昨天发现系统宕机了，查了下原因是mongodb服务停了，看了下日志：
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2017-10-18T18:09:59.181+0800 I CONTROL [signalProcessingThread] got signal 1 (Hangup), will terminate after current cmd ends
2017-10-18T18:09:59.181+0800 I FTDC [signalProcessingThread] Shutting down full-time diagnostic data capture
2017-10-18T18:09:59.225+0800 I CONTROL [signalProcessingThread] now exiting
2017-10-18T18:09:59.226+0800 I NETWORK [signalProcessingThread] shutdown: going to close listening sockets...
2017-10-18T18:09:59.226+0800 I NETWORK [signalProcessingThread] closing listening socket: 6
2017-10-18T18:09:59.226+0800 I NETWORK [signalProcessingThread] closing listening socket: 7
2017-10-18T18:09:59.226+0800 I NETWORK [signalProcessingThread] removing socket file: /tmp/mongodb-27017.sock
2017-10-18T18:09:59.226+0800 I NETWORK [signalProcessingThread] shutdown: going to flush diaglog...
2017-10-18T18:09:59.226+0800 I NETWORK [signalProcessingThread] shutdown: going to close sockets...
2017-10-18T18:09:59.226+0800 I STORAGE [signalProcessingThread] WiredTigerKVEngine shutting down
2017-10-18T18:09:59.226+0800 I NETWORK [conn14] end connection 127.0.0.1:37191 (3 connections now open)
2017-10-18T18:09:59.227+0800 I NETWORK [conn13] end connection 127.0.0.1:37190 (3 connections now open)
2017-10-18T18:09:59.227+0800 I NETWORK [conn12] end connection 127.0.0.1:37189 (3 connections now open)
2017-10-18T18:09:59.227+0800 I NETWORK [conn11] end connection 127.0.0.1:37188 (3 connections now open)
2017-10-18T18:09:59.258+0800 I STORAGE [signalProcessingThread] shutdown: removing fs lock...
2017-10-18T18:09:59.258+0800 I CONTROL [signalProcessingThread] dbexit: rc: 0
重新起mongodb的服务(命令：nohup /home/mongodb-linux-x86_64-rhel62-3.2.3/bin/mongod -dbpath /home/mongodb &)，系统又重新正常运行了。关闭Xshell之后系统又宕了。
查了下资料，原因是直接关闭Xshell窗口之后，linux认为是异常退出，这个shell进程会收到SIGNUP信号，然后给它所有的子进程发送SIGHUP信号。
但是在mongodb启动命令中加-fork参数，会自动将其挂到init进程(pid为1)下启动，如果不加的话ppid(父进程id)就是当前的shell进程，所以加上–fork问题就可以解决了。
启动命令：/home/mongodb-linux-x86_64-rhel62-3.2.3/bin/mongod --dbpath /home/mongodb --logpath=/home/mongodb/rs2.1.log --fork
参考：http://blog.csdn.net/shawyeok/article/details/46486683