zx

Makefile

.PHONY = inc

COUNTER_FILE = COUNTER

SOURCE_COUNTER = if [[ ! -e $(COUNTER_FILE) ]]; then touch $(COUNTER_FILE); fi; set -a; source $(COUNTER_FILE);

inc:
	@$(SOURCE_COUNTER) \
	echo $$COUNTER; \
	echo 'export COUNTER='$$((COUNTER + 1)) > $(COUNTER_FILE)

my solution:

COUNTER=$(shell make counter)
COUNTER_FILE=docker/counter

counter:
        @CURRENT_COUNTER=$(shell bc <<< "$(shell cat ${COUNTER_FILE}) + 1") && echo $${CURRENT_COUNTER} >| ${COUNTER_FILE}
        @cat ${COUNTER_FILE}

references:
https://stackoverflow.com/questions/1909188/define-make-variable-at-rule-execution-time
https://unix.stackexchange.com/questions/196521/anyway-to-pass-a-variable-to-bc-having-a-command-to-be-executed
https://askubuntu.com/questions/229446/how-to-pass-results-of-bc-to-a-variable
https://unix.stackexchange.com/questions/40786/how-to-do-integer-float-calculations-in-bash-or-other-languages-frameworks