diff --git a/gbsdk/rules.mk b/gbsdk/rules.mk
index 8e8d2a2..fba758e 100644
--- a/gbsdk/rules.mk
+++ b/gbsdk/rules.mk
@@ -135,8 +135,8 @@ $(BUILD)/assets/%.2bpp $(BUILD)/assets/%.map: assets/%.png
 	@echo Converting $<
 	@mkdir -p $(dir $@)
 	$(Q)rgbgfx \
-		-t "$(BUILD)/$(shell dirname $<)/$(shell basename --suffix=.png $<).map" \
-		-o "$(BUILD)/$(shell dirname $<)/$(shell basename --suffix=.png $<).2bpp" \
+		-t "$(BUILD)/$(shell dirname $<)/$(shell basename -s .png $<).map" \
+		-o "$(BUILD)/$(shell dirname $<)/$(shell basename -s .png $<).2bpp" \
 		-u \
 		$<
 
@@ -144,8 +144,8 @@ $(BUILD)/level/%.h $(BUILD)/level/%.c: level/%.png level/%_coll.png
 	@echo Generating level $<
 	@mkdir -p $(BUILD)/level
 	@cp $< $(BUILD)/level/
-	@cp $(shell dirname $<)/$(shell basename --suffix=.png $<)_coll.png $(BUILD)/level/
-	@python scripts/generate_map.py --compress 1 "$(BUILD)/$<"
+	@cp $(shell dirname $<)/$(shell basename -s .png $<)_coll.png $(BUILD)/level/
+	@python3 scripts/generate_map.py --compress 1 "$(BUILD)/$<"
 
 $(BUILD)/assets/%.oam.2bpp: assets/%.oam.png
 	@echo Converting $<