Replace placeholders from a file using environment variables

#!/bin/bash
# Entrypoint with password replacement based on ___PLACEHOLDERS___
# Only works in bash, does not work on sh (e.g.: Alpine)

FILE_TO_REPLACE="krakend.json"

passwords_to_replace="$(grep -o '___.*___' $FILE_TO_REPLACE | sed 's/___//g' | sort | uniq )"
for pass in $passwords_to_replace; do
  if [ "x${!pass}" = 'x' ]; then
	echo "[KO] Environment variable '$pass' is not declared and the configuration replacement failed!"
  else
    # Replace the ___PLACEHOLDER___ entry by its value
  	sed -i "s#___${pass}___#${!pass}#" $FILE_TO_REPLACE
	echo "[OK] ${pass} written"
  fi
done

# BONUS: Are there remaining placeholders?
PENDING_REPLACEMENTS="$(grep ___ $FILE_TO_REPLACE)"
if [ "x$PENDING_REPLACEMENTS" = 'x' ]; then
	echo "[OK] All passwords have been set"
else
	echo "[KO] The following passwords weren't retrieved:"
	echo "$PENDING_REPLACEMENTS"
	exit 1
fi