if impl == nil {
		return cmdError(cmd, errors.New("unknown command"))
	}

	async := impl.Usage().Async
	if cmd.background && !async {
		return cmdError(cmd, errors.New("command cannot be run in background"))
	}

	wait, runErr := impl.Run(s, cmd.args...)
	if wait == nil {
		if async && runErr == nil {
			return cmdError(cmd, errors.New("internal error: async command returned a nil WaitFunc"))

// A CommandError describes an error resulting from attempting to execute a
// specific command.
type CommandError struct {
	File string
	Line int
	Op   string
	Args []string
	Err  error
}

func cmdError(cmd *command, err error) *CommandError {
	return &CommandError{
		File: cmd.file,
		Line: cmd.line,
		Op:   cmd.name,
		Args: cmd.args,
		Err:  err,
	}
}

func (e *CommandError) Error() string {

		out = cgoTypeSigRe.ReplaceAllString(out, "C.")
	}

	// Usually desc is already p.Desc(), but if not, signal cmdError.Error to
	// add a line explicitly mentioning the import path.
	needsPath := importPath != "" && p != nil && desc != p.Desc()

	err := &cmdError{desc, out, importPath, needsPath}
	if cmdErr != nil {
		// The command failed. Report the output up as an error.
		return err
	}