"No rule to make target" with % in Makefile Error

I am writing my first Makefile. In it, I want all files that have been updated in a folder called FOLDER/assets to be copied to FOLDER/export_bin/application/assets. I've been using https://makefiletutorial.com/ as a reference.

Here's the code I wrote for it:

build_game:
	$(MAKE) $(export_folder)/application/assets/%
	...

$(export_folder)/application/assets/%: ../assets/%
	cp $@ $<

I get the error:

make[1]: *** No rule to make target 'debug/application/assets/%'.  Stop.

How should I do this instead?

Here you go:

base := FOLDER
export_folder := $(base)/export_bin
import_folder := $(base)/assets
input_files := $(wildcard $(import_folder)/*)
# See https://makefiletutorial.com/#string-substitution
output_files := $(patsubst $(import_folder)/%,$(export_folder)/application/assets/%,$(input_files))

# See https://makefiletutorial.com/#phony
.PHONY: build_game
build_game: $(output_files)

$(export_folder)/application/assets/%: $(import_folder)/%
	cp $< $@

The first few lines set up the directories we will be using. The key line is the input_files assignment calling $(wildcard). (This is a GNU extension.) The tutorial you went to doesn't appear to cover this feature.

Then we construct the output file list by substituting the import paths for the output path. The .PHONY: pseudo-target indicates that build_game is not intended to be a real file; otherwise make can get confused. We make the build_game target dependent on the output files; this triggers the rules for making the output files at the end. Finally we have the output file rule; it's essentially the same as what you used, except you swapped the $< and $@ and used a relative path, which might work but I wouldn't recommend it.

asset_files := %(notdir $(wildcard ../assets))
target_asset_files := $(addprefix $(export_folder)/application/assets/,$(asset_files))

build_game : $(target_asset_files)

$(export_folder)/application/assets/%: ../assets/%
    cp $@ $<

build_game is dependent on all the asset files, which will get copied over via the pattern rule if they are out of date.

One sharp stick here is that if you delete a file in the source directory (../assets/), it would not be deleted in the target directory, so your target directory will not necessarily be in sync with the source... If you need the two to be in sync, there's other ways to do that.

------ EDIT ---------

to use rsync, you would do something to this effect:

asset_src := ../assets
asset_trg := $(export_folder)/application/assets

build_game : rsync_sentinel

rsync_sentinel: $(wildcard $(asset_src)/*)
    rsync -a --delete $(asset_src)/ $(asset_trg)
    touch $@

Here, the sentinel file is a dummy file whos timestamp represents the last time it did a copy. If any of the files inside ../assets is newer than the sentinel, it will use rsync to copy any necessary files over, and then update the timestamp of the sentinel.