输入mongo，报如下错误：

root@instance-tbbjrcnc:/# mongo
MongoDB shell version v4.0.12
connecting to: mongodb://127.0.0.1:27017/?gssapiServiceName=mongodb
2019-08-12T10:40:58.930+0800 E QUERY    [js] Error: couldn't connect to server 127.0.0.1:27017, connection attempt failed: SocketException: Error connecting to 127.0.0.1:27017 :: caused by :: Connection refused :
connect@src/mongo/shell/mongo.js:344:17
@(connect):2:6
exception: connect failed

原因是mongo没有启动

解决方式一：

创建目录 /data/db

cd /
mkdir data
cd data
mkdir db

查找到mongo的安装路径

root@instance-tbbjrcnc:/# whereis mongo
mongo: /usr/bin/mongo 

然后手动启动mongo

root@instance-tbbjrcnc:/# cd /usr/bin/
root@instance-tbbjrcnc:/usr/bin# sudo ./mongod  -dbpath /data/db/

启动成功！

方法二：创建 service 启动，使用命令启动：service  mongodb restart

https://blog.csdn.net/cocos2dGirl/article/details/99300471

方法三：

https://stackoverflow.com/questions/45691032/mongod-service-start-exits-with-code-100

参考：https://blog.csdn.net/qq_39729527/article/details/81285405