Why is my envsubst command just inserting blank strings?

I’m trying to run the command envsubst < myfile to replace environment variables but instead of being replaced with their values, they are being replaced with blank strings.

Here’s my example command (split onto separate lines for clarity):

SIGNOFF="Goodbye"
&&
printf "nOkay, $SIGNOFF has been set to "$SIGNOFF"nnOriginal contents:nn"
&&
cat test.conf
&&
printf "nnContents after envsubst < test.confnn"
&&
envsubst < test.conf | cat
&&
printf "nn"

Here are my results:

$ SIGNOFF="Goodbye" && printf "nOkay, $SIGNOFF has been set to "$SIGNOFF"nnOriginal contents:nn" && cat test.conf && printf "nnContents after envsubst < test.confnn" && envsubst < test.conf | cat && printf "nn"

Okay, $SIGNOFF has been set to "Goodbye"

Original contents:

Hello world, let's change the variable in the quotes below to say Goodbye!
"$SIGNOFF" (it certainly shouldn't just be blank!)

Contents after envsubst < test.conf

Hello world, let's change the variable in the quotes below to say Goodbye!
"" (it certainly shouldn't just be blank!)

$ 

I’m clearly doing something obvious wrong, I just have a silly suspicion it’s one of those things that is so obvious that I must be overly complicating my own google searches in trying to find an answer 😅

Answer

The variable is not exported, so it’s not visible to any command. Export it.

SIGNOFF="Goodbye"
export SIGNOFF
envsubst < file