basic static config templater in bash

env_subst.sh

#!/bin/sh

# Basic shell templater script for static configs in docker
# made to be run before container start inside entrypoint.sh
# reads $1 file, replaces all variables, enclosed into <%= VAR %>
# with values of env variable with that name
# writes the new file into $2
# Fails if any of the matched variables is empty

set -o errexit
IFS=''

PATTERN='<%=[[:space:]]*([a-zA-Z_0-9]+)[[:space:]]*%>'
RESULT=""

while IFS='' read -r line || [[ -n "$line" ]]; do
  while [[ "$line" =~ $PATTERN ]] ; do
    VAR_NAME=${BASH_REMATCH[1]}
    VAR_VAL="`eval echo "$\"$VAR_NAME\""`"
    if [[ -z $VAR_VAL ]]; then
      echo "Variable $VAR_NAME is not set, quitting..."
      exit 1;
    fi
    TEMPLATE="`eval echo "\<%=[[:space:]]*\"$VAR_NAME\"[[:space:]]*%\>"`"
    SED_REGEX="s/$TEMPLATE/$VAR_VAL/"
    line="`IFS=''; echo $line | sed $SED_REGEX`"
  done
  RESULT+="$line\n"
done < "$1"

echo $RESULT > $2