Report an error if no flag(s) set in service update (#318)

For now if no flag(s) set, service update will still try to
do an update, it should return an error instead.

[issue 286](#286)

pkg/kn/commands/service/service_update.go

@@ -29,7 +29,7 @@ func NewServiceUpdateCommand(p *commands.KnParams) *cobra.Command {
 	var waitFlags commands.WaitFlags
 
 	serviceUpdateCommand := &cobra.Command{
-		Use:   "update NAME",
+		Use:   "update NAME [flags]",
 		Short: "Update a service.",
 		Example: `
   # Updates a service 'mysvc' with new environment variables
@@ -91,10 +91,21 @@ func NewServiceUpdateCommand(p *commands.KnParams) *cobra.Command {
 				return nil
 			}
 		},
+		PreRunE: func(cmd *cobra.Command, args []string) error {
+			return preCheck(cmd, args)
+		},
 	}
 
 	commands.AddNamespaceFlags(serviceUpdateCommand.Flags(), false)
 	editFlags.AddUpdateFlags(serviceUpdateCommand)
 	waitFlags.AddConditionWaitFlags(serviceUpdateCommand, 60, "Update", "service")
 	return serviceUpdateCommand
 }
+
+func preCheck(cmd *cobra.Command, args []string) error {
+	if cmd.Flags().NFlag() == 0 {
+		return errors.New(fmt.Sprintf("flag(s) not set\nUsage: %s", cmd.Use))
+	}
+
+	return nil
+}

pkg/kn/commands/service/service_update_test.go

@@ -88,6 +88,26 @@ func fakeServiceUpdate(original *v1alpha1.Service, args []string, sync bool) (
 	return
 }
 
+func TestServcieUpdateNoFlags(t *testing.T) {
+	orig := newEmptyService()
+
+	action, _, _, err := fakeServiceUpdate(orig, []string{
+		"service", "update", "foo"}, false)
+
+	if action != nil {
+		t.Errorf("Unexpected action if no flag(s) set")
+	}
+
+	if err == nil {
+		t.Fatal(err)
+	}
+
+	expectedErrMsg := "flag(s) not set"
+	if !strings.Contains(err.Error(), expectedErrMsg) {
+		t.Fatalf("Missing %s in %s", expectedErrMsg, err.Error())
+	}
+}
+
 func TestServiceUpdateImageSync(t *testing.T) {
 	orig := newEmptyService()