BASH: "local var=${3-16}" meaning not clear

Question

Trying to understand some BASH script I encountered this line

local var=${3-16}

I understand the assignment part and the local part - my question is what does the dash indicate in "${3-16}".

If I try:

 $ maxi=${1-45}; echo $maxi
 45 <-- result

Please explain the meaning of the dash. Thanks

Answer 1

When doing ${parameter-default} (or ${parameter:-default}), if parameter is not set, it will use the default value.

So in var=${3-16}, if $3 is not set, var will be 16, otherwise, var will be $3.

You can check Advanced Bash-Scripting Guide for more examples, and other substitutions.

Answer 2

It means "unless the parameter is unassigned, in which case use...". (:- would mean "unless the parameter is empty or unassigned".) So ${3-16} means "$3 if it exists, otherwise 16".