I'm trying to perform environment variable replacement through envsubst, but I want to only replace specific variables.

From the docs I should be able to tell envsubst to only replace certain variables but I'm failing to be able to do that.

For example, if I have a file containing:

VAR_1=${VAR_1}
VAR_2=${VAR_2}

how should I execute envsubst so that it only replaces the reference to ${VAR_1}?