首先按照官方文档：https://docs.mongodb.org/manual/tutorial/install-mongodb-on-ubuntu/
sudo service mongod start
在执行启动服务时会报错——
Failed to start mongod.service: Unit mongod.service failed to load: No such file or directory.
解决方法在这里：
http://askubuntu.com/questions/690993/mongodb-3-0-2-wont-start-after-upgrading-to-ubuntu-15-10
Create a file /lib/systemd/system/mongodb.service with the following content:
[Unit]
Description=High-performance, schema-free document-oriented database
After=network.target
[Service]
User=mongodb
ExecStart=/usr/bin/mongod –quiet –config /etc/mongod.conf
[Install]
WantedBy=multi-user.target
注意，/etc/mongod.conf是配置文件，原文是mongodb.conf，根据自己的情况修改。
问题原因：
在刚才帖子下方有人解释了——
This error occurred due to the problem with the new Ubuntu (15 and ahead).
Default init system is systemd which was Upstart previously.