Answer by Rory for How to determine if a bash variable is empty?

-z is a the best way.

Another options I've used is to set a variable, but it can be overridden by another variable eg

export PORT=${MY_PORT:-5432}

If the $MY_PORT variable is empty, then PORT gets set to 5432, otherwise PORT is set to the value of MY_PORT. Note the syntax include the colon and dash.