Nix Development Skill

Evaluation & Debugging

Evaluate flake outputs

# List available outputs
nix flake show

# Evaluate specific attribute
nix eval .#<attribute> --json

# Darwin config options
nix eval .#darwinConfigurations.<host>.options.<path>

# Home-manager config
nix eval .#homeConfigurations."<user>@<host>".config.<path>

Debug evaluation errors

# Show full trace
nix eval .#<attr> --show-trace

# Check flake validity
nix flake check

# Instantiate without building
nix-instantiate --eval -E '<expression>'

Building & Rebuilding

Darwin (macOS)

darwin-rebuild switch --flake .
darwin-rebuild switch --flake .#<hostname>

# Rollback
darwin-rebuild --list-generations
darwin-rebuild switch --rollback

NixOS

sudo nixos-rebuild switch --flake .#<hostname>
sudo nixos-rebuild boot --flake .#<hostname>

Home-manager standalone

home-manager switch --flake .#<user>@<host>

Service Management (launchd/systemd)

macOS launchd

# List services
launchctl list | grep -E "org.nixos|nix"

# Service status
launchctl print system/<label>
launchctl print gui/$(id -u)/<label>

# Control services
launchctl kickstart [-k] <domain>/<label>
launchctl kill SIGTERM <domain>/<label>

Linux systemd

systemctl --user list-units --type=service
systemctl --user status <service>
systemctl --user restart <service>
journalctl --user -u <service> -f

Launchd Configuration Options

For reducing CPU/IO priority in nix-darwin:

launchd.daemons.<name>.serviceConfig = {
  RunAtLoad = false;        # Don't start at boot
  KeepAlive = false;        # Don't auto-restart
  Nice = 5;                 # Lower CPU priority (1-20)
  ProcessType = "Background"; # Background scheduling
  LowPriorityIO = true;     # Lower I/O priority
  ThrottleInterval = 10;    # Min seconds between restarts
};

launchd.user.agents.<name>.serviceConfig = { /* same options */ };

Common Patterns

Override with mkForce

# When upstream sets a value you need to override
someOption = lib.mkForce false;

Conditional by platform

# At Nix level (preferred)
serviceCommands = if pkgs.stdenv.isDarwin
  then import ./darwin.nix
  else import ./linux.nix;

# In module
config = lib.mkIf pkgs.stdenv.isDarwin { ... };

Module structure

{ lib, config, pkgs, ... }:
let
  cfg = config.myModule;
in {
  options.myModule = {
    enable = lib.mkEnableOption "my module";
  };

  config = lib.mkIf cfg.enable {
    # implementation
  };
}

Flake Inputs Management

# Update all inputs
nix flake update

# Update specific input
nix flake lock --update-input <input-name>

# Show inputs
nix flake metadata

Troubleshooting

"infinite recursion" error

• Check for circular dependencies in imports
• Use lib.mkDefault or lib.mkForce to resolve conflicts

"attribute not found"

• Verify the attribute path with nix eval
• Check if module is properly imported

Service not starting

• Check plist/unit file generation
• Verify paths in ProgramArguments
• Check logs for errors