How do I print a message if a variable is not set and then exit the bash shell script?

shoebill

Note: Do not use for interactive shell scripts.

URL variable is not set.

WEBSITE_URL=${URL?"URL variable is not set"}
echo $WEBSITE_URL
echo $URL

$ ./varsubs.sh
./varsubs.sh: line 2: URL: URL variable is not set

When the URL variable is set.

URL=www.gnu.org
WEBSITE_URL=${URL?"URL variable is not set"}
echo $WEBSITE_URL
echo $URL

$ ./varsubs.sh
www.gnu.org
www.gnu.org