[Samsung Galaxy S9  Guides, News, & Discussion] AndroidGuy087:
As of the most recent Linux on DeX beta update (1.0.51) you can use Linux on DeX on the S9 and S9+ From 1.0.43 to 1.0.49 you could run it on the S9+ as well as the Note9 and Tab S4 but there was no S9 support and S9+ users had to sideload the app instead of install it directly from Google Play. As of 1.0.51 you can install it directly from the Google Play Store on both the S9 and S9+ and it will run on the S9 too. This update also includes support for the Tab S5e, S10 series, and (apparently) the Note8. So you can also run it on any of those devices.

Prerequisites:

A Galaxy S9 or S9+ (S10 series, Note9/8 and Tab S4/S5e also supported).

To be running any build of One UI (Android Pie) (S9+) or the April 2019 security patch or later (S9/Note8).

To be enrolled in the Linux on DeX beta program (if you're not you can just sideload the app manually, instructions further down below).

If you are in the Linux on DeX beta program, it's simple: just search for Linux on DeX in the Play Store and install it. You won't see the message 'Your device isn't compatible with this version' anymore because support for your device was added in the most recent update. S8 and S8+ still aren't supported at the moment and I don't know if or when they will be.

If you aren't in the Linux on DeX beta program, then you'll have to sideload the APK manually. Make sure that the website you download the APK from is safe and isn't known to provide infected versions of software. SamMobile is a good example of a safe website for APK downloads and it hosts the latest APK for Linux on DeX in its APK archive.

Once you've downloaded the APK click on it and allow installation of apps from unknown sources if you haven't already done. Click install and wait until it has finished installing and then open it.

You can now download the official Ubuntu 16.04 image and use it to create a container. I'd recommend using a microSD card of at least 16GB storage to store the image if using it on a device with 64GB storage. You can use terminal mode without DeX but to access the Ubuntu desktop you must run the application in DeX mode.

FAQ

What if I get the 'Update your software' error message?

Make sure that you downloaded Linux on DeX beta 1.0.51 or later (for future reference) if you sideloaded the APK. Make sure you're on any build of One UI (Android Pie) if you're on an S9+ or the April 2019 security patch or later (S9/Note8). On the Note9 and Tab S4 you should be on the October 2018 patch or later.

What if I get the 'Non-supported devices' error message?

If you're on an S9, Note8, or Tab S5e, you need to have at least version 1.0.51 installed for it to run. The S8/S8+ are still not supported and as far as I know the Tab S6 currently isn't supported either. A series phones don't even support regular DeX so there's no way Linux on DeX will run on any of them. Hopefully for A series owners this is only a software issue and Samsung will add DeX to some of the higher end A series phones later on with a software update.

Can I still sign up for the beta?

No, it closed ages ago. Hopefully the public release of Linux on DeX isn't too far away.

How will Linux on DeX work with the Note10's new DeX on laptop functionality?

Note10 support shouldn't be too far away for Linux on DeX. The new functionality should mirror the DeX UI to a Windows PC or a Mac so you should just be able to launch Linux on DeX from the UI. It will be interesting to see a video of it in action!