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87 lines
2.7 KiB
Markdown
87 lines
2.7 KiB
Markdown
(pinning-nixpkgs)=
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# Towards reproducibility: pinning Nixpkgs
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In various Nix examples, you'll often see references to [\<nixpkgs>](https://github.com/NixOS/nixpkgs), as follows.
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```nix
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{ pkgs ? import <nixpkgs> {}
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}:
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...
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```
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This is a **convenient** way to quickly demonstrate a Nix expression and get it working by importing Nix packages.
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However, <ref-search-path>**the resulting Nix expression is not fully reproducible**.
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## Pinning packages with URLs inside a Nix expression
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To create **fully reproducible** Nix expressions, we can pin an exact version of Nixpkgs.
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The simplest way to do this is to fetch the required Nixpkgs version as a tarball specified via the relevant Git commit hash:
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```nix
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{ pkgs ? import (fetchTarball "https://github.com/NixOS/nixpkgs/archive/06278c77b5d162e62df170fec307e83f1812d94b.tar.gz") {}
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}:
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...
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```
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Picking the commit can be done via [status.nixos.org](https://status.nixos.org/),
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which lists all the releases and the latest commit that has passed all tests.
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When choosing a commit, it is recommended to follow either
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- the **latest stable NixOS** release by using a specific version, such as `nixos-21.05`, **or**
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- the latest **unstable release** via `nixos-unstable`.
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## Dependency management with niv
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If you'd like a bit more automation around bumping dependencies, including Nixpkgs,
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[niv](https://github.com/nmattia/niv/) is made for exactly that. Niv itself is available
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in `nixpkgs` so using it is simple:
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```shell-session
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$ nix-shell -p niv --run "niv init"
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```
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This command will generate `nix/sources.json` with information about how and where
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dependencies are fetched. It will also create `nix/sources.nix`, which glues the sources together in Nix.
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By default, `niv` will use the **latest stable** NixOS release. However, you should check to see which version is currently specified in [the niv repository](https://github.com/nmattia/niv) if you require a specific release, as it might lag behind.
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You can see which version `niv` is tracking as follows:
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```shell-session
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$ niv show
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```
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And you can change the tracking branch to the one you want like this:
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```shell-session
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$ niv modify nixpkgs --branch nixos-21.05
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```
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You can use the generated `nix/sources.nix` with a top-level `default.nix`:
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```nix
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{ sources ? import ./nix/sources.nix
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, pkgs ? import sources.nixpkgs {}
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}:
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...
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```
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And you can update all the dependencies by running:
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```shell-session
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$ nix-shell -p niv --run "niv update"
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```
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## Next steps
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- For more examples and details of the different ways to pin `nixpkgs`, see {ref}`ref-pinning-nixpkgs`.
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- To quickly set up a Nix project, read through
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[Getting started Nix template](https://github.com/nix-dot-dev/getting-started-nix-template).
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