在 Ubuntu 20.04 系统中，使用 systemctl 命令启动 logstash，出现以下错误：

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$ systemctl start logstash
System has not been booted with systemd as init system (PID 1). Can't operate.
Failed to connect to bus: Host is down

这是因为 systemctl 不是 Ubuntu 系统自带的命令，可以通过 apt 命令进行安装：

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sudo apt install systemctl

安装过程中，会提示你输入 Yes, do as I say! 以继续安装。

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