What is the meaning of a question mark in bash variable parameter expansion as in ${var?}?

Question

What is the meaning of a bash variable used like this:

 ${Server?}

Answer 1

It means that the script should abort if the variable isn't defined

Example:

#!/bin/bash
echo We will see this
${Server?Oh no! server is undefined!}
echo Should not get here

This script will print out the first echo, and the "Oh no! ..." error message.

See all the variable substitutions for bash here: https://www.gnu.org/software/bash/manual/html_node/Shell-Parameter-Expansion.html