How can I determine the precise set of environment variables a systemd EnvironmentFile would set?

systemd has an EnvironmentFile directive, which sets environment variables from a file’s contents based on a number of rules, which are not quite equivalent to how a shell would parse that file.

How can I parse a systemd EnvironmentFile in exactly the same way that systemd itself would?

Answer

The surest thing is to let systemd parse the file itself, via a transient service. Thus:

# emit a NUL-delimited set of key=value definitions for environment variables defined by a set of files
newVarsForFile_nullsep() {
  local -a extraParams=( ); local file
  (( $# )) || return 0 # no files specified, nothing to do
  for file in "$@"; do
    extraParams+=( --property=EnvironmentFile="$file" )
  done
  comm -z -23 
    <(sort -z < <(systemd-run --user --pipe "${extraParams[@]}" grep -zvE '^INVOCATION_ID=' /proc/self/environ </dev/null)) 
    <(sort -z < <(systemd-run --user --pipe grep -zvE '^INVOCATION_ID=' /proc/self/environ </dev/null) )
}

# emit code that can be eval'd in an instance of bash to precisely define the exact variables
newVarsForFile_shellscript() {
  while IFS= read -r -d '' vardef; do
    printf '%s=%qn' "${vardef%%=*}" "${vardef#*=}"
  done < <(newVarsForFile_nullsep "$@")
}

Thereafter, one may invoke (as an example):

newVarsForFile_shellscript /etc/conf.d/*.conf

…to emit a shell script fragment which, when executed by bash, will set all the same environment variables that adding the relevant EnvironmentFiles to a service definition would set.