Make with dependencies from a file

Question

I want to write a Makefile that reads a file list.txt and produces result.tar containing the contents. If there is a change in either the list.txt file, or any of the files it points at, then result.tar should be rebuilt. How can I express this in a Makefile? The closest I have come is:

result.tar : list.txt
 cat list.txt | xargs tar -cf result.tar

But this omits the dependency on the contents of list.txt.

Answer 1

I think there should be something like this:

result.tar : list.txt $(shell cat list.txt)
 cat list.txt | xargs tar -cf result.tar

Or, a bit better (extracting list.txt to a variable and using automatic variables):

LIST_FILE := list.txt
result.tar : $(LIST_FILE) $(shell cat $(LIST_FILE))
 cat $< | xargs tar -cf $@

• Matthew Hannigan: This will break if xargs invokes tar more than once. Each invocation overwrites the tar file. Just remove the xargs to make it work correctly.