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80 lines
3.1 KiB
Markdown
80 lines
3.1 KiB
Markdown
# nixos-flake-example
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This shows how to build the same config, with and without flakes. And shows that both builds produce the same output.
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It also shows that `flake.nix` is basically just some syntax. In this case,
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we are using flakes to bring our NixOS system config (`configuration.nix` and `hardware-configuration.nix`)
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into a single repository *along with* the `nixpkgs` reference used to build the system. Without flakes,
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`nixpkgs` is ... well, potentially unknowable.
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The point of flakes is to:
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1. remove `NIX_PATH` and all of the indirection that comes with it
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2. encourages pinning by use/creation of `flake.lock` automatically with the use of `nix` commands
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3. enables fully hermetic Nix projects were all dependencies are specified
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4. allow pure evaluation (again, enabling hermetic projects)
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5. pure evaluation allows for Nix expression caching, leading to nice UX performance gains
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You can run `./check.sh` to show that this builds the same system config
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with/without flakes.
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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. See these examples:**
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These three examples, for me, are all the same source, but accessed in different ways:
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* `nix build '.#nixosConfigurations.mysystem'`
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(loads `flake.nix` from `.` (current dir))
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* `nix build '/home/cole/code/nixos-flake-example#nixosConfigurations.mysystem'`
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(loads `flake.nix` from `/home/cole/code/nixos-flake-example`)
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* `nix build 'github.com:colemickens/nixos-flake-example#nixosConfigurations.mysystem'`
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(nix will clone my github repo, then load `flake.nix` from `flake.nix` in the root of that repo checkout)
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## more tips
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1. `nixos-rebuild build --flake '.#'` will automatically try to find and build the attribute: `.#nixosConfigurations.your_hostname` (assuming your machines hostname is `your_hostname`)
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## overview
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The `./check.sh` script proves they give the same exact system toplevel outputs:
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```console
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cole@slynux ~/code/nixos-flake-example master* 7s
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❯ ./check.sh
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:: Updating the 'nixpkgs' input in flake.nix
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+ nix flake update --update-input nixpkgs
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+ set +x
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:: Using 'nixos-rebuild' to build the 'mysystem' toplevel
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+ nixos-rebuild build --flake .#mysystem
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warning: Git tree '/home/cole/code/nixos-flake-example' is dirty
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building the system configuration...
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warning: Git tree '/home/cole/code/nixos-flake-example' is dirty
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+ set +x
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:: Using rev=007126eef72271480cb7670e19e501a1ad2c1ff2 for <nixpkgs> (extracted from flake.nix)
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:: Setting NIX_PATH to the same values flakes is using
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+ NIX_PATH=nixpkgs=https://github.com/nixos/nixpkgs/archive/007126eef72271480cb7670e19e501a1ad2c1ff2.tar.gz:nixos-config=/home/cole/code/nixos-flake-example/configuration.nix
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+ nix-build '<nixpkgs/nixos>' -A config.system.build.toplevel
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/nix/store/gg1jhmzqndqa0rfnwfdbnzrn8f74ckr6-nixos-system-mysystem-21.03pre-git
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+ set +x
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flake: /nix/store/gg1jhmzqndqa0rfnwfdbnzrn8f74ckr6-nixos-system-mysystem-21.03pre-git
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clssc: /nix/store/gg1jhmzqndqa0rfnwfdbnzrn8f74ckr6-nixos-system-mysystem-21.03pre-git
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```
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