diff --git a/tests/test_jail.py b/tests/test_jail.py
new file mode 100644
index 0000000..dd45c5d
--- /dev/null
+++ b/tests/test_jail.py
@@ -0,0 +1,28 @@
+from focker.jail import backup_file
+import tempfile
+import os
+
+
+def test_backup_file():
+    with tempfile.TemporaryDirectory() as d:
+        fname = os.path.join(d, 'dummy.conf')
+        with open(fname, 'w') as f:
+            f.write('init')
+        nbackups = 10
+        for i in range(15):
+            backup_file(fname, nbackups=nbackups, chmod=0o640)
+            with open(fname, 'w') as f:
+                f.write(str(i))
+
+        fname = os.path.join(d, 'dummy.conf')
+        with open(fname, 'r') as f:
+            assert f.read() == '14'
+
+        for i in range(nbackups):
+            fname = os.path.join(d, 'dummy.conf.%d' % i)
+            assert os.path.exists(fname)
+            with open(fname, 'r') as f:
+                if i < 5:
+                    assert f.read() == str(i + 9)
+                else:
+                    assert f.read() == str(i - 1)