Simple template renderer in POSIX sh

template.sh

#!/bin/sh
# see https://serverfault.com/a/699377 and https://serverfault.com/a/925072

PROG=$(basename $0)

usage() {
	echo "${PROG} <template-file> [config-file]"
}

expand() {
	local template="$(cat $1)"
	template=$(sed 's/\([^\\]\)"/\1\\"/g; s/^"/\\"/g' <<< "$template")
	eval "echo \"${template}\""
}

case $# in
	1) expand "$1";;
	2) . "$2"; expand "$1";;
	*) usage; exit 0;;
esac