That goes fast...
FYI for Processing, when I install it with sudo in /opt using the install.sh script, everything went well, except it seems I had to run the following command as normal user to get Desktop icon. Not sure if this also applies to stable / former Arduino IDE. I'll check tonight or tomorrow.

xdg-desktop-icon install /usr/share/applications/processing-pde.desktop

Yes, it seems the script doesn't add a desktop shortcut icon when running the ./install.sh script of the stable IDE as root.

After running the install script, I had to run as normal user (not root):
xdg-desktop-icon install /usr/share/applications/arduino-arduinoide.desktop

I wonder if that shouldn't be /usr/local/share/applications though. It's a local install I would think.

I'm not an expert, but reading this tells me that if one installs in /opt, no files of that package should be placed outside of /opt
https://tldp.org/LDP/Linux-Filesystem-Hierarchy/html/opt.html

It could be that the xdg tool is placing the *.desktop file in /usr/share/applications, when used as root.

https://unix.stackexchange.com/questions/196636/system-wide-desktop-file-installation-for-unpackaged-software
https://wiki.archlinux.org/title/Desktop_entries

A bit confusing. I tend to think that those tools (xdg-tools, desktop-file-install) are for packaged packages (which you can install via apt etc.).

Maybe the best way for third party package is to copy the *.desktop file to the right place (/opt/arduino-/usr/share/applications/arduino-arduinoide.desktop) (?)

Or to arduino-/usr/share/applications/arduino-arduinoide.desktop (as it is now, its a users/ system administrators choice where to place the arduino folder. This probably should be in /opt, but it's not a requirement.

And then make a symbolic link from the Desktop folder to the Arduino *.desktop file in /opt/arduino....

But this has to be confirmed by someone more knowledgeable then I am on this topic.

A symlink from /usr/local/share/applications/arduino-arduinoide.desktop to /opt/arduino..../arduino-arduinoide.desktop, should add arduino to the menu.

A symlink from ~/Desktop to /opt/arduino..../arduino-arduinoide.desktop should add a shortcut on the Desktop to open Arduino.

Would be a solution, for a system wide installation probably. Like I said, needs confirmation though. I could be wrong.

I think I would install in /opt instead of /usr/local for a package like Arduino.
https://unix.stackexchange.com/questions/11544/what-is-the-difference-between-opt-and-usr-local

I just thought I'd share that i am helping with working on a flatpak (on flathub) release for the new Arduino IDE.
At the moment it only exists as a pull request and further decisions are to be made as to whether or not to add this to its own flatpak package (or have it in a beta branch then moving it to release upon full release of v2).
Here is the pull request as of right now: flathub/cc.arduino.arduinoide#42
I understand this doesn't solve the install script issue (which will be useful for those who would like to install on linux), but this allows an alternate installation method of the new IDE.

Related #356

Are you against installing using Flatpak?