Your particular distro might choose to include it. But it's certainly not part of Linux.
Your distro might include Libre Office too but that's not part of Linux either :)

I don't understand why there isn't a simple check to see if it NEEDS to be installed and it is instead just rammed through on install.

The DirectX installer does that already. They see if the product is installed, then they either quit or offer an option to repair the installation or upgrade it or whatever.
To prevent dialogs from showing, it offers a a silent mode controlled by programm parameters.


Early versions didn't do that - or better: could be configured not to do that. That's why the installation of Tomb Raider could mess up a system by overwriting newer files. The latest versions are a lot safer.

And yet, I have 15 installations of DirectX9.0c on my PC right now.

How?

It should only register as one. Besides, it's not 9.0c any longer as the full package includes 9, 10 and 11 (which is also called 10.1 extension).

You stumbled across a neccessity. Like C++, DX was updated often.

Game XY requires version 11.1234, game XZ requires version 11.1222 and is not compatible with 11.1234, will crash.
So both versions need to be accessible.

It really is just one installation, but it shows all different accessible versions, so games can find the right dlls.

edit: removed doppelmoppel

Also, it comes down to "better to be safe than sorry". It's better to just include all the dependencies and silently install them rather than having the check itself fail in any multiple ways, or leave them out and have the users creating a full wave of complaints because the game/app needs something, at least based on my own experience with slipstreaming and no-attended setups.

VC++ 2010-12 and onwards will also check if it's present if the silent switch is left out to do repair but you might as well install it in full, it's also faster. The older ones just unpacks and force installs regardless. .NET setups isn't any better.

These checks will not fix everything. And yes, it used to much worse (dll library hell), but thankfully Windows has tools to deal with them if they misbehave. :D

Yep, this was a very big issue in early Windows versions. They only allowed a single installation, which caused a lot of problems. Now different versions of certain APIs can be installed parallel. I don't know if they keep copies of identical dlls (edit: for DirectX, .Net and C++), but if they handle it like the system installers, they use dymanic and hard links instead.

By default the newest version is installed and used. If you look in WinSxS only one 32 bit version and one 64 bit version of the same file is retained (amd64 and wow64). Outside of that they use links between the folders, but not exactly sure how.

Wait, it gets better: \system32 is for 64-bit libraries and \WoW64 is for 32-bit libraries. Yes, gotta love Microsoft and it's "brainchild"...

No one can claim that they lack of immagination :)
Some genius had the idea to put everything into the system32 directory back in the old days and allowing direct access.

Only starting with the systems after XP they started to separate things, but the damage was already done.

And we haven't started talking about language dependent folder names in Windows 95 and XP yet ... luckily they found a good solution for that. Now everything is in english, the old language dependent names work as well however as aliases.

(edit: correceted wrong expression)

Then the exact necessary files that are different SHOULD be added/installed with the game's directory and not the system directory. Windows will search the local directory first for a particular dll file.

I remember the endless streams of 'this dll is v1.22, the old version is 1.10, do you want to replace?'. With no other information you just start bulk saying 'yes' to get it to finish, and later the system just becomes unstable...

As i recall there was also no file locking, as well as everyone ran as admin/privileged. Means some software won't work correctly unless you're admin today even if it doesn't need it because it used some higher privileged API calls or did things behind the scenes.

I'd think things are a bit better now, as they've basically copied unix systems in a lot of ways.

Does it really matter which version of Direct X 2010 or 2021 both are part of the "Legacy" pack wouldn't that be the same thing? the 2021 version has audio 2.7, and several versions of DX and D3 and Xinput 1.3 and even DirectX SDK. Is there much of a difference?

It's all the same installer, check the description for XAudio and XInput.
From the german site they removed "2010" from the title (not from the file name however).

Should I be concerned about some of the C++ distributable having some security issues from microsoft. I noticed a few of them are being issued with a security vulnerability warning. I Think I'll only need 2012, 2013 and 2015 to run a game. I got back into my original laptop and took a look at the visual programs it was running just those to run games.

Only if you don't trust the software that is using these runtimes. In the past access was less restricted, some software really did weird things with your system. Some of these are considered to be security issues these day (and rightfully so).

The warnings may be valid, but it depends on who is adressing the runtime. If you trust the software you use, then the runtime is ok as well.