public function fs($value = null)
    {
        if (null === $value) {
            return $this->fs;
        }
        if (is_string($value))
            $value = new Filesystem($value);
        $this->fs = $value;
        foreach ($this->commands as $name => $command) {
            $command->fs = $value;
        }
        return $this;
    }