Fix service/revision list output with '-o' param

CHANGELOG.adoc

@@ -40,6 +40,10 @@
 | 🎁
 | Add context.Context parameter to API function
 | https://github.com/knative/client/pull/1274[#1274]
+
+|🐛
+| Respect `-o` in `service list` and `revision list` in case if no services/revisions present
+| https://github.com/knative/client/pull/1276[#1276]
 |===
 
 ## v0.21.0 (2021-02-23)

pkg/kn/commands/revision/list.go

@@ -83,7 +83,7 @@ func NewRevisionListCommand(p *commands.KnParams) *cobra.Command {
 			}
 
 			// Stop if nothing found
-			if len(revisionList.Items) == 0 {
+			if !revisionListFlags.GenericPrintFlags.OutputFlagSpecified() && len(revisionList.Items) == 0 {
 				fmt.Fprintf(cmd.OutOrStdout(), "No revisions found.\n")
 				return nil
 			}

pkg/kn/commands/revision/list_test.go

@@ -15,6 +15,7 @@
 package revision
 
 import (
+	"encoding/json"
 	"strings"
 	"testing"
 
@@ -60,6 +61,27 @@ func TestRevisionListEmpty(t *testing.T) {
 	}
 }
 
+func TestRevisionListEmptyWithJSON(t *testing.T) {
+	action, output, err := fakeRevisionList([]string{"revision", "list", "-o", "json"}, &servingv1.RevisionList{})
+	assert.NilError(t, err)
+	if action == nil {
+		t.Errorf("No action")
+	} else if !action.Matches("list", "revisions") {
+		t.Errorf("Bad action %v", action)
+	}
+
+	var result servingv1.RevisionList
+	err = json.Unmarshal([]byte(strings.Join(output[:], "\n")), &result)
+	assert.NilError(t, err)
+	assert.DeepEqual(t, result, servingv1.RevisionList{
+		TypeMeta: metav1.TypeMeta{
+			APIVersion: "serving.knative.dev/v1",
+			Kind:       "RevisionList",
+		},
+		Items: []servingv1.Revision{},
+	})
+}
+
 func TestRevisionListEmptyByName(t *testing.T) {
 	action, _, err := fakeRevisionList([]string{"revision", "list", "name"}, &servingv1.RevisionList{})
 	assert.NilError(t, err)

pkg/kn/commands/service/list.go

@@ -64,7 +64,9 @@ func NewServiceListCommand(p *commands.KnParams) *cobra.Command {
 			if err != nil {
 				return err
 			}
-			if len(serviceList.Items) == 0 {
+
+			// Stop if nothing found
+			if !serviceListFlags.GenericPrintFlags.OutputFlagSpecified() && len(serviceList.Items) == 0 {
 				fmt.Fprintf(cmd.OutOrStdout(), "No services found.\n")
 				return nil
 			}

pkg/kn/commands/service/list_test.go

@@ -15,6 +15,7 @@
 package service
 
 import (
+	"encoding/json"
 	"strings"
 	"testing"
 
@@ -58,6 +59,27 @@ func TestListEmpty(t *testing.T) {
 	}
 }
 
+func TestListEmptyWithJSON(t *testing.T) {
+	action, output, err := fakeServiceList([]string{"service", "list", "-o", "json"}, &servingv1.ServiceList{})
+	assert.NilError(t, err)
+	if action == nil {
+		t.Errorf("No action")
+	} else if !action.Matches("list", "services") {
+		t.Errorf("Bad action %v", action)
+	}
+
+	var result servingv1.ServiceList
+	err = json.Unmarshal([]byte(strings.Join(output[:], "\n")), &result)
+	assert.NilError(t, err)
+	assert.DeepEqual(t, result, servingv1.ServiceList{
+		TypeMeta: metav1.TypeMeta{
+			APIVersion: "serving.knative.dev/v1",
+			Kind:       "ServiceList",
+		},
+		Items: []servingv1.Service{},
+	})
+}
+
 func TestGetEmpty(t *testing.T) {
 	action, _, err := fakeServiceList([]string{"service", "list", "name"}, &servingv1.ServiceList{})
 	assert.NilError(t, err)