NixOS notes

    Nix is an... interesting language, so these are my notes/thoughts along the way

    Right so nixos deserves its own file for all its... intricacies

    Syntax (i always forget)

    Angular bracket syntax

    Repl and Installables

    Repl

    According to the docs from nix repl --help

    Synopsis

    nix repl [option...] installables...

    The arg after nix repl is installables.

    Reading the docs for installables here, it is of the form `flakeref[ # attrpath ],

    e.g. nixpkgs#hello, or as the #attrpath is optional i guess just nixpkgs, where we can call nix repl 'nixpkgs'

    Flake show

    Now, nix flake show shows the outputs of a flake, so we can actually see with nixpkgs (i've trimmed the output as its long)

    ➜ joelyboy dev-setup (main) ✗ nix flake show nixpkgs
path:/nix/store/bgl6ldj5ihbwcq8p42z3a0qzgqafgk2b-source?lastModified=0&narHash=sha256-LwWRsENAZJKUdD3SpLluwDmdXY9F45ZEgCb0X%2BxgOL0%3D
├───htmlDocs: unknown
    ...
    ...
├───legacyPackages
│   ├───aarch64-darwin omitted (use '--legacy' to show)
│   ├───aarch64-linux omitted (use '--legacy' to show)
│   ├───armv6l-linux omitted (use '--legacy' to show)
│   ├───armv7l-linux omitted (use '--legacy' to show)
│   ├───i686-linux omitted (use '--legacy' to show)
│   ├───powerpc64le-linux omitted (use '--legacy' to show)
│   ├───riscv64-linux omitted (use '--legacy' to show)
│   ├───x86_64-darwin omitted (use '--legacy' to show)
│   ├───x86_64-freebsd omitted (use '--legacy' to show)
│   └───x86_64-linux omitted (use '--legacy' to show)
├───lib: unknown
└───nixosModules
    ├───notDetected: NixOS module
    └───readOnlyPkgs: NixOS module
➜ joelyboy dev-setup (main) ✗

    Can see now the legacyPackages attribute is loaded into the repl! which is from nixpkgs.

    ➜ joelyboy dev-setup (main) ✗ nix repl nixpkgs
Nix 2.28.4
Type :? for help.
Loading installable 'flake:nixpkgs#'...
Added 7 variables.
nix-repl> legacyPackages
{
  aarch64-darwin = { ... };
  aarch64-linux = { ... };
  armv6l-linux = { ... };
  armv7l-linux = { ... };
  i686-linux = { ... };
  powerpc64le-linux = { ... };
  riscv64-linux = { ... };
  x86_64-darwin = { ... };
  x86_64-freebsd = { ... };
  x86_64-linux = { ... };
}

nix-repl>

    (FROM CLAUDE)

    When you run nix repl nixpkgs, here's what happens:

    1. "nixpkgs" is a flake ID that gets resolved via the registry to github:NixOS/nixpkgs
    2. The flake itself has outputs like packages, legacyPackages, etc.
    3. In the REPL, you're interacting with the flake's outputs, not the flake itself

    So when you type nixpkgs in commands:

    • nix repl nixpkgs → loads the entire flake's outputs
    • nixpkgs#hello → means "the hello attribute from nixpkgs flake's outputs"
    • The # separates flake reference from attribute path

    The confusion comes because:

    • nixpkgs (the flake) contains legacyPackages.x86_64-linux with thousands of packages
    • When you load it in REPL, you see those packages directly
    • But nixpkgs itself is the flake, not an output attribute

    Lookup syntax

    Firstly, the nix "lookup syntax" <...> is sometimes used with nixpkgs, but I'm gonna write a simple example here to help myself understand it

    Nix expressions expect a default.nix file (or a flake.nix for newer ones?), see docs on default.nix here.

    This is a super simple example of looking up my example default.nix file with the greeting variable and evaluating it

    In this case, I did a load of my module mynixdir and then executed greeting (which was made available as a string from inside mynixdir)

    ➜ jollof dev-setup (main) ✗ mkdir -p /tmp/mynixdir
➜ jollof dev-setup (main) ✗ echo -e '{\n greeting = "Hello there!";\n }' > /tmp/mynixdir/default.nix
➜ jollof dev-setup (main) ✗ nix eval --file /tmp/mynixdir greeting
"Hello there!"
➜ jollof dev-setup (main) ✗

    Or, to demonstrate an import, and allocating it to a var and getting the value of greeting!

    ➜ joelyboy dev-setup (main) ✗ nix repl
Nix 2.28.4
Type :? for help.
nix-repl> import /tmp/mynixdir
{ greeting = "Hello there!"; }

nix-repl> myvar=(import /tmp/m
/tmp/mkdp-nvim.log  /tmp/mynixdir
nix-repl> myvar=(import /tmp/mynixdir)

nix-repl> myvar.greeting
"Hello there!"

nix-repl>

    Loading

    :l loads a file in the repl, AND makes all vars available (unlike import which must be assigned to a variable)

    e.g. see how myAge is loaded into available variables

    joelyboy@desktop-work ~/c/dev-setup (main)> echo "{myAge=31;}" > /tmp/testie.nix
joelyboy@desktop-work ~/c/dev-setup (main)> nix repl
Nix 2.28.4
Type :? for help.
nix-repl> :l /tmp/testie.nix
Added 1 variables.

nix-repl> myAge
31

nix-repl>

    Lambda functions

    e.g.

    joelyboy@desktop-work ~/c/dev-setup (main)> nix repl
Nix 2.28.4
Type :? for help.
nix-repl> myFunc = { name }: "Hello ${name}"

nix-repl> myFunc { name = "jollof"; }
"Hello jollof"

nix-repl>

    Nix override attributes

    the // operator overrides attributes e.g.

    joelyboy@desktop-work ~/c/dev-setup (main)> nix repl
Nix 2.28.4
Type :? for help.
nix-repl> me = { name="jollof"; age=31;}

nix-repl> me // {age=40;}
{
  age = 40;
  name = "jollof";
}

nix-repl>

    Curried functions

    Basically chained function creation e.g. create a function which creates ANOTHER function to add a set number

    joelyboy@desktop-work ~/c/dev-setup (main)> nix repl
Nix 2.28.4
Type :? for help.
nix-repl> createAdder = numberToAdd : value : value + numberToAdd

nix-repl> add5 = createAdder 5

nix-repl> add5 12
17

nix-repl> add3 = createAdder 3

nix-repl> add3 12
15

nix-repl>

    Nix derivations

    I'm not hoenstly sure exactly what they, except that they are some sort of enclased part of a function/package that runs/does something??

    Anyway, an example to create a derivation, "instantiate" it to the store

    A note on heredoc Anyway, continuing...

    ➜ jollof mynixdir cat > simple.nix << 'EOF'
with import <nixpkgs> {};
derivation {
  name = "simple";
  system = "x86_64-linux";
  builder = "${bash}/bin/bash";
  args = [ "-c" "echo hello > $out" ];
}
EOF
➜ jollof mynixdir nix-build simple.nix
this derivation will be built:
  /nix/store/ycdgnli9la2nhmrclldwv3mcckzb0684-simple.drv
building '/nix/store/ycdgnli9la2nhmrclldwv3mcckzb0684-simple.drv'...
/nix/store/i538l7xjp5d5sq5lr9v35pg34b6fq0mx-simple
➜ jollof mynixdir ll
lrwxrwxrwx   - jollof  9 Aug 19:24 result -> /nix/store/i538l7xjp5d5sq5lr9v35pg34b6fq0mx-simple
.rw-r--r-- 158 jollof  9 Aug 19:24 simple.nix
➜ jollof mynixdir cat result
hello
➜ jollof mynixdir readlink -f result
/nix/store/i538l7xjp5d5sq5lr9v35pg34b6fq0mx-simple
➜ jollof mynixdir cat $(readlink -f result)
hello
➜ jollof mynixdir

    Nix building

    mkdir -p /tmp/testieflake
cat > /tmp/testieflake/flake.nix << 'EOF'
{
  description = "Super simple flake";
  
  inputs = {};  # No inputs at all!
  
  outputs = { self }: {
    packages.x86_64-linux.hello = derivation {
      name = "hello";
      system = "x86_64-linux";
      builder = "/bin/sh";
      args = [ "-c" "echo 'Hello from a flake!' > $out" ];
    };
  };
}
EOF
cd /tmp/testieflake
nix build .#hello
cat result

    and then we get!

    ➜ joelyboy testieflake cd /tmp/testieflake
nix build .#hello
cat result

➜ joelyboy testieflake

    and then we can import it into another flake, this is where the url style syntax comes into flakes!

    ➜ joelyboy myotherflake pwd
/tmp/myotherflake
➜ joelyboy myotherflake cat flake.nix
# /tmp/myotherflake/flake.nix
{
  description = "Simpler version";

  inputs = {
    mytestie.url = "/tmp/testieflake";
  };

  outputs = { self, mytestie }: {
    packages.x86_64-linux.combined = mytestie.packages.x86_64-linux.hello;
  };
}
➜ joelyboy myotherflake nix build .#combined --impure
➜ joelyboy myotherflake cat result
Hello from a flake!
➜ joelyboy myotherflake cat flake.lock
{
  "nodes": {
    "mytestie": {
      "locked": {
        "lastModified": 1758017036,
        "narHash": "sha256-xgTGb89vbd5jNRibieQ6ksc2erfI32l0s1T/pL+AHi0=",
        "path": "/tmp/testieflake",
        "type": "path"
      },
      "original": {
        "path": "/tmp/testieflake",
        "type": "path"
      }
    },
    "root": {
      "inputs": {
        "mytestie": "mytestie"
      }
    }
  },
  "root": "root",
  "version": 7
}
➜ joelyboy myotherflake

    Nix inspection (nixpkgs and more)

    A good one is nix flake metadata after building this to see what nixpkgs is pinned to

    ➜ jollof dev-setup (main) ✗ nix flake metadata
warning: Git tree '/home/jollof/coding/dev-setup' is dirty
Resolved URL:  git+file:///home/jollof/coding/dev-setup
Description:   NixOS configuration of joel
Path:          /nix/store/p60sr5h6pgfa66b59n6cllmb4iixcz83-source
Revision:      635c008bcbb3f3080a9539f47c498da25b602375-dirty
Last modified: 2025-10-23 15:04:13
Fingerprint:   bc01275b322600e68894578c978ba61dc745fb529f73c90904a4271aab9b0dfe
Inputs:
├───nixpkgs: github:NixOS/nixpkgs/9a094440e02a699be5c57453a092a8baf569bdad?narHash=sha256-Vp9K5ol6h0J90jG7Rm4RWZsCB3x7v5VPx588TQ1dkfs%3D (2025-09-14 00:35:52)
├───nixpkgs-unstable: github:NixOS/nixpkgs/e643668fd71b949c53f8626614b21ff71a07379d?narHash=sha256-NY3kSorgqE5LMm1LqNwGne3ZLMF2/ILgLpFr1fS4X3o%3D (2025-09-24 05:06:22)
└───nur: github:nix-community/NUR/402529374b6642ed0a3c6e7cd26f47b475bb46ea?narHash=sha256-oP/0YzBq2TzFSgAOxKWxhnDgn1OYX0CfX6huYhdtXWE%3D (2025-10-24 12:40:00)
    ├───flake-parts: github:hercules-ci/flake-parts/205b12d8b7cd4802fbcb8e8ef6a0f1408781a4f9?narHash=sha256-4pDvzqnegAfRkPwO3wmwBhVi/Sye1mzps0zHWYnP88c%3D (2024-12-04 11:43:21)
    │   └───nixpkgs-lib follows input 'nur/nixpkgs'
    └───nixpkgs follows input 'nixpkgs'
➜ jollof dev-setup (main) ✗

    And can do registry list

    ➜ jollof dev-setup (main) ✗ nix registry list -v
system flake:nixpkgs path:/nix/store/805qxiz47zwd8hm35hbpk3ifr8slpj2r-source
global flake:agda github:agda/agda
global flake:arion github:hercules-ci/arion
global flake:blender-bin github:edolstra/nix-warez?dir=blender
global flake:bundlers github:NixOS/bundlers
global flake:cachix github:cachix/cachix
global flake:composable github:ComposableFi/composable
...