I think it is better to have somehthing like a default Linux.
But as you like .....
Winni
There is not such a thing, because Linux is only a kernel, you could probably replace the NT kernel with the Linux kernel in Windows and to the end user it would not be noticable (maybe in performance but who cares). And effectively make windows a Linux distro.
Every Linux distro is it's own OS, and assuming that an environment variable is set between Linux distros is like assuming the environment variables on Windows have the same name.
But there is the POSIX standard. While many Linux distros don't have their compliance certified (because that costs money), nearly all Linux distros are fully compatible with the POSIX standard.
So if you want to have something relyable, check out what is part of the POSIX standard and what not (it's mentioned in the man pages if this is part of any standard). Because this is something you can assume of nearly all linux distros (even though my windows example from above would not be POSIX compatible).
Bonus points: BSD and MacOS and other UNIX systems are also POSIX complient, meaning if you restrict yourself to the POSIX APIs, your program will automatically be cross plattform compatible.
Then there are APIs provided by the Linux Kernel, these are also available under every Linux system, but might change with kernel versions. This includes all the pseudo files located at /proc, which you can use for example to get the hardware spec, process list and so on