- Failed to restart mongod.service: Unit mongod.service not found.
解决方法：
Most probably unit mongodb.service is masked. Use following command to unmask it. sudo systemctl unmask mongodb and re-run sudo service mongod start
vagrant@homestead:~$ sudo service mongod restart
Failed to restart mongod.service: Unit mongod.service not found.
vagrant@homestead:~$ ^C
vagrant@homestead:~$ sudo systemctl unmask mongodb
Removed symlink /etc/systemd/system/mongodb.service.
vagrant@homestead:~$ sudo service mongod start
vagrant@homestead:~$ sudo service mongod status
● mongod.service - MongoDB Database Server
Loaded: loaded (/lib/systemd/system/mongod.service; disabled; vendor preset: enabled)
Active: active (running) since Thu -- :: UTC; 5s ago
Docs: https://docs.mongodb.org/manual
Main PID: (mongod)
Tasks:
Memory: 37.9M
CPU: 85ms
CGroup: /system.slice/mongod.service
└─ /usr/bin/mongod --config /etc/mongod.conf Oct :: homestead systemd[]: Started MongoDB Database Server.
Oct :: homestead mongod[]: --25T03::52.045+ I CONTROL [main] Automatically disabling TLS 1.0, to force-enable TLS 1.0 spe
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相关知识点：
屏蔽和取消屏蔽单位
我们的服务管理部分如何停止或禁用服务所看到的，但systemd还具有自动或手动标记机作为完全无法启动，通过其链接到的能力/dev/null 。 这就是所谓的掩蔽单元，并且可能用mask命令：
sudo systemctl mask nginx.service这将防止Nginx服务自动或手动启动，只要它被屏蔽。
如果选中list-unit-files ，你会看到该服务现已被列为蒙面：
systemctl list-unit-files. . .
kmod-static-nodes.service static
ldconfig.service static
mandb.service static
messagebus.service static
nginx.service masked
quotaon.service static
rc-local.service static
rdisc.service disabled
rescue.service static
. . .如果您尝试启动服务，您将看到如下所示的消息：
sudo systemctl start nginx.serviceFailed to start nginx.service: Unit nginx.service is masked.要再次揭露一个单元，使其可以使用，只要使用unmask命令：
sudo systemctl unmask nginx.service这将使单元返回到先前的状态，允许它被启动或启用。