dexing. When substring expansion of the form ${param:offset[:length} is used, an `offset' that evaluat... ts back from the end of the expanded value of $param.
When a negative `offset' begins with a minus s... four characters of $a. The problem is that ${param:-word} already has a well-defined meaning: expand to word if the expanded value of param is unset or null, and $param otherwise.
To use
