#!/bin/bash

checkForVariable() {
    if [[ -z ${!1+set} ]]; then
       echo "Error: Define $1 environment variable"
       # exit 1
    fi
}

# var not set
echo "case 1: "
checkForVariable var

# var set
echo "case 2: "
var='abcd'
checkForVariable var

# var set to empty
echo "case 3: "
var=
checkForVariable var