FAQ/en: Difference between revisions

   };

}</syntaxhighlight>

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

<syntaxhighlight lang="bash">nixos-rebuild switch</syntaxhighlight>

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

<build output and list of successfully realised paths>

</syntaxhighlight>

Repeat for your user and further profiles:

<syntaxhighlight lang="bash">nix-store -qR ~/.nix-profile | xargs nix-store -r</syntaxhighlight>

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

20  2014-08-12 19:09:20  (current)

</syntaxhighlight>

To rollback to the previous generation:

switching from generation 20 to 19

</syntaxhighlight>

To switch to a particular generation:

<syntaxhighlight lang="bash">nix-build -E 'with import <nixpkgs> { }; callPackage ./mypackage.nix { }'</syntaxhighlight>

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.

<syntaxhighlight lang="bash">sudo nix-build --check -A ncdu</syntaxhighlight>

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

=== 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 <code>/lib</code>. However this assumption is incorrect on NixOS systems due to the inner workings of <code>nix</code> - there is no default path, everything gets set to the corresponding version on compile time.

   '';

}</syntaxhighlight>

This can be built with:

<syntaxhighlight lang="bash">nix-build mybinaryprogram.nix</syntaxhighlight>

And run with:

<syntaxhighlight lang="bash">./result/bin/mybinaryprogram</syntaxhighlight>

Another possibility is using a FHS-compatible Sandbox with [https://nixos.org/nixpkgs/manual/#sec-fhs-environments buildFHSUserEnv]

   runScript = "bash";

}).env</syntaxhighlight>

the sandbox can be entered with

<syntaxhighlight lang="bash">nix-shell fhsUser.nix</syntaxhighlight>

If your target application can't find shared libraries inside buildFHSUserEnv, you may run [https://github.com/lexleogryfon/de-generate 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|Channel branches}}

* For nixos-small: the unstable-small [http://hydra.nixos.org/job/nixos/unstable-small/tested tested] job.

* For nixpkgs: the trunk [http://hydra.nixos.org/job/nixpkgs/trunk/unstable unstable] job, which contains some critical release packages.

<ol start="2" style="list-style-type: decimal;">

<li>Once the job succeeds at a particular nixpkgs commit, '''cache.nixos.org''' will download binaries from '''hydra.nixos.org'''.</li>

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.

<syntaxhighlight lang="bash">nix-instantiate --find-file nixpkgs</syntaxhighlight>

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.

<syntaxhighlight lang="bash">sudo unshare -m bash</syntaxhighlight>

remount the filesystem with write privileges (as root)

<syntaxhighlight lang="bash">mount -o remount,rw /nix/store</syntaxhighlight>

update the file

<syntaxhighlight lang="bash">nano <PATH_TO_PACKAGE>/default.nix</syntaxhighlight>

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

<syntaxhighlight lang="bash">exit</syntaxhighlight>

Be sure to [https://github.com/NixOS/nixpkgs/issues report the incorrect url] or [https://github.com/NixOS/nixpkgs/pulls fix it yourself].

build-sandbox-paths = $(nix-store -qR $(nix-build '<nixpkgs>' -A bash) | xargs echo /bin/sh=$(nix-build '<nixpkgs>' -A bash)/bin/bash)

</syntaxhighlight>

On NixOS set the following in ''configuration.nix'':

<syntaxhighlight lang="bash">sudo nix-channel --add https://nixos.org/channels/nixos-unstable nixos-unstable</syntaxhighlight>

After updating the channel

<syntaxhighlight lang="bash">sudo nix-channel --update nixos-unstable</syntaxhighlight>

queries via <code>nix-env</code> 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''.

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

<syntaxhighlight lang="nix">

{ config, pkgs, ... }:

}

</syntaxhighlight>

Note that this will rebuild all packages depending on the overlaid package, which may be a lot. Some modules offer a <code>services.foo.package</code> to change the actual derivation used by the module without and overlay, and without recompiling dependencies ([https://nixos.org/manual/nixos/stable/options.html#opt-services.gvfs.package example]).

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

<syntaxhighlight lang="nix">

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

Add this line to your configuration file.

<syntaxhighlight lang="bash">boot.extraModulePackages = [ config.boot.kernelPackages.exfat-nofuse ];</syntaxhighlight>

=== What does it mean to say that NixOS is "immutable" ===

Immutability is a property of data, in general, which means that the data cannot be modified after it is created. In the context of an operating system, it really means that certain parts of the system have this property. In the case of Nix and NixOS, that includes the Nix store, where files can be created but not modified after the time they are created. It does not apply to every part of the operating system, in that users can still modify their own files in their home directory, for example.