal Hell with Nix, which discusses the subject of Haskell development with Nix but also provides a basic introduction to Nix as well, i.e. it’s suitable for viewers with almost no prior Nix experience.

  • Oliver Charles wrote a very nice Tutorial how to develop Haskell packages with Nix.

  • The Journey into the Haskell NG infrastructure series of postings describe the new Haskell infrastructure in great detail:

    • Part 1 explains the differences between the old and the new code and gives instructions how to migrate to the new setup.

    • Part 2 looks in-depth at how to tweak and configure your setup by means of overrides.

    • Part 3 describes the infrastructure that keeps the Haskell package set in Nixpkgs up-to-date.

15.10. Idris

15.10.1. 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 ])'

15.10.2. 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

15.10.3. 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

15.10.4. 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.

15.11. 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.

15.11.1. 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

15.11.2. 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.

15.11.3. 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.

15.11.4. 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

15.12. 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.

15.13. Lua

Lua packages are built by the buildLuaPackage function. This function is implemented in pkgs/development/lua-modules/generic/default.nix and works similarly to buildPerlPackage. (See Section 15.16, “Perl” for details.)

Lua packages are defined in pkgs/top-level/lua-packages.nix. Most of them are simple. For example:

fileSystem = buildLuaPackage {
  name = "filesystem-1.6.2";
  src = fetchurl {
    url = "";
    sha256 = "1n8qdwa20ypbrny99vhkmx8q04zd2jjycdb5196xdhgvqzk10abz";
  meta = {
    homepage = "";
    hydraPlatforms = stdenv.lib.platforms.linux;
    maintainers = with maintainers; [ flosse ];

Though, more complicated package should be placed in a seperate file in pkgs/development/lua-modules.

Lua packages accept additional parameter disabled, which defines the condition of disabling package from luaPackages. For example, if package has disabled assigned to lua.luaversion != "5.1", it will not be included in any luaPackages except lua51Packages, making it only be built for lua 5.1.

15.14. 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 also provides support for multiple Node.js versions. The policy is that a new package should be added to the collection for the latest stable LTS release (which is currently 10.x), unless there is an explicit reason to support a different release.

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-v10.nix:

dat = nodePackages.dat.override (oldAttrs: {
  buildInputs = oldAttrs.buildInputs ++ [ nodePackages.node-gyp-build ];

To add a package from NPM to nixpkgs:

  1. Modify pkgs/development/node-packages/node-packages-v10.json to add, update or remove package entries. (Or pkgs/development/node-packages/node-packages-v8.json for packages depending on Node.js 8.x)

  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 a specific Node.js version (e.g. 10.x): nix-build -A nodePackages_10_x.<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.

15.15. 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 ];

15.16. Perl

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.

15.16.1. 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.

15.16.2. 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.

15.17. Python

15.17.1. 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. python35 refers to CPython 3.5, 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 3.5 version is python35.pkgs.toolz. The main package set contains aliases to these package sets, e.g. pythonPackages refers to python.pkgs and python35Packages to python35.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 and libraries. Python applications can be installed in your profile. 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 able to find each other and all of the modules.

In the following examples we create an environment with Python 3.5, numpy and toolz. As you may imagine, there is one limitation here, and that’s that you can install only one environment at a time. You will notice the complaints about collisions when you try to install a second environment. Environment defined in separate .nix file

Create a file, e.g. build.nix, with the following expression

with import <nixpkgs> {};

python35.withPackages (ps: with ps; [ numpy toolz ])

and install it in your profile with

nix-env -if build.nix

Now you can use the Python interpreter, as well as the extra packages (numpy, toolz) that you added to the environment. 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; {
    myEnv = python35.withPackages (ps: with ps; [ numpy toolz ]);

and install it in your profile with

nix-env -iA nixpkgs.myEnv

The environment is is installed by referring to the attribute, and considering the nixpkgs channel was used. 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; [
    (python35.withPackages(ps: with ps; [ numpy toolz ]))
} Temporary Python environment with nix-shell

The examples in the previous section showed how to install a Python environment into a profile. For development you may need to use multiple environments. nix-shell gives the possibility to temporarily load another environment, akin to virtualenv.

There are two methods for loading a shell with Python packages. The first and recommended method is to create an environment with python.buildEnv or python.withPackages and load that. E.g.

$ nix-shell -p 'python35.withPackages(ps: with ps; [ numpy toolz ])'

opens a shell from which you can launch the interpreter

[nix-shell:~] python3

The other method, which is not recommended, does not create an environment and requires you to list the packages directly,

$ nix-shell -p python35.pkgs.numpy python35.pkgs.toolz

Again, it is possible to launch the interpreter from the shell. The Python interpreter has the attribute pkgs which contains all Python libraries for that specific interpreter. Load environment from .nix expression

As explained in the Nix manual, nix-shell can also load an expression from a .nix file. Say we want to have Python 3.5, numpy and toolz, like before, in an environment. Consider a shell.nix file with

with import <nixpkgs> {};

(python35.withPackages (ps: [ps.numpy ps.toolz])).env

Executing nix-shell gives you again a Nix shell from which you can run Python.

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 3.5 environment with the withPackages function.

  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 = python35.withPackages (ps: [
in mkShell {
  buildInputs = [
} Execute command with --run

A convenient option with nix-shell is the --run option, with which you can execute a command in the nix-shell. We can e.g. directly open a Python shell

$ nix-shell -p python35Packages.numpy python35Packages.toolz --run "python3"

or run a script

$ nix-shell -p python35Packages.numpy python35Packages.toolz --run "python3" nix-shell as shebang

In fact, for the second use case, there is a more convenient method. You can add a shebang to your script specifying which dependencies nix-shell needs. With the following shebang, you can just execute ./, and it will make available all dependencies and run the script in the python3 shell.

#! /usr/bin/env nix-shell
#! nix-shell -i python3 -p "python3.withPackages(ps: [ps.numpy])"

import numpy

print(numpy.__version__) Developing with Python

Now that you know how to get a working Python environment with Nix, it is time to go forward and start actually developing with Python. We will first have a look at how Python packages are packaged on Nix. Then, we will look at how you can use development mode with your code. Packaging a library

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 = "0.7.4";

  src = fetchPypi {
    inherit pname version;
    sha256 = "43c2c9e5e7a16b6c88ba3088a9bfc82f7db8e13378be7c78d6c14a5f8ed05afd";

  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. python35.pkgs.toolz refers to the toolz derivation corresponding to the CPython 3.5 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 = python35.pkgs.buildPythonPackage rec {
      pname = "toolz";
      version = "0.7.4";

      src = python35.pkgs.fetchPypi {
        inherit pname version;
        sha256 = "43c2c9e5e7a16b6c88ba3088a9bfc82f7db8e13378be7c78d6c14a5f8ed05afd";

      doCheck = false;

      meta = {
        homepage = "";
        description = "List processing tools and functional utilities";

  in python35.withPackages (ps: [ps.numpy my_toolz])

Executing nix-shell will result in an environment in which you can use Python 3.5 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 as a buildInput, 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. 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 3.5 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 python35Packages;

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 = "0.7.4";

  src = fetchPypi {
    inherit pname version;
    sha256 = "43c2c9e5e7a16b6c88ba3088a9bfc82f7db8e13378be7c78d6c14a5f8ed05afd";

  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 = python35Packages.buildPythonPackage;
  in python35.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 python35Packages, and in this case the python35 interpreter is automatically used.

15.17.2. Reference Interpreters

Versions 2.7, 3.5, 3.6, 3.7 and 3.8 of the CPython interpreter are available as respectively python27, python35, python36, python37 and python38. The aliases python2 and python3 correspond to respectively python27 and python37. 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. python3.7.

  • 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.python35Packages

  • pkgs.python36Packages

  • pkgs.python37Packages

  • pkgs.pypyPackages

and the aliases

  • pkgs.python2Packages pointing to pkgs.python27Packages

  • pkgs.python3Packages pointing to pkgs.python37Packages

  • 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.

  • installFlags ? []: A list of strings. Arguments to be passed to pip install. To pass options to python install, use --install-option. E.g., installFlags=["--install-option='--cpp_implementation'"].

  • 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 "python3.5-" for Python 3.5, etc., and in case of applications to "".

  • 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 ];
  ignoreCollisions = true;

and running nix-build will create


with wrapped binaries in bin/.

You can also use the env attribute to create local environments with needed packages installed. This is somewhat comparable to virtualenv. For example, running nix-shell with the following shell.nix

with import <nixpkgs> {};

(python3.buildEnv.override {
  extraLibs = with python3Packages; [ numpy requests ];

will drop you into a shell where Python will have the specified packages in its path. python.buildEnv arguments
  • extraLibs: List of packages installed inside the environment.

  • postBuild: Shell command executed after the build of environment.

  • ignoreCollisions: Ignore file collisions inside the environment (default is false).

  • permitUserSite: Skip setting the PYTHONNOUSERSITE environment variable in wrapped binaries in the environment. python.withPackages function

The python.withPackages function provides a simpler interface to the python.buildEnv functionality. It takes a function as an argument that is passed the set of python packages and returns the list of the packages to be included in the environment. Using the withPackages function, the previous example for the Pyramid Web Framework environment can be written like this:

with import <nixpkgs> {};

python.withPackages (ps: [ps.pyramid])

withPackages passes the correct package set for the specific interpreter version as an argument to the function. In the above example, ps equals pythonPackages. But you can also easily switch to using python3:

with import <nixpkgs> {};

python3.withPackages (ps: [ps.pyramid])

Now, ps is set to python3Packages, matching the version of the interpreter.

As python.withPackages simply uses python.buildEnv under the hood, it also supports the env attribute. The shell.nix file from the previous section can thus be also written like this:

with import <nixpkgs> {};

(python36.withPackages (ps: [ps.numpy ps.requests])).env

In contrast to python.buildEnv, python.withPackages does not support the more advanced options such as ignoreCollisions = true or postBuild. If you need them, you have to use python.buildEnv.

Python 2 namespace packages may provide that collide. In that case python.buildEnv should be used with ignoreCollisions = true. Setup hooks

The following are setup hooks specifically for Python packages. Most of these are used in buildPythonPackage.

  • eggUnpackhook to move an egg to the correct folder so it can be installed with the eggInstallHook

  • eggBuildHook to skip building for eggs.

  • eggInstallHook to install eggs.

  • flitBuildHook to build a wheel using flit.

  • pipBuildHook to build a wheel using pip and PEP 517. Note a build system (e.g. setuptools or flit) should still be added as nativeBuildInput.

  • pipInstallHook to install wheels.

  • pytestCheckHook to run tests with pytest.

  • pythonCatchConflictsHook to check whether a Python package is not already existing.

  • pythonImportsCheckHook to check whether importing the listed modules works.

  • pythonRemoveBinBytecode to remove bytecode from the /bin folder.

  • setuptoolsBuildHook to build a wheel using setuptools.

  • setuptoolsCheckHook to run tests with python test.

  • venvShellHook to source a Python 3 venv at the venvDir location. A venv is created if it does not yet exist.

  • wheelUnpackHook to move a wheel to the correct folder so it can be installed with the pipInstallHook. Development mode

Development or editable mode is supported. To develop Python packages buildPythonPackage has additional logic inside shellPhase to run pip install -e . --prefix $TMPDIR/for the package.

Warning: shellPhase is executed only if exists.

Given a default.nix:

with import <nixpkgs> {};

pythonPackages.buildPythonPackage {
  name = "myproject";
  buildInputs = with pythonPackages; [ pyramid ];

  src = ./.;

Running nix-shell with no arguments should give you the environment in which the package would be built with nix-build.

Shortcut to setup environments with C headers/libraries and python packages:

nix-shell -p pythonPackages.pyramid zlib libjpeg git

Note: There is a boolean value lib.inNixShell set to true if nix-shell is invoked. Tools

Packages inside nixpkgs are written by hand. However many tools exist in community to help save time. No tool is preferred at the moment. Deterministic builds

The Python interpreters are now built deterministically. Minor modifications had to be made to the interpreters in order to generate deterministic bytecode. This has security implications and is relevant for those using Python in a nix-shell.

When the environment variable DETERMINISTIC_BUILD is set, all bytecode will have timestamp 1. The buildPythonPackage function sets DETERMINISTIC_BUILD=1 and PYTHONHASHSEED=0. Both are also exported in nix-shell. Automatic tests

It is recommended to test packages as part of the build process. Source distributions (sdist) often include test files, but not always.

By default the command python test is run as part of the checkPhase, but often it is necessary to pass a custom checkPhase. An example of such a situation is when py.test is used. Common issues
  • Non-working tests can often be deselected. By default buildPythonPackage runs python test. Most python modules follows the standard test protocol where the pytest runner can be used instead. py.test supports a -k parameter to ignore test methods or classes:

    buildPythonPackage {
      # ...
      # assumes the tests are located in tests
      checkInputs = [ pytest ];
      checkPhase = ''
        py.test -k 'not function_name and not other_function' tests
  • Tests that attempt to access $HOME can be fixed by using the following work-around before running tests (e.g. preCheck): export HOME=$(mktemp -d)

15.17.3. FAQ How to solve circular dependencies?

Consider the packages A and B that depend on each other. When packaging B, a solution is to override package A not to depend on B as an input. The same should also be done when packaging A. How to override a Python package?

We can override the interpreter and pass packageOverrides. In the following example we rename the pandas package and build it.

with import <nixpkgs> {};

  python = let
    packageOverrides = self: super: {
      pandas = super.pandas.overridePythonAttrs(old: {name="foo";});
  in pkgs.python35.override {inherit packageOverrides;};

in python.withPackages(ps: [ps.pandas])).env

Using nix-build on this expression will build an environment that contains the package pandas but with the new name foo.

All packages in the package set will use the renamed package. A typical use case is to switch to another version of a certain package. For example, in the Nixpkgs repository we have multiple versions of django and scipy. In the following example we use a different version of scipy and create an environment that uses it. All packages in the Python package set will now use the updated scipy version.

with import <nixpkgs> {};

( let
    packageOverrides = self: super: {
      scipy = super.scipy_0_17;
  in (pkgs.python35.override {inherit packageOverrides;}).withPackages (ps: [ps.blaze])

The requested package blaze depends on pandas which itself depends on scipy.

If you want the whole of Nixpkgs to use your modifications, then you can use overlays as explained in this manual. In the following example we build a inkscape using a different version of numpy.

  pkgs = import <nixpkgs> {};
  newpkgs = import pkgs.path { overlays = [ (pkgsself: pkgssuper: {
    python27 = let
      packageOverrides = self: super: {
        numpy = super.numpy_1_10;
    in pkgssuper.python27.override {inherit packageOverrides;};
  } ) ]; };
in newpkgs.inkscape python bdist_wheel cannot create .whl

Executing python bdist_wheel in a nix-shellfails with

ValueError: ZIP does not support timestamps before 1980

This is because files from the Nix store (which have a timestamp of the UNIX epoch of January 1, 1970) are included in the .ZIP, but .ZIP archives follow the DOS convention of counting timestamps from 1980.

The command bdist_wheel reads the SOURCE_DATE_EPOCH environment variable, which nix-shell sets to 1. Unsetting this variable or giving it a value corresponding to 1980 or later enables building wheels.

Use 1980 as timestamp:

nix-shell --run "SOURCE_DATE_EPOCH=315532800 python3 bdist_wheel"

or the current time:

nix-shell --run "SOURCE_DATE_EPOCH=$(date +%s) python3 bdist_wheel"


nix-shell --run "unset SOURCE_DATE_EPOCH; python3 bdist_wheel" install_data / data_files problems

If you get the following error:

could not create '/nix/store/6l1bvljpy8gazlsw2aw9skwwp4pmvyxw-python-2.7.8/etc':
Permission denied

This is a known bug in setuptools. Setuptools install_data does not respect --prefix. An example of such package using the feature is pkgs/tools/X11/xpra/default.nix. As workaround install it as an extra preInstall step:

${python.interpreter} install_data --install-dir=$out --root=$out
sed -i '/ = data\_files/d' Rationale of non-existent global site-packages

On most operating systems a global site-packages is maintained. This however becomes problematic if you want to run multiple Python versions or have multiple versions of certain libraries for your projects. Generally, you would solve such issues by creating virtual environments using virtualenv.

On Nix each package has an isolated dependency tree which, in the case of Python, guarantees the right versions of the interpreter and libraries or packages are available. There is therefore no need to maintain a global site-packages.

If you want to create a Python environment for development, then the recommended method is to use nix-shell, either with or without the python.buildEnv function. How to consume python modules using pip in a virtual environment like I am used to on other Operating Systems?

While this approach is not very idiomatic from Nix perspective, it can still be useful when dealing with pre-existing projects or in situations where it’s not feasible or desired to write derivations for all required dependencies.

This is an example of a default.nix for a nix-shell, which allows to consume a virtual environment created by venv, and install python modules through pip the traditional way.

Create this default.nix file, together with a requirements.txt and simply execute nix-shell.

with import <nixpkgs> { };

  pythonPackages = python3Packages;
in pkgs.mkShell rec {
  name = "impurePythonEnv";
  venvDir = "./.venv";
  buildInputs = [
    # A python interpreter including the 'venv' module is required to bootstrap
    # the environment.

    # This execute some shell code to initialize a venv in $venvDir before
    # dropping into the shell

    # Those are dependencies that we would like to use from nixpkgs, which will
    # add them to PYTHONPATH and thus make them accessible from within the venv.

    # In this particular example, in order to compile any binary extensions they may
    # require, the python modules listed in the hypothetical requirements.txt need
    # the following packages to be installed locally:

  # Now we can execute any commands within the virtual environment.
  # This is optional and can be left out to run pip manually.
  postShellHook = ''
    pip install -r requirements.txt


In case the supplied venvShellHook is insufficient, or when python 2 support is needed, you can define your own shell hook and adapt to your needs like in the following example:

with import <nixpkgs> { };

  venvDir = "./.venv";
  pythonPackages = python3Packages;
in pkgs.mkShell rec {
  name = "impurePythonEnv";
  buildInputs = [
    # Needed when using python 2.7
    # pythonPackages.virtualenv
    # ...

  # This is very close to how venvShellHook is implemented, but
  # adapted to use 'virtualenv'
  shellHook = ''
    SOURCE_DATE_EPOCH=$(date +%s)

    if [ -d "${venvDir}" ]; then
      echo "Skipping venv creation, '${venvDir}' already exists"
      echo "Creating new venv environment in path: '${venvDir}'"
      # Note that the module venv was only introduced in python 3, so for 2.7
      # this needs to be replaced with a call to virtualenv
      ${pythonPackages.python.interpreter} -m venv "${venvDir}"

    # Under some circumstances it might be necessary to add your virtual
    # environment to PYTHONPATH, which you can do here too;
    # PYTHONPATH=$PWD/${venvDir}/${pythonPackages.python.sitePackages}/:$PYTHONPATH

    source "${venvDir}/bin/activate"

    # As in the previous example, this is optional.
    pip install -r requirements.txt

Note that the pip install is an imperative action. So every time nix-shell is executed it will attempt to download the python modules listed in requirements.txt. However these will be cached locally within the virtualenv folder and not downloaded again. How to override a Python package from configuration.nix?

If you need to change a package’s attribute(s) from configuration.nix you could do:

  nixpkgs.config.packageOverrides = super: {
    python = super.python.override {
      packageOverrides = python-self: python-super: {
        zerobin = python-super.zerobin.overrideAttrs (oldAttrs: {
          src = super.fetchgit {
            url = "";
            rev = "a344dbb18fe7a855d0742b9a1cede7ce423b34ec";
            sha256 = "16d769kmnrpbdr0ph0whyf4yff5df6zi4kmwx7sz1d3r6c8p6xji";

pythonPackages.zerobin is now globally overridden. All packages and also the zerobin NixOS service use the new definition. Note that python-super refers to the old package set and python-self to the new, overridden version.

To modify only a Python package set instead of a whole Python derivation, use this snippet:

  myPythonPackages = pythonPackages.override {
    overrides = self: super: {
      zerobin = ...;
  } How to override a Python package using overlays?

Use the following overlay template:

self: super: {
  python = super.python.override {
    packageOverrides = python-self: python-super: {
      zerobin = python-super.zerobin.overrideAttrs (oldAttrs: {
        src = super.fetchgit {
          url = "";
          rev = "a344dbb18fe7a855d0742b9a1cede7ce423b34ec";
          sha256 = "16d769kmnrpbdr0ph0whyf4yff5df6zi4kmwx7sz1d3r6c8p6xji";
} How to use Intel’s MKL with numpy and scipy?

A site.cfg is created that configures BLAS based on the blas parameter of the numpy derivation. By passing in mkl, numpy and packages depending on numpy will be built with mkl.

The following is an overlay that configures numpy to use mkl:

self: super: {
  python37 = super.python37.override {
    packageOverrides = python-self: python-super: {
      numpy = python-super.numpy.override {
        blas = super.pkgs.mkl;

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,darwin} platforms; moreover, Hydra is not building and distributing pre-compiled binaries using it. What inputs do setup_requires, install_requires and tests_require map to?

In a or setup.cfg it is common to declare dependencies:

  • setup_requires corresponds to nativeBuildInputs

  • install_requires corresponds to propagatedBuildInputs

  • tests_require corresponds to checkInputs

15.17.4. Contributing Contributing guidelines

Following rules are desired to be respected:

  • Python libraries are called from python-packages.nix and packaged with buildPythonPackage. The expression of a library should be in pkgs/development/python-modules/<name>/default.nix. Libraries in pkgs/top-level/python-packages.nix are sorted quasi-alphabetically to avoid merge conflicts.

  • Python applications live outside of python-packages.nix and are packaged with buildPythonApplication.

  • Make sure libraries build for all Python interpreters.

  • By default we enable tests. Make sure the tests are found and, in the case of libraries, are passing for all interpreters. If certain tests fail they can be disabled individually. Try to avoid disabling the tests altogether. In any case, when you disable tests, leave a comment explaining why.

  • Commit names of Python libraries should reflect that they are Python libraries, so write for example pythonPackages.numpy: 1.11 -> 1.12.

  • Attribute names in python-packages.nix should be normalized according to PEP 0503. This means that characters should be converted to lowercase and . and _ should be replaced by a single - (foo-bar-baz instead of Foo__Bar.baz )

15.18. Qt

This section describes the differences between Nix expressions for Qt libraries and applications and Nix expressions for other C++ software. Some knowledge of the latter is assumed. There are primarily two problems which the Qt infrastructure is designed to address: ensuring consistent versioning of all dependencies and finding dependencies at runtime.

Example 15.8. Nix expression for a Qt package (default.nix)

{ mkDerivation, lib, qtbase }: 1

mkDerivation { 2
  pname = "myapp";
  version = "1.0";

  buildInputs = [ qtbase ]; 3


Import mkDerivation and Qt (such as qtbase modules directly. Do not import Qt package sets; the Qt versions of dependencies may not be coherent, causing build and runtime failures.


Use mkDerivation instead of stdenv.mkDerivation. mkDerivation is a wrapper around stdenv.mkDerivation which applies some Qt-specific settings. This deriver accepts the same arguments as stdenv.mkDerivation; refer to Chapter 6, The Standard Environment for details.

To use another deriver instead of stdenv.mkDerivation, use mkDerivationWith:

mkDerivationWith myDeriver {
  # ...

If you cannot use mkDerivationWith, please refer to Locating runtime dependencies.


mkDerivation accepts the same arguments as stdenv.mkDerivation, such as buildInputs.

Locating runtime dependencies.  Qt applications need to be wrapped to find runtime dependencies. If you cannot use mkDerivation or mkDerivationWith above, include wrapQtAppsHook in nativeBuildInputs:

stdenv.mkDerivation {
  # ...

  nativeBuildInputs = [ wrapQtAppsHook ];

Entries added to qtWrapperArgs are used to modify the wrappers created by wrapQtAppsHook. The entries are passed as arguments to wrapProgram executable makeWrapperArgs .

mkDerivation {
  # ...

  qtWrapperArgs = [ ''--prefix PATH : /path/to/bin'' ];

Set dontWrapQtApps to stop applications from being wrapped automatically. It is required to wrap applications manually with wrapQtApp, using the syntax of wrapProgram executable makeWrapperArgs :

mkDerivation {
  # ...

  dontWrapQtApps = true;
  preFixup = ''
      wrapQtApp "$out/bin/myapp" --prefix PATH : /path/to/bin

Note: wrapQtAppsHook ignores files that are non-ELF executables. This means that scripts won't be automatically wrapped so you'll need to manually wrap them as previously mentioned. An example of when you'd always need to do this is with Python applications that use PyQT.

Libraries are built with every available version of Qt. Use the meta.broken attribute to disable the package for unsupported Qt versions:

mkDerivation {
  # ...

  # Disable this library with Qt < 5.9.0
  meta.broken = builtins.compareVersions qtbase.version "5.9.0" < 0;

Adding a library to Nixpkgs.  Add a Qt library to all-packages.nix by adding it to the collection inside mkLibsForQt5. This ensures that the library is built with every available version of Qt as needed.

Example 15.9. Adding a Qt library to all-packages.nix

  # ...

  mkLibsForQt5 = self: with self; {
    # ...

    mylib = callPackage ../path/to/mylib {};

  # ...

Adding an application to Nixpkgs.  Add a Qt application to all-packages.nix using libsForQt5.callPackage instead of the usual callPackage. The former ensures that all dependencies are built with the same version of Qt.

Example 15.10. Adding a Qt application to all-packages.nix

  # ...

  myapp = libsForQt5.callPackage ../path/to/myapp/ {};

  # ...

15.19. R

15.19.1. Installation

Define an environment for R that contains all the libraries that you’d like to use by adding the following snippet to your $HOME/.config/nixpkgs/config.nix file:

    packageOverrides = super: let self = super.pkgs; in

        rEnv = super.rWrapper.override {
            packages = with self.rPackages; [

Then you can use nix-env -f "<nixpkgs>" -iA rEnv to install it into your user profile. The set of available libraries can be discovered by running the command nix-env -f "<nixpkgs>" -qaP -A rPackages. The first column from that output is the name that has to be passed to rWrapper in the code snipped above.

However, if you’d like to add a file to your project source to make the environment available for other contributors, you can create a default.nix file like so:

  pkgs = import <nixpkgs> {};
  stdenv = pkgs.stdenv;
in with pkgs; {
  myProject = stdenv.mkDerivation {
    name = "myProject";
    version = "1";
    src = if pkgs.lib.inNixShell then null else nix;

    buildInputs = with rPackages; [

and then run nix-shell . to be dropped into a shell with those packages available.

15.19.2. RStudio

RStudio uses a standard set of packages and ignores any custom R environments or installed packages you may have. To create a custom environment, see rstudioWrapper, which functions similarly to rWrapper:

    packageOverrides = super: let self = super.pkgs; in

        rstudioEnv = super.rstudioWrapper.override {
            packages = with self.rPackages; [

Then like above, nix-env -f "<nixpkgs>" -iA rstudioEnv will install this into your user profile.

Alternatively, you can create a self-contained shell.nix without the need to modify any configuration files:

{ pkgs ? import <nixpkgs> {}

pkgs.rstudioWrapper.override {
  packages = with pkgs.rPackages; [ dplyr ggplot2 reshape2 ];

Executing nix-shell will then drop you into an environment equivalent to the one above. If you need additional packages just add them to the list and re-enter the shell.

15.19.3. Updating the package set

nix-shell generate-shell.nix

Rscript generate-r-packages.R cran  >
mv cran-packages.nix

Rscript generate-r-packages.R bioc  >
mv bioc-packages.nix

generate-r-packages.R <repo> reads <repo>-packages.nix, therefor the renaming.

15.19.4. Testing if the Nix-expression could be evaluated

nix-build test-evaluation.nix --dry-run

If this exits fine, the expression is ok. If not, you have to edit default.nix

15.20. Ruby

There currently is support to bundle applications that are packaged as Ruby gems. The utility "bundix" allows you to write a Gemfile, let bundler create a Gemfile.lock, and then convert this into a nix expression that contains all Gem dependencies automatically.

For example, to package sensu, we did:

$ cd pkgs/servers/monitoring
$ mkdir sensu
$ cd sensu
$ cat > Gemfile
source ''
gem 'sensu'
$ $(nix-build '<nixpkgs>' -A bundix --no-out-link)/bin/bundix --magic
$ cat > default.nix
{ lib, bundlerEnv, ruby }:

bundlerEnv rec {
  name = "sensu-${version}";

  version = (import gemset).sensu.version;
  inherit ruby;
  # expects Gemfile, Gemfile.lock and gemset.nix in the same directory
  gemdir = ./.;

  meta = with lib; {
    description = "A monitoring framework that aims to be simple, malleable, and scalable";
    homepage    =;
    license     = with licenses; mit;
    maintainers = with maintainers; [ theuni ];
    platforms   = platforms.unix;

Please check in the Gemfile, Gemfile.lock and the gemset.nix so future updates can be run easily.

Updating Ruby packages can then be done like this:

$ cd pkgs/servers/monitoring/sensu
$ nix-shell -p bundler --run 'bundle lock --update'
$ nix-shell -p bundix --run 'bundix'

For tools written in Ruby - i.e. where the desire is to install a package and then execute e.g. rake at the command line, there is an alternative builder called bundlerApp. Set up the gemset.nix the same way, and then, for example:

{ lib, bundlerApp }:

bundlerApp {
  pname = "corundum";
  gemdir = ./.;
  exes = [ "corundum-skel" ];

  meta = with lib; {
    description = "Tool and libraries for maintaining Ruby gems.";
    homepage    =;
    license     =;
    maintainers = [ maintainers.nyarly ];
    platforms   = platforms.unix;

The chief advantage of bundlerApp over bundlerEnv is the executables introduced in the environment are precisely those selected in the exes list, as opposed to bundlerEnv which adds all the executables made available by gems in the gemset, which can mean e.g. rspec or rake in unpredictable versions available from various packages.

Resulting derivations for both builders also have two helpful attributes, env and wrappedRuby. The first one allows one to quickly drop into nix-shell with the specified environment present. E.g. nix-shell -A sensu.env would give you an environment with Ruby preset so it has all the libraries necessary for sensu in its paths. The second one can be used to make derivations from custom Ruby scripts which have Gemfiles with their dependencies specified. It is a derivation with ruby wrapped so it can find all the needed dependencies. For example, to make a derivation my-script for a my-script.rb (which should be placed in bin) you should run bundix as specified above and then use bundlerEnv like this:

let env = bundlerEnv {
  name = "my-script-env";

  inherit ruby;
  gemfile = ./Gemfile;
  lockfile = ./Gemfile.lock;
  gemset = ./gemset.nix;

in stdenv.mkDerivation {
  name = "my-script";
  buildInputs = [ env.wrappedRuby ];
  script = ./my-script.rb;
  buildCommand = ''
    install -D -m755 $script $out/bin/my-script
    patchShebangs $out/bin/my-script

15.21. Rust

To install the rust compiler and cargo put


into the environment.systemPackages or bring them into scope with nix-shell -p rustc cargo.

For daily builds (beta and nightly) use either rustup from nixpkgs or use the Rust nightlies overlay.

15.21.1. Compiling Rust applications with Cargo

Rust applications are packaged by using the buildRustPackage helper from rustPlatform:

rustPlatform.buildRustPackage rec {
  pname = "ripgrep";
  version = "11.0.2";

  src = fetchFromGitHub {
    owner = "BurntSushi";
    repo = pname;
    rev = version;
    sha256 = "1iga3320mgi7m853la55xip514a3chqsdi1a1rwv25lr9b1p7vd3";

  cargoSha256 = "17ldqr3asrdcsh4l29m3b5r37r5d0b3npq1lrgjmxb6vlx6a36qh";
  verifyCargoDeps = true;

  meta = with stdenv.lib; {
    description = "A fast line-oriented regex search tool, similar to ag and ack";
    homepage =;
    license = licenses.unlicense;
    maintainers = [ maintainers.tailhook ];
    platforms = platforms.all;

buildRustPackage requires a cargoSha256 attribute which is computed over all crate sources of this package. Currently it is obtained by inserting a fake checksum into the expression and building the package once. The correct checksum can be then take from the failed build.

When the Cargo.lock, provided by upstream, is not in sync with the Cargo.toml, it is possible to use cargoPatches to update it. All patches added in cargoPatches will also be prepended to the patches in patches at build-time.

When verifyCargoDeps is set to true, the build will also verify that the cargoSha256 is not out of date by comparing the Cargo.lock file in both the cargoDeps and src. Note that this option changes the value of cargoSha256 since it also copies the Cargo.lock in it. To avoid breaking backward-compatibility this option is not enabled by default but hopefully will be in the future. Building a crate for a different target

To build your crate with a different cargo --target simply specify the target attribute:

pkgs.rustPlatform.buildRustPackage {
  target = "x86_64-fortanix-unknown-sgx";

15.21.2. Compiling Rust crates using Nix instead of Cargo Simple operation

When run, cargo build produces a file called Cargo.lock, containing pinned versions of all dependencies. Nixpkgs contains a tool called carnix (nix-env -iA nixos.carnix), which can be used to turn a Cargo.lock into a Nix expression.

That Nix expression calls rustc directly (hence bypassing Cargo), and can be used to compile a crate and all its dependencies. Here is an example for a minimal hello crate:

$ cargo new hello
$ cd hello
$ cargo build
 Compiling hello v0.1.0 (file:///tmp/hello)
  Finished dev [unoptimized + debuginfo] target(s) in 0.20 secs
$ carnix -o hello.nix --src ./. Cargo.lock --standalone
$ nix-build hello.nix -A hello_0_1_0

Now, the file produced by the call to carnix, called hello.nix, looks like:

# Generated by carnix 0.6.5: carnix -o hello.nix --src ./. Cargo.lock --standalone
{ lib, stdenv, buildRustCrate, fetchgit }:
let kernel =;
    # ... (content skipped)
rec {
  hello = f: hello_0_1_0 { features = hello_0_1_0_features { hello_0_1_0 = f; }; };
  hello_0_1_0_ = { dependencies?[], buildDependencies?[], features?[] }: buildRustCrate {
    crateName = "hello";
    version = "0.1.0";
    authors = [ " <>" ];
    src = ./.;
    inherit dependencies buildDependencies features;
  hello_0_1_0 = { features?(hello_0_1_0_features {}) }: hello_0_1_0_ {};
  hello_0_1_0_features = f: updateFeatures f (rec {
        hello_0_1_0.default = (f.hello_0_1_0.default or true);
    }) [ ];

In particular, note that the argument given as --src is copied verbatim to the source. If we look at a more complicated dependencies, for instance by adding a single line libc="*" to our Cargo.toml, we first need to run cargo build to update the Cargo.lock. Then, carnix needs to be run again, and produces the following nix file:

# Generated by carnix 0.6.5: carnix -o hello.nix --src ./. Cargo.lock --standalone
{ lib, stdenv, buildRustCrate, fetchgit }:
let kernel =;
    # ... (content skipped)
rec {
  hello = f: hello_0_1_0 { features = hello_0_1_0_features { hello_0_1_0 = f; }; };
  hello_0_1_0_ = { dependencies?[], buildDependencies?[], features?[] }: buildRustCrate {
    crateName = "hello";
    version = "0.1.0";
    authors = [ " <>" ];
    src = ./.;
    inherit dependencies buildDependencies features;
  libc_0_2_36_ = { dependencies?[], buildDependencies?[], features?[] }: buildRustCrate {
    crateName = "libc";
    version = "0.2.36";
    authors = [ "The Rust Project Developers" ];
    sha256 = "01633h4yfqm0s302fm0dlba469bx8y6cs4nqc8bqrmjqxfxn515l";
    inherit dependencies buildDependencies features;
  hello_0_1_0 = { features?(hello_0_1_0_features {}) }: hello_0_1_0_ {
    dependencies = mapFeatures features ([ libc_0_2_36 ]);
  hello_0_1_0_features = f: updateFeatures f (rec {
    hello_0_1_0.default = (f.hello_0_1_0.default or true);
    libc_0_2_36.default = true;
  }) [ libc_0_2_36_features ];
  libc_0_2_36 = { features?(libc_0_2_36_features {}) }: libc_0_2_36_ {
    features = mkFeatures (features.libc_0_2_36 or {});
  libc_0_2_36_features = f: updateFeatures f (rec {
    libc_0_2_36.default = (f.libc_0_2_36.default or true);
    libc_0_2_36.use_std =
      (f.libc_0_2_36.use_std or false) ||
      (f.libc_0_2_36.default or false) ||
      (libc_0_2_36.default or false);
  }) [];

Here, the libc crate has no src attribute, so buildRustCrate will fetch it from A sha256 attribute is still needed for Nix purity. Handling external dependencies

Some crates require external libraries. For crates from, such libraries can be specified in defaultCrateOverrides package in nixpkgs itself.

Starting from that file, one can add more overrides, to add features or build inputs by overriding the hello crate in a seperate file.

with import <nixpkgs> {};
((import ./hello.nix).hello {}).override {
  crateOverrides = defaultCrateOverrides // {
    hello = attrs: { buildInputs = [ openssl ]; };

Here, crateOverrides is expected to be a attribute set, where the key is the crate name without version number and the value a function. The function gets all attributes passed to buildRustCrate as first argument and returns a set that contains all attribute that should be overwritten.

For more complicated cases, such as when parts of the crate’s derivation depend on the crate’s version, the attrs argument of the override above can be read, as in the following example, which patches the derivation:

with import <nixpkgs> {};
((import ./hello.nix).hello {}).override {
  crateOverrides = defaultCrateOverrides // {
    hello = attrs: lib.optionalAttrs (lib.versionAtLeast attrs.version "1.0")  {
      postPatch = ''
        substituteInPlace lib/ \
          --replace "/usr/share/zoneinfo" "${tzdata}/share/zoneinfo"

Another situation is when we want to override a nested dependency. This actually works in the exact same way, since the crateOverrides parameter is forwarded to the crate’s dependencies. For instance, to override the build inputs for crate libc in the example above, where libc is a dependency of the main crate, we could do:

with import <nixpkgs> {};
((import hello.nix).hello {}).override {
  crateOverrides = defaultCrateOverrides // {
    libc = attrs: { buildInputs = []; };
} Options and phases configuration

Actually, the overrides introduced in the previous section are more general. A number of other parameters can be overridden:

  • The version of rustc used to compile the crate:

    (hello {}).override { rust = pkgs.rust; };
  • Whether to build in release mode or debug mode (release mode by default):

    (hello {}).override { release = false; };
  • Whether to print the commands sent to rustc when building (equivalent to --verbose in cargo:

    (hello {}).override { verbose = false; };
  • Extra arguments to be passed to rustc:

    (hello {}).override { extraRustcOpts = "-Z debuginfo=2"; };
  • Phases, just like in any other derivation, can be specified using the following attributes: preUnpack, postUnpack, prePatch, patches, postPatch, preConfigure (in the case of a Rust crate, this is run before calling the build script), postConfigure (after the build script),preBuild, postBuild, preInstall and postInstall. As an example, here is how to create a new module before running the build script:

    (hello {}).override {
      preConfigure = ''
         echo "pub const PATH=\"${hi.out}\";" >> src/"
    }; Features

One can also supply features switches. For example, if we want to compile diesel_cli only with the postgres feature, and no default features, we would write:

(callPackage ./diesel.nix {}).diesel {
  default = false;
  postgres = true;

Where diesel.nix is the file generated by Carnix, as explained above.

15.21.3. Setting Up nix-shell

Oftentimes you want to develop code from within nix-shell. Unfortunately buildRustCrate does not support common nix-shell operations directly (see this issue) so we will use stdenv.mkDerivation instead.

Using the example hello project above, we want to do the following: - Have access to cargo and rustc - Have the openssl library available to a crate through it’s normal compilation mechanism (pkg-config).

A typical shell.nix might look like:

with import <nixpkgs> {};

stdenv.mkDerivation {
  name = "rust-env";
  nativeBuildInputs = [
    rustc cargo

    # Example Build-time Additional Dependencies
  buildInputs = [
    # Example Run-time Additional Dependencies

  # Set Environment Variables

You should now be able to run the following:

$ nix-shell --pure
$ cargo build
$ cargo test Controlling Rust Version Inside nix-shell

To control your rust version (i.e. use nightly) from within shell.nix (or other nix expressions) you can use the following shell.nix

# Latest Nightly
with import <nixpkgs> {};
let src = fetchFromGitHub {
      owner = "mozilla";
      repo = "nixpkgs-mozilla";
      # commit from: 2019-05-15
      rev = "9f35c4b09fd44a77227e79ff0c1b4b6a69dff533";
      sha256 = "18h0nvh55b5an4gmlgfbvwbyqj91bklf1zymis6lbdh75571qaz0";
with import "${src.out}/rust-overlay.nix" pkgs pkgs;
stdenv.mkDerivation {
  name = "rust-env";
  buildInputs = [
    # Note: to use use stable, just replace `nightly` with `stable`

    # Add some extra dependencies from `pkgs`
    pkgconfig openssl

  # Set Environment Variables

Now run:

$ rustc --version
rustc 1.26.0-nightly (188e693b3 2018-03-26)

To see that you are using nightly.

15.21.4. Using the Rust nightlies overlay

Mozilla provides an overlay for nixpkgs to bring a nightly version of Rust into scope. This overlay can also be used to install recent unstable or stable versions of Rust, if desired.

To use this overlay, clone nixpkgs-mozilla, and create a symbolic link to the file rust-overlay.nix in the ~/.config/nixpkgs/overlays directory.

$ git clone
$ mkdir -p ~/.config/nixpkgs/overlays
$ ln -s $(pwd)/nixpkgs-mozilla/rust-overlay.nix ~/.config/nixpkgs/overlays/rust-overlay.nix

The latest version can be installed with the following command:

$ nix-env -Ai nixos.latest.rustChannels.stable.rust

Or using the attribute with nix-shell:

$ nix-shell -p nixos.latest.rustChannels.stable.rust

To install the beta or nightly channel, stable should be substituted by nightly or beta, or use the function provided by this overlay to pull a version based on a build date.

The overlay automatically updates itself as it uses the same source as rustup.

15.22. TeX Live

Since release 15.09 there is a new TeX Live packaging that lives entirely under attribute texlive.

15.22.1. User's guide

  • For basic usage just pull texlive.combined.scheme-basic for an environment with basic LaTeX support.

  • It typically won't work to use separately installed packages together. Instead, you can build a custom set of packages like this:

    texlive.combine {
      inherit (texlive) scheme-small collection-langkorean algorithms cm-super;

    There are all the schemes, collections and a few thousand packages, as defined upstream (perhaps with tiny differences).

  • By default you only get executables and files needed during runtime, and a little documentation for the core packages. To change that, you need to add pkgFilter function to combine.

    texlive.combine {
      # inherit (texlive) whatever-you-want;
      pkgFilter = pkg:
        pkg.tlType == "run" || pkg.tlType == "bin" || pkg.pname == "cm-super";
      # elem tlType [ "run" "bin" "doc" "source" ]
      # there are also other attributes: version, name

  • You can list packages e.g. by nix repl.

    $ nix repl
    nix-repl> :l <nixpkgs>
    nix-repl> texlive.collection-<TAB>

  • Note that the wrapper assumes that the result has a chance to be useful. For example, the core executables should be present, as well as some core data files. The supported way of ensuring this is by including some scheme, for example scheme-basic, into the combination.

15.22.2. Custom packages

You may find that you need to use an external TeX package. A derivation for such package has to provide contents of the "texmf" directory in its output and provide the tlType attribute. Here is a (very verbose) example:

with import <nixpkgs> {};

  foiltex_run = stdenvNoCC.mkDerivation {
    pname = "latex-foiltex";
    version = "2.1.4b";
    passthru.tlType = "run";

    srcs = [
      (fetchurl {
        url = "";
        sha256 = "07frz0krpz7kkcwlayrwrj2a2pixmv0icbngyw92srp9fp23cqpz";
      (fetchurl {
        url = "";
        sha256 = "09wkyidxk3n3zvqxfs61wlypmbhi1pxmjdi1kns9n2ky8ykbff99";

    unpackPhase = ''
      runHook preUnpack

      for _src in $srcs; do
        cp "$_src" $(stripHash "$_src")

      runHook postUnpack

    nativeBuildInputs = [ texlive.combined.scheme-small ];

    dontConfigure = true;

    buildPhase = ''
      runHook preBuild

      # Generate the style files
      latex foiltex.ins

      runHook postBuild

    installPhase = ''
      runHook preInstall

      mkdir -p "$path"
      cp *.{cls,def,clo} "$path/"

      runHook postInstall

    meta = with lib; {
      description = "A LaTeX2e class for overhead transparencies";
      license = licenses.unfreeRedistributable;
      maintainers = with maintainers; [ veprbl ];
      platforms = platforms.all;
  foiltex = { pkgs = [ foiltex_run ]; };

  latex_with_foiltex = texlive.combine {
    inherit (texlive) scheme-small;
    inherit foiltex;
  runCommand "test.pdf" {
    nativeBuildInputs = [ latex_with_foiltex ];
  } ''
cat >test.tex <<EOF

\title{Presentation title}

  pdflatex test.tex
  cp test.pdf $out

15.22.3. Known problems

  • Some tools are still missing, e.g. luajittex;

  • some apps aren't packaged/tested yet (asymptote, biber, etc.);

  • feature/bug: when a package is rejected by pkgFilter, its dependencies are still propagated;

  • in case of any bugs or feature requests, file a github issue or better a pull request and /cc @vcunat.

15.23. Titanium

The Nixpkgs repository contains facilities to deploy a variety of versions of the Titanium SDK versions, a cross-platform mobile app development framework using JavaScript as an implementation language, and includes a function abstraction making it possible to build Titanium applications for Android and iOS devices from source code.

Not all Titanium features supported – currently, it can only be used to build Android and iOS apps.

15.23.1. Building a Titanium app

We can build a Titanium app from source for Android or iOS and for debugging or release purposes by invoking the titaniumenv.buildApp {} function:

titaniumenv.buildApp {
  name = "myapp";
  src = ./myappsource;

  preBuild = "";
  target = "android"; # or 'iphone'
  tiVersion = "7.1.0.GA";
  release = true;

  androidsdkArgs = {
    platformVersions = [ "25" "26" ];
  androidKeyStore = ./keystore;
  androidKeyAlias = "myfirstapp";
  androidKeyStorePassword = "secret";

  xcodeBaseDir = "/Applications/";
  xcodewrapperArgs = {
    version = "9.3";
  iosMobileProvisioningProfile = ./myprovisioning.profile;
  iosCertificateName = "My Company";
  iosCertificate = ./mycertificate.p12;
  iosCertificatePassword = "secret";
  iosVersion = "11.3";
  iosBuildStore = false;

  enableWirelessDistribution = true;
  installURL = "/installipa.php";

The titaniumenv.buildApp {} function takes the following parameters:

  • The name parameter refers to the name in the Nix store.

  • The src parameter refers to the source code location of the app that needs to be built.

  • preRebuild contains optional build instructions that are carried out before the build starts.

  • target indicates for which device the app must be built. Currently only android and iphone (for iOS) are supported.

  • tiVersion can be used to optionally override the requested Titanium version in tiapp.xml. If not specified, it will use the version in tiapp.xml.

  • release should be set to true when building an app for submission to the Google Playstore or Apple Appstore. Otherwise, it should be false.

When the target has been set to android, we can configure the following parameters:

  • The androidSdkArgs parameter refers to an attribute set that propagates all parameters to the androidenv.composeAndroidPackages {} function. This can be used to install all relevant Android plugins that may be needed to perform the Android build. If no parameters are given, it will deploy the platform SDKs for API-levels 25 and 26 by default.

When the release parameter has been set to true, you need to provide parameters to sign the app:

  • androidKeyStore is the path to the keystore file

  • androidKeyAlias is the key alias

  • androidKeyStorePassword refers to the password to open the keystore file.

When the target has been set to iphone, we can configure the following parameters:

  • The xcodeBaseDir parameter refers to the location where Xcode has been installed. When none value is given, the above value is the default.

  • The xcodewrapperArgs parameter passes arbitrary parameters to the xcodeenv.composeXcodeWrapper {} function. This can, for example, be used to adjust the default version of Xcode.

When release has been set to true, you also need to provide the following parameters:

  • iosMobileProvisioningProfile refers to a mobile provisioning profile needed for signing.

  • iosCertificateName refers to the company name in the P12 certificate.

  • iosCertificate refers to the path to the P12 file.

  • iosCertificatePassword contains the password to open the P12 file.

  • iosVersion refers to the iOS SDK version to use. It defaults to the latest version.

  • iosBuildStore should be set to true when building for the Apple Appstore submission. For enterprise or ad-hoc builds it should be set to false.

When enableWirelessDistribution has been enabled, you must also provide the path of the PHP script (installURL) (that is included with the iOS build environment) to enable wireless ad-hoc installations.

15.23.2. Emulating or simulating the app

It is also possible to simulate the correspond iOS simulator build by using xcodeenv.simulateApp {} and emulate an Android APK by using androidenv.emulateApp {}.

15.24. Vim

Both Neovim and Vim can be configured to include your favorite plugins and additional libraries.

Loading can be deferred; see examples.

At the moment we support three different methods for managing plugins:

  • Vim packages (recommend)

  • VAM (=vim-addon-manager)

  • Pathogen

  • vim-plug

15.24.1. Custom configuration

Adding custom .vimrc lines can be done using the following code:

vim_configurable.customize {
  # `name` specifies the name of the executable and package
  name = "vim-with-plugins";

  vimrcConfig.customRC = ''
    set hidden

This configuration is used when Vim is invoked with the command specified as name, in this case vim-with-plugins.

For Neovim the configure argument can be overridden to achieve the same:

neovim.override {
  configure = {
    customRC = ''
      # here your custom configuration goes!

If you want to use neovim-qt as a graphical editor, you can configure it by overriding Neovim in an overlay or passing it an overridden Neovimn:

neovim-qt.override {
  neovim = neovim.override {
    configure = {
      customRC = ''
        # your custom configuration

15.24.2. Managing plugins with Vim packages

To store you plugins in Vim packages (the native Vim plugin manager, see :help packages) the following example can be used:

vim_configurable.customize {
  vimrcConfig.packages.myVimPackage = with pkgs.vimPlugins; {
    # loaded on launch
    start = [ youcompleteme fugitive ];
    # manually loadable by calling `:packadd $plugin-name`
    # however, if a Vim plugin has a dependency that is not explicitly listed in
    # opt that dependency will always be added to start to avoid confusion.
    opt = [ phpCompletion elm-vim ];
    # To automatically load a plugin when opening a filetype, add vimrc lines like:
    # autocmd FileType php :packadd phpCompletion

myVimPackage is an arbitrary name for the generated package. You can choose any name you like. For Neovim the syntax is:

neovim.override {
  configure = {
    customRC = ''
      # here your custom configuration goes!
    packages.myVimPackage = with pkgs.vimPlugins; {
      # see examples below how to use custom packages
      start = [ ];
      # If a Vim plugin has a dependency that is not explicitly listed in
      # opt that dependency will always be added to start to avoid confusion.
      opt = [ ];

The resulting package can be added to packageOverrides in ~/.nixpkgs/config.nix to make it installable:

  packageOverrides = pkgs: with pkgs; {
    myVim = vim_configurable.customize {
      # `name` specifies the name of the executable and package
      name = "vim-with-plugins";
      # add here code from the example section
    myNeovim = neovim.override {
      configure = {
      # add here code from the example section

After that you can install your special grafted myVim or myNeovim packages.

15.24.3. Managing plugins with vim-plug

To use vim-plug to manage your Vim plugins the following example can be used:

vim_configurable.customize {
  vimrcConfig.packages.myVimPackage = with pkgs.vimPlugins; {
    # loaded on launch
    plug.plugins = [ youcompleteme fugitive phpCompletion elm-vim ];

For Neovim the syntax is:

neovim.override {
  configure = {
    customRC = ''
      # here your custom configuration goes!
    plug.plugins = with pkgs.vimPlugins; [

15.24.4. Managing plugins with VAM Handling dependencies of Vim plugins

VAM introduced .json files supporting dependencies without versioning assuming that using latest version is ok most of the time. Example

First create a vim-scripts file having one plugin name per line. Example:

{'name': 'vim-addon-sql'}
{'filetype_regex': '\%(vim)$', 'names': ['reload', 'vim-dev-plugin']}

Such vim-scripts file can be read by VAM as well like this:

call vam#Scripts(expand('~/.vim-scripts'), {})

Create a default.nix file:

{ nixpkgs ? import <nixpkgs> {}, compiler ? "ghc7102" }:
nixpkgs.vim_configurable.customize { name = "vim"; vimrcConfig.vam.pluginDictionaries = [ "vim-addon-vim2nix" ]; }

Create a generate.vim file:

ActivateAddons vim-addon-vim2nix
let vim_scripts = "vim-scripts"
call nix#ExportPluginsForNix({
\  'path_to_nixpkgs': eval('{"'.substitute(substitute(substitute($NIX_PATH, ':', ',', 'g'), '=',':', 'g'), '\([:,]\)', '"\1"',"g").'"}')["nixpkgs"],
\  'cache_file': '/tmp/vim2nix-cache',
\  'try_catch': 0,
\  'plugin_dictionaries': ["vim-addon-manager"]+map(readfile(vim_scripts), 'eval(v:val)')
\ })

Then run

nix-shell -p vimUtils.vim_with_vim2nix --command "vim -c 'source generate.vim'"

You should get a Vim buffer with the nix derivations (output1) and vam.pluginDictionaries (output2). You can add your Vim to your system’s configuration file like this and start it by vim-my:

my-vim =
  let plugins = let inherit (vimUtils) buildVimPluginFrom2Nix; in {
    copy paste output1 here
  }; in vim_configurable.customize {
    name = "vim-my";

    vimrcConfig.vam.knownPlugins = plugins; # optional
    vimrcConfig.vam.pluginDictionaries = [
       copy paste output2 here

    # Pathogen would be
    # vimrcConfig.pathogen.knownPlugins = plugins; # plugins
    # vimrcConfig.pathogen.pluginNames = ["tlib"];

Sample output1:

"reload" = buildVimPluginFrom2Nix { # created by nix#NixDerivation
  name = "reload";
  src = fetchgit {
    url = "git://";
    rev = "0a601a668727f5b675cb1ddc19f6861f3f7ab9e1";
    sha256 = "0vb832l9yxj919f5hfg6qj6bn9ni57gnjd3bj7zpq7d4iv2s4wdh";
  dependencies = ["nim-misc"];


Sample output2:

  { "name" = ''vim-addon-sql''; }
  { "filetype_regex" = ''\%(vim)$$''; "names" = [ ''reload'' ''vim-dev-plugin'' ]; }

15.24.5. Adding new plugins to nixpkgs

Nix expressions for Vim plugins are stored in pkgs/misc/vim-plugins. For the vast majority of plugins, Nix expressions are automatically generated by running ./ This creates a generated.nix file based on the plugins listed in vim-plugin-names. Plugins are listed in alphabetical order in vim-plugin-names using the format [github username]/[repository]. For example becomes scrooloose/nerdtree.

Some plugins require overrides in order to function properly. Overrides are placed in overrides.nix. Overrides are most often required when a plugin requires some dependencies, or extra steps are required during the build process. For example deoplete-fish requires both deoplete-nvim and vim-fish, and so the following override was added:

deoplete-fish = super.deoplete-fish.overrideAttrs(old: {
  dependencies = with super; [ deoplete-nvim vim-fish ];

Sometimes plugins require an override that must be changed when the plugin is updated. This can cause issues when Vim plugins are auto-updated but the associated override isn’t updated. For these plugins, the override should be written so that it specifies all information required to install the plugin, and running ./ doesn’t change the derivation for the plugin. Manually updating the override is required to update these types of plugins. An example of such a plugin is LanguageClient-neovim.

To add a new plugin:

  1. run ./ and create a commit named vimPlugins: Update,

  2. add the new plugin to vim-plugin-names and add overrides if required to overrides.nix,

  3. run ./ again and create a commit named vimPlugins.[name]: init at [version] (where name and version can be found in generated.nix), and

  4. create a pull request.

15.24.6. Important repositories

  • vim-pi is a plugin repository from VAM plugin manager meant to be used by others as well used by

  • vim2nix which generates the .nix code

Chapter 16. Packages

This chapter contains information about how to use and maintain the Nix expressions for a number of specific packages, such as the Linux kernel or

16.1. Citrix Workspace

Note: Please note that the citrix_receiver package has been deprecated since its development was discontinued by upstream and has been replaced by the citrix workspace app.

Citrix Receiver and Citrix Workspace App are a remote desktop viewers which provide access to XenDesktop installations.

16.1.1. Basic usage

The tarball archive needs to be downloaded manually as the license agreements of the vendor for Citrix Receiver or Citrix Workspace need to be accepted first. Then run nix-prefetch-url file://$PWD/linuxx64-$version.tar.gz. With the archive available in the store the package can be built and installed with Nix.

Warning: It's recommended to install Citrix Receiver and/or Citrix Workspace using nix-env -i or globally to ensure that the .desktop files are installed properly into $XDG_CONFIG_DIRS. Otherwise it won't be possible to open .ica files automatically from the browser to start a Citrix connection.

16.1.2. Custom certificates

The Citrix Workspace App in nixpkgs trust several certificates from the Mozilla database by default. However several companies using Citrix might require their own corporate certificate. On distros with imperative packaging these certs can be stored easily in $ICAROOT, however this directory is a store path in nixpkgs. In order to work around this issue the package provides a simple mechanism to add custom certificates without rebuilding the entire package using symlinkJoin:

with import <nixpkgs> { config.allowUnfree = true; };
let extraCerts = [ ./custom-cert-1.pem ./custom-cert-2.pem /* ... */ ]; in
citrix_workspace.override {
  inherit extraCerts;

16.2. DLib

DLib is a modern, C++-based toolkit which provides several machine learning algorithms.

16.2.1. Compiling without AVX support

Especially older CPUs don't support AVX (Advanced Vector Extensions) instructions that are used by DLib to optimize their algorithms.

On the affected hardware errors like Illegal instruction will occur. In those cases AVX support needs to be disabled:

self: super: {
  dlib = super.dlib.override { avxSupport = false; };

16.3. Eclipse

The Nix expressions related to the Eclipse platform and IDE are in pkgs/applications/editors/eclipse.

Nixpkgs provides a number of packages that will install Eclipse in its various forms. These range from the bare-bones Eclipse Platform to the more fully featured Eclipse SDK or Scala-IDE packages and multiple version are often available. It is possible to list available Eclipse packages by issuing the command:

$ nix-env -f '<nixpkgs>' -qaP -A eclipses --description

Once an Eclipse variant is installed it can be run using the eclipse command, as expected. From within Eclipse it is then possible to install plugins in the usual manner by either manually specifying an Eclipse update site or by installing the Marketplace Client plugin and using it to discover and install other plugins. This installation method provides an Eclipse installation that closely resemble a manually installed Eclipse.

If you prefer to install plugins in a more declarative manner then Nixpkgs also offer a number of Eclipse plugins that can be installed in an Eclipse environment. This type of environment is created using the function eclipseWithPlugins found inside the nixpkgs.eclipses attribute set. This function takes as argument { eclipse, plugins ? [], jvmArgs ? [] } where eclipse is a one of the Eclipse packages described above, plugins is a list of plugin derivations, and jvmArgs is a list of arguments given to the JVM running the Eclipse. For example, say you wish to install the latest Eclipse Platform with the popular Eclipse Color Theme plugin and also allow Eclipse to use more RAM. You could then add

packageOverrides = pkgs: {
  myEclipse = with pkgs.eclipses; eclipseWithPlugins {
    eclipse = eclipse-platform;
    jvmArgs = [ "-Xmx2048m" ];
    plugins = [ plugins.color-theme ];

to your Nixpkgs configuration (~/.config/nixpkgs/config.nix) and install it by running nix-env -f '<nixpkgs>' -iA myEclipse and afterward run Eclipse as usual. It is possible to find out which plugins are available for installation using eclipseWithPlugins by running

$ nix-env -f '<nixpkgs>' -qaP -A eclipses.plugins --description

If there is a need to install plugins that are not available in Nixpkgs then it may be possible to define these plugins outside Nixpkgs using the buildEclipseUpdateSite and buildEclipsePlugin functions found in the nixpkgs.eclipses.plugins attribute set. Use the buildEclipseUpdateSite function to install a plugin distributed as an Eclipse update site. This function takes { name, src } as argument where src indicates the Eclipse update site archive. All Eclipse features and plugins within the downloaded update site will be installed. When an update site archive is not available then the buildEclipsePlugin function can be used to install a plugin that consists of a pair of feature and plugin JARs. This function takes an argument { name, srcFeature, srcPlugin } where srcFeature and srcPlugin are the feature and plugin JARs, respectively.

Expanding the previous example with two plugins using the above functions we have

packageOverrides = pkgs: {
  myEclipse = with pkgs.eclipses; eclipseWithPlugins {
    eclipse = eclipse-platform;
    jvmArgs = [ "-Xmx2048m" ];
    plugins = [
      (plugins.buildEclipsePlugin {
        name = "myplugin1-1.0";
        srcFeature = fetchurl {
          url = "http://…/features/myplugin1.jar";
          sha256 = "123…";
        srcPlugin = fetchurl {
          url = "http://…/plugins/myplugin1.jar";
          sha256 = "123…";
      (plugins.buildEclipseUpdateSite {
        name = "myplugin2-1.0";
        src = fetchurl {
          stripRoot = false;
          url = "http://…/";
          sha256 = "123…";

16.4. Elm

To start a development environment do nix-shell -p elmPackages.elm elmPackages.elm-format

To update Elm compiler, see nixpkgs/pkgs/development/compilers/elm/

To package Elm applications, read about elm2nix.

16.5. Emacs

16.5.1. Configuring Emacs

The Emacs package comes with some extra helpers to make it easier to configure. emacsWithPackages allows you to manage packages from ELPA. This means that you will not have to install that packages from within Emacs. For instance, if you wanted to use company, counsel, flycheck, ivy, magit, projectile, and use-package you could use this as a ~/.config/nixpkgs/config.nix override:

  packageOverrides = pkgs: with pkgs; {
    myEmacs = emacsWithPackages (epkgs: (with epkgs.melpaStablePackages; [

You can install it like any other packages via nix-env -iA myEmacs. However, this will only install those packages. It will not configure them for us. To do this, we need to provide a configuration file. Luckily, it is possible to do this from within Nix! By modifying the above example, we can make Emacs load a custom config file. The key is to create a package that provide a default.el file in /share/emacs/site-start/. Emacs knows to load this file automatically when it starts.

  packageOverrides = pkgs: with pkgs; rec {
    myEmacsConfig = writeText "default.el" ''
;; initialize package

(require 'package)
(package-initialize 'noactivate)
  (require 'use-package))

;; load some packages

(use-package company
  :bind ("<C-tab>" . company-complete)
  :diminish company-mode
  :commands (company-mode global-company-mode)
  :defer 1

(use-package counsel
  :commands (counsel-descbinds)
  :bind (([remap execute-extended-command] . counsel-M-x)
         ("C-x C-f" . counsel-find-file)
         ("C-c g" . counsel-git)
         ("C-c j" . counsel-git-grep)
         ("C-c k" . counsel-ag)
         ("C-x l" . counsel-locate)
         ("M-y" . counsel-yank-pop)))

(use-package flycheck
  :defer 2
  :config (global-flycheck-mode))

(use-package ivy
  :defer 1
  :bind (("C-c C-r" . ivy-resume)
         ("C-x C-b" . ivy-switch-buffer)
         :map ivy-minibuffer-map
         ("C-j" . ivy-call))
  :diminish ivy-mode
  :commands ivy-mode
  (ivy-mode 1))

(use-package magit
  :if (executable-find "git")
  :bind (("C-x g" . magit-status)
         ("C-x G" . magit-dispatch-popup))
  (setq magit-completing-read-function 'ivy-completing-read))

(use-package projectile
  :commands projectile-mode
  :bind-keymap ("C-c p" . projectile-command-map)
  :defer 5
    myEmacs = emacsWithPackages (epkgs: (with epkgs.melpaStablePackages; [
      (runCommand "default.el" {} ''
mkdir -p $out/share/emacs/site-lisp
cp ${myEmacsConfig} $out/share/emacs/site-lisp/default.el

This provides a fairly full Emacs start file. It will load in addition to the user's presonal config. You can always disable it by passing -q to the Emacs command.

Sometimes emacsWithPackages is not enough, as this package set has some priorities imposed on packages (with the lowest priority assigned to Melpa Unstable, and the highest for packages manually defined in pkgs/top-level/emacs-packages.nix). But you can't control this priorities when some package is installed as a dependency. You can override it on per-package-basis, providing all the required dependencies manually - but it's tedious and there is always a possibility that an unwanted dependency will sneak in through some other package. To completely override such a package you can use overrideScope'.

overrides = self: super: rec {
  haskell-mode = self.melpaPackages.haskell-mode;
((emacsPackagesGen emacs).overrideScope' overrides).emacsWithPackages (p: with p; [
  # here both these package will use haskell-mode of our own choice

16.6. ibus-engines.typing-booster

This package is an ibus-based completion method to speed up typing.

16.6.1. Activating the engine

IBus needs to be configured accordingly to activate typing-booster. The configuration depends on the desktop manager in use. For detailed instructions, please refer to the upstream docs.

On NixOS you need to explicitly enable ibus with given engines before customizing your desktop to use typing-booster. This can be achieved using the ibus module:

{ pkgs, ... }: {
  i18n.inputMethod = {
    enabled = "ibus";
    ibus.engines = with pkgs.ibus-engines; [ typing-booster ];

16.6.2. Using custom hunspell dictionaries

The IBus engine is based on hunspell to support completion in many languages. By default the dictionaries de-de, en-us, fr-moderne es-es, it-it, sv-se and sv-fi are in use. To add another dictionary, the package can be overridden like this:

ibus-engines.typing-booster.override {
  langs = [ "de-at" "en-gb" ];

Note: each language passed to langs must be an attribute name in pkgs.hunspellDicts.

16.6.3. Built-in emoji picker

The ibus-engines.typing-booster package contains a program named emoji-picker. To display all emojis correctly, a special font such as noto-fonts-emoji is needed:

On NixOS it can be installed using the following expression:

{ pkgs, ... }: {
  fonts.fonts = with pkgs; [ noto-fonts-emoji ];

16.7. Kakoune

Kakoune can be built to autoload plugins:

(kakoune.override {
  configure = {
    plugins = with pkgs.kakounePlugins; [ parinfer-rust ];

16.8. Linux kernel

The Nix expressions to build the Linux kernel are in pkgs/os-specific/linux/kernel.

The function that builds the kernel has an argument kernelPatches which should be a list of {name, patch, extraConfig} attribute sets, where name is the name of the patch (which is included in the kernel’s meta.description attribute), patch is the patch itself (possibly compressed), and extraConfig (optional) is a string specifying extra options to be concatenated to the kernel configuration file (.config).

The kernel derivation exports an attribute features specifying whether optional functionality is or isn’t enabled. This is used in NixOS to implement kernel-specific behaviour. For instance, if the kernel has the iwlwifi feature (i.e. has built-in support for Intel wireless chipsets), then NixOS doesn’t have to build the external iwlwifi package:

modulesTree = [kernel]
  ++ pkgs.lib.optional (!kernel.features ? iwlwifi) kernelPackages.iwlwifi
  ++ ...;

How to add a new (major) version of the Linux kernel to Nixpkgs:

  1. Copy the old Nix expression (e.g. linux-2.6.21.nix) to the new one (e.g. linux-2.6.22.nix) and update it.

  2. Add the new kernel to all-packages.nix (e.g., create an attribute kernel_2_6_22).

  3. Now we’re going to update the kernel configuration. First unpack the kernel. Then for each supported platform (i686, x86_64, uml) do the following:

    1. Make an copy from the old config (e.g. config-2.6.21-i686-smp) to the new one (e.g. config-2.6.22-i686-smp).

    2. Copy the config file for this platform (e.g. config-2.6.22-i686-smp) to .config in the kernel source tree.

    3. Run make oldconfig ARCH={i386,x86_64,um} and answer all questions. (For the uml configuration, also add SHELL=bash.) Make sure to keep the configuration consistent between platforms (i.e. don’t enable some feature on i686 and disable it on x86_64).

    4. If needed you can also run make menuconfig:

      $ nix-env -i ncurses
      $ export NIX_CFLAGS_LINK=-lncurses
      $ make menuconfig ARCH=arch

    5. Copy .config over the new config file (e.g. config-2.6.22-i686-smp).

  4. Test building the kernel: nix-build -A kernel_2_6_22. If it compiles, ship it! For extra credit, try booting NixOS with it.

  5. It may be that the new kernel requires updating the external kernel modules and kernel-dependent packages listed in the linuxPackagesFor function in all-packages.nix (such as the NVIDIA drivers, AUFS, etc.). If the updated packages aren’t backwards compatible with older kernels, you may need to keep the older versions around.

16.9. Locales

To allow simultaneous use of packages linked against different versions of glibc with different locale archive formats Nixpkgs patches glibc to rely on LOCALE_ARCHIVE environment variable.

On non-NixOS distributions this variable is obviously not set. This can cause regressions in language support or even crashes in some Nixpkgs-provided programs. The simplest way to mitigate this problem is exporting the LOCALE_ARCHIVE variable pointing to ${glibcLocales}/lib/locale/locale-archive. The drawback (and the reason this is not the default) is the relatively large (a hundred MiB) size of the full set of locales. It is possible to build a custom set of locales by overriding parameters allLocales and locales of the package.

16.10. Nginx

Nginx is a reverse proxy and lightweight webserver.

16.10.1. ETags on static files served from the Nix store

HTTP has a couple different mechanisms for caching to prevent clients from having to download the same content repeatedly if a resource has not changed since the last time it was requested. When nginx is used as a server for static files, it implements the caching mechanism based on the Last-Modified response header automatically; unfortunately, it works by using filesystem timestamps to determine the value of the Last-Modified header. This doesn't give the desired behavior when the file is in the Nix store, because all file timestamps are set to 0 (for reasons related to build reproducibility).

Fortunately, HTTP supports an alternative (and more effective) caching mechanism: the ETag response header. The value of the ETag header specifies some identifier for the particular content that the server is sending (e.g. a hash). When a client makes a second request for the same resource, it sends that value back in an If-None-Match header. If the ETag value is unchanged, then the server does not need to resend the content.

As of NixOS 19.09, the nginx package in Nixpkgs is patched such that when nginx serves a file out of /nix/store, the hash in the store path is used as the ETag header in the HTTP response, thus providing proper caching functionality. This happens automatically; you do not need to do modify any configuration to get this behavior.

16.11. OpenGL

Packages that use OpenGL have NixOS desktop as their primary target. The current solution for loading the GPU-specific drivers is based on libglvnd and looks for the driver implementation in LD_LIBRARY_PATH. If you are using a non-NixOS GNU/Linux/X11 desktop with free software video drivers, consider launching OpenGL-dependent programs from Nixpkgs with Nixpkgs versions of libglvnd and mesa_drivers in LD_LIBRARY_PATH. For proprietary video drivers you might have luck with also adding the corresponding video driver package.

16.12. Interactive shell helpers

Some packages provide the shell integration to be more useful. But unlike other systems, nix doesn't have a standard share directory location. This is why a bunch PACKAGE-share scripts are shipped that print the location of the corresponding shared folder. Current list of such packages is as following:

  • autojump: autojump-share

  • fzf: fzf-share

E.g. autojump can then used in the .bashrc like this:

  source "$(autojump-share)/autojump.bash"

16.13. Steam

16.13.1. Steam in Nix

Steam is distributed as a .deb file, for now only as an i686 package (the amd64 package only has documentation). When unpacked, it has a script called steam that in Ubuntu (their target distro) would go to /usr/bin . When run for the first time, this script copies some files to the user's home, which include another script that is the ultimate responsible for launching the steam binary, which is also in $HOME.

Nix problems and constraints:

  • We don't have /bin/bash and many scripts point there. Similarly for /usr/bin/python .

  • We don't have the dynamic loader in /lib .

  • The script in $HOME can not be patched, as it is checked and rewritten by steam.

  • The steam binary cannot be patched, it's also checked.

The current approach to deploy Steam in NixOS is composing a FHS-compatible chroot environment, as documented here. This allows us to have binaries in the expected paths without disrupting the system, and to avoid patching them to work in a non FHS environment.

16.13.2. How to play

For 64-bit systems it's important to have

hardware.opengl.driSupport32Bit = true;

in your /etc/nixos/configuration.nix. You'll also need

hardware.pulseaudio.support32Bit = true;

if you are using PulseAudio - this will enable 32bit ALSA apps integration. To use the Steam controller or other Steam supported controllers such as the DualShock 4 or Nintendo Switch Pro, you need to add

hardware.steam-hardware.enable = true;

to your configuration.

16.13.3. Troubleshooting

Steam fails to start. What do I do?

Try to run

strace steam

to see what is causing steam to fail.

Using the FOSS Radeon or nouveau (nvidia) drivers
  • The newStdcpp parameter was removed since NixOS 17.09 and should not be needed anymore.

  • Steam ships statically linked with a version of libcrypto that conflics with the one dynamically loaded by If you get the error line 713: 7842 Segmentation fault (core dumped)

    have a look at this pull request.

  1. There is no java in steam chrootenv by default. If you get a message like

    /home/foo/.local/share/Steam/SteamApps/common/towns/ line 1: java: command not found

    You need to add

     steam.override { withJava = true; };

    to your configuration.

16.13.4. steam-run

The FHS-compatible chroot used for steam can also be used to run other linux games that expect a FHS environment. To do it, add

pkgs.(steam.override {
          nativeOnly = true;
          newStdcpp = true;

to your configuration, rebuild, and run the game with

steam-run ./foo

16.14. Urxvt

Urxvt, also known as rxvt-unicode, is a highly customizable terminal emulator.

16.14.1. Configuring urxvt

In nixpkgs, urxvt is provided by the package rxvt-unicode. It can be configured to include your choice of plugins, reducing its closure size from the default configuration which includes all available plugins. To make use of this functionality, use an overlay or directly install an expression that overrides its configuration, such as

rxvt-unicode.override { configure = { availablePlugins, ... }: {
    plugins = with availablePlugins; [ perls resize-font vtwheel ];

If the configure function returns an attrset without the plugins attribute, availablePlugins will be used automatically.

In order to add plugins but also keep all default plugins installed, it is possible to use the following method:

rxvt-unicode.override { configure = { availablePlugins, ... }: {
     plugins = (builtins.attrValues availablePlugins) ++ [ custom-plugin ];

To get a list of all the plugins available, open the Nix REPL and run

$ nix repl
:l <nixpkgs>
map (p: pkgs.rxvt-unicode.plugins

Alternatively, if your shell is bash or zsh and have completion enabled, simply type nixpkgs.rxvt-unicode.plugins.<tab>.

In addition to plugins the options extraDeps and perlDeps can be used to install extra packages. extraDeps can be used, for example, to provide xsel (a clipboard manager) to the clipboard plugin, without installing it globally:

rxvt-unicode.override { configure = { availablePlugins, ... }: {
     pluginsDeps = [ xsel ];

perlDeps is a handy way to provide Perl packages to your custom plugins (in $HOME/.urxvt/ext). For example, if you need AnyEvent you can do:

rxvt-unicode.override { configure = { availablePlugins, ... }: {
     perlDeps = with perlPackages; [ AnyEvent ];

16.14.2. Packaging urxvt plugins

Urxvt plugins resides in pkgs/applications/misc/rxvt-unicode-plugins. To add a new plugin create an expression in a subdirectory and add the package to the set in pkgs/applications/misc/rxvt-unicode-plugins/default.nix.

A plugin can be any kind of derivation, the only requirement is that it should always install perl scripts in $out/lib/urxvt/perl. Look for existing plugins for examples.

If the plugin is itself a perl package that needs to be imported from other plugins or scripts, add the following passthrough:

passthru.perlPackages = [ "self" ];

This will make the urxvt wrapper pick up the dependency and set up the perl path accordingly.

16.15. Weechat

Weechat can be configured to include your choice of plugins, reducing its closure size from the default configuration which includes all available plugins. To make use of this functionality, install an expression that overrides its configuration such as

weechat.override {configure = {availablePlugins, ...}: {
    plugins = with availablePlugins; [ python perl ];

If the configure function returns an attrset without the plugins attribute, availablePlugins will be used automatically.

The plugins currently available are python, perl, ruby, guile, tcl and lua.

The python and perl plugins allows the addition of extra libraries. For instance, the script in weechat-scripts requires D-Bus or libnotify, and the script requires pycrypto. To use these scripts, use the plugin's withPackages attribute:

weechat.override { configure = {availablePlugins, ...}: {
    plugins = with availablePlugins; [
            (python.withPackages (ps: with ps; [ pycrypto python-dbus ]))

In order to also keep all default plugins installed, it is possible to use the following method:

weechat.override { configure = { availablePlugins, ... }: {
  plugins = builtins.attrValues (availablePlugins // {
    python = availablePlugins.python.withPackages (ps: with ps; [ pycrypto python-dbus ]);
}; }

WeeChat allows to set defaults on startup using the --run-command. The configure method can be used to pass commands to the program:

weechat.override {
  configure = { availablePlugins, ... }: {
    init = ''
      /set foo bar
      /server add freenode

Further values can be added to the list of commands when running weechat --run-command "your-commands".

Additionally it's possible to specify scripts to be loaded when starting weechat. These will be loaded before the commands from init:

weechat.override {
  configure = { availablePlugins, ... }: {
    scripts = with pkgs.weechatScripts; [
      weechat-xmpp weechat-matrix-bridge wee-slack
    init = ''
      /set plugins.var.python.jabber.key "val"

In nixpkgs there's a subpackage which contains derivations for WeeChat scripts. Such derivations expect a passthru.scripts attribute which contains a list of all scripts inside the store path. Furthermore all scripts have to live in $out/share. An exemplary derivation looks like this:

{ stdenv, fetchurl }:

stdenv.mkDerivation {
  name = "exemplary-weechat-script";
  src = fetchurl {
    url = "https://scripts.tld/your-scripts.tar.gz";
    sha256 = "...";
  passthru.scripts = [ "" "bar.lua" ];
  installPhase = ''
    mkdir $out/share
    cp $out/share
    cp bar.lua $out/share


The Nix expressions for the packages reside in pkgs/servers/x11/xorg/default.nix. This file is automatically generated from lists of tarballs in an release. As such it should not be modified directly; rather, you should modify the lists, the generator script or the file pkgs/servers/x11/xorg/overrides.nix, in which you can override or add to the derivations produced by the generator.

The generator is invoked as follows:

$ cd pkgs/servers/x11/xorg
$ cat tarballs-7.5.list extra.list old.list \
  | perl ./

For each of the tarballs in the .list files, the script downloads it, unpacks it, and searches its and * files for dependencies. This information is used to generate default.nix. The generator caches downloaded tarballs between runs. Pay close attention to the NOT FOUND: name messages at the end of the run, since they may indicate missing dependencies. (Some might be optional dependencies, however.)

A file like tarballs-7.5.list contains all tarballs in a release. It can be generated like this:

$ export i="mirror://xorg/X11R7.4/src/everything/"
$ cat $(PRINT_PATH=1 nix-prefetch-url $i | tail -n 1) \
  | perl -e 'while (<>) { if (/(href|HREF)="([^"]*.bz2)"/) { print "$ENV{'i'}$2\n"; }; }' \
  | sort > tarballs-7.4.list

extra.list contains libraries that aren’t part of proper, but are closely related to it, such as libxcb. old.list contains some packages that were removed from, but are still needed by some people or by other packages (such as imake).

If the expression for a package requires derivation attributes that the generator cannot figure out automatically (say, patches or a postInstall hook), you should modify pkgs/servers/x11/xorg/overrides.nix.

Chapter 17. Quick Start to Adding a Package

To add a package to Nixpkgs:

  1. Checkout the Nixpkgs source tree:

    $ git clone
    $ cd nixpkgs

  2. Find a good place in the Nixpkgs tree to add the Nix expression for your package. For instance, a library package typically goes into pkgs/development/libraries/pkgname, while a web browser goes into pkgs/applications/networking/browsers/pkgname. See Section 18.3, “File naming and organisation” for some hints on the tree organisation. Create a directory for your package, e.g.

    $ mkdir pkgs/development/libraries/libfoo

  3. In the package directory, create a Nix expression — a piece of code that describes how to build the package. In this case, it should be a function that is called with the package dependencies as arguments, and returns a build of the package in the Nix store. The expression should usually be called default.nix.

    $ emacs pkgs/development/libraries/libfoo/default.nix
    $ git add pkgs/development/libraries/libfoo/default.nix

    You can have a look at the existing Nix expressions under pkgs/ to see how it’s done. Here are some good ones:

    Some notes:

    • All meta attributes are optional, but it’s still a good idea to provide at least the description, homepage and license.

    • You can use nix-prefetch-url url to get the SHA-256 hash of source distributions. There are similar commands as nix-prefetch-git and nix-prefetch-hg available in nix-prefetch-scripts package.

    • A list of schemes for mirror:// URLs can be found in pkgs/build-support/fetchurl/mirrors.nix.

    The exact syntax and semantics of the Nix expression language, including the built-in function, are described in the Nix manual in the chapter on writing Nix expressions.

  4. Add a call to the function defined in the previous step to pkgs/top-level/all-packages.nix with some descriptive name for the variable, e.g. libfoo.

    $ emacs pkgs/top-level/all-packages.nix

    The attributes in that file are sorted by category (like “Development / Libraries”) that more-or-less correspond to the directory structure of Nixpkgs, and then by attribute name.

  5. To test whether the package builds, run the following command from the root of the nixpkgs source tree:

    $ nix-build -A libfoo

    where libfoo should be the variable name defined in the previous step. You may want to add the flag -K to keep the temporary build directory in case something fails. If the build succeeds, a symlink ./result to the package in the Nix store is created.

  6. If you want to install the package into your profile (optional), do

    $ nix-env -f . -iA libfoo

  7. Optionally commit the new package and open a pull request to nixpkgs, or use the Patches category on Discourse for sending a patch without a GitHub account.

Chapter 18. Coding conventions

18.1. Syntax

  • Use 2 spaces of indentation per indentation level in Nix expressions, 4 spaces in shell scripts.

  • Do not use tab characters, i.e. configure your editor to use soft tabs. For instance, use (setq-default indent-tabs-mode nil) in Emacs. Everybody has different tab settings so it’s asking for trouble.

  • Use lowerCamelCase for variable names, not UpperCamelCase. Note, this rule does not apply to package attribute names, which instead follow the rules in Section 18.2, “Package naming”.

  • Function calls with attribute set arguments are written as

    foo {
      arg = ...;


      arg = ...;

    Also fine is

    foo { arg = ...; }

    if it's a short call.

  • In attribute sets or lists that span multiple lines, the attribute names or list elements should be aligned:

    # A long list.
    list = [
    # A long attribute set.
    attrs = {
      attr1 = short_expr;
      attr2 =
        if true then big_expr else big_expr;
    # Combined
    listOfAttrs = [
        attr1 = 3;
        attr2 = "fff";
        attr1 = 5;
        attr2 = "ggg";

  • Short lists or attribute sets can be written on one line:

    # A short list.
    list = [ elem1 elem2 elem3 ];
    # A short set.
    attrs = { x = 1280; y = 1024; };

  • Breaking in the middle of a function argument can give hard-to-read code, like

    someFunction { x = 1280;
      y = 1024; } otherArg

    (especially if the argument is very large, spanning multiple lines).


      { x = 1280; y = 1024; }


    let res = { x = 1280; y = 1024; };
    in someFunction res otherArg yetAnotherArg

  • The bodies of functions, asserts, and withs are not indented to prevent a lot of superfluous indentation levels, i.e.

    { arg1, arg2 }:
    assert system == "i686-linux";
    stdenv.mkDerivation { ...


    { arg1, arg2 }:
      assert system == "i686-linux";
        stdenv.mkDerivation { ...

  • Function formal arguments are written as:

    { arg1, arg2, arg3 }:

    but if they don't fit on one line they're written as:

    { arg1, arg2, arg3
    , arg4, ...
    , # Some comment...

  • Functions should list their expected arguments as precisely as possible. That is, write

    { stdenv, fetchurl, perl }: ...

    instead of

    args: with args; ...


    { stdenv, fetchurl, perl, ... }: ...

    For functions that are truly generic in the number of arguments (such as wrappers around mkDerivation) that have some required arguments, you should write them using an @-pattern:

    { stdenv, doCoverageAnalysis ? false, ... } @ args:
    stdenv.mkDerivation (args // {
      ... if doCoverageAnalysis then "bla" else "" ...

    instead of

    args.stdenv.mkDerivation (args // {
      ... if args ? doCoverageAnalysis && args.doCoverageAnalysis then "bla" else "" ...

18.2. Package naming

The key words must, must not, required, shall, shall not, should, should not, recommended, may, and optional in this section are to be interpreted as described in RFC 2119. Only emphasized words are to be interpreted in this way.

In Nixpkgs, there are generally three different names associated with a package:

  • The name attribute of the derivation (excluding the version part). This is what most users see, in particular when using nix-env.

  • The variable name used for the instantiated package in all-packages.nix, and when passing it as a dependency to other functions. Typically this is called the package attribute name. This is what Nix expression authors see. It can also be used when installing using nix-env -iA.

  • The filename for (the directory containing) the Nix expression.

Most of the time, these are the same. For instance, the package e2fsprogs has a name attribute "e2fsprogs-version", is bound to the variable name e2fsprogs in all-packages.nix, and the Nix expression is in pkgs/os-specific/linux/e2fsprogs/default.nix.

There are a few naming guidelines:

  • The name attribute should be identical to the upstream package name.

  • The name attribute must not contain uppercase letters — e.g., "mplayer-1.0rc2" instead of "MPlayer-1.0rc2".

  • The version part of the name attribute must start with a digit (following a dash) — e.g., "hello-0.3.1rc2".

  • If a package is not a release but a commit from a repository, then the version part of the name must be the date of that (fetched) commit. The date must be in "YYYY-MM-DD" format. Also append "unstable" to the name - e.g., "pkgname-unstable-2014-09-23".

  • Dashes in the package name should be preserved in new variable names, rather than converted to underscores or camel cased — e.g., http-parser instead of http_parser or httpParser. The hyphenated style is preferred in all three package names.

  • If there are multiple versions of a package, this should be reflected in the variable names in all-packages.nix, e.g. json-c-0-9 and json-c-0-11. If there is an obvious “default” version, make an attribute like json-c = json-c-0-9;. See also Section 18.3.2, “Versioning”

18.3. File naming and organisation

Names of files and directories should be in lowercase, with dashes between words — not in camel case. For instance, it should be all-packages.nix, not allPackages.nix or AllPackages.nix.

18.3.1. Hierarchy

Each package should be stored in its own directory somewhere in the pkgs/ tree, i.e. in pkgs/category/subcategory/.../pkgname. Below are some rules for picking the right category for a package. Many packages fall under several categories; what matters is the primary purpose of a package. For example, the libxml2 package builds both a library and some tools; but it’s a library foremost, so it goes under pkgs/development/libraries.

When in doubt, consider refactoring the pkgs/ tree, e.g. creating new categories or splitting up an existing category.

If it’s used to support software development:
If it’s a library used by other packages:

development/libraries (e.g. libxml2)

If it’s a compiler:

development/compilers (e.g. gcc)

If it’s an interpreter:

development/interpreters (e.g. guile)

If it’s a (set of) development tool(s):
If it’s a parser generator (including lexers):

development/tools/parsing (e.g. bison, flex)

If it’s a build manager:

development/tools/build-managers (e.g. gnumake)


development/tools/misc (e.g. binutils)



If it’s a (set of) tool(s):

(A tool is a relatively small program, especially one intended to be used non-interactively.)

If it’s for networking:

tools/networking (e.g. wget)

If it’s for text processing:

tools/text (e.g. diffutils)

If it’s a system utility, i.e., something related or essential to the operation of a system:

tools/system (e.g. cron)

If it’s an archiver (which may include a compression function):

tools/archivers (e.g. zip, tar)

If it’s a compression program:

tools/compression (e.g. gzip, bzip2)

If it’s a security-related program:

tools/security (e.g. nmap, gnupg)



If it’s a shell:

shells (e.g. bash)

If it’s a server:
If it’s a web server:

servers/http (e.g. apache-httpd)

If it’s an implementation of the X Windowing System:

servers/x11 (e.g. xorg — this includes the client libraries and programs)



If it’s a desktop environment:

desktops (e.g. kde, gnome, enlightenment)

If it’s a window manager:

applications/window-managers (e.g. awesome, stumpwm)

If it’s an application:

A (typically large) program with a distinct user interface, primarily used interactively.

If it’s a version management system:

applications/version-management (e.g. subversion)

If it’s for video playback / editing:

applications/video (e.g. vlc)

If it’s for graphics viewing / editing:

applications/graphics (e.g. gimp)

If it’s for networking:
If it’s a mailreader:

applications/networking/mailreaders (e.g. thunderbird)

If it’s a newsreader:

applications/networking/newsreaders (e.g. pan)

If it’s a web browser:

applications/networking/browsers (e.g. firefox)





If it’s data (i.e., does not have a straight-forward executable semantics):
If it’s a font:


If it’s an icon theme:


If it’s related to SGML/XML processing:
If it’s an XML DTD:

data/sgml+xml/schemas/xml-dtd (e.g. docbook)

If it’s an XSLT stylesheet:

(Okay, these are executable...)

data/sgml+xml/stylesheets/xslt (e.g. docbook-xsl)

If it’s a theme for a desktop environment, a window manager or a display manager:


If it’s a game:




18.3.2. Versioning

Because every version of a package in Nixpkgs creates a potential maintenance burden, old versions of a package should not be kept unless there is a good reason to do so. For instance, Nixpkgs contains several versions of GCC because other packages don’t build with the latest version of GCC. Other examples are having both the latest stable and latest pre-release version of a package, or to keep several major releases of an application that differ significantly in functionality.

If there is only one version of a package, its Nix expression should be named e2fsprogs/default.nix. If there are multiple versions, this should be reflected in the filename, e.g. e2fsprogs/1.41.8.nix and e2fsprogs/1.41.9.nix. The version in the filename should leave out unnecessary detail. For instance, if we keep the latest Firefox 2.0.x and 3.5.x versions in Nixpkgs, they should be named firefox/2.0.nix and firefox/3.5.nix, respectively (which, at a given point, might contain versions and 3.5.4). If a version requires many auxiliary files, you can use a subdirectory for each version, e.g. firefox/2.0/default.nix and firefox/3.5/default.nix.

All versions of a package must be included in all-packages.nix to make sure that they evaluate correctly.

18.4. Fetching Sources

There are multiple ways to fetch a package source in nixpkgs. The general guideline is that you should package reproducible sources with a high degree of availability. Right now there is only one fetcher which has mirroring support and that is fetchurl. Note that you should also prefer protocols which have a corresponding proxy environment variable.

You can find many source fetch helpers in pkgs/build-support/fetch*.

In the file pkgs/top-level/all-packages.nix you can find fetch helpers, these have names on the form fetchFrom*. The intention of these are to provide snapshot fetches but using the same api as some of the version controlled fetchers from pkgs/build-support/. As an example going from bad to good:

  • Bad: Uses git:// which won't be proxied.

    src = fetchgit {
      url = "git://";
      rev = "1f795f9f44607cc5bec70d1300150bfefcef2aae";
      sha256 = "1cw5fszffl5pkpa6s6wjnkiv6lm5k618s32sp60kvmvpy7a2v9kg";

  • Better: This is ok, but an archive fetch will still be faster.

    src = fetchgit {
      url = "";
      rev = "1f795f9f44607cc5bec70d1300150bfefcef2aae";
      sha256 = "1cw5fszffl5pkpa6s6wjnkiv6lm5k618s32sp60kvmvpy7a2v9kg";

  • Best: Fetches a snapshot archive and you get the rev you want.

    src = fetchFromGitHub {
      owner = "NixOS";
      repo = "nix";
      rev = "1f795f9f44607cc5bec70d1300150bfefcef2aae";
      sha256 = "1i2yxndxb6yc9l6c99pypbd92lfq5aac4klq7y2v93c9qvx2cgpc";

    Find the value to put as sha256 by running nix run -f '<nixpkgs>' nix-prefetch-github -c nix-prefetch-github --rev 1f795f9f44607cc5bec70d1300150bfefcef2aae NixOS nix or nix-prefetch-url --unpack

18.5. Obtaining source hash

Preferred source hash type is sha256. There are several ways to get it.

  1. Prefetch URL (with nix-prefetch-XXX URL, where XXX is one of url, git, hg, cvs, bzr, svn). Hash is printed to stdout.

  2. Prefetch by package source (with nix-prefetch-url '<nixpkgs>' -A PACKAGE.src, where PACKAGE is package attribute name). Hash is printed to stdout.

    This works well when you've upgraded existing package version and want to find out new hash, but is useless if package can't be accessed by attribute or package has multiple sources (.srcs, architecture-dependent sources, etc).

  3. Upstream provided hash: use it when upstream provides sha256 or sha512 (when upstream provides md5, don't use it, compute sha256 instead).

    A little nuance is that nix-prefetch-* tools produce hash encoded with base32, but upstream usually provides hexadecimal (base16) encoding. Fetchers understand both formats. Nixpkgs does not standardize on any one format.

    You can convert between formats with nix-hash, for example:

    $ nix-hash --type sha256 --to-base32 HASH

  4. Extracting hash from local source tarball can be done with sha256sum. Use nix-prefetch-url file:///path/to/tarball if you want base32 hash.

  5. Fake hash: set fake hash in package expression, perform build and extract correct hash from error Nix prints.

    For package updates it is enough to change one symbol to make hash fake. For new packages, you can use lib.fakeSha256, lib.fakeSha512 or any other fake hash.

    This is last resort method when reconstructing source URL is non-trivial and nix-prefetch-url -A isn't applicable (for example, one of kodi dependencies). The easiest way then would be replace hash with a fake one and rebuild. Nix build will fail and error message will contain desired hash.

    Warning: This method has security problems. Check below for details.

18.5.1. Obtaining hashes securely

Let's say Man-in-the-Middle (MITM) sits close to your network. Then instead of fetching source you can fetch malware, and instead of source hash you get hash of malware. Here are security considerations for this scenario:

  • http:// URLs are not secure to prefetch hash from;

  • hashes from upstream (in method 3) should be obtained via secure protocol;

  • https:// URLs are secure in methods 1, 2, 3;

  • https:// URLs are not secure in method 5. When obtaining hashes with fake hash method, TLS checks are disabled. So refetch source hash from several different networks to exclude MITM scenario. Alternatively, use fake hash method to make Nix error, but instead of extracting hash from error, extract https:// URL and prefetch it with method 1.

18.6. Patches

Patches available online should be retrieved using fetchpatch.

patches = [
  (fetchpatch {
    name = "fix-check-for-using-shared-freetype-lib.patch";
    url = ";a=patch;h=8f5d285";
    sha256 = "1f0k043rng7f0rfl9hhb89qzvvksqmkrikmm38p61yfx51l325xr";

Otherwise, you can add a .patch file to the nixpkgs repository. In the interest of keeping our maintenance burden to a minimum, only patches that are unique to nixpkgs should be added in this way.

patches = [ ./0001-changes.patch ];

If you do need to do create this sort of patch file, one way to do so is with git:

  1. Move to the root directory of the source code you're patching.

    $ cd the/program/source

  2. If a git repository is not already present, create one and stage all of the source files.

    $ git init
    $ git add .

  3. Edit some files to make whatever changes need to be included in the patch.

  4. Use git to create a diff, and pipe the output to a patch file:

    $ git diff > nixpkgs/pkgs/the/package/0001-changes.patch

Chapter 19. Submitting changes

19.1. Making patches

  • Read Manual (How to write packages for Nix).

  • Fork the Nixpkgs repository on GitHub.

  • Create a branch for your future fix.

    • You can make branch from a commit of your local nixos-version. That will help you to avoid additional local compilations. Because you will receive packages from binary cache. For example

      $ nixos-version --hash
      $ git checkout 0998212
      $ git checkout -b 'fix/pkg-name-update'

    • Please avoid working directly on the master branch.

  • Make commits of logical units.

  • If you removed pkgs or made some major NixOS changes, write about it in the release notes for the next stable release. For example nixos/doc/manual/release-notes/rl-2003.xml.

  • Check for unnecessary whitespace with git diff --check before committing.

  • Format the commit in a following way:

    (pkg-name | nixos/<module>): (from -> to | init at version | refactor | etc)
    Additional information.
    • Examples:

      • nginx: init at 2.0.1

      • firefox: 54.0.1 -> 55.0

      • nixos/hydra: add bazBaz option

      • nixos/nginx: refactor config generation

  • Test your changes. If you work with

    • nixpkgs:

      • update pkg ->

        • nix-env -i pkg-name -f <path to your local nixpkgs folder>

      • add pkg ->

        • Make sure it's in pkgs/top-level/all-packages.nix

        • nix-env -i pkg-name -f <path to your local nixpkgs folder>

      • If you don't want to install pkg in you profile.

        • nix-build -A pkg-attribute-name <path to your local nixpkgs folder>/default.nix and check results in the folder result. It will appear in the same directory where you did nix-build.

      • If you did nix-env -i pkg-name you can do nix-env -e pkg-name to uninstall it from your system.

    • NixOS and its modules:

      • You can add new module to your NixOS configuration file (usually it's /etc/nixos/configuration.nix). And do sudo nixos-rebuild test -I nixpkgs=<path to your local nixpkgs folder> --fast.

  • If you have commits pkg-name: oh, forgot to insert whitespace: squash commits in this case. Use git rebase -i.

  • Rebase your branch against current master.

19.2. Submitting changes

19.3. Submitting security fixes

Security fixes are submitted in the same way as other changes and thus the same guidelines apply.

If the security fix comes in the form of a patch and a CVE is available, then the name of the patch should be the CVE identifier, so e.g. CVE-2019-13636.patch in the case of a patch that is included in the Nixpkgs tree. If a patch is fetched the name needs to be set as well, e.g.:

   (fetchpatch {
     name = "CVE-2019-11068.patch";
     url = "";
     sha256 = "0pkpb4837km15zgg6h57bncp66d5lwrlvkr73h0lanywq7zrwhj8";

If a security fix applies to both master and a stable release then, similar to regular changes, they are preferably delivered via master first and cherry-picked to the release branch.

Critical security fixes may by-pass the staging branches and be delivered directly to release branches such as master and release-*.

19.4. Pull Request Template

The pull request template helps determine what steps have been made for a contribution so far, and will help guide maintainers on the status of a change. The motivation section of the PR should include any extra details the title does not address and link any existing issues related to the pull request.

When a PR is created, it will be pre-populated with some checkboxes detailed below:

19.4.1. Tested using sandboxing

When sandbox builds are enabled, Nix will setup an isolated environment for each build process. It is used to remove further hidden dependencies set by the build environment to improve reproducibility. This includes access to the network during the build outside of fetch* functions and files outside the Nix store. Depending on the operating system access to other resources are blocked as well (ex. inter process communication is isolated on Linux); see sandbox in Nix manual for details.

Sandboxing is not enabled by default in Nix due to a small performance hit on each build. In pull requests for nixpkgs people are asked to test builds with sandboxing enabled (see Tested using sandboxing in the pull request template) because in sandboxing is also used.

Depending if you use NixOS or other platforms you can use one of the following methods to enable sandboxing before building the package:

  • Globally enable sandboxing on NixOS: add the following to configuration.nix

    nix.useSandbox = true;

  • Globally enable sandboxing on non-NixOS platforms: add the following to: /etc/nix/nix.conf

    sandbox = true

19.4.2. Built on platform(s)

Many Nix packages are designed to run on multiple platforms. As such, it's important to let the maintainer know which platforms your changes have been tested on. It's not always practical to test a change on all platforms, and is not required for a pull request to be merged. Only check the systems you tested the build on in this section.

19.4.3. Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)

Packages with automated tests are much more likely to be merged in a timely fashion because it doesn't require as much manual testing by the maintainer to verify the functionality of the package. If there are existing tests for the package, they should be run to verify your changes do not break the tests. Tests only apply to packages with NixOS modules defined and can only be run on Linux. For more details on writing and running tests, see the section in the NixOS manual.

19.4.4. Tested compilation of all pkgs that depend on this change using nixpkgs-review

If you are updating a package's version, you can use nixpkgs-review to make sure all packages that depend on the updated package still compile correctly. The nixpkgs-review utility can look for and build all dependencies either based on uncommited changes with the wip option or specifying a github pull request number.

review changes from pull request number 12345:

nix run nixpkgs.nixpkgs-review -c nixpkgs-review pr 12345

review uncommitted changes:

nix run nixpkgs.nixpkgs-review -c nixpkgs-review wip

review changes from last commit:

nix run nixpkgs.nixpkgs-review -c nixpkgs-review rev HEAD

19.4.5. Tested execution of all binary files (usually in ./result/bin/)

It's important to test any executables generated by a build when you change or create a package in nixpkgs. This can be done by looking in ./result/bin and running any files in there, or at a minimum, the main executable for the package. For example, if you make a change to texlive, you probably would only check the binaries associated with the change you made rather than testing all of them.

19.4.6. Meets Nixpkgs contribution standards

The last checkbox is fits The contributing document has detailed information on standards the Nix community has for commit messages, reviews, licensing of contributions you make to the project, etc... Everyone should read and understand the standards the community has for contributing before submitting a pull request.

19.5. Hotfixing pull requests

  • Make the appropriate changes in you branch.

  • Don't create additional commits, do

    • git rebase -i

    • git push --force to your branch.

19.6. Commit policy

  • Commits must be sufficiently tested before being merged, both for the master and staging branches.

  • Hydra builds for master and staging should not be used as testing platform, it's a build farm for changes that have been already tested.

  • When changing the bootloader installation process, extra care must be taken. Grub installations cannot be rolled back, hence changes may break people's installations forever. For any non-trivial change to the bootloader please file a PR asking for review, especially from @edolstra.

19.6.1. Master branch

The master branch is the main development branch. It should only see non-breaking commits that do not cause mass rebuilds.

19.6.2. Staging branch

The staging branch is a development branch where mass-rebuilds go. It should only see non-breaking mass-rebuild commits. That means it is not to be used for testing, and changes must have been well tested already. If the branch is already in a broken state, please refrain from adding extra new breakages.

19.6.3. Staging-next branch

The staging-next branch is for stabilizing mass-rebuilds submitted to the staging branch prior to merging them into master. Mass-rebuilds should go via the staging branch. It should only see non-breaking commits that are fixing issues blocking it from being merged into the master branch.

If the branch is already in a broken state, please refrain from adding extra new breakages. Stabilize it for a few days and then merge into master.

19.6.4. Stable release branches

For cherry-picking a commit to a stable release branch (backporting), use git cherry-pick -x <original commit> so that the original commit id is included in the commit.

Add a reason for the backport by using git cherry-pick -xe <original commit> instead when it is not obvious from the original commit message. It is not needed when it’s a minor version update that includes security and bug fixes but don’t add new features or when the commit fixes an otherwise broken package.

Here is an example of a cherry-picked commit message with good reason description:

zfs: Keep trying root import until it works

Works around #11003.

(cherry picked from commit 98b213a11041af39b39473906b595290e2a4e2f9)

Reason: several people cannot boot with ZFS on NVMe

Other examples of reasons are:

  • Previously the build would fail due to, e.g., getaddrinfo not being defined

  • The previous download links were all broken

  • Crash when starting on some X11 systems

Chapter 20. Reviewing contributions

Warning: The following section is a draft, and the policy for reviewing is still being discussed in issues such as #11166 and #20836 .

The Nixpkgs project receives a fairly high number of contributions via GitHub pull requests. Reviewing and approving these is an important task and a way to contribute to the project.

The high change rate of Nixpkgs makes any pull request that remains open for too long subject to conflicts that will require extra work from the submitter or the merger. Reviewing pull requests in a timely manner and being responsive to the comments is the key to avoid this issue. GitHub provides sort filters that can be used to see the most recently and the least recently updated pull requests. We highly encourage looking at this list of ready to merge, unreviewed pull requests.

When reviewing a pull request, please always be nice and polite. Controversial changes can lead to controversial opinions, but it is important to respect every community member and their work.

GitHub provides reactions as a simple and quick way to provide feedback to pull requests or any comments. The thumb-down reaction should be used with care and if possible accompanied with some explanation so the submitter has directions to improve their contribution.

pull request reviews should include a list of what has been reviewed in a comment, so other reviewers and mergers can know the state of the review.

All the review template samples provided in this section are generic and meant as examples. Their usage is optional and the reviewer is free to adapt them to their liking.

20.1. Package updates

A package update is the most trivial and common type of pull request. These pull requests mainly consist of updating the version part of the package name and the source hash.

It can happen that non-trivial updates include patches or more complex changes.

Reviewing process:

  • Add labels to the pull request. (Requires commit rights)

    • 8.has: package (update) and any topic label that fit the updated package.

  • Ensure that the package versioning fits the guidelines.

  • Ensure that the commit text fits the guidelines.

  • Ensure that the package maintainers are notified.

    • CODEOWNERS will make GitHub notify users based on the submitted changes, but it can happen that it misses some of the package maintainers.

  • Ensure that the meta field information is correct.

    • License can change with version updates, so it should be checked to match the upstream license.

    • If the package has no maintainer, a maintainer must be set. This can be the update submitter or a community member that accepts to take maintainership of the package.

  • Ensure that the code contains no typos.

  • Building the package locally.

    • pull requests are often targeted to the master or staging branch, and building the pull request locally when it is submitted can trigger many source builds.

      It is possible to rebase the changes on nixos-unstable or nixpkgs-unstable for easier review by running the following commands from a nixpkgs clone.

      $ git fetch origin nixos-unstable 1
      $ git fetch origin pull/PRNUMBER/head 2
      $ git rebase --onto nixos-unstable BASEBRANCH FETCH_HEAD 3


      Fetching the nixos-unstable branch.


      Fetching the pull request changes, PRNUMBER is the number at the end of the pull request title and BASEBRANCH the base branch of the pull request.


      Rebasing the pull request changes to the nixos-unstable branch.

    • The nixpkgs-review tool can be used to review a pull request content in a single command. PRNUMBER should be replaced by the number at the end of the pull request title. You can also provide the full github pull request url.

      $ nix-shell -p nixpkgs-review --run "nixpkgs-review pr PRNUMBER"
  • Running every binary.

Example 20.1. Sample template for a package update review

##### Reviewed points

- [ ] package name fits guidelines
- [ ] package version fits guidelines
- [ ] package build on ARCHITECTURE
- [ ] executables tested on ARCHITECTURE
- [ ] all depending packages build

##### Possible improvements

##### Comments

20.2. New packages

New packages are a common type of pull requests. These pull requests consists in adding a new nix-expression for a package.

Reviewing process:

  • Add labels to the pull request. (Requires commit rights)

    • 8.has: package (new) and any topic label that fit the new package.

  • Ensure that the package versioning is fitting the guidelines.

  • Ensure that the commit name is fitting the guidelines.

  • Ensure that the meta field contains correct information.

    • License must be checked to be fitting upstream license.

    • Platforms should be set or the package will not get binary substitutes.

    • A maintainer must be set. This can be the package submitter or a community member that accepts to take maintainership of the package.

  • Ensure that the code contains no typos.

  • Ensure the package source.

    • Mirrors urls should be used when available.

    • The most appropriate function should be used (e.g. packages from GitHub should use fetchFromGitHub).

  • Building the package locally.

  • Running every binary.

Example 20.2. Sample template for a new package review

##### Reviewed points

- [ ] package path fits guidelines
- [ ] package name fits guidelines
- [ ] package version fits guidelines
- [ ] package build on ARCHITECTURE
- [ ] executables tested on ARCHITECTURE
- [ ] `meta.description` is set and fits guidelines
- [ ] `meta.license` fits upstream license
- [ ] `meta.platforms` is set
- [ ] `meta.maintainers` is set
- [ ] build time only dependencies are declared in `nativeBuildInputs`
- [ ] source is fetched using the appropriate function
- [ ] phases are respected
- [ ] patches that are remotely available are fetched with `fetchpatch`

##### Possible improvements

##### Comments

20.3. Module updates

Module updates are submissions changing modules in some ways. These often contains changes to the options or introduce new options.

Reviewing process

  • Add labels to the pull request. (Requires commit rights)

    • 8.has: module (update) and any topic label that fit the module.

  • Ensure that the module maintainers are notified.

    • CODEOWNERS will make GitHub notify users based on the submitted changes, but it can happen that it misses some of the package maintainers.

  • Ensure that the module tests, if any, are succeeding.

  • Ensure that the introduced options are correct.

    • Type should be appropriate (string related types differs in their merging capabilities, optionSet and string types are deprecated).

    • Description, default and example should be provided.

  • Ensure that option changes are backward compatible.

    • mkRenamedOptionModule and mkAliasOptionModule functions provide way to make option changes backward compatible.

  • Ensure that removed options are declared with mkRemovedOptionModule

  • Ensure that changes that are not backward compatible are mentioned in release notes.

  • Ensure that documentations affected by the change is updated.

Example 20.3. Sample template for a module update review

##### Reviewed points

- [ ] changes are backward compatible
- [ ] removed options are declared with `mkRemovedOptionModule`
- [ ] changes that are not backward compatible are documented in release notes
- [ ] module tests succeed on ARCHITECTURE
- [ ] options types are appropriate
- [ ] options description is set
- [ ] options example is provided
- [ ] documentation affected by the changes is updated

##### Possible improvements

##### Comments

20.4. New modules

New modules submissions introduce a new module to NixOS.

  • Add labels to the pull request. (Requires commit rights)

    • 8.has: module (new) and any topic label that fit the module.

  • Ensure that the module tests, if any, are succeeding.

  • Ensure that the introduced options are correct.

    • Type should be appropriate (string related types differs in their merging capabilities, optionSet and string types are deprecated).

    • Description, default and example should be provided.

  • Ensure that module meta field is present

    • Maintainers should be declared in meta.maintainers.

    • Module documentation should be declared with meta.doc.

  • Ensure that the module respect other modules functionality.

    • For example, enabling a module should not open firewall ports by default.

Example 20.4. Sample template for a new module review

##### Reviewed points

- [ ] module path fits the guidelines
- [ ] module tests succeed on ARCHITECTURE
- [ ] options have appropriate types
- [ ] options have default
- [ ] options have example
- [ ] options have descriptions
- [ ] No unneeded package is added to environment.systemPackages
- [ ] meta.maintainers is set
- [ ] module documentation is declared in meta.doc

##### Possible improvements

##### Comments

20.5. Other submissions

Other type of submissions requires different reviewing steps.

If you consider having enough knowledge and experience in a topic and would like to be a long-term reviewer for related submissions, please contact the current reviewers for that topic. They will give you information about the reviewing process. The main reviewers for a topic can be hard to find as there is no list, but checking past pull requests to see who reviewed or git-blaming the code to see who committed to that topic can give some hints.

Container system, boot system and library changes are some examples of the pull requests fitting this category.

20.6. Merging pull requests

It is possible for community members that have enough knowledge and experience on a special topic to contribute by merging pull requests.

TODO: add the procedure to request merging rights.

In a case a contributor definitively leaves the Nix community, they should create an issue or post on Discourse with references of packages and modules they maintain so the maintainership can be taken over by other contributors.

Chapter 21. Contributing to this documentation

The DocBook sources of the Nixpkgs manual are in the doc subdirectory of the Nixpkgs repository.

You can quickly check your edits with make:

$ cd /path/to/nixpkgs/doc
$ nix-shell
[nix-shell]$ make

If you experience problems, run make debug to help understand the docbook errors.

After making modifications to the manual, it's important to build it before committing. You can do that as follows:

$ cd /path/to/nixpkgs/doc
$ nix-shell
[nix-shell]$ make clean
[nix-shell]$ nix-build .

If the build succeeds, the manual will be in ./result/share/doc/nixpkgs/manual.html.