Flakes

Updated: July 17, 2026

Flakes help create standalone builds whether a single program or an entire OS.

Table of Contents

• Basic Flake
• Lock
• Hashes

It is the replacement of using channels. You are using either one or the other.
Channels work fine but flakes gives more granular control.

There are a few things to note about flakes:

• Beware of perpetuation, may cause having many instances of nixpkgs.
• dependencies should not have their own nixpkgs.
• Avoid using overlays, use composition over inheritance instead.
• Use nix follows will reduce nixpkgs instances. Less compute if less sources.

Basic flake

{
  description = "flake for cross platform";
  inputs = {
    nixpkgs.url = "githut:NixOS/nixpkgs/nixpkgs-unstable";
  };

  outputs = { self, nixpkgs }:
    let
      supportedSystems = [ "x86_64-darwin" "x86_64-linux" "aarch64-darwin" "aarch64-linux" ]:
      forAllSystems = nixpkgs.lib.genAttrs supportedSystems;
  };
}

In regard to items above: when we declare a default it is referring to self and pointing to an input.

{ }:              # set and semicolon means it is a function
self              # is a flake listed as a parameter, currently looking at
nixpkgs           # also a flake listed as a parameter, but comes from inputs

Lock

# update all flake inputs
nix flake update

# update inputs for commit changes
nix flake update --commit-lock-file

# update a specific input in the flake
nix flake lock --update-input <input>

Hashes

Sometimes you need a hash during the build of a flake.
You cant get ahead of time so use lib.fakeHash.

{lib, ...}:
{
  services.somesuch = {
    hash = lib.fakeHash;
  };
}

This will produce an error that is actually helpful. It will say while building
that your hash is wrong and got out:
place the correct hash where lib.fakeHash is and remove lib if nothing else uses it.