private function defineArgument($key)
    {
        $shellArgumentFound = false;
        foreach ($this->shellArguments as $shellArgument) {
            if ($shellArgument['key'] === $key) {
                $shellArgumentFound = true;
            }
        }

        if (! $shellArgumentFound) {
            array_push($this->shellArguments, ['key' => $key, 'value' => '']);
        }

        return $this;
    }