Use this gist to install MongoDB Community Edition on LTS Ubuntu Linux systems in one shell script.

installMongo.sh

if [ ! -f /usr/bin/mongod ]; then
  sudo apt-key adv --keyserver hkp://keyserver.ubuntu.com:80 --recv 2930ADAE8CAF5059EE73BB4B58712A2291FA4AD5
  echo "deb [ arch=amd64,arm64 ] https://repo.mongodb.org/apt/ubuntu xenial/mongodb-org/3.6 multiverse" | sudo tee /etc/apt/sources.list.d/mongodb-org-3.6.list
  sudo apt-get update
  sudo apt-get install -y mongodb-org
else
  echo "mongo db already installed.  Skipping..."
  sudo service mongod stop 
  sudo apt-get purge mongodb-org* 
  sudo rm -r /var/log/mongodb 
  sudo rm -r /var/lib/mongodb
  sudo apt-key adv --keyserver hkp://keyserver.ubuntu.com:80 --recv 2930ADAE8CAF5059EE73BB4B58712A2291FA4AD5
  echo "deb [ arch=amd64,arm64 ] https://repo.mongodb.org/apt/ubuntu xenial/mongodb-org/3.6 multiverse" | sudo tee /etc/apt/sources.list.d/mongodb-org-3.6.list
  sudo apt-get update
  sudo apt-get install -y mongodb-org
fi

echo "mongo installed... Starting the mongodb"
sudo systemctl enable mongod
sudo service mongod restart