diff --git a/knowledge base/nix.md b/knowledge base/nix.md
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+# The Nix package manager
+
+## Table of contents <!-- omit in toc -->
+
+1. [TL;DR](#tldr)
+1. [Further readings](#further-readings)
+1. [Sources](#sources)
+
+## TL;DR
+
+```sh
+# Install Nix.
+# Works on Linux *using systemd* with SELinux *disabled*. Or MacOSX.
+curl -L 'https://nixos.org/nix/install' | sh
+bash <(curl -L 'https://nixos.org/nix/install') --daemon
+bash <(curl -L 'https://nixos.org/nix/install') --no-daemon
+
+
+# List configured channels.
+nix-channel --list
+
+# Update channels.
+nix-channel --update
+nix-channel --update 'nixpkgs'
+
+
+# List installed packages.
+nix-env -q
+nix-env --query --installed
+
+# List available packages.
+# Without '--attr' it takes a worrying amount of resources at the time of
+# writing, and froze a VM multiple times during experimentation.
+nix-env -qa --attr 'nixpkgs'
+nix-env --query --available --attr 'nixpkgs'
+
+# Install packages.
+nix-env -i 'coreutils'
+nix-env --install --attr 'nixpkgs.parallel'
+
+# Update packages.
+# After a channel update.
+nix-env -u '*'
+nix-env --upgrade --attr 'nixpkgs.firefox'
+
+# Remove packages.
+nix-env -e 'coreutils'
+nix-env --uninstall --dry-run 'parallel'
+
+# Rollback `nix-env` actions.
+nix-env --rollback
+
+# Create a new shell environment with specific programs on top of the current
+# state of the user environment.
+# Useful to test packages without installing them system-wide.
+nix-shell -p 'cowsay' 'lolcat'
+nix-shell --packages 'cowsay' 'lolcat'
+
+# Create a new shell environment with specific programs on top of the current
+# state of the user environment, then run specific commands in it and exit.
+nix-shell -p 'cowsay' 'lolcat' --run 'cowsay "something" | lolcat'
+
+# Free up space occupied by unreachable store objects like packages used in
+# temporary shell environments.
+nix-collect-garbage
+nix-collect-garbage --delete-old
+nix-collect-garbage -d --dry-run
+nix-store --gc
+
+
+# Evaluate Nix expressions in an interactive session.
+nix repl
+
+# Evaluate Nix expressions from files.
+# The file path defaults to 'default.nix'.
+nix-instantiate --eval
+nix-instantiate --eval 'path/to/file.nix'
+```
+
+## Further readings
+
+- [Website]
+- [NixOS]
+
+## Sources
+
+All the references in the [further readings] section, plus the following:
+
+- [Manual]
+
+<!--
+  References
+  -->
+
+<!-- Upstream -->
+[manual]: https://nix.dev/manual/nix/2.19/
+[website]: https://nix.dev
+
+<!-- In-article sections -->
+[further readings]: #further-readings
+
+<!-- Knowledge base -->
+[nixos]: nixos.placeholder
diff --git a/knowledge base/nixos.placeholder b/knowledge base/nixos.placeholder
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+https://nixos.org