Say you have a text file with variables or commands in it:

Today is $(date).
I have $n things to do.

Store text file contents in a variable and expand variables and commands in the text with:

# Init variables
n=25
# Store file in a variable
text="$(echo EOF;cat $filename;echo EOF)"
# Expand text (and print it to stdout)
eval "cat <<$text"

That’s it! You will see something like:

Today is Thu Oct 15 12:04:56 JST 2015.
You have 25 things to do.

Note, that without echo EOF bash will use first line of text as a limit string for heredoc.