Nix Cook Book


                           tropf


                          ABSTRACT

          My personal recipes when using nix <https://
     nixos.org/>.


1.  Intro

     Nix refers to the operating system NixOS,  the  package
manager  and  associated  repository  nixpkgs, and the func-
tional language nix itself.  I use all three, because I like
its focus towards reproducability.

     It  declarative  approach  breaks  with many habits and
also tools, so in this document I attempt to make some notes
how to tackle challenges I encountered in using nix.

2.  Parameterized Packages in Flakes

     To  create multiple versions of the same package (deri-
vation) with different parameters use callPackage.  Note the
trailing {} to immedeately create a valid derivation.


     mypkg = pkgs.callPackage ({doCheck ? false}: stdenv.mkDerivation rec {
       inherit doCheck;
       checkTarget = "test";

       cmakeFlags = (if doCheck then [
         "-DBUILD_TESTING=TRUE"
       ] else []);
     }) {};

     mypkg_with_tests = selfpkgs.mypkg.override {doCheck = true;};


3.  Include nix Include Dirs in Exported Compile Commands

     When  exporting  cmake compile commands the nix-defined
include dirs (extracted by querying g++)  will  not  be  in-
cluded.   You  can manually add them with a (hacky) sed com-
mand.





                      13 September 2023





                             -2-


     > This modifies JSON with sed and is evil. Use  at
     your own risk.


     nix develop && cd build
     cmake -DCMAKE_EXPORT_COMPILE_COMMANDS=ON .
     sed -e "s,$(which g++),$(which g++) $(g++ -xc++ -E -Wp,-v /dev/null 2>&1 | sed -n '/^ /{s/^ / -I/;H};${x;s/\n//g;p}'),g" -i compile_commands.json

     # note: this is maybe already called automatically by your editor
     rc -J .


4.  Creating Debuggable Builds

     By  default,  nix  will  enable  all sorts of optimiza-
tions/security options ("fortifications") in C/C++ programs.
This  can  make  debugging  very  hard, as it optimizes code
out/reorders your code, even using Debug cmake build type.

     To disable these modifications set: hardeningDisable  =
[ "all" ]
In full context this may be used as such:


     buildInputs = [
         cmake
         gcc
     ] ++ (if doCheck then [
         catch2
         cmakeCurses
         gdb
     ] else []);

     cmakeFlags = (if doCheck then [
         "-DBUILD_TESTING=TRUE"
         "-DCMAKE_BUILD_TYPE=Debug"
     ] else []);

     hardeningDisable = if doCheck then [ "all" ] else [];


5.  Build without Tests, but Develop with Tests

     You can specify different targets for nix build and nix
develop such that they refer to different deviations:


     defaultPackage.x86_64-linux = mypkg;
     devShells.x86_64-linux.default = mypkg_with_tests;








                      13 September 2023





                             -3-


6.  Ensure Locales Exist

     The CI infrastructure I use lacks some  locales.   When
building  plots  etc., the font handling doesn't work due to
these missing locales.  Disturbingly, my local nix build in-
vocation  finds the locales and looks completely fine, while
the CI version does not.  To  fix  this  set  the  following
variables inside the stdenv invocation:


     LOCALE_ARCHIVE = "${glibcLocales.override {allLocales = false; locales=["en_US.UTF-8/UTF-8"];}}/lib/locale/locale-archive";
     LC_ALL = "en_US.utf8";


7.  Load System Config From Flake

     To  investigate  the configuration generated by a flake
run (in the directory with the system's flake.nix):


     $ nix repl
     nix-repl> :lf .
     Added 13 variables.
     nix-repl> nixosConfigurations.kurt.config.programs.git.enable
     true


8.  nixos-shell Base Image

     The command nixos-shell spawns a shell with a nixos VM.
One might view it as nix shell on steroids.  It is available
on github <https://github.com/Mic92/nixos-shell/> and can be
installed  directly through nixpkgs.  (I.e., there is no ex-
tra option/module required.)

     The following is a base configuration that  I  commonly
use.  It mounts the current directory under /mnt, configures
nix to enable flake support, and adds some packages.  It in-
cludes  a  normal  (non-root)  user  account (useful for the
mount).

















                      13 September 2023





                             -4-


     { pkgs, ... }: {
       boot.kernelPackages = pkgs.linuxPackages_latest;

       nixos-shell.mounts = {
         mountHome = false;
         extraMounts = {
           "/mnt" = ./.;
         };
       };

       users.users.mensch.isNormalUser = true;
       nix = {
         package = pkgs.nixUnstable;
         settings.trusted-users = [ "root" "user" ];
         extraOptions = ''
           experimental-features = nix-command flakes
         '';
       };
       programs.git.enable = true;
       environment.systemPackages = with pkgs; [
         moreutils tree jq htop
       ];
     }


































                      13 September 2023