Modifying file extensions using Makefiles - makefile

Question

I'm new to Makefiles and I want to modify the extension of a set of files. The following command works on the shell:
for file in path/*.ext1; do j=`echo $file | cut -d . -f 1`;j=$j".ext2";echo mv $file $j; done
However, I'm not sure how to run this in a Makefile. I tried running
$(shell for file in path/*.ext1; do j=`echo $file | cut -d . -f 1`;j=$j".ext2";echo mv $file $j; done)
But this never did what I needed it to do. What do I need to do to make this work on the Makefile? How do I call it in a section?

Answer 1

The immediate answer to your question is that the $ character is special to make: it introduces a make variable. If you want to pass a $ to the shell, you'll have to write two of them: $$.
So, your shell function invocation would have to be written as:
$(shell for file in path/*.ext1; do j=`echo $$file | cut -d . -f 1`;j=$$j".ext2";echo mv $$file $$j; done)
However, this is almost certainly not a good way to do what you want. You don't really describe clearly what you want to do, however. If you just want to have a target in a makefile that can be invoked to make this change, you can use:
fixext:
for file in path/*.ext1; do \
j=`echo $$file | cut -d . -f 1`; \
j=$$j".ext2"; \
echo mv $$file $$j; \
done
Or, taking advantage of some useful shell shortcuts, you could just run:
fixext:
for file in path/*.ext1; do \
echo mv $$file $${file%.*}.ext2; \
done
Now if you run make fixext it will perform those steps.
But, a much more make-like way to do it would be to write a single rule that knows how to rename one file, then use prerequisites to have them all renamed:
TARGETS = $(patsubst %.ext1,%.ext2,$(wildcard path/*.ext1))
fixext: $(TARGETS)
%.ext2 : %.ext1
mv $< $#
Now you can even run make -j5 and do 5 of the move commands in parallel...

Answer 2

you can also add rename blocks at the top of your file eg to change a suffix
output := $(input:.mov=.mp4)
but this won't work inside a make command as far as I can see
check:
output := $(input:.mov=.mp4)
gives
$ input=walkthrough.mov make check
output := walkthrough.mp4
make: output: No such file or directory
make: *** [check] Error 1