大致如下，无论重启或停止都会报错：
[root@123 ~]# service nginx restart
Stoping nginx… nginx: [error] invalid PID number "" in "/usr/local/nginx/logs/nginx.pid"
failed. Use force-quit
Starting nginx… nginx (pid 11124 599) already running.
原因，猜测可能是nginx卡死：
ps -ef|grep nginx
kill -9 599 11124 #杀死主进程
再次重启，正常：
service nginx start
或
service nginx restart
