func (e Variable) ValueOrDefault(defaultValue string) string {
	return e.ValueOrDefaultFunc(func() string {
		return defaultValue
	})
}

// ValueOrDefaultFunc returns the value of the environment variable if it is non-empty. Otherwise returns the value function provided.
func (e Variable) ValueOrDefaultFunc(defaultValueFunc func() string) string {
	if value := e.Value(); value != "" {
		return value
	}

	KUBECONFIG Variable = "KUBECONFIG"

	// IstioSrc is the location of istio source ($TOP/src/istio.io/istio
	IstioSrc = REPO_ROOT.ValueOrDefaultFunc(getDefaultIstioSrc)

	// IstioOut is the location of the output directory ($TOP/out)
	IstioOut = verifyFile(TARGET_OUT, TARGET_OUT.ValueOrDefaultFunc(getDefaultIstioOut))

	// LocalOut is the location of the output directory for the OS we are running in,