Creates a configuration file from a template where environment variables are replaced with values. See command `createFromTpl.sh -h`

createFromTpl.sh

#!/bin/sh

# Some of the code was borrowed from the NGINX docker image 
# https://github.com/nginxinc/docker-nginx/blob/master/mainline/debian/20-envsubst-on-templates.sh

set -e

ME=$(basename "$0")

entrypoint_log() {
  echo "$@"
}
auto_envsubst() {
  local template_dir="$(pwd)/env_tpls"
  local suffix=".tpl"
  local output_dir="$(pwd)"
  local filter=""
  while getopts "f:o:t:h" options; do
      case "${options}" in
          h)
          echo "Replaces variables (all or by filter \"${filter}\") in \"${template_dir}/*${suffix}\" files and saves them into files of the same name in the output directory (${output_dir})."
          echo "[-f \"\\\${env_var} \\\${env_var2}\"] - Set variables filter"
          echo "[-o \"${output_dir}\"]- Set output directory for parsed files"
          echo "[-t \"${template_dir}\"] - Set templates directory"
          echo "----"
          echo "Template file sufix (extension) - ${suffix}"
          echo "Templates directory  -  ${template_dir}"
          echo "Output directory  -  ${output_dir}"
          echo "Designation of variables in  template file - \${variable}"
          return 0
          ;;
          f)
          filter="${OPTARG}"
          ;;
          t)
          template_dir="${OPTARG}"
          ;;
          o)
          output_dir="${OPTARG}"
          ;;
      esac
  done

  local template defined_envs relative_path output_path subdir
  defined_envs=$(printf '${%s} ' $(awk "END { for (name in ENVIRON) { print ( name ~ /${filter}/ ) ? name : \"\" } }" < /dev/null ))
  [ -d "$template_dir" ] || return 0
 if [ ! -w "$output_dir" ]; then
    entrypoint_log "$ME: ERROR: $template_dir exists, but $output_dir is not writable"
    return 0
  fi
  find "$template_dir" -follow -type f -name "*$suffix" -print | while read -r template; do
    relative_path="${template#"$template_dir/"}"
    output_path="$output_dir/${relative_path%"$suffix"}"
    subdir=$(dirname "$relative_path")
    # create a subdirectory where the template file exists
    mkdir -p "$output_dir/$subdir"
    entrypoint_log "$ME: Running envsubst on $template to $output_path"
    envsubst "$defined_envs" < "$template" > "$output_path"
  done
}

auto_envsubst "$@"

exit 0