${var:-padrão}
If the variable is empty, the default takes over — without touching the variable.
The most used operator of parameter expansion: it returns the value of var, or the default when it is empty or does not exist. The variable itself is unchanged.
The colon is the fine aim: ${var-default}, without it, applies the default only when the variable does not exist — empty passes through as empty. The distinction holds for the whole family: :-, :=, :? and :+.
$ unset EDITOR
$ echo "abrindo com ${EDITOR:-nano}"
abrindo com nano
$ EDITOR=vim
$ echo "abrindo com ${EDITOR:-nano}"
abrindo com vim