diff --git a/.editorconfig b/.editorconfig
index ea3959a..d4c724e 100644
--- a/.editorconfig
+++ b/.editorconfig
@@ -10,6 +10,9 @@ trim_trailing_whitespace = true
 [*.py]
 indent_size = 4
 
+[*.bats]
+indent_size = 2
+
 [*.nix]
 indent_size = 2
 
diff --git a/default.nix b/default.nix
index 5044cbf..b5f2646 100644
--- a/default.nix
+++ b/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 $out/bin/mechanicaldiff
   '';
 }
diff --git a/flake.nix b/flake.nix
index d97cd4f..e386431 100644
--- a/flake.nix
+++ b/flake.nix
@@ -15,6 +15,19 @@
       in
       {
         packages.default = pkgs.callPackage ./default.nix { };
+        devShells.default = pkgs.mkShell {
+          packages = [
+            (pkgs.bats.withLibraries (p: [
+              p.bats-assert
+              p.bats-support
+              p.bats-file
+            ]))
+            pkgs.python3
+            pkgs.python3Packages.flake8
+            pkgs.shellcheck
+            pkgs.shfmt
+          ];
+        };
       }
     );
 }
diff --git a/test/mechanicaldiff.bats b/test/mechanicaldiff.bats
new file mode 100755
index 0000000..c415c7d
--- /dev/null
+++ b/test/mechanicaldiff.bats
@@ -0,0 +1,50 @@
+#!/usr/bin/env bats
+
+setup() {
+  set -euo pipefail
+
+  bats_load_library bats-support
+  bats_load_library bats-assert
+
+  DIR="$(cd "$(dirname "$BATS_TEST_FILENAME")" >/dev/null 2>&1 && pwd)"
+  PATH="$DIR/../bin:$PATH"
+}
+
+@test "keeps hunk when replacement matches" {
+  run bash -c 'cat <<"EOF" | mechanicaldiff.py foo bar
+diff --git a/file.txt b/file.txt
+index 1111111..2222222 100644
+--- a/file.txt
++++ b/file.txt
+@@ -1,1 +1,1 @@
+-foo
++bar
+EOF'
+  assert_success
+  expected="$(
+    cat <<'EOF'
+diff --git a/file.txt b/file.txt
+index 1111111..2222222 100644
+--- a/file.txt
++++ b/file.txt
+@@ -1,1 +1,1 @@
+-foo
++bar
+EOF
+  )"
+  assert_output "$expected"
+}
+
+@test "drops diff when replacement does not match" {
+  run bash -c 'cat <<"EOF" | mechanicaldiff.py foo bar
+diff --git a/file.txt b/file.txt
+index 1111111..2222222 100644
+--- a/file.txt
++++ b/file.txt
+@@ -1,1 +1,1 @@
+-foo
++baz
+EOF'
+  assert_success
+  assert_output ""
+}