wiki/structured/tools.nix.org

: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: "2021-06-09 17:39:23 +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 with direnv, lorri, and [[id:c05e1aa9-0619-4617-abb6-870fceca3430][Niv]]

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

#+begin_src bash
#!/usr/bin/env nix-shell
#! nix-shell --pure -i bash -p curl 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]] (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: { } ); };~.