Minimal working example for stackoverlfow question

Makefile

DIRS := $(filter dir%, $(shell ls))

foo_sources := $(wildcard */source/foo.a)
foo_targets_prt := $(patsubst %.a, %.b, $(foo_sources))
foo_targets := $(subst source,target, $(foo_targets_prt))

bar_sources := $(wildcard */source/bar.a)
bar_x := $(patsubst %/bar.a, %/Y.a, $(bar_sources))
bar_y := $(patsubst %/bar.a, %/Z.a, $(bar_sources))
bar_targets :=  $(bar_x) $(bar_y)

all: 
	$(MAKE) rebuild
	$(MAKE) foos
	$(MAKE) bars

##################################################
#### 1. dir%/source/foo.a > dir%/target/foo.b ####
##################################################

foos: $(foo_targets)

### This is the general idea
#target/%.b: source/%.a
#	operation1.sh $< $@

#$(foo_targets):$(foo_sources)
#	operation1.sh $< $@

#$(DIRS)/target/%.b: $(DIRS)/source/%.a
#	operation1.sh $< $@

#$(foo_targets): $$(@D)/foo.a
#	operation1.sh $< $@

$(foo_targets): $(subst target,source, $(patsubst %.b, %.a, $@))
	$(eval dep := $(subst target,source, $(patsubst %.b, %.a, $@)) )
	echo $(dep)
	./operation1.sh $(dep) $@

################################################
#### 2. dir%/source/bar.a > dir%/source/Y.a ####
####                      > dir%/source/Z.a ####
################################################

bars: $(bar_targets)

### This is the general idea
#%/source/Y.a %/source/Z.a: %/source/bar.a
#	operation2.sh $< $@

#$(DIRS)/source/Y.a: $(patsubst %/source/Y.a, %/source/bar.a, $@)
#	operation2.sh $< $(patsubst %/source/bar.a, %, $@)


### $(dirname $@) doesn't evaluate to anything
#$(bar_targets): $(dirname $@)/source/bar.a
#	./operation2.sh $< $@
    
$(bar_targets): $(filter %bar.a, $(patsubst dir%/source/Y.a, dir%/source/bar.a, $@) $(patsubst dir%/source/Z.a, dir%/source/bar.a, $@))
	$(eval dep := $(filter %bar.a, $(patsubst dir%/source/Y.a, dir%/source/bar.a, $@) $(patsubst dir%/source/Z.a, dir%/source/bar.a, $@)) )
	./operation2.sh $(dep) $@

##################################################################

build_demo:
	mkdir dir0
	mkdir dir1
	mkdir dir2
	mkdir dir0/source
	mkdir dir1/source
	mkdir dir2/source
	mkdir dir0/target
	mkdir dir1/target
	mkdir dir2/target
	head -c 32 /dev/urandom > dir0/source/foo.a
	head -c 32 /dev/urandom > dir1/source/foo.a
#	head -c 32 /dev/urandom > dir2/source/foo.a
#	head -c 32 /dev/urandom > dir0/source/bar.a
	head -c 32 /dev/urandom > dir1/source/bar.a
	head -c 32 /dev/urandom > dir2/source/bar.a
	echo -e $$'#!/bin/sh\n\nhead $$1 -c 10 > $$2' > operation1.sh
	echo -e $$'#!/bin/sh\n\nhead $$1 -c 10 > $$(dirname $$2)/Y.a\ntail $$1 -c 10 > $$(dirname $$2)/Z.a' > operation2.sh
	echo -e $$'#!/bin/sh\n\necho "1:" $$1\necho "2:" $$2' > operation3.sh
	chmod +x operation*.sh

delete:
	find . -type d -name 'dir*' -exec rm -r {} +
	find . -type f ! -name 'Makefile' -exec rm {} +

rebuild: 
	$(MAKE) delete
	$(MAKE) build_demo

debug:
	echo
	echo $(foo_sources)
	echo $(foo_targets_prt)
	echo $(foo_targets)