Nix Cookbook

From NixOS Wiki
Note: This is a Cookbook page. It serves as a collection of topical, often question and answer style, information. Information should be well categorized for easy reference.

Managing storage

Reclaim space on Nix install?

TL&DR: nix-collect-garbage --delete-older-than 7d

Remove old generations

When you make changes to your system, Nix creates a new system generation. All of the changes to the system since the previous generation are stored there. Old generations can add up and will not be removed automatically by default. You can see your saved system generations with:

  # nix-env --profile /nix/var/nix/profiles/system --list-generations

To keep just your current generation and the two older than it:

  # nix-env --profile  --delete-generations +3

To remove all but your current generation:

  # nix-env --profile  --delete-generations old

Apart from the system profile in /nix/var/nix/profiles/system, every user has profiles for their user environment and channels. The operations above may be repeated for those profiles. By default, they are located at ~/.nix-profile and ~/.nix-defexpr/channels. For more information on profile locations, see Nix Manual - nix-env - Files.

Generation trimmer script

For a smart interactive script which can handle all the normally available profile types across NixOS and be more conservative and safe than the built-in Nix generations deletion commands, see NixOS Generations Trimmer.

Garbage collection

As you work with your system (installs, uninstalls, upgrades), files in the Nix store are not automatically removed, even when no longer needed. Nix instead has a garbage collector which must be run periodically (you could set up, e.g., a cron to do this).

  $ nix-collect-garbage

This is safe so long as everything you need is listed in an existing generation or garbage collector root (gcroot).

If you are sure you only need your current generation, this will delete all old generations and then do garbage collection:

  $ nix-collect-garbage -d

On NixOS, you can enable a service to automatically do daily garbage collection:

  nix.gc.automatic = true;


You may wind up with duplicate files in your Nix store. Data deduplication is a resource intense process while running, so is not done automatically by default. Often you can save about 25-35% of your store space by optimizing the store though. This will perform a deduplication process on your Nix store (hard link duplicates together):

  $ nix store optimise

With standalone Nix, you can set the nix.conf option below to set this to happen periodically:

  auto-optimise-store = true

For NixOS, the option to set this is:

/etc/nixos/configuration.nix = true;

NixOS will update nix.conf for you, with that setting.

Nix manual references

Deeper cleaning

Environment tasks

Creating shell scripts

Arbitrary system shell scripts can be created with pkgs.writeShellScriptBin. It creates a derivation which you add to environment.systemPackages.

{ pkgs, ... }:

  helloWorld = pkgs.writeShellScriptBin "helloWorld" ''
    echo Hello World

in {
  environment.systemPackages = [ helloWorld ];

Creating periodic services

Using the systemd support periodic services can be defined. In this case a service named simple-timer writes out the current time to /tmp/simple-timer.log every minute.

{ pkgs, ... }:

  systemd = {
    timers.simple-timer = {
      wantedBy = [ "" ];
      partOf = [ "simple-timer.service" ];
      timerConfig.OnCalendar = "minutely";
    services.simple-timer = {
      serviceConfig.Type = "oneshot";
      script = ''
        echo "Time: $(date)." >> /tmp/simple-timer.log

Wrapping packages

If you need to wrap a binary of a package (or a non-binary), there are a few ways of doing it. The simplest of which is just creating a new binary that calls the old one:

pkgs.writeShellScriptBin "hello" ''
  # Call hello with a traditional greeting 
  exec ${pkgs.hello}/bin/hello -t

The disadvantage of this way is that it doesn't propagate man pages and other paths from the old derivation. There are multiple ways of solving that:

  wrapped = pkgs.writeShellScriptBin "hello" ''
    exec ${pkgs.hello}/bin/hello -t

pkgs.symlinkJoin {
  name = "hello";
  paths = [

Similarly the following works too:

pkgs.symlinkJoin {
  name = "hello";
  paths = [ pkgs.hello ];
  buildInputs = [ pkgs.makeWrapper ];
  postBuild = ''
    wrapProgram $out/bin/hello \
      --add-flags "-t"

If you prefer not to have every file symlinked and have a cleaner result, the following is also possible:

pkgs.runCommand "hello" {
  buildInputs = [ pkgs.makeWrapper ];
} ''
  mkdir $out
  # Link every top-level folder from pkgs.hello to our new target
  ln -s ${pkgs.hello}/* $out
  # Except the bin folder
  rm $out/bin
  mkdir $out/bin
  # We create the bin folder ourselves and link every binary in it
  ln -s ${pkgs.hello}/bin/* $out/bin
  # Except the hello binary
  rm $out/bin/hello
  # Because we create this ourself, by creating a wrapper
  makeWrapper ${pkgs.hello}/bin/hello $out/bin/hello \
    --add-flags "-t"

And lastly, there is the possibility of wrapping things right inside the derivation you want to wrap, this is however discouraged and impractical in most cases, as it requires recompilation of it:

pkgs.hello.overrideAttrs (oldAttrs: {
  buildInputs = oldAttrs.buildInputs or [] ++ [ pkgs.makeWrapper ];
  postInstall = oldAttrs.postInstall or "" + ''
    wrapProgram $out/bin/hello \
      --add-flags "-t"

Securing Nix

See Security


Common errors

Bad configuration option: gssapikexalgorithms

Found when using an SSH binary from Nix on typically RPM-based distros like CentOS, Fedora, Scientific Linux, Redhat, etc. Possible fixes, from least to most invasive:

  1. The quick fix: Just comment out the configuration option in the ssh config file
  2. If you want to keep the option in but don't need it to work (e.g., you're sharing a config across systems, but only use GSSAPI/Kerberos on another system): add IgnoreUnknown GSSAPI* to your ssh configuration
  3. Install the openssh_gssapi package instead of openssh. This will fix ssh used directly, but some dependencies may still use the non-GSSAPI package.
  4. Force specific other packages to build with the GSSAPI version: for example, you might add (git-repo.override { openssh = openssh_gssapi; }) to your environment.systemPackages list (if git-repo is the problematic package), or use overlays like:
      nixpkgs.overlays = [
        (final: prev: {
          mosh = prev.mosh.override { openssh = prev.openssh_gssapi; };
    (which will fix mosh used as a dependency too)
  5. Force all packages that depend on openssh to use openssh_gssapi instead:
     nixpkgs.overlays = [
       (final: prev: { openssh = prev.openssh_gssapi; } )

Desktop environment does not find .desktop files

IF your DE does not look in $HOME/.nix-profile/share for .desktop files. You need to add that path to the XDG_DATA_DIRS, the position reflects precedence so files in earlier directories shadow files in later directories. This can be accomplished in various ways depending on your login manager, see Arch wiki: Xprofile for more information. For example using ~/.xprofile as follows:

$ export XDG_DATA_DIRS=$HOME/.nix-profile/share:/usr/local/share:/usr/share

Notice that you have to include the default locations on your system, otherwise they will be overwritten. Find out the proper paths using echo $XDG_DATA_DIRS. (Note: export XDG_DATA_DIRS=$HOME/.nix-profile/share:$XDG_DATA_DIRS did not work, XDG_DATA_DIRS ended up containing only $HOME/.nix-profile/share: which isn't even a valid path.)

NOTE: The above fix will make your programs installed by nix visible in your application menu, but you still will not be able to run them, because they are symlinked outside your XDG_DATA_DIRS paths, and are not executable (one or the other criteria must be met to run the program from a menu). This impacts KDE users, and potentially others. I noticed that on native NixOS with KDE, NixOS adds all these paths for each application to one's XDG_DATA_DIRS variable.

Error: the option has conflicting definitions

If while doing a

nixos-rebuild switch

you see an error like:

building Nix...
building the system configuration...
error: The option `' has conflicting definitions, in `/nix/var/nix/profiles/per-user/root/channels/nixos/nixos/modules/
services/mail/postfix.nix' and `/etc/nixos/configuration.nix'.
(use '--show-trace' to show detailed location information)

This means exactly what it says, but how to fix it? Assuming one of the nix files is your configuration file, then you want your version to stick, and not the version from some maintainer somewhere, you can use


, which is a nix property defined in lib. so in the example above, the option in conflict is '', so to override it you would do something like: = pkgs.lib.mkOverride 0 "mynewvalue";

which will override the other value, and force yours to have priority.


License stance

Example on how to check if a given list of packages (as returned by the pkgs.nix derivation) conforms to permitted licenses criteria:

with rec {
  # Incomplete list, customize to your policies.
  permissiveLicense = v: v.license == "bsd3" || v.license == "mit" || v.license == "bsd2" || v.license == "publicDomain" || v.license == "asl20" || v.license == "zlib" || v.license == "bsdOriginal" || v.license == "openssl";

  # Omit some false-positive buildInputs like bash and perl.. those should be nativeBuildInputs rather?
  saneDep = d: d ? meta.license
      && builtins.substring 0 5 != "bash-"
      && builtins.substring 0 5 != "perl-";

  # Keep if the license is not allowed, or if has any (transitive) dep with a license that is not allowed.
  keepBadDeps = ds: builtins.filter (n: !(permissiveLicense n) || n.baddeps != []) (map derivToNode (builtins.filter saneDep ds));

  derivToNode = d: 
    { license = if builtins.typeOf d.meta.license == "string" 
                then d.meta.license
                else if builtins.typeOf d.meta.license == "list"  # can happen sometimes, could concat.. but have a look rather
                     then "MULTI"
                     else d.meta.license.shortName;
      name =;
      baddeps = keepBadDeps (builtins.filter saneDep d.buildInputs);
let ps = import ./pkgs.nix;  # pkgs.nix should result in a list of derivations to check
in keepBadDeps ps

Then exercise it in nix repl, using :p to force the result so we can actually see it:

nix-repl> xs = import ./lic.nix
nix-repl> :p xs

This will print a (somewhat unreadable) nested tree of derivation names and their licences, where (at least) at the roots there are not-allowed licenses.

Be sure to manually check them for being false positives - navigate to the derivation in the nixpkgs repo and eyeball the license info (it is updated every now and then), also cross-check with the original source to make sure.


See Vulnix.