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https://www.enteroa.com/2015/02/07/%ec%89%98-%ec%8a%a4%ed%81%ac%eb%a6%bd%ed%8a%b8%ec%9d%98-%ec%a4%91%eb%b3%b5%ec%8b%a4%ed%96%89-%eb%b0%a9%ec%a7%80%eb%a5%bc-%ec%9c%84%ed%95%b4-pid-%eb%a5%bc-%ec%83%9d%ec%84%b1%ed%95%98%ec%9e%90/
쉘 스크립트의 중복실행 방지를 위해 PID 를 생성