Add a new ValidateRequiredFlagsFunc to Command to customize the required flag validation behavior. This is useful to customize the validation and messaging when you have to set, say, either flag1 or flag2, but not both.

command.go

@@ -143,6 +143,9 @@ type Command struct {
 	//FParseErrWhitelist flag parse errors to be ignored
 	FParseErrWhitelist FParseErrWhitelist
 
+	// ValidateRequiredFlagsFunc is called to validate if all required flags are set
+	ValidateRequiredFlagsFunc func(*Command) error
+
 	// commands is the list of commands supported by this program.
 	commands []*Command
 	// parent is a parent command for this command.
@@ -880,7 +883,8 @@ func (c *Command) ValidateArgs(args []string) error {
 	return c.Args(c, args)
 }
 
-func (c *Command) validateRequiredFlags() error {
+// GetMissingRequiredFlags returns the names of the missing required flags
+func (c *Command) GetMissingRequiredFlags() []string {
 	flags := c.Flags()
 	missingFlagNames := []string{}
 	flags.VisitAll(func(pflag *flag.Flag) {
@@ -892,7 +896,18 @@ func (c *Command) validateRequiredFlags() error {
 			missingFlagNames = append(missingFlagNames, pflag.Name)
 		}
 	})
+	return missingFlagNames
+}
 
+func (c *Command) validateRequiredFlags() error {
+	if c.ValidateRequiredFlagsFunc != nil {
+		return c.ValidateRequiredFlagsFunc(c)
+	}
+	return c.defaultValidateRequiredFlags()
+}
+
+func (c *Command) defaultValidateRequiredFlags() error {
+	missingFlagNames := c.GetMissingRequiredFlags()
 	if len(missingFlagNames) > 0 {
 		return fmt.Errorf(`required flag(s) "%s" not set`, strings.Join(missingFlagNames, `", "`))
 	}

command_test.go

@@ -691,6 +691,27 @@ func TestRequiredFlags(t *testing.T) {
 	}
 }
 
+func TestRequiredFlagsFunc(t *testing.T) {
+	c := &Command{Use: "c", Run: emptyRun, ValidateRequiredFlagsFunc: func(c *Command) error {
+		missing := c.GetMissingRequiredFlags()
+		return fmt.Errorf(strings.Join(missing, "|"))
+	}}
+	c.Flags().String("foo1", "", "")
+	c.MarkFlagRequired("foo1")
+	c.Flags().String("foo2", "", "")
+	c.MarkFlagRequired("foo2")
+	c.Flags().String("bar", "", "")
+
+	expected := "foo1|foo2"
+
+	_, err := executeCommand(c)
+	got := err.Error()
+
+	if got != expected {
+		t.Errorf("Expected error: %q, got: %q", expected, got)
+	}
+}
+
 func TestPersistentRequiredFlags(t *testing.T) {
 	parent := &Command{Use: "parent", Run: emptyRun}
 	parent.PersistentFlags().String("foo1", "", "")
@@ -716,6 +737,34 @@ func TestPersistentRequiredFlags(t *testing.T) {
 	}
 }
 
+func TestPersistentRequiredFlagsFunc(t *testing.T) {
+	parent := &Command{Use: "parent", Run: emptyRun}
+	parent.PersistentFlags().String("foo1", "", "")
+	parent.MarkPersistentFlagRequired("foo1")
+	parent.PersistentFlags().String("foo2", "", "")
+	parent.MarkPersistentFlagRequired("foo2")
+	parent.Flags().String("foo3", "", "")
+
+	child := &Command{Use: "child", Run: emptyRun, ValidateRequiredFlagsFunc: func(c *Command) error {
+		missing := c.GetMissingRequiredFlags()
+		return fmt.Errorf(strings.Join(missing, "|"))
+	}}
+	child.Flags().String("bar1", "", "")
+	child.MarkFlagRequired("bar1")
+	child.Flags().String("bar2", "", "")
+	child.MarkFlagRequired("bar2")
+	child.Flags().String("bar3", "", "")
+
+	parent.AddCommand(child)
+
+	expected := "bar1|bar2|foo1|foo2"
+
+	_, err := executeCommand(parent, "child")
+	if err.Error() != expected {
+		t.Errorf("Expected %q, got %q", expected, err.Error())
+	}
+}
+
 func TestInitHelpFlagMergesFlags(t *testing.T) {
 	usage := "custom flag"
 	rootCmd := &Command{Use: "root"}