Hello https://git.savannah.gnu.org/cgit/bash.git/tree/doc/bash.info#n1842
'${PARAMETER:-WORD}'
If PARAMETER is unset or null, the expansion of WORD is
substituted. Otherwise, the value of PARAMETER is substituted.
$ v=123
$ echo ${v-unset}
123
The heading says :-, but the example just says -. Which one is intended?
Or is it supposed to say that both are equivalent?
-- Alfred Agrell
