file.template

function render {
cat << END_RENDER

#begin the "real" content
blah $FOO
foo $BAR
smack $HEAD
for $USER
#end the "real" content

END_RENDER
}

jim.sh

#!/bin/bash

source variables
source file.template

render

variables

#These are defaults but can be overridden by a like-named environment variable

FOO=${FOO:="Some Value"}
BAR=${BAR:="Another Value"}
USER=${USER:="Jimbo"}
HEAD=${HEAD:="I dunno"}