2020-10-24 22:55:15 -05:00
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# nixos-flake-example
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This shows how to build the same config, with and without flakes.
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It also shows that `flake.nix` is basically just some syntax.
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2020-10-24 23:28:56 -05:00
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## flake fundamentals
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Nix is in flakes mode when:
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1. `nixos-rebuild <cmd> --flake '.#'` is used
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2. `nix build '.#something'` the hash-tag syntax is used
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**This automatically loads from a `flake.nix` in the specified dir.**
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If the flake.nix were located elsewhere you could do this too:
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* `nix build '/home/cole/code/nixos-flake-example#something` (loads from `/home/cole/code/nixos-flake-example`)
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* `nix build 'github.com:colemickens/nixos-flake-example#something` (loads from `flake.nix` in the root of that repo checkout)
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Those are just examples of syntax. Again,
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2020-10-24 22:55:15 -05:00
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## overview
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Note that these produce the same output:
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1. with flakes:
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```shell
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nix build '.#nixosConfigurations.mysystem.config.system.build.toplevel'
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readlink -f result
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/nix/store/0imi716z1qd04pfh4zdw6mb0gnxmakjs-nixos-system-nixos-21.03.20201020.007126e
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nixos-rebuild build --flake '.#mysystem'
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readlink -f result
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/nix/store/0imi716z1qd04pfh4zdw6mb0gnxmakjs-nixos-system-nixos-21.03.20201020.007126e
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```
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Note, nixos-rebuild is basically just some magic to build the right derivation
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and then set it as a system profile, and activate it.
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2. without flakes:
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```shell
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export NIX_PATH=nixos-config=$(pwd)/configuration.nix:nixpkgs=https://github.com/nixos/nixpkgs/archive/nixos-unstable.tar.gz
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/nix/store/zidq625i13hvbbs8alkklj8k6a191xix-nixos-system-nixos-21.03pre-git
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```
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**Note**, ~~same path~~ same inner system, just much slower due to no eval cache.
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(they should be identical, but the flake version suffix is slightly different)
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They build the same thing, the flake.nix just moves the redirection from the NixOS channel system
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into the flake instead.
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Note, if you come back and run this later, you may need to tell nix to update the `nixpkgs` that it
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has pinned in `flake.lock` by running `nix flake update --update-input nixpkgs`. The non-flake example
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is going to re-download the nixos-unstable build when the cache expires. This could cause any hash differences
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if they're on different revs. (again, another reason to have control of it via flakes, and can lock it directly in the source.)
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