You can use the following function to check if an environment variable is set via parameter expansion. Early exit if it's not.

environment-variable-check.sh

#!/usr/bin/env bash

function environ-check() {
    local environ_tag="${1}"
    local environ_val="${!1}"
    if [[ -z "${environ_val}" ]]
      then echo "Environ variable is not set ${environ_tag}"; exit 1
    fi
}

environ-check ENVIRON_VARIABLE_NAME