To get the assigned value, or default if it's missing: FOO="${VARIABLE:-default}" # FOO will be assigned 'default' value if VARIABLE not set or null. # The value of VARIABLE remains untouched. To do the same, as well as assign default to VARIABLE: FOO="${VARIABLE:=default}" # If VARIABLE not set or null, set its value to 'default'. More @Wikipedia
Hover over any link to get a description of the article. Please note that search keywords are sometimes hidden within the full article and don't appear in the description or title.