Makefile.bad

.PHONY: tag
SHELL=bash

# assuming v0.1.0 is the only tag in the repo
tag: nr_of_existing_tags := $(shell git tag | grep "\<${TAG}\>" | wc -l)
tag:
        @echo ${nr_of_existing_tags}
ifeq (1,$(nr_of_existing_tags))
        @echo tag ${TAG} already present ${nr_of_existing_tags}
else
        @echo tag ${TAG} NOT yet present ${nr_of_existing_tags}
endif
ifeq (0,${nr_of_existing_tags})
        @echo tag ${TAG} NOT yet present ${nr_of_existing_tags}
else
        @echo tag ${TAG} already present ${nr_of_existing_tags}
endif

Makefile.good

.PHONY: tag
SHELL=bash

# assuming v0.1.0 is the only tag in the repo
nr_of_existing_tags := $(shell git tag | grep "\<${TAG}\>" | wc -l)
tag:
        @echo ${nr_of_existing_tags}
ifeq (1,$(nr_of_existing_tags))
        @echo tag ${TAG} already present ${nr_of_existing_tags}
else
        @echo tag ${TAG} NOT yet present ${nr_of_existing_tags}
endif
ifeq (0,${nr_of_existing_tags})
        @echo tag ${TAG} NOT yet present ${nr_of_existing_tags}
else
        @echo tag ${TAG} already present ${nr_of_existing_tags}
endif

output

$ make -f Makefile.bad TAG=0.1.0
0
tag 0.1.0 NOT yet present 0
tag 0.1.0 already present 0
$ make -f Makefile.bad TAG=0.2.0
0
tag 0.2.0 NOT yet present 0
tag 0.2.0 already present 0
$ make -f Makefile.bad TAG=0.1.0
0
tag 0.1.0 NOT yet present 0
tag 0.1.0 NOT yet present 0
$ make -f Makefile.bad TAG=0.2.0
0
tag 0.2.0 NOT yet present 0
tag 0.2.0 NOT yet present 0