subcommands.go
func (cdr *Commander) Execute(ctx context.Context, args ...interface{}) ExitStatus {
	if cdr.topFlags.NArg() < 1 {
		cdr.topFlags.Usage()
		return ExitUsageError
	}

	name := cdr.topFlags.Arg(0)

	for _, group := range cdr.commands {
		for _, cmd := range group.commands {
			if name != cmd.Name() {
				continue
			}
			f := flag.NewFlagSet(name, flag.ContinueOnError)
			f.Usage = func() { explain(cdr.Error, cmd) }
			cmd.SetFlags(f)
			if f.Parse(cdr.topFlags.Args()[1:]) != nil {
				return ExitUsageError
			}
			return cmd.Execute(ctx, f, args...)
		}
	}

	// Cannot find this command.
	cdr.topFlags.Usage()
	return ExitUsageError
}
func (cdr *Commander) Execute(ctx context.Context, args ...interface{}) ExitStatus {
	if cdr.topFlags.NArg() < 1 {
		cdr.topFlags.Usage()
		return ExitUsageError
	}

	name := cdr.topFlags.Arg(0)

	for _, group := range cdr.commands {
		for _, cmd := range group.commands {
			if name != cmd.Name() {
				continue
			}
			f := flag.NewFlagSet(name, flag.ContinueOnError)
			f.Usage = func() { explain(cdr.Error, cmd) }
			cmd.SetFlags(f)
			if f.Parse(cdr.topFlags.Args()[1:]) != nil {
				return ExitUsageError
			}
			return cmd.Execute(ctx, f, args...)
		}
	}

	// Cannot find this command.
	cdr.topFlags.Usage()
	return ExitUsageError
}
// Execute should be called once the top-level-flags on a Commander
// have been initialized. It finds the correct subcommand and executes
// it, and returns an ExitStatus with the result. On a usage error, an
// appropriate message is printed to os.Stderr, and ExitUsageError is
// returned. The additional args are provided as-is to the Execute method
// of the selected Command.