Support OnlySubCommands for validation of positional arguments

Currently, suggestions for "unknown command" are not working for
subcommands.

This patch supports new validation of positional arguments as
`OnlySubCommands`. It accepts only subcommands if enabled, and also
`findSuggestions()` will be called.

README.md

@@ -392,6 +392,7 @@ The following validators are built in:
 - `NoArgs` - the command will report an error if there are any positional args.
 - `ArbitraryArgs` - the command will accept any args.
 - `OnlyValidArgs` - the command will report an error if there are any positional args that are not in the `ValidArgs` field of `Command`.
+- `OnlySubCommands` - the command will report an error if if any args are included except for subcommands.
 - `MinimumNArgs(int)` - the command will report an error if there are not at least N positional args.
 - `MaximumNArgs(int)` - the command will report an error if there are more than N positional args.
 - `ExactArgs(int)` - the command will report an error if there are not exactly N positional args.

args.go

@@ -43,6 +43,17 @@ func OnlyValidArgs(cmd *Command, args []string) error {
 	return nil
 }
 
+// OnlySubCommands returns an error if any args are included except for subcommands.
+func OnlySubCommands(cmd *Command, args []string) error {
+	if !cmd.HasSubCommands() {
+		return nil
+	}
+	if len(args) > 0 {
+		return fmt.Errorf("unknown command %q for %q%s", args[0], cmd.CommandPath(), cmd.findSuggestions(args[0]))
+	}
+	return nil
+}
+
 // ArbitraryArgs never returns an error.
 func ArbitraryArgs(cmd *Command, args []string) error {
 	return nil

args_test.go

@@ -69,6 +69,46 @@ func TestOnlyValidArgsWithInvalidArgs(t *testing.T) {
 	}
 }
 
+func TestOnlySubCommandsArgs(t *testing.T) {
+	rootCmd := &Command{
+		Use:  "root",
+		Args: OnlySubCommands,
+		Run:  emptyRun,
+	}
+	subCmd := &Command{Use: "sub", Run: emptyRun}
+	rootCmd.AddCommand(subCmd)
+
+	output, err := executeCommand(rootCmd, "sub")
+	if output != "" {
+		t.Errorf("Unexpected output: %v", output)
+	}
+
+	if err != nil {
+		t.Fatalf("Unexpected error: %v", err)
+	}
+}
+
+func TestOnlySubCommandsArgsWithInvalidArgs(t *testing.T) {
+	rootCmd := &Command{
+		Use:  "root",
+		Args: OnlySubCommands,
+		Run:  emptyRun,
+	}
+	subCmd := &Command{Use: "sub", Run: emptyRun}
+	rootCmd.AddCommand(subCmd)
+
+	_, err := executeCommand(rootCmd, "notsub")
+	if err == nil {
+		t.Fatal("Expected an error")
+	}
+
+	got := err.Error()
+	expected := `unknown command "notsub" for "root"`
+	if got != expected {
+		t.Errorf("Expected: %q, got: %q", expected, got)
+	}
+}
+
 func TestArbitraryArgs(t *testing.T) {
 	c := &Command{Use: "c", Args: ArbitraryArgs, Run: emptyRun}
 	output, err := executeCommand(c, "a", "b")