Add devshell and initial bats tests

default.nix

@@ -1,28 +1,60 @@
 {
   stdenv,
+  lib,
+  bats,
   python3,
   python3Packages,
+  shellcheck-minimal,
+  shfmt,
 }:
 
+let
+  fs = lib.fileset;
+in
 stdenv.mkDerivation {
   pname = "mechanicaldiff";
   version = "0.0.1";
 
-  src = ./bin;
+  src = fs.toSource {
+    root = ./.;
+    fileset = fs.unions [
+      ./.editorconfig
+      ./bin
+      ./test
+    ];
+  };
 
   buildInputs = [ python3 ];
 
+  postPatch = ''
+    patchShebangs .
+  '';
+
   doCheck = true;
 
-  nativeCheckInputs = [ python3Packages.flake8 ];
+  nativeCheckInputs = [
+    (bats.withLibraries (p: [
+      p.bats-assert
+      p.bats-support
+      p.bats-file
+    ]))
+    python3Packages.flake8
+    shellcheck-minimal
+    shfmt
+  ];
 
   checkPhase = ''
-    flake8 mechanicaldiff.py
+    runHook preCheck
+    flake8 bin/mechanicaldiff.py
+    shellcheck test/mechanicaldiff.bats
+    shfmt -d test/mechanicaldiff.bats
+    bats test
+    runHook postCheck
   '';
 
   installPhase = ''
     mkdir -p $out/bin
-    cp mechanicaldiff.py $out/bin/mechanicaldiff
+    cp $src/bin/mechanicaldiff.py $out/bin/mechanicaldiff
     chmod +x $out/bin/mechanicaldiff
   '';
 }