Nix Development Skill

Evaluation & Debugging

Evaluate flake outputs

bash

# 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

bash

# 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)

bash

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

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

NixOS

bash

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

Home-manager standalone

bash

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

Service Management (launchd/systemd)

macOS launchd

bash

# 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

bash

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:

nix

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

nix

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

Conditional by platform

nix

# 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

nix

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

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

Flake Inputs Management

bash

# 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