@@ -45,7 +45,7 @@ func runUpdate(dockerCli *client.DockerCli, flags *pflag.FlagSet, serviceID stri

 		return err

 	}

-	err = updateService(&service.Spec, flags)

+	err = updateService(flags, &service.Spec)

 	if err != nil {

 		return err

 	}

@@ -58,7 +58,7 @@ func runUpdate(dockerCli *client.DockerCli, flags *pflag.FlagSet, serviceID stri

 	return nil

 }

-func updateService(spec *swarm.ServiceSpec, flags *pflag.FlagSet) error {

+func updateService(flags *pflag.FlagSet, spec *swarm.ServiceSpec) error {

 	updateString := func(flag string, field *string) {

 		if flags.Changed(flag) {

@@ -123,7 +123,7 @@ func updateService(spec *swarm.ServiceSpec, flags *pflag.FlagSet) error {

 	updateLabels(flags, &spec.Labels)

 	updateString("image", &cspec.Image)

 	updateSlice("command", &cspec.Command)

-	updateSlice("arg", &cspec.Command)

+	updateSlice("arg", &cspec.Args)

 	updateListOpts("env", &cspec.Env)

 	updateString("workdir", &cspec.Dir)

 	updateString(flagUser, &cspec.User)

new file mode 100644

@@ -0,0 +1,26 @@

+package service

+

+import (

+	"testing"

+

+	"github.com/docker/docker/pkg/testutil/assert"

+	"github.com/docker/engine-api/types/swarm"

+)

+

+func TestUpdateServiceCommandAndArgs(t *testing.T) {

+	flags := newUpdateCommand(nil).Flags()

+	flags.Set("command", "the")

+	flags.Set("command", "new")

+	flags.Set("command", "command")

+	flags.Set("arg", "the")

+	flags.Set("arg", "new args")

+

+	spec := &swarm.ServiceSpec{}

+	cspec := &spec.TaskTemplate.ContainerSpec

+	cspec.Command = []string{"old", "command"}

+	cspec.Args = []string{"old", "args"}

+

+	updateService(flags, spec)

+	assert.EqualStringSlice(t, cspec.Command, []string{"the", "new", "command"})

+	assert.EqualStringSlice(t, cspec.Args, []string{"the", "new args"})

+}

@@ -19,6 +19,21 @@ func Equal(t TestingT, actual, expected interface{}) {

 	}

 }

+//EqualStringSlice compares two slices and fails the test if they do not contain

+// the same items.

+func EqualStringSlice(t TestingT, actual, expected []string) {

+	if len(actual) != len(expected) {

+		t.Fatalf("Expected (length %d): %q\nActual (length %d): %q",

+			len(expected), expected, len(actual), actual)

+	}

+	for i, item := range actual {

+		if item != expected[i] {

+			t.Fatalf("Slices differ at element %d, expected %q got %q",

+				i, expected[i], item)

+		}

+	}

+}

+

 // NilError asserts that the error is nil, otherwise it fails the test.

 func NilError(t TestingT, err error) {

 	if err != nil {