How do I assign a value to variable when a different variable has no default value in a bash shell script?

[shoebill]

Here the variable URL is not set so, WEBSITE_URL becomes www.gnu.org

Using the syntax ${name-word}

WEBSITE_URL=${URL-"www.gnu.org"}
echo $WEBSITE_URL

Running the script give the following results:

$ ./varsubs.sh
www.gnu.org