Install Elasticsearch 6 on Debian for Magento

To install Elasticsearch for Magento on Debian, install Java 1.8 and apt-transport-https

apt install -y openjdk-8-jdk-headless
apt install -y apt-transport-https

Add key

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

Add repository

echo "deb https://artifacts.elastic.co/packages/6.x/apt stable main" | tee -a /etc/apt/sources.list.d/elastic-6.x.list

Install Elasticsearch

apt update
apt install -y elasticsearch

Enable and start Elasticsearch

systemctl enable elasticsearch
systemctl restart elasticsearch