Two mechanisms, matching what the make engine actually did:
Lazy defaults. A stack setting is a function myos_default_<VAR>, called only
when the variable has no value, and called again at every reference. That is
exactly a recursive ?=: an explicit value wins, and the default follows a
DOMAIN that a .env changes later. The prefix is what makes it safe; the first
version used a bare function named after the variable, and the test suite
caught it running /usr/bin/host for a stack group called host.
Templates. myos env-update fills a .env from the .env.dist files, expanding
${VAR} against the current values and running $(command), forward references
included.
Also fixed: the project .env now wins over /etc/conf.d/myos, which is what the
documentation claimed and the code did not.
share/make/shim.mk lets a project keep make as a front end: every myos command
becomes a target that shells out to bin/myos, and the project keeps its own
targets and its stack .mk files. It sits outside make/ because the legacy
engine globs every .mk in there.
holcommon's empty .env made the loader evaluate one blank line, which
tripped set -u. Verified afterwards on holcommon's real host stack: the CLI
and the make engine resolve the same five files and render a byte-identical
241-line compose config.
config: dotenv files are parsed, never sourced, so a value can hold a # or a
$(...) without breaking or executing. The make engine included .env with
make syntax, which neither allowed.
compose: one call per project instead of one per sub-stack.
Yann Autissier