That file is a shell script. You have to set the permissions to "executable" and then run it, like so:
chmod +x chmod +x FTBA_unix_202007161516-479dedc615-release.sh
then
./FTBA_unix_202007161516-479dedc615-release.sh
This will open the FTBA installer.
I just ran it, installed the "Sky" variant and launched it. Seems to work fine.

edit: ninja'd by @pomon 😀
edit2: nevermind, actually launching Minecraft failed. Something went wrong along the way.
Pretty sure it has to do with it trying to use its own Java, as far as I can see. Not sure how to fix it though.

    Thanks a lot for the super quick aid guys! It appears I chose the right distro and community. 😄 I don't have time to try out the solutions yet. But when I do I will let you know how it went.

    Staudey Perhaps I would have to edit the exec path to launch FTB with Java, as I had to with the Minecraft launcher?

      pomon I did get to the part where I can open the FTB app and download the mods. However, when I click play I get the following Java error:

      Unable to locate the Java runtime.
      Error details: Suksess
      Filename on disk: java
      Path: /usr/bin/java
      Exists: Nonexistent

      I don't get this error when I use the normal Minecraft launcher, after having used the tutorial which you are refering to.

      Edit: I might have figured it out. In the launcher where I select "edit" on the selected mod, I could then again click more options and locate OPENjdk-8 in the "java executable" options from there. Seems to work now, but will have to see once the mod is finished updating. At least it launched the updater.

        show eopkg li | grep openjdk and ls -l ~/FTBA

        I'm running MultiMC, which can install Feed The Beast (and other) modpacks from CurseForge.

        I've had the most success with openjdk-8. It looks like pointing the MultiMC Settings at the /usr/lib64/openjdk-8/jre/bin/java binary will make all profiles inherit it?

        I'm currently making do with a close-to-vanilla Fabric modpack called Fabulously Optimised where I've added a few extra convenience mods on top (map, more info, more convenient tree chopping etc.) and the PureBDCraft 128x resource pack(s).

          An update on the progress.
          I was able to start the loader (not the updater as I previously called it) when changing the settings of the FTB mod in the launcher: - edit - more options - java executable and locating the openjdk-8 executable.
          However, it appears that the system I was using for this is too old and outdated to handle the FTB mod, as the loader is having a real hard time lanching the game (been loading for over an hour now, with the memory used/total bar at maximum/red capacity).

          TL;DR my problem is no longer OS related but hardware related

          So in conclusion I suppose this thread can be frozen until I've upgraded my system with better hardware. I still want to thanks you a lot for all of your help @pomon @Staudey and @ermo! I am confident I chose the right Linux distro. 😀

          ermo Very interesting. I will be sure to look into MultiMc as an option, soon as I have the system to run the mod tough. 🤣

          • ermo replied to this.

              xaldron You could just choose a lighter modpack? There are many different FTB modpacks on CurseForge.

              FTB modpacks can use up a lot of memory too -- 4-8GB for the java runtime engine alone. So if your PC only has 4GB of RAM, you'll need to load a lighter modpack and configure java to only use up to 2GB of memory for instance.

                  ermo Sorry for the late reply. Iv'e been travelling a lot lately. The computer upon I was installing minecraft and FTB was on my old system which I gave away to my nephew, and it was for him I installed those games. He was however, happy and content with the original Minecraft for now and we will look into FTB more next time I visit them and I bring him some new and improved RAM. But, your tip is very handy and I will look into it/ask more about it next time I am going there to try and get it up and working again. 🙂