Templatey

subst.sh

#!/bin/sh

# cat <<EOF | ./subst.sh zzzlucy
# name=Lucy
# weather="Good, how about you?!"
# family=Smith
# EOF

set -a

sourceDir="example-files"
targetDir="target"
dir=$1

input=${2:-/dev/stdin}

while read line
do
    varslist="$varslist\$$(echo $line | cut -d= -f1)"
    export "$line"
done < $input

mkdir -p $targetDir/$dir
envsubst "$varslist" < $sourceDir/demo.txt > $targetDir/$dir/demo.txt