解决方法：
Most probably unit mongodb.service is masked. Use following command to unmask it.sudo systemctl unmask mongodband re-runsudo service mongod start
1 vagrant@homestead:~$ sudo service mongod restart 2 Failed to restart mongod.service: Unit mongod.service not found. 3 vagrant@homestead:~$ ^C 4 vagrant@homestead:~$ sudo systemctl unmask mongodb 5 Removed symlink /etc/systemd/system/mongodb.service. 6 vagrant@homestead:~$ sudo service mongod start 7 vagrant@homestead:~$ sudo service mongod status 8 ● mongod.service - MongoDB Database Server 9 Loaded: loaded (/lib/systemd/system/mongod.service; disabled; vendor preset: enabled)10 Active: active (running) since Thu 2018-10-25 03:50:52 UTC; 5s ago11 Docs: https://docs.mongodb.org/manual12 Main PID: 6669 (mongod)13 Tasks: 1914 Memory: 37.9M15 CPU: 85ms16 CGroup: /system.slice/mongod.service17 └─6669 /usr/bin/mongod --config /etc/mongod.conf18 19 Oct 25 03:50:52 homestead systemd[1]: Started MongoDB Database Server.20 Oct 25 03:50:52 homestead mongod[6669]: 2018-10-25T03:50:52.045+0000 I CONTROL [main] Automatically disabling TLS 1.0, to force-enable TLS 1.0 spe21 lines 1-13/13 (END)
