Nix_repl_tips

May 14, 2025

Nix Repl List available commands

Nix Reference Manual Nix Repl
The command: nix repl - starts an interactive environment for evaluating Nix expressions.
This command provides an interactive environment for evaluating Nix expressions. (REPL stands for ‘read–eval–print loop’.)

On startup, it loads the Nix expressions named files and adds them into the lexical scope. You can load addition files using the :l <filename> command, or reload all files using :r.

List available commands with :?:

nix repl
Nix 2.24.11
Type :? for help.
nix-repl> :?

Load Nix expressions Directly

You can quickly evaluate a random Nix expression:

nix repl --expr '{a = { b = 3; c = 4; }; }'

Welcome to Nix 2.11.0. Type :? for help.

Loading installable ''...
Added 1 variables.
nix-repl> a
{ b = 3; c = 4; }

The --expr flag is helpful to prime directly the Nix REPL with valuable data or values.

nix repl --expr 'import <nixpkgs>{}'
nix-repl> helix.name
"helix-25.01.1"
nix-repl> helix.drvPath
"/nix/store/1rxncc2jk2l2gc1f8m0442ak1yn1s5gj-helix-25.01.1.drv"
nix-repl> drv = runCommand "hello" { buildInputs = [ hello ]; } "hello; hello > $out"
nix-repl> :b drv
nix-repl> builtins.readFile drv
"Hello, world!\n"
nix-repl> :log drv
Hello, world!

:b is for build derivation

Load Flakes

We can use the --expr flag to load a random Nix Flake directly:

nix repl --expr 'builtins.getFlake "github:nix-community/ethereum.nix"'

Also, you can load a flake directly inside the REPL with :load-flake or :lf:

nix repl

nix-repl> :lf github:nix-community/home-manager
# or
nix-repl> :lf /path/to/your/flake

Debugging with a Flake REPL output

One way to do this is to launch the repl with nix repl and inside the repl type :lf /path/to/flake. Or nixos-rebuild repl --flake /path/to/flake the latter provides a helpful welcome script showing what is loaded into your repl’s scope.

I like to create a simple repl output to load your flake into the environment with nix repl .#repl.

First, we’ll create a REPL environment to inspect and debug our flake’s outputs, packages, and configurations. Define a repl output in flake.nix for easy access with nix repl .#repl:

# flake.nix
outputs = { self, nixpkgs, ... }: let
  pkgs = import nixpkgs { system = "x86_64-linux"; };
in {
  repl = import ./repl.nix {
    inherit (pkgs) lib;
    flake = self;
    pkgs = pkgs;
  };
};

And in repl.nix:

# repl.nix
{
  lib,
  flake,
  pkgs,
}: {
  inherit flake pkgs lib;
  configs = flake.nixosConfigurations;
  # inherit (flake.outputs) userVars;
}
# Accepts `lib`, `flake`, `pkgs` from `flake.nix` as arguments
# Attributes: flake: all flake outputs (flake.outputs, flake.inputs)
# run `nix repl .#repl` to load the REPL environment
# :l <nixpkgs>  # load additional Nixpkgs if needed
# :p flake.inputs.nixpkgs.rev # nixpkgs revision
# :p flake.inputs.home-manager.rev
# flake.outputs.packages.x86_64-linux.default # inspect default package
# pkgs.helix # access helix package
# lib.version # check lib version
# configs.magic.config.environment.systemPackages # list packages
# configs.magic.config.home-manager.users.jr.home.packages # home packages
# :p configs.magic.config.home-manager.users.jr.programs.git.userName
# Debugging
# :p builtins.typeOf configs.magic (should be `set`)
# :p builtins.attrNames configs.magic
# :p configs.magic.config # errors indicate issues
# :p configs.magic.config.environment # isolate the module or issue
# :p builtins.attrNames configs.magic.config.home-manager.users.jr # home attrs
# :p configs.magic.config.home-manager.users.jr.programs.git.enable # true/false
#  :p lib.filterAttrs (n: v: lib.hasPrefix "firefox" n) pkgs
# :p configs.magic.config.stylix # check theming
# :p configs.magic.config.home-manager.users.jr.stylix
# :p lib.mapAttrsToList (name: cfg: name) configs

❗: Replace magic with your host name

Usage

Load REPL environment with: nix repl .#repl

Attributes:

nix-repl> builtins.attrNames flake.inputs
[
  "dont-track-me"
  "helix"
  "home-manager"
  "hyprland"
  "neovim-nightly-overlay"
  "nixpkgs"
  "nvf"
  "rose-pine-hyprcursor"
  "stylix"
  "treefmt-nix"
  "wallpapers"
  "wezterm"
  "yazi"
]
nix-repl> builtins.attrNames flake.outputs
[
  "checks"
  "devShells"
  "formatter"
  "nixosConfigurations"
  "packages"
  "repl"
]
nix-repl> flake.outputs.formatter
{
  x86_64-linux = «derivation /nix/store/q71q00wmh1gnjzdrw5nrvwbr6k414036-treefmt.drv»;
}

Inspect the default package output:

nix-repl> flake.outputs.packages.x86_64-linux.default
«derivation /nix/store/6kp660mm62saryskpa1f2p6zwfalcx2w-default-tools.drv»

From here out I’ll leave out the nix-repl> prefix just know that it’s there.
Check lib version(Nixpkgs lib attribute):

lib.version
"25.05pre-git"

List systemPackages and home.packages, my hostname is magic list yours in its place:

configs.magic.config.environment.systemPackages
# list home.packages
configs.magic.config.home-manager.users.jr.home.packages

Or an individual value:

:p configs.magic.config.home-manager.users.jr.programs.git.userName
TSawyer87

Debugging

Check if the module system is fully evaluating, anything other than a “set” the configuration isn’t fully evaluated (e.g. “lambda” might indicate an unevaluated thunk):

:p builtins.typeOf configs.magic
set

Debugging Module System:
Check if configs.magic is a valid configuration:

:p builtins.attrNames configs.magic