How to get the hadoop home environment variable?

Any of those help So the answer is, there isn't any! Intentionally! And once I read the reasoning, it actually made sense. Though it is a bummer I can't do local nginx installs to people's $HOME dir, but I can live with that.
See this stackoverflow answer:

help you fix your problem Q: Why does my php $HOME environment variable not match my bash shell $HOME?
A: Because your web server is running in a different user context than the one you log in as.

 ps -eaf|grep http
root       856     1  0 Mar24 ?        00:04:10 /usr/sbin/httpd
apache    4960   856  0 Aug02 ?        00:00:02 /usr/sbin/httpd
apache    4961   856  0 Aug02 ?        00:00:00 /usr/sbin/httpd
apache    7411   856  0 Aug03 ?        00:00:00 /usr/sbin/httpd
apache    7413   856  0 Aug03 ?        00:00:00 /usr/sbin/httpd
apache    7416   856  0 Aug03 ?        00:00:00 /usr/sbin/httpd
apache    7420   856  0 Aug03 ?        00:00:00 /usr/sbin/httpd
<= In this case, the web server is run as daemon user "apache"

To fix the issue you can do The answer is here!
http://www.timehat.com/javas-user-home-is-wrong-on-windows/

To fix the issue you can do It's because your string is in double quotes and PHP is substituting the value of the variable $HOME. Try changing the string to use single quotes. I.e.

exec('echo $HOME', $out);

like below fixes the issue R startup is somewhat complicated, but it is pretty well documented. The usual starting place is help("Startup"). The answer to your question is not documented there, but you will find this clue in the See also section: