diff --git a/cmd/sbox/sbox_test.go b/cmd/sbox/sbox_test.go
new file mode 100644
index 0000000..3f13d2f
--- /dev/null
+++ b/cmd/sbox/sbox_test.go
@@ -0,0 +1,127 @@
+// Copyright 2022 Google Inc. All rights reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package main
+
+import (
+	"io/ioutil"
+	"os"
+	"path/filepath"
+	"strings"
+	"testing"
+)
+
+func Test_filesHaveSameContents(t *testing.T) {
+
+	tests := []struct {
+		name     string
+		a        string
+		b        string
+		missingA bool
+		missingB bool
+
+		equal bool
+	}{
+		{
+			name:  "empty",
+			a:     "",
+			b:     "",
+			equal: true,
+		},
+		{
+			name:  "equal",
+			a:     "foo",
+			b:     "foo",
+			equal: true,
+		},
+		{
+			name:  "unequal",
+			a:     "foo",
+			b:     "bar",
+			equal: false,
+		},
+		{
+			name:  "unequal different sizes",
+			a:     "foo",
+			b:     "foobar",
+			equal: false,
+		},
+		{
+			name:  "equal large",
+			a:     strings.Repeat("a", 2*1024*1024),
+			b:     strings.Repeat("a", 2*1024*1024),
+			equal: true,
+		},
+		{
+			name:  "equal large unaligned",
+			a:     strings.Repeat("a", 2*1024*1024+10),
+			b:     strings.Repeat("a", 2*1024*1024+10),
+			equal: true,
+		},
+		{
+			name:  "unequal large",
+			a:     strings.Repeat("a", 2*1024*1024),
+			b:     strings.Repeat("a", 2*1024*1024-1) + "b",
+			equal: false,
+		},
+		{
+			name:  "unequal large unaligned",
+			a:     strings.Repeat("a", 2*1024*1024+10),
+			b:     strings.Repeat("a", 2*1024*1024+9) + "b",
+			equal: false,
+		},
+		{
+			name:     "missing a",
+			missingA: true,
+			b:        "foo",
+			equal:    false,
+		},
+		{
+			name:     "missing b",
+			a:        "foo",
+			missingB: true,
+			equal:    false,
+		},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			tempDir, err := os.MkdirTemp("", "testFilesHaveSameContents")
+			if err != nil {
+				t.Fatalf("failed to create temp dir: %s", err)
+			}
+			defer os.RemoveAll(tempDir)
+
+			fileA := filepath.Join(tempDir, "a")
+			fileB := filepath.Join(tempDir, "b")
+
+			if !tt.missingA {
+				err := ioutil.WriteFile(fileA, []byte(tt.a), 0666)
+				if err != nil {
+					t.Fatalf("failed to write %s: %s", fileA, err)
+				}
+			}
+
+			if !tt.missingB {
+				err := ioutil.WriteFile(fileB, []byte(tt.b), 0666)
+				if err != nil {
+					t.Fatalf("failed to write %s: %s", fileB, err)
+				}
+			}
+
+			if got := filesHaveSameContents(fileA, fileB); got != tt.equal {
+				t.Errorf("filesHaveSameContents() = %v, want %v", got, tt.equal)
+			}
+		})
+	}
+}