Nix Manager

name: nix-manager description: Manage Nix packages, flakes, and configurations using Determinate Nix installer patterns. Use when installing/updating packages, creating flakes, troubleshooting Nix issues, or optimizing Nix workflows. Keywords: nix, flake, package, nixpkgs, nix profile, flake.nix, flake.lock, determinate, nix-installer

Nix Package & Configuration Manager

Comprehensive Nix management following Determinate Systems best practices and this repository's patterns.

Instructions

1. Understand Repository Context

Check current Nix setup:

• Flake location: /Users/wcygan/Development/dotfiles/flake.nix
• Installation script: scripts/install-packages.sh
• Package management: nix profile (user-scoped, modern approach)
• Installer: Determinate Systems installer (macOS/Linux)
• Update mechanism: make update or nix flake update && nix profile upgrade

Read current flake.nix to understand:

• Input sources (currently: nixpkgs.url = "github:NixOS/nixpkgs/nixos-unstable")
• Package definitions (buildEnv with ~60+ packages)
• Outputs: packages, devShells, formatter
• Supported systems: x86_64-linux, aarch64-linux, x86_64-darwin, aarch64-darwin

2. Package Management Operations

Install New Package

Process:

1. Add package to flake.nix in appropriate category
2. Run nix flake check to validate
3. Run nix profile upgrade dotfiles to apply changes
4. Test package availability

Example:

# flake.nix packages section
paths = [
  # ... existing packages ...

  # New package
  cowsay  # Fun terminal tool
];

# Validate and install
nix flake check
nix profile upgrade dotfiles
which cowsay  # Verify installation

Update All Packages

Process:

# Update flake inputs (updates nixpkgs revision)
nix flake update

# Apply updates to installed profile
nix profile upgrade dotfiles

# Verify no breakage
nix profile list

Or use Makefile shortcut:

make update  # Runs both commands above

Remove Package

Process:

1. Remove from flake.nix
2. Run nix flake check
3. Run nix profile upgrade dotfiles
4. Old package remains in store but not in PATH

Note: Garbage collection removes unreferenced packages:

make clean  # or: nix-collect-garbage -d

3. Flake Configuration

Modify flake.nix

Common operations:

Add new input:

inputs = {
  nixpkgs.url = "github:NixOS/nixpkgs/nixos-unstable";

  # Add new input
  home-manager.url = "github:nix-community/home-manager";
  home-manager.inputs.nixpkgs.follows = "nixpkgs";  # Prevent duplicate nixpkgs
};

Add platform-specific packages:

paths = [
  # Universal packages
  git gh lazygit
] ++ lib.optionals stdenv.isDarwin [
  # macOS-only
  darwin.apple_sdk.frameworks.Security
] ++ lib.optionals stdenv.isLinux [
  # Linux-only
  libnotify
];

Modify devShell:

devShells = forAllSystems ({ pkgs }: {
  default = pkgs.mkShell {
    packages = with pkgs; [
      fish
      nixpkgs-fmt
      shellcheck
      # Add development tools here
    ];
    inputsFrom = [ self.packages.${pkgs.system}.default ];
    shellHook = ''
      echo "🐠 Dotfiles development environment"
      echo "Run: make test-pre"
    '';
  };
});

Validate Flake

Always validate before applying:

nix flake check        # Full validation (slow, builds everything)
nix flake metadata     # Quick metadata check
nix flake show         # Show outputs without building

Common issues:

• Package renamed in nixpkgs (e.g., du-dust → dust)
• Missing comma in package list
• Invalid attribute path
• Syntax errors in Nix expressions

Update Lock File

When to update:

• Regular maintenance (weekly/monthly)
• Security updates needed
• Specific package version required

How:

# Update all inputs
nix flake update

# Update specific input only
nix flake lock --update-input nixpkgs

# Verify changes
git diff flake.lock

4. Troubleshooting

Slow Nix Operations

Diagnosis:

nix store info             # Check store size
nix store gc --dry-run     # See what can be cleaned

Solutions:

• Run nix-collect-garbage -d to remove old generations
• Run nix store optimise to deduplicate files
• Check network connectivity (binary cache downloads)

Package Not Found

Error: error: attribute 'package-name' missing

Solutions:

1. Check nixpkgs version: some packages only in unstable
2. Search for package: nix search nixpkgs package-name
3. Check if package was renamed
4. Try alternative package names

Evaluation Errors

Error: error: ... while evaluating ...

Common causes:

• Syntax error in flake.nix
• Recursive attribute access
• Type mismatch (string vs list)

Debug:

nix eval .#packages.aarch64-darwin.default.name  # Test specific attribute
nix repl                                          # Interactive REPL
:lf .                                             # Load flake in REPL
packages.aarch64-darwin.default.name              # Evaluate in REPL

Lock File Conflicts

Error: error: flake.lock is dirty

Solutions:

# Regenerate lock file
rm flake.lock
nix flake update

# Or accept uncommitted changes
nix flake check --impure  # NOT recommended for reproducibility

Profile Issues

List installed profiles:

nix profile list

Output format:

Index:              0
Flake attribute:    legacyPackages.aarch64-darwin.dotfiles
Original flake URL: git+file:///Users/wcygan/Development/dotfiles
Locked flake URL:   git+file:///Users/wcygan/Development/dotfiles?rev=...
Store paths:        /nix/store/...-system-packages

Rollback to previous generation:

nix profile rollback

Remove specific profile:

nix profile remove <index-number>

5. CI/CD Integration

This repository uses Determinate Systems GitHub Actions for CI.

GitHub Actions setup (.github/workflows/ci.yml):

- name: Setup Nix cache
  uses: DeterminateSystems/magic-nix-cache-action@v2

- name: Install Nix
  uses: DeterminateSystems/nix-installer-action@v14
  with:
    extra-conf: |
      experimental-features = nix-command flakes

- name: Run install script
  run: ./install.sh

Benefits:

• Magic Nix Cache: ~90% faster CI (uses GitHub Actions cache)
• Automatic cache population
• No configuration required

Local equivalent:

# Test installation in Docker
make test-docker

# Test idempotency
./install.sh && ./install.sh  # Should succeed twice

6. Best Practices (Determinate Nix Patterns)

Use nixos-unstable Instead of master

Reasoning:

• nixos-unstable: Tested, passes Hydra CI
• master: Untested, may have broken packages

Current setup:

inputs.nixpkgs.url = "github:NixOS/nixpkgs/nixos-unstable";

Never Use --impure

Problem: Breaks reproducibility by allowing environment variable access

Correct:

nix profile add .              # Pure evaluation
nix profile upgrade dotfiles   # Pure evaluation

Incorrect:

nix profile add . --impure     # BAD: non-reproducible

Exception: Only use --impure if flake explicitly uses getEnv or similar

Pin Dependencies in Lock File

Why:

• Ensures reproducible builds across machines
• Prevents "works on my machine" issues
• Required for CI/CD reliability

How:

# Always commit flake.lock
git add flake.lock
git commit -m "chore: update flake lock"

Use buildEnv for Package Groups

Pattern in this repo:

packages.default = pkgs.buildEnv {
  name = "system-packages";
  paths = [ git gh lazygit ... ];
};

Benefits:

• Single derivation for all packages
• Atomic updates (all packages succeed or fail together)
• Easier to manage than individual nix profile install calls

Enable Flakes in Config

User-level config (~/.config/nix/nix.conf):

experimental-features = nix-command flakes

This is automatically set by scripts/install-packages.sh

7. Development Workflows

Create New Project Flake

Use repository root as template:

# Copy flake structure
cp flake.nix /path/to/new-project/

# Customize for project needs
cd /path/to/new-project
$EDITOR flake.nix

Or use templates directory:

# Use template (if available in .claude/skills/nix-manager/templates/)
nix flake init -t .#template-name

Test Flake Locally

Without installing:

# Enter dev shell
nix develop

# Build without installing
nix build

# Run specific package
nix run .#package-name

Format Nix Code

Using formatter output:

nix fmt  # Uses nixpkgs-fmt (defined in flake.nix)

Manual formatting:

nixpkgs-fmt flake.nix

8. Migration Guidance

From Homebrew

Don't uninstall Homebrew—it coexists peacefully. Fish PATH priority:

1. Homebrew (/opt/homebrew/bin) - highest priority
2. User bins (~/.local/bin, ~/bin)
3. Language toolchains (~/.cargo/bin, ~/go/bin)
4. Nix (~/.nix-profile/bin) - lowest priority

Migration strategy:

# 1. Install package via Nix
# (add to flake.nix and run nix profile upgrade)

# 2. Test package works
which package-name  # Should show Homebrew path (higher priority)

# 3. Uninstall from Homebrew
brew uninstall package-name

# 4. Verify Nix version now active
which package-name  # Should show /nix/store/... path

From apt/dnf

Linux distros:

• Nix coexists with system package managers
• System packages have priority over Nix (via PATH ordering)
• Use Nix for tools not in distro repos or needing newer versions

9. Output Format

When modifying flake.nix:

Use Edit tool for existing files:

• Modify specific sections
• Preserve comments and formatting
• Minimize diff size

Use Write tool for new files:

• Complete flake.nix from scratch
• Include comments explaining choices
• Follow repository formatting style

After changes, always:

1. Validate: nix flake check
2. Test build: nix build --dry-run
3. Apply: nix profile upgrade dotfiles
4. Verify: nix profile list

Include testing commands:

# Validate changes
nix flake check

# Show what changed
nix flake show

# Apply updates
nix profile upgrade dotfiles

Repository Patterns

This dotfiles repository follows these conventions:

File Structure:

• flake.nix - Package definitions and outputs
• flake.lock - Pinned dependency versions
• scripts/install-packages.sh - Installation wrapper
• scripts/link-config.sh - Dotfile symlinking
• config/ - Dotfile configurations (fish, starship, etc.)

Package Organization: Packages grouped by purpose with comments:

paths = [
  # Version control
  git gh lazygit

  # Build tools
  gnumake cmake pkg-config

  # Programming languages
  rustup go python3 deno

  # ... etc
];

Testing:

• make test-pre - Pre-flight validation
• make test-local - Ephemeral HOME test
• make test-docker - Multi-distro Docker matrix

Common Commands:

• make install - Run full installation
• make update - Update flake + upgrade packages
• make clean - Garbage collect
• make verify - Check Nix installation health

Reference Documentation

• Determinate Installer: https://determinate.systems/blog/determinate-nix-installer/
• Zero to Nix (Flakes): https://zero-to-nix.com/concepts/flakes/
• Nix.dev (Flakes): https://nix.dev/concepts/flakes.html
• NixOS Wiki: https://nixos.wiki/wiki/Flakes
• Repository: /Users/wcygan/Development/dotfiles/

Quick Reference

Essential Commands:

# Package management
nix search nixpkgs <package>     # Search for package
nix profile list                 # List installed packages
nix profile upgrade dotfiles     # Apply flake changes
nix-collect-garbage -d           # Clean old generations

# Flake management
nix flake update                 # Update all inputs
nix flake check                  # Validate flake
nix flake show                   # Display outputs
nix flake metadata               # Show metadata

# Development
nix develop                      # Enter dev shell
nix build                        # Build package
nix fmt                          # Format Nix code
nix run .#package                # Run package

# Troubleshooting
nix store info                   # Store statistics
nix store gc --dry-run           # Preview cleanup
nix profile rollback             # Revert to previous