启动mongodb报错，无法连接mongodb

报错原因如下：

MongoDB shell version v3.4.2
connecting to: mongodb://127.0.0.1:27017
2017-03-01T12:06:59.013+0800 W NETWORK [thread1] Failed to connect to 127.0.0.1:27017, in(checking socket for error after poll), reason: Connection refused
2017-03-01T12:06:59.015+0800 E QUERY [thread1] Error: couldn't connect to server 127.0.0.1:27017, connection attempt failed :
connect@src/mongo/shell/mongo.js:237:13
@(connect):1:6
exception: connect failed

试了好多方法都没有解决，最后解决办法如下：

1> 进入终端，执行以下命令：

brew services start mongodb

2>再次启动即可：

mongo

查看mongodb安装目录：

which mongodb