Recommended Drush version to use and install for Ubuntu 19 with PHP 7.2 and running Drupal 7
Drush 9 does not work with drupal 7, but drush 8 does work with it. I installed it by creating a new ubuntu user and making sure composer is installed globally then install cgr via:

composer global require consolidation/cgr

then add to your ~/.bashrc file

export PATH="$HOME/.config/composer/vendor/bin:$PATH"

then

cgr drush/drush:8.x-dev --prefer-source

link: https://github.com/consolidation/cgr#installation-and-usage

Then I followed the tutorial here: https://docs.drush.org/en/8.x/install-alternative/


  Install a global Drush via Composer
  
  Follow the instructions below:
  
  Install Composer globally.
  
  Install the cgr tool following the instructions in that project.
  
  Add composer's bin directory to the system path by placing export 
  PATH="$HOME/.composer/vendor/bin:$PATH" into your ~/.bash_profile (Mac
  OS users) or into your ~/.bashrc (Linux users).
  
  Install latest stable Drush: cgr drush/drush.
  
  Verify that Drush works: drush status
  
  Please do not install Drush using composer global require. See Fixing
  the Composer Global command for more information.