Implement exec.

Change-Id: I20329bc9b378479d745b498d6a00eca0872cd5ab
diff --git a/init/util_test.cpp b/init/util_test.cpp
index e9a1581..5b3ab50 100644
--- a/init/util_test.cpp
+++ b/init/util_test.cpp
@@ -35,3 +35,9 @@
   s[5] = 0;
   EXPECT_STREQ("Linux", s.c_str());
 }
+
+TEST(util, decode_uid) {
+  EXPECT_EQ(0U, decode_uid("root"));
+  EXPECT_EQ(-1U, decode_uid("toot"));
+  EXPECT_EQ(123U, decode_uid("123"));
+}