The documentation makes it sound as though you can set Environment Variables and use them in the /etc/elasticsearch/elasticsearch.yml file. It even gives ${HOSTNAME} as an example. According to Ubuntu, if I set a variable in /etc/environment, that variable name is available every time I log in.
When I set ES_HEAP_SIZE=4g or FOO=bar in my /etc/environment file I can see it with echo ${ES_HEAP_SIZE} or export -p but Elasticsearch completes ignores these variables.
I've seen some references to /etc/default/servicename via this post ([THIS POST]http://unix.stackexchange.com/questions/44370/how-to-make-unix-service-see-environment-variables)
AM I completely misreading the documentation? Does ES allow for reading/Importing of Environment Variables other than ${HOSTNAME} via the service?