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https://github.com/colemickens/nixos-flake-example
This is a demo NixOS config, with optional flakes support. Along with notes on why flakes is useful and worth adopting.
https://github.com/colemickens/nixos-flake-example
evangelism flakes nix
Last synced: about 2 months ago
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This is a demo NixOS config, with optional flakes support. Along with notes on why flakes is useful and worth adopting.
- Host: GitHub
- URL: https://github.com/colemickens/nixos-flake-example
- Owner: colemickens
- Created: 2020-10-25T03:55:40.000Z (almost 4 years ago)
- Default Branch: master
- Last Pushed: 2021-05-31T00:34:14.000Z (over 3 years ago)
- Last Synced: 2024-05-31T08:35:11.159Z (4 months ago)
- Topics: evangelism, flakes, nix
- Language: Nix
- Homepage:
- Size: 48.8 KB
- Stars: 167
- Watchers: 6
- Forks: 16
- Open Issues: 2
-
Metadata Files:
- Readme: README.md
Awesome Lists containing this project
README
# nixos-flake-example
## warning
**WARNING**: You should understand that:
- there is currently no path to flakes being stable
- we can't even experiment with flakes alternatives without committing to pure-eval as a stepping stone
- the people that CARE about the problems solved by flakes seem to care about solving this problem
- the people using impurity everywhere don't seem very motivated to dive into this problem-space
- as a result, the tooling is completely fractured and stagnant
- besides some UX fixes, bugfixes, and other feature-work in Nix, this has more or less been the case for years
- I'm tired of seeing (often self-proclaimed, repeated-from-others) FUD by non-flakes users
Anyway, now you know. Proceed at your own caution.
## overview
**NOTE**: [nixflk](https://github.com/nrdxp/nixflk) is a better example repo for a full
NixOS config layout, this repo is mostly to provide more context+examples around flakes,
and to show that you can produce the same EXACT system with flakes as with nix-build,
if you know what to do.
This readme starts out with an attempt to explain and justify flakes. It also contains
some examples of `nix` cli flakes syntax and tips for adopting flakes in your project.
Finally, [at the end of the readme](#example-nixos-config-with-optional-flake-support)
is an example NixOS config with a supporting `flake.nix`,
and instructions to build it with *and without* flakes support at the same time.
- [Overview of Flakes (and why you want it)](#overview-of-flakes-and-why-you-want-it)
- [Important Related Reading](#important-related-reading)
- [Nix CLI - Flakes Usage](#nix-cli---flakes-usage)
- [Useful Commands and Examples](#useful-commands-and-examples)
- [nixos-rebuild](#nixos-rebuild)
- [nix build](#nix-build)
- [nix flake](#nix-flake)
- [Auto-coercion examples](#auto-coercion-examples)
- [Tips for Porting to Flakes](#tips-for-porting-to-flakes)
- [Example NixOS Config with optional Flake support](#example-nixos-config-with-optional-flake-support)
## Overview of Flakes (and why you want it)
Flakes is a few things:
* `flake.nix`: a Nix file, with a specific structure to describe inputs and outputs for a Nix project
* See [NixOS Wiki - Flakes - Input Schema](https://nixos.wiki/wiki/Flakes#Input_schema) for flake input examples
* See [NixOS Wiki - Flakes - Output Schema](https://nixos.wiki/wiki/Flakes#Input_schema) for flake output examples
* `flake.lock`: a manifest that "locks" inputs and records the exact versions in use
* CLI support for flake-related features
* pure (by default) evaluations
This ultimately enables:
* properly hermetic builds
* fully reproducable and portable Nix projects
* faster Nix operations due to evaluation caching enabled by pure evaluations)
This removes the need for:
* using `niv` or other tooling to lock dependencies
* manually documenting or scripting to ensure `NIX_PATH` is set consistently for your team
* the need for the *"the impure eval tree of sorrow"* that comes with all of today's Nix impurities
## Important Related Reading
* [NixOS Wiki - Flakes](https://nixos.wiki/wiki/Flakes)
* a somewhat haphazard collection of factoids/snippets related to flakes
* particularly look at: **[Flake Schema](https://nixos.wiki/wiki/Flakes#Flake_schema)**, and it's two sections: **[Input Schema](https://nixos.wiki/wiki/Flakes#Input_schema)**, **[Output Schema](https://nixos.wiki/wiki/Flakes#Output_schema)**
* [Tweag - NixOS flakes](https://www.tweag.io/blog/2020-07-31-nixos-flakes/)
* this article describes how to enable flake support in `nix` and `nix-daemon`
* reading this article is a **pre-requisite**
* this README.md assumes you've enabled flakes system-wide
* omit using `boot.isContainer = true;` on `configuration.nix` (as the article suggests) if you want to use `nixos-rebuild` rather than `nixos-container`
## Nix CLI - Flakes Usage
Nix is in flakes mode when:
* `--flake` is used with the `nixos-rebuild` command
* or, when `nix build` is used with an argument like `'.#something'` (the hash symbol separates the flake source from the attribute to build)
When in this mode:
* Nix flake commands will implicitly take a directory path, it expects a `flake.nix` inside
* when you see: `nix build '.#something'`, the `.` means current directory, and `#something` means to build the `something` output attribute
### Useful Commands and Examples
#### nixos-rebuild
* `nixos-rebuild build --flake '.#'`
* looks for `flake.nix` in `.` (current dir)
* since it's `nixos-rebuild`, it automatically tries to build:
* `#nixosConfigurations.{hostname}.config.system.build.toplevel`
* `nixos-rebuild build --flake '/code/nixos-config#mysystem'`
* looks for `flake.nix` in `/code/nixos-config`
* since it's `nixos-rebuild`, it automatically tries to build:
* `#nixosConfigurations.mysystem.config.system.build.toplevel`
* (note that this time we specifically asked, and got to build the `mysystem` config)
#### nix build
* `nix build 'github:colemickens/nixpkgs-wayland#obs-studio'`
* looks for `flake.nix` in (a checkout of `github.com/colemickens/nixpkgs-wayland`)
* builds and run the first attribute found:
* `#obs-studio`
* `#packages.{currentSystem}.obs-studio`
* TODO: finish fleshing out this list
#### nix flake
* `nix flake update --recreate-lock-file`
* updates all inputs and recreating `flake.lock`
* `nix flake update --update-input nixpkgs`
* updates a single input to latest and recording it in `flake.lock`
### Auto-coercion examples
Nix CLI will try to be ... smart and auto-coerce some output attribute paths for you.
* `nix build '/some/path#obs-studio'`:
* builds and run the first attribute found:
* `/some/path#obs-studio`
* `/some/path#packages.x86_64-linux.obs-studio`
* `/some/path#legacyPackages.x86_64-linux.obs-studio`
* TODO: finish fleshing out this list
* TODO: not sure about search order, presumably the bare one would be priority
## Tips for Porting to Flakes
**Remove Impurities** - Since nix flakes does a 'pure' build by default,
* `NIX_PATH` is ignored
* `` imports do not work, and explicitly error
* local user nixpkgs config (`~/.config/{nix,nixpkgs}`) are ignore
* unpinned imports (aka, `fetchTarball` without a pinned `rev`+`sha256`) are forbidden
To fix these:
* specify all remote imports in `flake.nix` instead of using `fetchTarball`
* the config in this repo shows an example of using the overlay from
`nixpkgs-wayland`.
* TODO: investigate `getFlake` vs passing `inputs` in `specialArgs`
## Example NixOS Config with optional Flake support
Consider the nixos configuration in this repo:
* [./configuration.nix](./configuration.nix)
* [./hardware-configuration.nix](./hardware-configuration.nix)
These represent an example, minimal NixOS system configuration.
The easiest way to build it, without cloning this repo:
```
nix build 'github:colemickens/nixos-flake-example#nixosConfigurations.mysystem.config.system.build.toplevel'
```
Let's prove that we can build this config, with and without flakes:
* Using `nixos-rebuild`:
```shell
# with flakes
unset NIX_PATH
nixos-rebuild build --flake '.#mysystem'
readlink -f ./result
/nix/store/gg1jhmzqndqa0rfnwfdbnzrn8f74ckr6-nixos-system-mysystem-21.03pre-git
# !! for this next step, match the git SHA1 to what the flake.lock uses
# otherwise you'll have a hash mismatch due to different nixpkgs
# without flakes
export NIX_PATH=nixpkgs=https://github.com/nixos/nixpkgs/archive/007126eef72271480cb7670e19e501a1ad2c1ff2.tar.gz:nixos-config=/home/cole/code/nixos-flake-example/configuration.nix
nixos-rebuild build
readlink -f ./result
/nix/store/gg1jhmzqndqa0rfnwfdbnzrn8f74ckr6-nixos-system-mysystem-21.03pre-git
```
* Using `nix build`:
```shell
# with flakes
unset NIX_PATH
nix build '.#nixosConfigurations.mysystem.config.system.build.toplevel
readlink -f ./result
/nix/store/gg1jhmzqndqa0rfnwfdbnzrn8f74ckr6-nixos-system-mysystem-21.03pre-git
# without flakes
export NIX_PATH=nixpkgs=https://github.com/nixos/nixpkgs/archive/007126eef72271480cb7670e19e501a1ad2c1ff2.tar.gz:nixos-config=/home/cole/code/nixos-flake-example/configuration.nix
nix-build '' -A config.system.build.toplevel
readlink -f ./result
/nix/store/gg1jhmzqndqa0rfnwfdbnzrn8f74ckr6-nixos-system-mysystem-21.03pre-git
```
* The `./check.sh` script automates this process:
```shell
cole@slynux ~/code/nixos-flake-example master* 7s
❯ ./check.sh
:: Updating the 'nixpkgs' input in flake.nix
+ nix flake update --update-input nixpkgs
+ set +x
:: Using 'nixos-rebuild' to build the 'mysystem' toplevel
+ nixos-rebuild build --flake '.#mysystem'
warning: Git tree '/home/cole/code/nixos-flake-example' is dirty
building the system configuration...
warning: Git tree '/home/cole/code/nixos-flake-example' is dirty
+ set +x
:: Using rev=007126eef72271480cb7670e19e501a1ad2c1ff2 for (extracted from flake.nix)
:: Setting NIX_PATH to the same values flakes is using
+ NIX_PATH=nixpkgs=https://github.com/nixos/nixpkgs/archive/007126eef72271480cb7670e19e501a1ad2c1ff2.tar.gz:nixos-config=/home/cole/code/nixos-flake-example/configuration.nix
+ nix-build '' -A config.system.build.toplevel
/nix/store/gg1jhmzqndqa0rfnwfdbnzrn8f74ckr6-nixos-system-mysystem-21.03pre-git
+ set +x
flake: /nix/store/gg1jhmzqndqa0rfnwfdbnzrn8f74ckr6-nixos-system-mysystem-21.03pre-git
clssc: /nix/store/gg1jhmzqndqa0rfnwfdbnzrn8f74ckr6-nixos-system-mysystem-21.03pre-git
```
# Flake Feedback/Ponderings
- Is the hash tag syntax really worth it?
- For example, is:
- `nix build 'github:colemickens/nixpkgs-wayland#obs-studio'`
- really better than:
- `nix build --flake 'github:colemickens/nixpkgs-wayland' 'obs-studio'` ?
- Are the auto-coercion rules for attribute paths worth it?
They definitely add some mental overhead...
-