连接MongoDB时出现错误：SHARDING [initandlisten] Marking collection local.system.replset as collection version: <unsharded>

问题详述：当输入mongo连接时自动退出，显示Error: couldn't connect to server 127.0.0.1:27017, connection attempt failed

于是输入mongod，出现如上问题SHARDING [initandlisten] Marking collection local.system.replset as collection version: <unsharded>

解决办法：再另外打开一个终端shell，打开对应bin目录，然后mongo就能连接了

原理：在运行了mongod命令之后，系统输出的日志会停在I SHARDING [ftdc] Marking collection local.oplog.rs as collection version: <unsharded>，