env substitution in bash

gistfile1.txt

tpl::render() {
  local assign_re='@@([A-Za-z0-9_]+)@@'

  while IFS='' read -r line || [[ -n "${line}" ]]; do
    while [[ "${line}" =~ $assign_re ]]; do
      line="${line/${BASH_REMATCH[0]}/${!BASH_REMATCH[1]}}"
    done
    echo "${line}"
  done < "${1}"
}