diff --git a/internal/app/sysconf_test.go b/internal/app/sysconf_test.go
new file mode 100644
index 0000000..b73f2d0
--- /dev/null
+++ b/internal/app/sysconf_test.go
@@ -0,0 +1,15 @@
+package app
+
+import "testing"
+
+const (
+	_POSIX_LOGIN_NAME_MAX = 9
+)
+
+func TestSysconf(t *testing.T) {
+	t.Run("LOGIN_NAME_MAX", func(t *testing.T) {
+		if got := sysconf(_SC_LOGIN_NAME_MAX); got < _POSIX_LOGIN_NAME_MAX {
+			t.Errorf("sysconf(_SC_LOGIN_NAME_MAX): %d < _POSIX_LOGIN_NAME_MAX", got)
+		}
+	})
+}
diff --git a/internal/app/username_test.go b/internal/app/username_test.go
new file mode 100644
index 0000000..15f15a4
--- /dev/null
+++ b/internal/app/username_test.go
@@ -0,0 +1,26 @@
+package app
+
+import (
+	"strings"
+	"testing"
+)
+
+func TestIsValidUsername(t *testing.T) {
+	t.Run("long", func(t *testing.T) {
+		if isValidUsername(strings.Repeat("a", sysconf(_SC_LOGIN_NAME_MAX))) {
+			t.Errorf("isValidUsername unexpected true")
+		}
+	})
+
+	t.Run("regexp", func(t *testing.T) {
+		if isValidUsername("0") {
+			t.Errorf("isValidUsername unexpected true")
+		}
+	})
+
+	t.Run("valid", func(t *testing.T) {
+		if !isValidUsername("alice") {
+			t.Errorf("isValidUsername unexpected false")
+		}
+	})
+}