https://blog.gongquiz.com/2021/10/17/%eb%8b%a4%ec%9d%8c-%ec%a4%91-%ea%b0%80%ec%9e%a5-%ed%81%b0-%eb%b2%88%ed%98%b8-%ea%b0%92%ec%9d%84-%ea%b0%96%eb%8a%94-%ec%8b%9c%ea%b7%b8%eb%84%90signal%eb%a1%9c-%ec%95%8c%eb%a7%9e%ec%9d%80-%ea%b2%83/