问题描述：
使用命令：mongod --config /usr/local/etc/mongod.conf 无法启动mongodb，也不打印任何信息。之前用的都好好的，请问是什么原因呢？
输入mongod 打印：
liudeMacBook-Air:~ liu$ mongod
2016-05-09T15:05:42.632+0800 I CONTROL [initandlisten] MongoDB starting : pid=867 port=27017 dbpath=/data/db 64-bit host=liudeMacBook-Air.local
2016-05-09T15:05:42.633+0800 I CONTROL [initandlisten] db version v3.2.3
2016-05-09T15:05:42.633+0800 I CONTROL [initandlisten] git version: b326ba837cf6f49d65c2f85e1b70f6f31ece7937
2016-05-09T15:05:42.633+0800 I CONTROL [initandlisten] allocator: system
2016-05-09T15:05:42.633+0800 I CONTROL [initandlisten] modules: none
2016-05-09T15:05:42.633+0800 I CONTROL [initandlisten] build environment:
2016-05-09T15:05:42.633+0800 I CONTROL [initandlisten] distarch: x86_64
2016-05-09T15:05:42.633+0800 I CONTROL [initandlisten] target_arch: x86_64
2016-05-09T15:05:42.633+0800 I CONTROL [initandlisten] options: {}
2016-05-09T15:05:42.633+0800 I STORAGE [initandlisten] exception in initAndListen: 29 Data directory /data/db not found., terminating
2016-05-09T15:05:42.633+0800 I CONTROL [initandlisten] dbexit: rc: 100环境：osx（mac）
从错误消息来看，很明确地说明了/data/db不存在，应该先检查这点。
2016-05-09T15:05:42.633+0800 I STORAGE [initandlisten] exception in initAndListen: 29 Data directory /data/db not found., terminating
如有必要先建立这个目录再试
sudo mkdir /data/db