Nix Init

Nix Init

nix-init scaffolds a complete Nix package from a repository URL: it picks the fetcher, prefetches the source and dependency hashes via nurl, infers the build system (Rust, Go, Python) and its dependencies, and detects the license.

The generated file is a strong starting point, not a finished package — upstream warns that tweaks are usually required. Always verify the result before handing it back (see Verification).

Running nix-init

Check if nix-init is on PATH first. If not, run it through nix:

nix run nixpkgs#nix-init -- <args>        # latest packaged release

Non-interactive invocation

Agents cannot answer nix-init's fuzzy interactive prompts, so always pass --headless. It requires --url; everything else gets inferred unless you specify it explicitly.

nix run nixpkgs#nix-init -- --headless \
  -u 'https://github.com/owner/repo' \
  --overwrite=true \
  /path/to/package.nix

Flags worth knowing

• -u/--url — always pass full HTTPS URLs (https://github.com/owner/repo, https://gitlab.com/..., https://codeberg.org/...). Short forms like github:owner/repo get mangled into an invalid git+github: fetcher and fail.
• --rev + -V/--version — pin a specific tag and its version. Pass both together or neither: -V 1.2.3 without --rev leaves the source pinned to the latest tag while claiming version 1.2.3. With neither, nix-init tracks the latest release and usually emits dynamic tags like tag = "v${finalAttrs.version}", which is ideal for updates.
• --pname — only needed when the derived name would be wrong (e.g. repo named foo.rs should be packaged as foo).
• --builder — forces a builder; otherwise inferred from repo contents (see below). Note buildNpmPackage is auto-detected but cannot be forced.
• --cargo-vendor fetchCargoVendor|importCargoLock — how cargo deps are vendored. Prefer the default (fetchCargoVendor) for new packages.
• -S true — fetch git submodules when the project needs them.
• --overwrite=true — long form only; -y true swallows true as a positional argument and breaks the command.
• Output position — pass an explicit file path. A directory makes nix-init write default.nix inside it, which is wrong for pkgs/by-name layout.

Builder inference

nix-init also fills in what it can detect: license(s), description, homepage, changelog link, mainProgram, Go ldflags, Python build-system, dependencies, optional-dependencies, and pythonImportsCheck.

Verification

Treat the generated file as unverified until proven otherwise. Work through these steps every time — they catch most real-world breakage:

1. Read the whole file. Sanity-check pname/version against the actual tag, and whether Application vs Package was chosen correctly for Python projects (libraries used as dependencies want buildPythonPackage).
2. Build it. For a quick check of a standalone file:

   nix-build -E 'with import <nixpkgs> {}; callPackage ./package.nix { }'
   

   In a nixpkgs checkout, build from the repo root with nix-build -A <pname> (or nix build .#<pname> in a flake).
3. Fix hash mismatches. If the build fails with a hash mismatch, set the offending attribute to lib.fakeHash, rebuild, and copy the got: value. The nix-hash skill covers conversions if needed.
4. Smoke test. Run ./result/bin/<mainProgram> --version (or equivalent) so "it built" doesn't hide "it crashes on startup".
5. Review meta. Double-check license and description even when the build succeeds — detection can pick up the wrong LICENSE variant or copy a typo from the upstream repo.

Common per-language fixes

• Rust: a failing cargoHash after fixing code changes means the lockfile inputs changed; re-prefetch rather than hand-editing. If the crate uses workspace features or git dependencies, consider importCargoLock.
• Go: vendorHash mismatches are routine; take the got: value. Projects with no module dependencies need vendorHash = null.
• Python: dependency names must exist under python3Packages — rename or add overrides where they don't. Missing native libraries show up as build failures; add them to buildInputs. If pythonImportsCheck fails, either fix missing runtime deps or drop the check if it tests optional extras.
• stdenv: everything meaningful (deps, install phase) still needs manual work; consult similar existing packages in nixpkgs.

Where to put the result

nixpkgs contributions (pkgs/by-name)

RFC 140 layout: pkgs/by-name/<first-two-letters>/<name>/package.nix (e.g. pkgs/by-name/vi/vivid/package.nix). Pass that exact file path as the output argument — remember a directory argument produces default.nix, which by-name rejects. Keep the generated structure intact (finalAttrs, updateScript, meta); reviewers expect it. nix-init's -C true can commit the change, but do not commit without being asked.

Local flakes and configs

Write the file somewhere durable (e.g. pkgs/<name>.nix) and wire it up with callPackage semantics:

# flake.nix
outputs = { nixpkgs, ... }: {
  packages.x86_64-linux.vivid = nixpkgs.legacyPackages.x86_64-linux.callPackage ./pkgs/vivid.nix { };
};

For home-manager/NixOS, expose it through a packages overlay or reference the derivation path in home.packages / environment.systemPackages. When adding several generated packages, an overlay returning callPackage results keeps things tidy.

When generation falls short

• Rate limits / private repos — configure access tokens in ~/.config/nix-init/config.toml ([access-tokens]), or ask the user for one.
• Monorepos — generate, then adjust src with the needed subpath handling; nurl supports extra args if you must stay automated.
• Unsupported ecosystems — the stdenv.mkDerivation scaffold is minimal; look for a comparable language builder in nixpkgs (e.g. buildNpmPackage, buildMaven, composerRepository) and model the file on an existing package.