POSIX environment variable

This section is about POSIX environment variable that have special effects.

They are documented by POSIX at: pubs.opengroup.org/onlinepubs/9699919799/basedefs/V1_chap08.html#tag_08

Some mentions by Ciro Santilli at: superuser.com/questions/284342/what-are-path-and-other-environment-variables-and-how-can-i-set-or-use-them/1864687#1864687