Can envsubst be used to replace environment variables in a given variable, not file?

I know that I can use envsubst command to replace environment variables inside a file and write it to an output file:

envsubst < input.txt > output.txt

However, now I only have a variable and it's not a file. But I still need to replace any environment variable inside it.

export original_text="Hello $name, please come here $date"
# I want to be able to replace $name and $date, in the RAM and not on disk and files
export $name="John"
export $date="tomorrow"
output=$(envsubst < $original_text) # this is a pseudo-command
echo $output
# prints => Hello John, please come here tomorrow

Is it possible? How can do this?

Yes, you can use a here-string: <<< "$variable"

Your original_text must be single quoted, otherwise the variables will be replaced on creating original_text.

$ original_text='Hello $name, please come here $date'
$ export name="John"
$ export date="tomorrow"
$ envsubst <<< "$original_text"
Hello John, please come here tomorrow

Of course, you can save it in a variable like you'd always do:

output=$(envsubst <<< "$original_text")

You can also pipe to envsubst, e.g.:

$ printf '%s\n' "$var" | envsubst
Hello John, please come here tomorrow