In most cases [Super User][1] is the place to ask.  There are also sites specifically for [Unix & Linux][2], [Ubuntu][3], and [Apple][4] questions, so you might try one of those if they apply.  If it's an environment variable specifically for a programming tool (e.g., JAVA_HOME) you should be able to ask on [Stack Overflow][5].


  [1]: https://superuser.com/
  [2]: https://unix.stackexchange.com/
  [3]: https://askubuntu.com/
  [4]: https://apple.stackexchange.com/
  [5]: http://stackoverflow.com/