You can test to see if a variable is not defined, or is set to a null string for example the construction:-
${var-word}
returns the value of var if defined otherwise it returns the value of word:-
> unset var
> echo "${var-jim}"
jim
> var="fred"
> echo "${var-jim}"
fred
Similar constructions:-
${var=word} |
If var unset, set it equal to word then return var |
${var+word} |
if var unset return nothing otherwise return word |
${var?word} |
if var unset print word to standard error and exit. |