.env.sample generation from app Makefile vars with notes prefixed by '#- ...' comment

config.Makefile

# app custom Makefile

SHELL         = /bin/sh
CFG           = .env

# ------------------------------------------------------------------------------
# app custom config
# comments prefixed with '#- ' will be copied to $(CFG).sample

#- Postgresql container name (access via docker)
PG_CONTAINER ?= dcape_db_1

#- PowerDNS DB user name
PGUSER       ?= pdns

#- PowerDNS DB name
PGDATABASE   ?= pdns

# ------------------------------------------------------------------------------

-include $(CFG)
export

# This code generates $(CFG).sample from Makefile vars with previous comment line(s)

# Internal: generate config sample data
.env.temp.mk:
	@echo "define CFVAR" > $@
	@grep -A 1 -h "^#- " $(MAKEFILE_LIST) | grep -vE "^--" \
	  | sed -E 's/^([^\n ]+)\ *\??=([^\n]*)$$/\1=$$(\1)\n/ ; s/^(#)-/\1/' >> $@
	@echo "endef" >> $@

ifneq ($(findstring $(MAKECMDGOALS),config $(CFG).sample),)
include .env.temp.mk
endif

# Internal: generate config sample
$(CFG).sample: .env.temp.mk
	@echo -e "# app config file, generated by 'make config'\n" > $@
	@echo "$$CFVAR" >> $@
	@rm -f $<

## generate sample config
config: $(CFG).sample