nix

Nix Skill

Nix is a powerful package manager and functional programming language. This skill covers common operations like running apps on-the-fly and managing environments.

Running Applications

You can run any application from nixpkgs without installing it permanently.

# Run a package once
nix run nixpkgs#hello

# Run a package with specific arguments
nix run nixpkgs#cowsay -- "Hello from Nix!"

# Run a command within a shell environment (non-interactive)
nix shell nixpkgs#git nixpkgs#vim --command git --version

# Run long-running applications (e.g., servers): `tmux new -d 'nix run nixpkgs#some-server'`

Formatting

Format Nix files in your project:

# Format current flake
nix fmt

# Check formatting
nix fmt -- --check

Evaluating Expressions (Debugging)

Since the environment is headless and non-interactive, use nix eval instead of the REPL for debugging.

# Evaluate a simple expression
nix eval --expr '1 + 2'

# Inspect an attribute from nixpkgs
nix eval nixpkgs#hello.name

# Evaluate a local nix file
nix eval --file ./default.nix

# List keys in a set (useful for exploration)
nix eval --expr 'builtins.attrNames (import <nixpkgs> {})'

Searching for Packages

# Search for a package by name or description
nix search nixpkgs python3

Common Nix Language Patterns

Variables and Functions

# Let binding
let
  name = "Nix";
in
  "Hello ${name}"

# Function definition
let
  multiply = x: y: x * y;
in
  multiply 2 3

Attribute Sets

{
  a = 1;
  b = "foo";
}

Shebang Scripts

Use Nix as a script interpreter:

#!/usr/bin/env nix
#! nix shell nixpkgs#bash nixpkgs#curl --command bash

curl -s https://example.com

Or with flakes:

#!/usr/bin/env nix
#! nix shell nixpkgs#python3 --command python3

print("Hello from Nix!")

Troubleshooting

• Broken Builds: Use nix log to see the build output of a derivation.
• Dependency Issues: Use nix-store -q --references $(which program) to see what a program depends on.
• Cache issues: Add --no-substitute to force a local build if you suspect a bad binary cache.
• Why depends: Use nix why-depends nixpkgs#hello nixpkgs#glibc to see dependency chain.

Related Skills

• nix-flakes: Use Nix Flakes for reproducible builds and dependency management in Nix projects.
• nh: Manage NixOS and Home Manager operations with improved output using nh.

Related Tools

• search-nix-packages: Search for packages available in the NixOS package repository.
• search-nix-options: Find configuration options available in NixOS.
• search-home-manager-options: Find configuration options for Home Manager.