What is `:-` doing in `FOO=${BAR:-localhost}`?

Question

I can see that the following code is initializing FOO to localhost if BAR is empty, but what's exactly happening with the :-? What is it?

Answer 1

If BAR had a non-empty value, FOO would be assigned that value. If not, FOO would be assigned localhost. You can find more ways in Shell Parameter Expansion.

${parameter:-word}

If parameter is unset or null, the expansion of word is substituted.
Otherwise, the value of parameter is substituted.