nix

name: nix description: "Runs packages temporarily, creates isolated shell environments, and evaluates Nix expressions. Use when executing tools without installing, debugging derivations, or working with nixpkgs." token_cost: 210 related: [nix-flake, nh] keywords: ["nix", "shell", "env", "evaluate", "derivation", "nixpkgs", "run", "package"] requires_tools: [bash]

Package manager and functional language for reproducible environments. Run any tool once without installing it permanently.

Running Packages

Execute a package from nixpkgs directly:

nix run nixpkgs#cowsay -- "Hello!"    # Run with arguments
nix run nixpkgs#hello                  # Simple command

For long-running services, wrap in tmux: tmux new -d 'nix run nixpkgs#some-server'.

Shell Environments

Create a temporary shell with specific tools:

nix shell nixpkgs#git nixpkgs#vim --command git --version

Evaluating Expressions

Debug and inspect Nix expressions in headless environments:

nix eval --expr '1 + 2'              # Simple expression
nix eval nixpkgs#hello.name          # Inspect an attribute
nix eval --file ./default.nix        # Evaluate a local file
nix eval --expr 'builtins.attrNames (import <nixpkgs> {})'  # List keys

Searching Packages

nix search nixpkgs python3           # Search by name or description

Formatting Nix Files

nix fmt                              # Format current directory
nix fmt -- --check                   # Check formatting without changes

Hashes

Never pre-compute hashes with nix-prefetch-url, nix hash (file, path, convert, etc.), or nix-hash. These commands are blocked.

Set the hash to "" in your fetchSource and let the build fail with a got: error that reports the correct hash:

fetchFromGitHub {
  owner = "nixos";
  repo = "nixpkgs";
  rev = "abc123";
  hash = "";  # let the build tell you the correct hash
}

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

Rules

Use nix log <derivation> to debug broken builds
Use nix why-depends to trace dependency chains
Add --no-substitute to force local build when cache is bad
Use shebang scripts (#! nix shell ... --command) for inline nix scripts