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Frequently asked questions and common newcomer trouble should be put here so that we can point to this page instead of answering the same question over and over again.

http://unix.stackexchange.com/questions/tagged/nixos can also be used for questions.


Why is Nix written in C++ rather than a functional language like Haskell?

Mainly because Nix is intended to be lightweight, easy to learn and portable (zero dependencies). Since 24. April 2017 thanks to Shea Levy and the crowdfunding of 54 community members, nix does not have Perl as dependency anymore.

I installed a library but my compiler is not finding it. Why?

With nix, only applications should be installed into profiles. Libraries are used using nix-shell. If you want to compile a piece of software that requires zlib (or openssl, sqlite etc.) and uses pkg-config to discover it, run

$ nix-shell -p gcc pkg-config zlib

to get into a shell with the appropriate environment variables set. In there, a configure script (with C Autotools, C++ CMake, Rust Cargo etc.) will work as expected.

This applies to other language environments too. In some cases the expressions to use are a bit different, e.g. because the interpreter needs to be wrapped to have some additional environment variables passed to it. The manual has a section on the subject.

If you have a lot of dependencies, you may want to write a nix expression that includes your dependencies so that you can simply use nix-shell rather than writing out each dependency every time or keeping your development environment in your shell history. A minimal example looks like this:

# default.nix
with import <nixpkgs> {};
stdenv.mkDerivation {
    name = "dev-environment"; # Probably put a more meaningful name here
    buildInputs = [ pkg-config zlib ];

Why does it work like that?

This helps ensure purity of builds: on other distributions, the result of building a piece of software may depend on which other software you have installed. Nix attempts to avoid this to the greatest degree possible, which allows builds of a piece of software to be identical (in the ideal case) no matter where they're built, by requiring all dependencies to be declared.

Why not use nix-env -i hello?

nix-env -i hello is slower and tends to be less precise than nix-env -f '<nixpkgs>' -iA hello. This is because it will evaluate all of nixpkgs searching for packages with the name hello, and install the one determined to be the latest (which may not even be the one that you want). Meanwhile, with -A, nix-env will evaluate only the given attribute in nixpkgs. This will be significantly faster, consume significantly less memory, and more likely get you what you want.

nix-env -u has the same problem, searching for all the packages in the user environment by name and upgrading them. This may lead to unwanted major-version upgrades like JDK 8 → JDK 9. If you want to have a declarative user environment, you may wish to use Home Manager. It is also possible to home-bake a pure nix solution like LnL's. With this setup, you can update your packages by simply running nix-rebuild.

When do I update stateVersion

Keep stateVersion to the version you originally installed.[1]

The system.stateVersion option is described as such:

Every once in a while, a new NixOS release may change configuration defaults in a way incompatible with stateful data. For instance, if the default version of PostgreSQL changes, the new version will probably be unable to read your existing databases. To prevent such breakage, you can set the value of this option to the NixOS release with which you want to be compatible. The effect is that NixOS will option defaults corresponding to the specified release (such as using an older version of PostgreSQL).

Frequent answers:

  • stateVersion has nothing to do with the current version of the system[2]
  • Do NOT change the stateVersion in the configuration; [it] tells nixos what version your state is; changing it will break the things [it is] meant to fix.[3]

When can I update stateVersion?


  1. You have read all release notes starting from your stateVersion.
  2. You have verified all instances of stateVersion in the code in <nixpkgs/nixos>.
  3. You have made all manual interventions as required by the changes previously inventoried.

How to keep build-time dependencies around / be able to rebuild while being offline?

# /etc/nixos/configuration.nix
{ config, pkgs, lib, ... }:
  nix.extraOptions = ''
    keep-outputs = true
    keep-derivations = true

Check 'man configuration.nix' for these options. Rebuild for these options to take effect:

nixos-rebuild switch

List all store paths that form the system closure and realise them:

nix-store -qR $(nix-instantiate '<nixpkgs/nixos>' -A system) | xargs nix-store -r
warning: you did not specify `--add-root'; the result might be removed by the garbage collector

<build output and list of successfully realised paths>

Repeat for your user and further profiles:

nix-store -qR ~/.nix-profile | xargs nix-store -r

The warning can be ignored for profiles that are listed/linked in /nix/var/nix/profiles/ or one of its subdirectories.

Consult man pages of nix-store and nix-instantiate for further information.

Why <hash>-<name> instead of <name>-<hash>?

For the rare cases where we have to dig into the /nix/store it is more practical to keep in mind the first few letters at the beginning than finding a package by name. Ie, you can uniquely identify almost any storepath with just the first 4-5 characters of the hash. (Rather than having to type out the full package name, then 4-5 characters of the hash.)

Also, since the initial part is all of the same length, visually parsing a list of packages is easier.

If you still wonder why, run ls -1 /nix/store | sort -R -t - -k 2 | less in your shell. (? unclear)

This is what might happen if you don't garbage collect frequently, or if you are testing compilation variants:


I've updated my channel and something is broken, how can I rollback to an earlier channel?

View the available generations of your channel:

nix-env --list-generations -p /nix/var/nix/profiles/per-user/root/channels
18   2014-04-17 09:16:28
19   2014-06-13 10:31:24 
20   2014-08-12 19:09:20   (current)

To rollback to the previous generation:

nix-env --rollback -p /nix/var/nix/profiles/per-user/root/channels
switching from generation 20 to 19

To switch to a particular generation:

nix-env --switch-generation 18 -p /nix/var/nix/profiles/per-user/root/channels
switching from generation 20 to 18

I'm working on a new package, how can I build it without adding it to nixpkgs?

nix-build -E 'with import <nixpkgs> { }; callPackage ./mypackage.nix { }'

You can replace callPackage with callPackage_i686 to build the 32-bit version of your package on a 64-bit system if you want to test that.

How can I compile a package with debugging symbols included?

To build a package with -Og and -g, and without stripping debug symbols use:

nix-build -E 'with import <nixpkgs> { }; enableDebugging fooPackage'

See also Debug Symbols

How can I force a rebuild from source even without modifying the nix expression?

As root you can run nix-build with the --check flag:

sudo nix-build --check -A ncdu

How can I manage software with nix-env like with configuration.nix?

There are many ways, one is the following:

  1. Create a meta package called userPackages your ~/.config/nixpkgs/config.nix file with the packages you would like to have in your environment:

    with (import <nixpkgs> {});
      packageOverrides = pkgs: with pkgs; {
        userPackages = buildEnv {
          inherit ((import <nixpkgs/nixos> {}).config.system.path)
          pathsToLink ignoreCollisions postBuild;
          extraOutputsToInstall = [ "man" ];
          name = "user-packages";
          paths = [ vim git wget ];
  2. Install all specified packages using this command:

    nix-env -iA userPackages -f '<nixpkgs>'

Now you can add and remove packages from the paths list and rerun nix-env to update your user local packages.

Another way is using Home Manager.

I've downloaded a binary, but I can't run it, what can I do?

Binaries normally do not work out of the box when you download them because they normally just assume that libraries can be found in hardcoded paths such as /lib. However this assumption is incorrect on NixOS systems due to the inner workings of nix - there is no default path, everything gets set to the corresponding version on compile time.

If you are new to packaging proprietary software you should check out the Packaging Binaries Tutorial.

If you are in a hurry and just want to get shit running, continue reading:
Compiled binaries have hard-coded interpreter and require certain dynamic libraries. You can use patchelf to set the library path and dynamic linker appropriately:

# mybinaryprogram.nix
with import <nixpkgs> {}; 
stdenv.mkDerivation rec {
  name = "somename";
  buildInputs = [ makeWrapper ];
  buildPhase = "true";
  libPath = lib.makeLibraryPath with xlibs;[ libXrandr libXinerama libXcursor ];
  unpackPhase = "true";
  installPhase = ''
    mkdir -p $out/bin
    cp ${./mybinaryprogram} $out/bin/mybinaryprogram
  postFixup = ''
    patchelf \
      --set-interpreter "$(cat $NIX_CC/nix-support/dynamic-linker)" \
      --set-rpath "${libPath}" \

This can be built with:

nix-build mybinaryprogram.nix

And run with:


Another possibility is using a FHS-compatible Sandbox with buildFHSUserEnv

# fhsUser.nix
{ pkgs ? import <nixpkgs> {} }:
(pkgs.buildFHSUserEnv {
  name = "example-env";
  targetPkgs = pkgs: with pkgs; [
  multiPkgs = pkgs: with pkgs; [
  runScript = "bash";

the sandbox can be entered with

nix-shell fhsUser.nix

If your target application can't find shared libraries inside buildFHSUserEnv, you may run nix-de-generate for target application inside FHS, which will generate newenv.nix file, an nix-expression of buildFHSUserEnv with resolved dependencies for shared libraries.

What are channels and how do they get updated?

Main article: Nix Channels

Nixpkgs is the git repository containing all packages and NixOS modules/expressions. Installing packages directly from Nixpkgs master branch is possible but a bit risky as git commits are merged into master before being heavily tested. That's where channels are useful.

A "channel" is a name for the latest "verified" git commits in Nixpkgs. Each channel has a different definition of what "verified" means. Each time a new git commit is verified, the channel declaring this verification gets updated. Contrary to an user of the git master branch, a channel user will benefit both from verified commits and binary packages from the binary cache.

Channels are reified as git branches in the nixpkgs repository and as disk images in the channels webpage. There are several channels, each with its own use case and verification phase:

  • nixos-unstable
    • description Use this when you want the latest package and module versions while still benefiting from the binary cache. You can use this channel on non-NixOS systems. This channel corresponds to NixOS’s main development branch, and may thus see radical changes between channel updates. This channel is not recommended for production systems.
    • definition this channel is updated depending on release.nix and release-lib.nix
  • nixos-unstable-small
    • description This channel is identical to nixos-unstable described above, except that this channel contains fewer binary packages. This means the channel gets updated faster than nixos-unstable (for instance, when a critical security patch is committed to NixOS’s source tree). However, the binary cache may contain less binary packages and thus using this channel may require building more packages from source than nixos-unstable. This channel is mostly intended for server environments and as such contains few GUI applications.
    • definition this channel is updated depending on release-small.nix and release-lib.nix
  • nixos-YY.MM (where YY is a 2-digit year and MM is a 2-digit month, such as nixos-17.03)
    • description These channels are called stable and only get conservative bug fixes and package upgrades. For instance, a channel update may cause the Linux kernel on your system to be upgraded from 3.4.66 to 3.4.67 (a minor bug fix), but not from 3.4.x to 3.11.x (a major change that has the potential to break things). Stable channels are generally maintained until the next stable branch is created.
    • definition this channel is updated depending on release.nix and release-lib.nix
  • nixos-YY.MM-small (where YY is a 2-digit year and MM is a 2-digit month, such as nixos-15.09-small)
    • description The difference between nixos-YY.MM-small and nixos-YY.MM is the name as the one between nixos-unstable-small and nixos-unstable (see above)

Channel update works as follows:

  1. Each channel has a particular job at hydra.nixos.org which must succeed:
  • For NixOS: the trunk-combined tested job, which contains some automated NixOS tests.
  • For nixos-small: the unstable-small tested job.
  • For nixpkgs: the trunk unstable job, which contains some critical release packages.
  1. Once the job succeeds at a particular nixpkgs commit, cache.nixos.org will download binaries from hydra.nixos.org.
  2. Once the above download completes, the channel updates.

You can checkout the nixpkgs git and reset it to a particular commit of a channel. This will not affect your access to the binary cache.

How do I know where's nixpkgs channel located and at which commit?

First echo $NIX_PATH to see where nix looks for the expressions. Note that nix-env uses ~/.nix-defexpr regardless of $NIX_PATH.

If you want to know where <nixpkgs> is located:

nix-instantiate --find-file nixpkgs

To know the commit, open the .version-suffix file in the nixpkgs location. The hash after the dot is the git commit.

Nixpkgs branches

Branches on the nixpkgs repo have a relationship with channels, but that relationship is not 1:1.

Some branches are reified as channels (e.g. the nixos-XX.YY branches, or nix(os|pkgs)-unstable), whereas others are the starting point for those branches (e.g. the master or release-XX.YY branches). For example:

  • When a change in master needs to be backported to the current NixOS release, it is cherry-picked into the current release-XX.YY branch
  • Hydra picks up this change, runs tests, and if those tests pass, updates the corresponding nixos-XX.YY branch, which is then reified as a channel.

So in short, the relase-XX.YY branches have not been run through Hydra yet, whereas the nixos-XX.YY ones have.

There's an updated version for $software on nixpkgs but not in channels, how can I use it?

You can jump the queue and use nix-shell with a NIX_PATH pointing to a tarball of the channel to get a shell for that software. Some building may occur. This will not work for system services.

NIX_PATH=nixpkgs=https://github.com/NixOS/nixpkgs/archive/release-17.09.tar.gz nix-shell -p $software

How do I install a specific version of a package for build reproducibility etc.?

See FAQ/Pinning Nixpkgs and How to fetch Nixpkgs with an empty NIX PATH. Find the version of nixpkgs with the package version you want and pin nixpkgs to that. However, be aware that the pinning of a package of another nixpkgs version results in a much larger package size as not only the package itself but all dependencies (down to libc) have older versions.

if you just want the old version of the single package but with new dependencies it is often easier to copy the package description into your scope and add it to your configuration.nix via: mypackage-old = pkgs.callPackage ./mypackage-old.nix {};.You can try to build the package as described in the FAQ: building a single derivation.

An error occurs while fetching sources from an url, how do I fix it?

First try to update the local nixpkgs expressions with nix-channel --update (these describe where to download sources from and how to build them). Try your build again and the url might have already been correctly updated for the package in question. You can also subscribe the unstable channel (which includes the most up-to-date expressions) with nix-channel --add http://nixos.org/channels/nixpkgs-unstable, update and try the build again.

If that fails you can update the url in the nix expression yourself. Navigate to your channel's expressions and find the package in one of the subdirectories. Edit the respective default.nix file by altering the url and sha256. You can use nix-prefetch-url url to get the SHA-256 hash of source distributions.

If the shell complains that you do not have write privileges for the file system, you will have to enable them.

start a new shell with a private mount namespace (Linux-only)

sudo unshare -m bash

remount the filesystem with write privileges (as root)

mount -o remount,rw /nix/store

update the file

nano <PATH_TO_PACKAGE>/default.nix

exit to shell where /nix/store is still mounted read-only


Be sure to report the incorrect url or fix it yourself.

How do I know the sha256 to use with fetchgit, fetchsvn, fetchbzr or fetchcvs?

Install nix-prefetch-scripts and use the corresponding nix prefetch helper.

For instance to get the checksum of a git repository use:

nix-prefetch-git https://git.zx2c4.com/password-store

Should I use http://hydra.nixos.org/ as a binary cache?

No. As of 2017, all build artifacts are directly pushed to http://cache.nixos.org/ and are available there, therefore setting http://hydra.nixos.org/ as a binary cache no longer serves any function.

I'm trying to install NixOS but my Wifi isn't working and I don't have an ethernet port

First of all, NixOS cannot be installed without an internet connection. However, most phones will allow you to share your Wifi connection over USB. On Android you can enable this setting via Settings > Wireless & Networks / More ... > Tethering & portable hotspot > USB tethering. This should be enough to allow you to install NixOS, and then fix your Wifi. iPhones only let you tether using your data connection rather than WiFi.

For connecting to your wifi, see NixOS_Installation_Guide#Wireless

How can I disable the binary cache and build everything locally?

Set the binary caches to an empty list: nix.binaryCaches = []; in configuration.nix or pass ad-hoc --option binary-caches '' as parameter to nix-build or its wrappers.

This is also useful to make simple configuration changes in NixOS (ex.: network related), when no network connectivity is available:

nixos-rebuild switch --option binary-caches ''

How do I enable sandboxed builds on non-NixOS?

Two options have to be added to make sandboxed builds work on Nix, build-use-sandbox and build-sandbox-paths:

# /etc/nix/nix.conf
build-use-sandbox = true
build-sandbox-paths = $(nix-store -qR $(nix-build '<nixpkgs>' -A bash) | xargs echo /bin/sh=$(nix-build '<nixpkgs>' -A bash)/bin/bash)

On NixOS set the following in configuration.nix:

nix.settings.sandbox = true;

See Nix Package Manager#Sandbox_builds for more details.

I cannot find $package when running nix-env -qaP even with channels configured

Not all packages are listed. Packages may not be listed because:

  • the package is unfree, like e.g. unrar and teamspeak_client; see The appropriate FAQ entry for installing unfree packages
  • the package is part of an attribute set and nix-env doesn't recurse into this set (see pkgs.recurseIntoAttrs), use nix-env -qaP -A haskellPackages for listing these entries

How can I install a proprietary or unfree package?

Configuration for a user

You won't be able to install or search for an unfree package as a user, unless you explicitly enable it in ~/.config/nixpkgs/config.nix (the file and folder may need to be created):

  # [...]
  allowUnfree = true;

System-level configuration (NixOS)

When using NixOS, it is possible to enable unfree for eg. environment.systemPackages with the following setting in your /etc/nixos/configuration.nix:

  # [...]
  nixpkgs.config.allowUnfree = true;

Note that this alone will not allow you to search for or install packages with nix-env without the user configured. See the previous section. No reboot is required for unfree to take effect, however you need to run nixos-rebuild switch just like for all other changes to your systems configuration.nix

Temporary allowing

For temporary allowing unfree packages you can set the environment variable NIXPKGS_ALLOW_UNFREE, e.g.


More precision

For whitelisting only specific unfree packages or unfree licenses see the "Installing unfree packages" section of the Nixpkgs manual.

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.

How can I install a package from unstable while remaining on the stable channel?

If you simply want to run a nix-shell with a package from unstable, you can run a command like the following:

nix-shell -I nixpkgs=channel:nixpkgs-unstable -p somepackage

It is possible to have multiple nix-channels simultaneously. To add the unstable channel with the specifier unstable,

sudo nix-channel --add https://nixos.org/channels/nixos-unstable nixos-unstable

After updating the channel

sudo nix-channel --update nixos-unstable

queries via nix-env will show packages from both stable and unstable. Use this to install unstable packages into your user environment. The following snippet shows how this can be done in configuration.nix.

{ config, pkgs, ... }:
  unstable = import <nixos-unstable> {};
in {
  environment.systemPackages = [ unstable.PACKAGE_NAME ];

This only changes what version of PACKAGE_NAME is available on $PATH. If the package you want to take from unstable is installed through a NixOS module, you must use overlays:

{ config, pkgs, ... }:
  unstable = import <nixos-unstable> {};
in {
  nixpkgs.overlays = [
    (self: super: {
       PACKAGE_NAME = unstable.PACKAGE_NAME;

Note that this will rebuild all packages depending on the overlaid package, which may be a lot. Some modules offer a services.foo.package to change the actual derivation used by the module without and overlay, and without recompiling dependencies (example).

If you want to install unfree packages from unstable you need to also set allowUnfree by replacing the import statment above with:

import <nixos-unstable> { config = { allowUnfree = true; }; }

I'm unable to connect my USB HDD | External HDD is failing to mount automatically

Note: If you're using a kernel with at least version 5.6, you don't need to explicitly add this.

exfat is not supported in NixOS by default - since there are legality issues still with exFAT filesystem.

su nano /etc/nixos/configuration.nix

Add this line to your configuration file.

boot.extraModulePackages = [ config.boot.kernelPackages.exfat-nofuse ];

After saving the file rebuild NixOS:

nixos-rebuild switch

Restart NixOS.

What is the origin of the name Nix

The name Nix is derived from the Dutch word niks, meaning nothing;build actions do not see anything that has not been explicitly declared as an input > Nix: A Safe and Policy-Free System for Software Deployment, page 2


  1. https://logs.nix.samueldr.com/nixos/2017-12-05#745406
  2. https://logs.nix.samueldr.com/nixos/2018-04-18#1524080358-1524080429;
  3. https://logs.nix.samueldr.com/nixos/2017-10-18#1508341107-1508341130;