Bash function for JSON templating with environment variable substitution

json_scripts.sh

#!/usr/bin/env bash

json_template_to_canon() {
  # stdin: template with ${VARS}; output: canonical (minified) json
  # Substitutes environment variables in a JSON template.
  #
  # NOTE: Variables MUST be exported (not just set) for this to work.
  # NOTE: Unset variables are left as-is (safe_substitute behavior).
  #
  # Example 1 - single line:
  #   export PORT=8088 FLAGS="--foo=bar"
  #   echo '{"port":"${PORT}","flags":"${FLAGS}"}' | json_template_to_canon
  #   # Output: {"port":"8088","flags":"--foo=bar"}
  #
  # Example 2 - multiline with heredoc:
  #   export PORT=8088 NAME="my-server"
  #   cat << 'EOF' | json_template_to_canon
  #   {
  #     "server": {
  #       "port": "${PORT}",
  #       "name": "${NAME}"
  #     },
  #     "enabled": true
  #   }
  #   EOF
  #   # Output: {"server":{"port":"8088","name":"my-server"},"enabled":true}
  #
  # Example 3 - capture to variable:
  #   export URL="http://localhost:8088"
  #   payload="$(cat << 'EOF' | json_template_to_canon
  #   {"base_url": "${URL}"}
  #   EOF
  #   )"
  #   echo "$payload"
  #
  local input
  input="$(cat)"
  python3 - "$input" <<'PY'
import os, sys, json
from string import Template
tmpl = Template(sys.argv[1])
rendered = tmpl.safe_substitute(os.environ)
obj = json.loads(rendered)
print(json.dumps(obj, separators=(",", ":"), ensure_ascii=False))
PY
}