How did I install Elasticsearch 6 for Magento 2.3 to Debian 9?

Step 1

Install Java.

Step 2

wget -qO - https://artifacts.elastic.co/GPG-KEY-elasticsearch | sudo apt-key add -

Step 3

aptitude install apt-transport-https

Step 4

Add to /etc/apt/sources.list:

deb https://artifacts.elastic.co/packages/6.x/apt stable main

Step 5

aptitude update && aptitude install elasticsearch

Step 6

update-rc.d elasticsearch defaults 95 10
service elasticsearch start

devdocs.magento.com/guides/v2.3/config-guide/elasticsearch/es-overview.html#es-spt-versions

See also: How did I install Elasticsearch to Windows?