[TD-1656] <doc>: fix command to detect systemd in documentation.

documentation20/webdocs/markdowndocs/Getting Started-ch.md

@@ -30,13 +30,13 @@ TDengine软件分为服务器、客户端和报警模块三部分，目前2.0版
 
 - TDengine-alert-2.0.0-Linux-x64.tar.gz (8.1M)
 
-目前，TDengine只支持在使用[`systemd`](https://en.wikipedia.org/wiki/Systemd)做进程服务管理的linux系统上安装。其他linux系统的支持正在开发中。用`which`命令来检测系统中是否存在`systemd`:
+目前，TDengine只支持在使用[`systemd`](https://en.wikipedia.org/wiki/Systemd)做进程服务管理的linux系统上安装。其他linux系统的支持正在开发中。用`which systemctl`命令来检测系统中是否存在`systemd`包:
 
 ```cmd
-which systemd
+which systemctl
 ```
 
-如果系统中不存在`systemd`命令，请考虑[通过源码安装](#通过源码安装)TDengine。
+如果系统中不存在`systemd`包，请考虑[通过源码安装](#通过源码安装)TDengine。
 
 具体的安装过程，请参见<a href="https://www.taosdata.com/blog/2019/08/09/566.html">TDengine多种安装包的安装和卸载</a>。
 

documentation20/webdocs/markdowndocs/Getting Started.md

@@ -16,13 +16,13 @@ Three different packages are provided, please pick up the one you like.
 <li><a id='tdengine-deb' style='color:var(--b2)'>TDengine DEB package (1.7M)</a></li>
 <li><a id='tdengine-tar' style='color:var(--b2)'>TDengine Tarball (3.0M)</a></li>
 </ul>
-For the time being, TDengine only supports installation on Linux systems using [`systemd`](https://en.wikipedia.org/wiki/Systemd) as the service manager. To check if your system has *systemd*, use the _which_ command.
+For the time being, TDengine only supports installation on Linux systems using [`systemd`](https://en.wikipedia.org/wiki/Systemd) as the service manager. To check if your system has *systemd* package, use the _which systemctl_ command.
 
 ```cmd
-which systemd
+which systemctl
 ```
 
-If the `systemd` command is not found, please [install from source code](#Install-from-Source). 
+If the `systemd` package is not found, please [install from source code](#Install-from-Source). 
 
 ### Running TDengine