EnvSubst references don't fail when the environment variable doesn't exist

[MattHodge]

When using the EnvSubst provider, if an environment variable doesn't exist, it will just pass a blank string to the evaluated file and not error.

To replicate the issue:

echo 'foo: ref+envsubst://$VARIABLE' | vals eval -f -

Result: foo: ""

export VARIABLE=bar
echo 'foo: ref+envsubst://$VARIABLE' | vals eval -f -
unset VARIABLE

Result: foo: bar

2 potential ideas for a backwards compatible way to address this:

• Add a new provider called "mustenvsubst" eg: ref+mustenvsubst://$VARIABLE which would fail if the variable is not set
• Add a query string like other providers, eg: ref+envsubst://$VARIABLE?must_exist=true

Happy to take a stab at the PR once given guidance on the preferred direction to address this :)

I am performing a vals eval on a JSON source file, so using this script as a workaround:

#!/bin/bash
set -eo pipefail
IFS=$'\n\t'

# This script will parse the JSON file and fail if the requested vals environment variables don't exist.

if [ $# -ne 1 ]; then
    cat <<< "Usage: check_vals_envsubst_exist.sh <json-file-to-be-evaled-by-vals>" 1>&2;
    exit 1
fi

missing_vars=false

while read -r var; do
  if [ -z "${!var}" ]; then
    echo "[MISSING] Environment Variable '$var'"
    missing_vars=true
  else
    echo "[FOUND] Environment Variable '$var'"
  fi
done < <(jq -r '.. | select(type == "string" and test("^ref\\+envsubst://\\$\\w+")) | sub("^ref\\+envsubst://\\$"; "")' $1)

if [ "$missing_vars" = true ]; then
  echo "The [MISSING] environment variables are missing. Make sure they are set so vals can use them. Exiting."
  exit 1
fi

[yxxhero]

@mumoshu WDYT?

Add a query string like other providers, eg: ref+envsubst://$VARIABLE?must_exist=true

let must_exist defautl be true will be better?

@MattHodge