Redundant use of :- in Bash?

Question

I have this code inside a function:

local var="$1"
local fileVar="${var}_FILE"
local def="${2:-}"
if [ "${!var:-}" ] && [ "${!fileVar:-}" ]; then
    echo >&2 "error: both $var and $fileVar are set (but are exclusive)"
    exit 1
fi

What is the role of :- here? Isn't it redundant? To be specific, could we not write the if statement this way?

if [ "${!var}" ] && [ "${!fileVar}" ]; then

How does it help to have an empty "word" on the right side of :-?

Answer 1

Figured it out. The :- construct in indirect parameter expansion prevents the script from failing when run with set -u.

Here is an example:

set -u
var=x
[[ ${!var:-} ]] && echo This works
[[ ${!var} ]]   && echo This should fail
echo "This should print only when run without 'set -u'"

which gives this output:

line 6: !var: unbound variable

If the same statements are run without set -u, we get:

This should print only when run without 'set -u'

However, this trick wouldn't work in case we are using direct parameter expansion. So,

set -u
[[ ${var:-} ]] && echo Does this work

still throws the error:

tt.sh: line 6: var: unbound variable