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I found the Ubuntu app button in Microsoft App Store and attached it. There were limited instructions: I had Microsoft Windows help with their side. Ubuntu did not boot up as option. Windows 10 S booted again, but I had a Linux command window. That's it.

Below are instructions from the official Get Ubuntu 18.04 – Microsoft Store website.

To launch, use "ubuntu 1804" on the command line prompt (cmd.exe), or click on the Ubuntu icon tile in the Windows 10 Start menu.

To use this feature, one first needs to use "Turn Windows features on or off" and select "Windows Subsystem for Linux", click OK, reboot, and use this app.

Over 214MB of data was installed when I clicked on the Ubuntu icon.

Then I opened the Windows command prompt window and typed ubuntu 1804, but nothing happened. My Windows Command Prompt results are shown below.

C:Usersphoebedavis>UBUNTU 1804

'UBUNTU' is not recognized as an internal or external command,

operable program or batch file.



C:Usersphoebedavis>

Can someone assist this totally new noob? I want to run Ubuntu in Windows Subsystem for Linux in Windows 10 S before buying multiple single board computers and installing Ubuntu on them as workstations.

Ubuntu 18.04 on Windows allows one to use Ubuntu Terminal and run Ubuntu command line utilities including bash, ssh, git, apt and many more. Ubuntu 18.04 WSL is published by Canonical. The Windows 10 April 2019 Update will give Windows Subsystem for Linux the new ability to let you access Linux files safely from Windows.

Please note that Windows 10 S does not support running this app. Windows 10 in S Mode can’t use command-line shells like PowerShell, Command Prompt, or Bash which are commonly used in WSL. Various other developer tools are also off limits.^*

The options for running Ubuntu in Windows 10 S are either in a virtual machine like VirtualBox or as a dual boot alongside Windows 10 S.

Install the Windows Subsystem for Linux

1. 
   

   To use this feature, one first needs to use Turn Windows features on or off and select Windows Subsystem for Linux, click OK, reboot, and use this app. This step can also be performed using Administrator PowerShell prompt:

   
   
   

   Enable-WindowsOptionalFeature -Online -FeatureName "Microsoft-Windows-Subsystem-Linux"
   
   

   
   



2. Restart your computer when prompted.

Install Ubuntu 18.04

1. Check that Windows 10 build 16215 or later is installed.




2. Open the Microsoft Store and choose Ubuntu.




3. From the Ubuntu page, click the blue Get button.

To launch Ubuntu, type ubuntu1804 on the command-line prompt (cmd.exe), or click on the Ubuntu tile in the Start Menu. Note that the command to launch Ubuntu is ubuntu1804, not ubuntu 1804 as was written in your question. The explanation of this error message that you received:

'UBUNTU' is not recognized as an internal or external command, operable program or batch file.  

...is that the Windows Command Prompt interprets UBUNTU as a separate command because of the space character that separates it from 1804, so instead you need to type ubuntu1804 without including the unnecessary space character.

According to Microsoft: Windows 10S is not able to use command prompt or any settings that can alter system files for security purposes. The only way for you to do this is to upgrade/switch to Windows 10 Pro.
(Source: Microsoft Answers)

Moreover, for WSL to work you need to have cmd or powershell. So, I would suggest you to either install Ubuntu in VMWare or upgrade to Windows 10 Home/Pro.