:PROPERTIES:
:ID:       892676b3-76cb-4cd4-9689-910c1fe6587a
:END:
#+title: The basics of Nix package manager
#+date: "2021-06-05 12:34:49 +08:00"
#+date_modified: "2021-06-30 22:36:30 +08:00"
#+language: en



[[id:3b3fdcbf-eb40-4c89-81f3-9d937a0be53c][Nix package manager]] is a great tool for reproducibility as you can easily set up your environment.
Taking it up to the next level with NixOS, your whole installation.




* Ecosystem

Nix has tools to make setting up environments easier.

- [[https://direnv.net/][direnv]] has [[https://github.com/direnv/direnv/wiki/Nix][integration with Nix]] as well as a lot of editors
- [[https://github.com/nix-community/lorri][lorri]] replaces nix-shell integrating with direnv
- [[https://github.com/nmattia/niv][niv]] provides a easier way to manage dependencies though it will be easier with Nix flakes
- [[Nix flakes]] is an upcoming feature for Nix, replacing the traditional Nix channels into a decentralized set of derivations that can be retrieved from anywhere similar to Go modules [fn:: At a glance, anyways. I'm not experienced enough with Go to say that with utmost confidence.]
- [[https://cachix.org/][Cachix]] is a cache service enabling to easily distribute binaries built with Nix.




* Reproducible executables

You can create a [[https://nix.dev/tutorials/ad-hoc-developer-environments#reproducible-executables][reproducible executable]] that only requires Nix.

Here's a sample script that uses multiple dependencies.

#+begin_tip
If the script interact with the network (e.g., =curl=, =wget=) and the environment is completely pure, don't forget to install public Certificate Authorities with =cacert=.
#+end_tip

#+begin_src bash
#!/usr/bin/env nix-shell
#! nix-shell --pure -i bash -p coreutils curl cacert jq fzf findutils

# A quick command line interface for creating a gitignore with the API from https://gitignore.io.
# This script comes with a simple caching to avoid creating too much requests.

set -eo pipefail

CACHE_FILE="${XDG_CACHE_DIR:-$HOME/.cache}/gitignore-io.langs.json"

# Check if the language list is downloaded for the last hour (3600 seconds).
if [ ! -e $CACHE_FILE ] || test $(expr $(date "+%s") - $(date -r $CACHE_FILE "+%s")) -gt 3600
then
    ping "gitignore.io" --count 4 && curl --silent --location --output $CACHE_FILE "https://gitignore.io/api/list?format=json"
fi

KEYS=$(jq 'keys | .[] | @text' --raw-output $CACHE_FILE | fzf --multi | while read lang; do echo " .[\"$lang\"].contents"; done | paste -s -d ',')

jq "$KEYS" --raw-output $CACHE_FILE
#+end_src




* Components of the package manager

Holistically, Nix is made up of at least four components: the store, the language, the derivations, and the sandbox.

- The store is a immutable centralized location where all of the outputs are placed.
- The derivations are essentially build instructions.
- The language (also called as Nix but we'll refer to it as Nixlang) is a domain-specific language for creating derivations.
- The build process can be locked in a sandbox, improving the reproducibility of a setup and lowering the attack surface for a malicious package.




* Overlays

You can override values in Nix as a way to customize nixpkgs.
For example, if you want to use a different version from the nixpkgs channel, you can change the appropriate value.

#+begin_src nix
let overlay = self: super:
      {
        ncmpcpp = super.ncmpcpp.override { visualizerSupport = true; };
      }
#+end_src

# TODO: Bring more examples
For another example, you can see some examples from [[https://github.com/neovim/neovim/blob/f695457f815544d0dc16469569c70556e3165bb6/contrib/flake.nix][Neovim]] and [[https://gitlab.com/veloren/veloren/-/tree/685f4971ac0deb31b301e9d2bc0201d2531fd895/nix][Veloren]] (which also uses Nix flakes).

You can set overlays automatically either by setting =nixpkgs.overlays= from your system configuration or =~/.config/nixpkgs/overlays/= folder for user-specific settings.
You could also set overlays for standalone Nix code similarly through the =overlays= key — e.g., ~import <nixpkgs> ? { overlays = (self: super: { } ); };~.




* TODO Nix flakes

#+begin_note
As of 2021-06-30, the version used for this note is at v2.3 so it needs to be invoked with the unstable version.
#+end_note

- similar to [[https://guix.gnu.org/manual/en/html_node/Channels.html][Guix channels]]
- a collection of packages and functions while making it easy to configure Nix declaratively
- replaces the traditional Nix channels since fully reproducing an environment with Nix requires special care in practice;
  plus, there's no standard way of composing projects with Nix
- as of 2021-06-30, this is on the unstable version of the Nix package manager and needs some additional configuration

why flakes?

- provides a structure for discoverability
- makes 100% reproducibility a little easier with Nix
- in case you're using NixOS, it also provides an easier way to extend it with third-party custom modules

Here's an example to interact with a flake.
It will show the entire outputs of a flake as well as the normalized version of the flake object.

#+name: flake-sample-object
#+begin_src python  :results value silent :exports none
return "github:edolstra/dwarffs"
#+end_src

#+begin_src shell  :shebang "#!/usr/bin/env nix-shell"
#! nix-shell -i bash -p nixUnstable
nix --experimental-features 'nix-command flakes' flake show <<flake-sample-object()>> | sed -e "s/\x1b\[.\{1,5\}m//g"
#+end_src

#+results:
#+begin_example
github:edolstra/dwarffs/f691e2c991e75edb22836f1dbe632c40324215c5
├───checks
│   ├───aarch64-linux
│   │   ├───build: derivation 'dwarffs-0.1.20210121.f691e2c'
│   │   └───test: derivation 'vm-test-run-unnamed'
│   ├───i686-linux
│   │   ├───build: derivation 'dwarffs-0.1.20210121.f691e2c'
│   │   └───test: derivation 'vm-test-run-unnamed'
│   └───x86_64-linux
│       ├───build: derivation 'dwarffs-0.1.20210121.f691e2c'
│       └───test: derivation 'vm-test-run-unnamed'
├───defaultPackage
│   ├───aarch64-linux: package 'dwarffs-0.1.20210121.f691e2c'
│   ├───i686-linux: package 'dwarffs-0.1.20210121.f691e2c'
│   └───x86_64-linux: package 'dwarffs-0.1.20210121.f691e2c'
├───nixosModules
│   └───dwarffs: NixOS module
└───overlay: Nixpkgs overlay
#+end_example

Let's build from one of the outputs of call_flake-sample-object().

#+begin_src shell  :shebang "#!/usr/bin/env nix-shell"  :results silent  :exports none
#! nix-shell -i bash -p nixUnstable
nix --experimental-features 'nix-command flakes' build '<<flake-sample-object()>>#checks.aarch64-linux.build'
#+end_src