diff --git a/.rubocop.yml b/.rubocop.yml
index e55c5d1..72f47ae 100644
--- a/.rubocop.yml
+++ b/.rubocop.yml
@@ -10,4 +10,4 @@ Metrics/MethodLength:
 Metrics/BlockLength:
     Exclude:
         - bin/git-format-staged  # Long block for option parser is ok
-        
\ No newline at end of file
+        - spec/**/*.rb
diff --git a/spec/git.rb b/spec/git.rb
index 6d5ec56..17cad1f 100644
--- a/spec/git.rb
+++ b/spec/git.rb
@@ -31,6 +31,14 @@ module Git
       File.write absolute, content.end_with?("\n") ? content : "#{content}\n"
     end
 
+    def get_content(name)
+      File.read(Pathname.new(path) + name).chomp
+    end
+
+    def get_staged(name)
+      git 'show', ":#{name}"
+    end
+
     def stage(file)
       git 'add', file
     end
@@ -62,6 +70,12 @@ module Git
     def in_repo(&block)
       Dir.chdir path, &block
     end
+
+    def run_formatter
+      in_repo do
+        FormatStaged.run formatter: "#{__dir__}/test_hook.rb {}", patterns: ['*.test']
+      end
+    end
   end
 
   def self.new_repo
diff --git a/spec/test_hook.rb b/spec/test_hook.rb
new file mode 100755
index 0000000..0dd2109
--- /dev/null
+++ b/spec/test_hook.rb
@@ -0,0 +1,6 @@
+#!/usr/bin/env ruby
+# frozen_string_literal: true
+
+$stdin.readlines.each do |line|
+  puts line.gsub(/([^\s]*)\s*=\s*(.*)/, '\1 = \2')
+end
diff --git a/spec/test_spec.rb b/spec/test_spec.rb
index 778f761..2b3ef4c 100644
--- a/spec/test_spec.rb
+++ b/spec/test_spec.rb
@@ -6,12 +6,8 @@ require 'format_staged'
 
 describe FormatStaged do
   def repo
-    @repo ||= Git.new_repo do |r|
-      r.file_in_tree 'index.js', <<~CONTENT
-        function foo () {
-          return 'foo'
-        }
-      CONTENT
+    @repo ||= Git.new_repo do |repo|
+      repo.file_in_tree 'origin.test', 'x = y'
     end
   end
 
@@ -19,4 +15,41 @@ describe FormatStaged do
     @repo&.cleanup
     @repo = nil
   end
+
+  it 'updates staged file and working copy' do
+    repo.set_content 'test.test', 'a=b'
+    repo.stage 'test.test'
+    repo.run_formatter
+
+    expect(repo.get_staged('test.test')).to eq('a = b')
+    expect(repo.get_content('test.test')).to eq('a = b')
+  end
+
+  it 'leaves other changes in working copy' do
+    repo.set_content 'test.test', "x=y\na=b\n"
+    repo.stage 'test.test'
+    repo.set_content 'test.test', 'abc'
+    repo.run_formatter
+
+    expect(repo.get_content('test.test')).to eq('abc')
+    expect(repo.get_staged('test.test')).to eq("x = y\na = b")
+  end
+
+  it 'merges update to working copy' do
+    repo.set_content 'test.test', "x=y\n#stuff\n"
+    repo.stage 'test.test'
+    repo.set_content 'test.test', "x=y\n#stuff\nmore stuff\n"
+    repo.run_formatter
+
+    expect(repo.get_content('test.test')).to eq("x = y\n#stuff\nmore stuff")
+    expect(repo.get_staged('test.test')).to eq("x = y\n#stuff")
+  end
+
+  it 'only touches files matching the given pattern' do
+    repo.set_content 'test.other', 'x=y'
+    repo.stage 'test.other'
+    repo.run_formatter
+
+    expect(repo.get_content('test.other')).to eq('x=y')
+  end
 end