make wildcard not finding files I just copied - makefile

Question

I would like to copy pdf files from several directories into a build directory, then use pdfunite to compile them into one pdf. The following make recipe works, but I have to run it twice because the first time through, I get an error from pdfunite - no files are found in the build directory (the PDFS variable is empty) even though they were just copied in the previous lines. How can I fix this so it works in one pass? I have simplified the recipe for clarity; I am actually pulling from various folders and making some pdfs on the fly as well, so I can't easily concatenate a full list of files from various subfolders (folder1 and folder2 in the example) to pass to pdfunite.
notebook:
mkdir -p $(out)
mkdir -p $(build)/notebook
$(eval PR := $(sort $(wildcard $(data)/folder1/*.pdf)) )
cp $(PR) $(build)/notebook
$(eval SR := $(sort $(wildcard $(data)/folder2/*.pdf)) )
cp $(SR) $(build)/notebook
$(eval PDFS := $(sort $(wildcard $(build)/notebook/*.pdf)) )
pdfunite $(PDFS) $(out)/notebook.pdf

Answer 1

Your Makefile is not in line with make's philosophy. You are using make as another scripting language, while make is more than this. It compares targets and prerequisites dates, based on this decides which must be built or re-built, and passes the recipes to the shell. So, for your particular problem, you should rather try something like:
PR := $(wildcard $(data)/folder1/*.pdf)
SR := $(wildcard $(data)/folder2/*.pdf)
PDFS1 := $(patsubst $(data)/folder1/%.pdf,$(build)/notebook/%.pdf,$(PR))
PDFS2 := $(patsubst $(data)/folder2/%.pdf,$(build)/notebook/%.pdf,$(SR))
PDFS := $(sort $(PDFS1) $(PDFS2))
.PHONY: notebook
notebook: $(out)/notebook.pdf
$(PDFS1): $(build)/notebook/%.pdf: $(data)/folder1/%.pdf | $(build)/notebook
cp $< $#
$(PDFS2): $(build)/notebook/%.pdf: $(data)/folder2/%.pdf | $(build)/notebook
cp $< $#
$(build)/notebook $(out):
mkdir -p $#
$(out)/notebook.pdf: $(PDFS) | $(out)
pdfunite $(PDFS) $#
The variables definitions are reasonably straightforward: patsubst, as its name says, substitutes strings. The target: pattern: prerequisites is a static pattern rule. And the prerequisites after | are order-only prerequisites.
What this makefile says, basically, is that $(out)/notebook.pdf depends on a set of pdf files in $(build)/notebook/ and that these pdf files depend on source pdf files with the same basenames in $(data)/folder1/ and $(data)/folder2/. It also says that directories must be created before being populated. Thanks to all this only what needs to be done will be done, no more, no less. And it is more in line with make's philosophy.
If you have many source folders and do not want to replicate the copying rules, you can use more advanced features like:
FOLDERS := folder1 folder2
.PHONY: notebook
notebook: $(out)/notebook.pdf
define MY_rule
$(1)_SRCS := $$(wildcard $$(data)/$(1)/*.pdf)
$(1)_DSTS := $$(patsubst $$(data)/$(1)/%.pdf,$$(build)/notebook/%.pdf,$$($(1)_SRCS))
PDFS += $$($(1)_DSTS)
$(1)_DSTS: $$(build)/notebook/%.pdf: $$(data)/$(1)/%.pdf | $$(build)/notebook
cp $$< $$#
endef
$(foreach f,$(FOLDERS),$(eval $(call MY_rule,$(f))))
$(build)/notebook $(out):
mkdir -p $#
$(out)/notebook.pdf: $(PDFS) | $(out)
pdfunite $(PDFS) $#

Answer 2

I would do another way.
PR:=$(sort $(wildcard $(data)/folder1/*.pdf))
SR:=$(sort $(wildcard $(data)/folder2/*.pdf))
PDFS=$(sort $(wildcard $(build)/notebook/*.pdf))
all: copy
pdfunite $(PDFS) $(out)/notebook.pdf
copy:
mkdir -p $(out)
mkdir -p $(build)/notebook
cp $(PR) $(build)/notebook
cp $(SR) $(build)/notebook
.PHONY: all copy
Please check: PDFS= and not PDFS:=. If you use simple = the variable's value will calculate when it needed (not sooner!).
When you run make it want to build all. The all's requirement is copy - so make does some mkdir and cp. After it return the all: the value of PDFS is needed so will evalute now - we have many-many pdf in $(build)/notebook :)