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`JAVA_HOME`::

  Set a custom Java path to be used.

`ES_PATH_CONF`::

    Configuration file directory (which needs to include `elasticsearch.yml`,
    `jvm.options`, and `log4j2.properties` files); defaults to
    `/etc/elasticsearch`.

`ES_JAVA_OPTS`::

    Any additional JVM system properties you may want to apply.

`RESTART_ON_UPGRADE`::

    Configure restart on package upgrade, defaults to `false`. This means you
    will have to restart your Elasticsearch instance after installing a
    package manually. The reason for this is to ensure, that upgrades in a
    cluster do not result in a continuous shard reallocation resulting in high
    network traffic and reducing the response times of your cluster.