evix

Experimental CLI utility using nix-bindings for the Nix C API, and a Rust replacement for nix-eval-jobs in Rust to leverage language features, and evaluate Nix expressions and stream derivation info as JSON lines in style.

Why evix?

• Faster evaluation by using threads
• Memory used for evaluation is reclaimed after nix-eval-jobs finish, so that the build can use it.
• Evaluation of jobs can fail individually
• It's really cool!

Usage

Exactly one input is required:

evix --flake .#hydraJobs
evix --expr 'import <nixpkgs> {}'
evix --file ./default.nix

# Evaluate the hydraJobs attribute of the patchelf flake
$ evix --flake 'github:NixOS/patchelf#hydraJobs'
copying path '/nix/store/jfdpyszsgvsnz68y36qi65irx7r6a52q-source' from 'https://cache.nixos.org'...
{"attr":"tarball","attrPath":["tarball"],"drvPath":"/nix/store/dbhsb9ji8ya2js87v1q5621lx87smw3l-patchelf-tarball-0.18.0.drv","name":"patchelf-tarball-0.18.0","outputs":{"out":null},"system":"x86_64-linux"}
{"attr":"coverage","attrPath":["coverage"],"drvPath":"/nix/store/h8fzgxxddi1470vad93j2y5s1lyxsii8-patchelf-coverage-0.18.0.drv","name":"patchelf-coverage-0.18.0","outputs":{"out":null},"system":"x86_64-linux"}
{"attr":"patchelf-win32","attrPath":["patchelf-win32"],"drvPath":"/nix/store/8kbg5mf09zyykcjvkmwna621ja8vm5pr-patchelf-i686-w64-mingw32-0.18.0.drv","name":"patchelf-i686-w64-mingw32-0.18.0","outputs":{"out":null},"system":"x86_64-linux"}
{"attr":"patchelf-win64","attrPath":["patchelf-win64"],"drvPath":"/nix/store/5zvjbw8y4k1fs3vhbb465ixhl032imgg-patchelf-x86_64-w64-mingw32-0.18.0.drv","name":"patchelf-x86_64-w64-mingw32-0.18.0","outputs":{"out":null},"system":"x86_64-linux"}
{"attr":"release","attrPath":["release"],"drvPath":"/nix/store/4l4cl6w4afn4g4bha5h6z0nm16vddnph-patchelf-0.18.0.drv","name":"patchelf-0.18.0","outputs":{"out":null},"system":"x86_64-linux"}

Options

Flag	Description
--flake REF	Evaluate a flake output
--expr EXPR	Evaluate an inline Nix expression
--file PATH	Evaluate a Nix file
--arg NAME EXPR	Pass a Nix expression argument
--argstr NAME VALUE	Pass a string argument
--workers N	Worker processes (default: 1)
--max-memory-size MB	Memory limit per worker; restarts when exceeded (default: 4096)
--force-recurse	Recurse into all attrsets, ignoring recurseForDerivations
--gc-roots-dir DIR	Register GC root symlinks for evaluated derivations

Output

Each line is a JSON object. Derivation attributes emit:

{
  "attr": "packages.x86_64-linux.hello",
  "attrPath": ["packages", "x86_64-linux", "hello"],
  "name": "hello-2.12.1",
  "system": "x86_64-linux",
  "drvPath": "/nix/store/...",
  "outputs": { "out": "/nix/store/..." }
}

Non-derivation attrsets emit child attribute names for further recursion:

{
  "attr": "packages.x86_64-linux",
  "attrPath": ["packages", "x86_64-linux"],
  "attrs": ["hello", "git", "vim"]
}

Errors are non-fatal unless "fatal": true:

{"attr": "...", "attrPath": [...], "error": "...", "fatal": false}

Architecture

The binary operates in two modes based on the _NEJ_WORKER environment variable:

• Master (default): Spawns worker subprocesses, manages a work queue of attr paths, distributes work via stdin/stdout, and collects results.
• Worker (_NEJ_WORKER=1): Evaluates Nix expressions, processes individual attr paths, and signals when memory limits are reached.

Workers are restarted automatically when they exceed the memory limit.

Building

Requires Rust 1.90.0+. Supported on x86_64-linux and aarch64-linux.

License

EUPL-1.2