check for available parameter

tests/performance/Makefile

@@ -3,6 +3,19 @@
 #
 include ../../scripts/common.Makefile
 
+# Check that given variables are set and all have non-empty values,
+# die with an error otherwise.
+#
+# Params:
+#   1. Variable name(s) to test.
+#   2. (optional) Error message to print.
+check_defined = \
+    $(strip $(foreach 1,$1, \
+        $(call __check_defined,$1,$(strip $(value 2)))))
+__check_defined = \
+    $(if $(value $1),, \
+      $(error Undefined $1$(if $2, ($2))))
+
 .PHONY: requirements
 requirements: ## compiles pip requirements (.in -> .txt)
 	@$(MAKE_C) requirements reqs
@@ -18,7 +31,8 @@ build:
 	docker buildx build --tag local/locust:latest .
 
 .PHONY: up
-up: build
+up:
+	@$(call check_defined, target, please define target file when calling $@ - e.g. ```make $@ target=MY_LOCUST_FILE.py```)
 	@export TARGET=$(target); \
 	docker-compose --file docker-compose.yml up