Existing shell.nix didn't work for me.

Rakefile

@@ -0,0 +1,8 @@
+require 'rake/testtask'
+
+Rake::TestTask.new do |t|
+  t.pattern = 'test/*.rb'
+  t.warning = false
+end
+
+task :default => :test

shell.nix

@@ -1,19 +1,36 @@
 with (import <nixpkgs> {});
 let
-  ruby = pkgs.ruby_2_4_1;
-  bundler = pkgs.bundler.override { inherit ruby; };
-  gems = bundlerEnv {
+  minitest = buildRubyGem {
     inherit ruby;
-    name = "bundix-gems";
-    gemdir = ./.;
+    gemName = "minitest";
+    type = "gem";
+    version = "5.10.1";
+    sha256 = "1yk2m8sp0p5m1niawa3ncg157a4i0594cg7z91rzjxv963rzrwab";
+    gemPath = [];
+  };
+
+  rake = buildRubyGem {
+    inherit ruby;
+    gemName = "rake";
+    type = "gem";
+    version = "12.0.0";
+    sha256 = "01j8fc9bqjnrsxbppncai05h43315vmz9fwg28qdsgcjw9ck1d7n";
+    gemPath = [];
   };
 in
-stdenv.mkDerivation {
-  name = "bundix-shell";
-  buildInputs = [
-    bundler
-    (lowPrio gems)
-    ruby
-    postgresql96
-  ];
+  stdenv.mkDerivation {
+  name = "bundix";
+  src = ./.;
+  phases = "installPhase";
+  installPhase = ''
+    mkdir -p $out
+    makeWrapper $src/bin/bundix $out/bin/bundix \
+      --prefix PATH : "${nix.out}/bin" \
+      --prefix PATH : "${nix-prefetch-git.out}/bin" \
+      --set GEM_PATH "${bundler}/${bundler.ruby.gemPath}"
+  '';
+
+  nativeBuildInputs = [makeWrapper];
+
+  buildInputs = [bundler ruby minitest rake nix-prefetch-scripts];
 }

test/commandline.rb

@@ -0,0 +1,34 @@
+require 'minitest/autorun'
+require 'bundix/commandline'
+
+class CommandLineTest < Minitest::Test
+  def setup
+    @cli = Bundix::CommandLine.new
+    @options = {
+      project: "test-project",
+      ruby: "test-ruby",
+      gemfile: "test-gemfile",
+      lockfile: "test-lockfile",
+      gemset: "test-gemset",
+    }
+  end
+
+  def test_shell_nix
+    assert_equal(@cli.shell_nix_string(@options), <<SHELLNIX)
+with (import <nixpkgs> {});
+let
+  env = bundlerEnv {
+    name = "test-project-bundler-env";
+    inherit test-ruby;
+    gemfile  = ./test-gemfile;
+    lockfile = ./test-lockfile;
+    gemset   = ./test-gemset;
+  };
+in stdenv.mkDerivation {
+  name = "test-project";
+  buildInputs = [ env ];
+}
+SHELLNIX
+
+  end
+end