Installing Jenkins on Ubuntu 18.04LTS

Step 1: Install Java
sudo apt update && sudo apt install openjdk-11-jdk
As of August 2019, Jenkins only supports Java 8 or 11.

Step 2: Add Jenkins repo
wget -q -O - https://pkg.jenkins.io/debian/jenkins.io.key | sudo apt-key add -
sudo sh -c 'echo deb http://pkg.jenkins.io/debian-stable binary/ > /etc/apt/sources.list.d/jenkins.list'

Step 3: Install Jenkins
sudo apt update && sudo apt install jenkins

Step 4: Edit Jenkins config
For me, port 8080 is taken so I generally use port 9000.
sudo emacs /etc/default/jenkins
sudo systemctl restart jenkins.service