FAQ: Difference between revisions

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.

* https://greasyfork.org/en/scripts/495011-redirect-to-wiki-nixos-org (trivial userscript to redirect nixos.wiki links here)

== 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).

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

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

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

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

switching from generation 20 to 18

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

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? ==

See also [[Debug Symbols]]

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

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

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== What are channels and how do they get updated? ==

{{main|Channel branches}}

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? ==

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.

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

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

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

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 <code>configuration.nix</code> via: <code> mypackage-old = pkgs.callPackage ./mypackage-old.nix {};</code>.You can try to build the package as described in [[FAQ#I.27m_working_on_a_new_package.2C_how_can_I_build_it_without_adding_it_to_nixpkgs.3F|the FAQ: building a single derivation]].

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

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

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

Or, use <code>lib.fakeHash</code> as the fetcher's hash argument, and attempt to build; Nix will tell you the actual and expected hash's mismatch, and you may copy the actual hash.

== 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 ==

For connecting to your WiFi, see [[NixOS_Installation_Guide#Wireless]]

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

<syntaxhighlight lang="bash">nixos-rebuild switch --option binary-caches ''</syntaxhighlight>

== How do I enable sandboxed builds on non-NixOS? ==

See [[Nix package manager#Sandbox_builds]] for more details.

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

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

Restart NixOS.

== What is the origin of the name "Nix" ==

[[Category:Cookbook]]