Part 5: Essential NixOS Commands

You have your config, and you know the language. Now, let's master the tools. These are the four commands you will use 99% of the time.

Applying changes (nixos-rebuild switch)

This is your daily driver. Whenever you edit configuration.nix (to add a package, change a user, or tweak a setting), you run this:

sudo nixos-rebuild switch

It does three things atomically:

  1. Builds your new configuration.
  2. Creates a new "Generation" in the boot menu.
  3. Switches the running system to the new config immediately.

The "Test" Variant: If you are tweaking something risky (like network drivers or kernel modules), use:

sudo nixos-rebuild test

This switches to the new configuration without adding it to the boot loader. If you break your WiFi, just reboot, and you are back to the previous stable state automatically.

Updating packages (nix-channel)

This is where Nix differs from Arch or Debian. Running nixos-rebuild switch does not update your package versions. It only rebuilds your config based on the current definition of packages it downloaded weeks ago.

To actually update your system (get the latest Firefox, Kernel, etc.), you need to update the definitions first:

# Download the latest package list
sudo nix-channel --update

# Rebuild your system against this new list
sudo nixos-rebuild switch

The Shorthand: You can do both in one command:

sudo nixos-rebuild switch --upgrade

Temporary shells (nix-shell)

Need htop for five minutes? Need python to run one script? Do not install them globally.

Polluting your global config with one-off tools defeats the purpose of Nix. Instead, pop into a temporary shell:

nix-shell -p htop

This downloads htop, drops you into a shell where it exists, and when you type exit, it is gone.

The shell.nix file

You can save these environments into a file called shell.nix to share with your team. This is the ancestor of Flakes (which we will cover next), but it's still useful to know.

Create a file named shell.nix:

{ pkgs ? import <nixpkgs> {} }:

pkgs.mkShell {
  # List packages available in this shell
  packages = with pkgs; [
    python3
    python311Packages.requests
    git
    neofetch
  ];

  # Set environment variables
  MY_PROJECT_ROOT = "/tmp/my-project";

  # Run this automatically when entering the shell
  shellHook = ''
    echo "------------------------------------------------"
    echo "Welcome to the custom dev environment!"
    echo "------------------------------------------------"
    alias run="python3 app.py"
  '';
}

Now, navigate to that folder and run just nix-shell. Nix detects the file, builds the environment, and drops you in.

The script magic

You can even write standalone scripts that fetch their own dependencies. This is magic for portability.

Create a file myscript.sh:

#! /usr/bin/env nix-shell
#! nix-shell -i bash -p cowsay figlet

# The lines above tell Nix to fetch 'cowsay' and 'figlet'
# BEFORE running the rest of the script.

figlet "It Works!"
cowsay "No global install needed!"

Cleaning up (nix-collect-garbage)

Because Nix never deletes files (it just creates new versions), your disk will fill up. Old versions of Firefox and Kernel modules stick around in the Nix Store to allow rollbacks.

To delete old generations and free up space:

# Delete everything not currently in use by the running system
sudo nix-collect-garbage -d

Warning: Once you run this, you cannot rollback to yesterday’s config anymore. Only do this when your system is stable!

Next step: 06 Flakes
Previous step: 04 Configuration File
Go back to the index: Nixology