How to write a Makefile rule to remove files (clean) only if the files are present?

Question

It looks to me like Makefile rules can be roughly classified into "positive" and "negative" ones: "positive" rules create missing or update outdated files, while "negative" ones remove files.

Writing prerequisites for "positive" rules is quite easy: if the target and the prerequisite are file names, make by default runs the recipe if the target is missing or outdated (a missing file in this context may be viewed as an infinitely old file).

However, consider a "negative" rule, for example for target clean. A usual way to write it seems to be something like this:

clean:
    rm -f *.log *.synctex.gz *.aux *.out *.toc

This is clearly not the best way to do:

1. rm is executed even when there is nothing to do,

2. its error messages and exit status need to be suppressed with -f flag, which has other (possibly undesirable) effects, and

3. the fact that there were nothing to do for target clean is not reported to the user, unlike what is normal for "positive" targets.

My question is: how to write a Makefile rule that shall be processed by make only if certain files are present? (Like what would be useful for make clean.)

Answer 1

how to write a Makefile rule that shall be processed by make only if certain files are present? (Like what would be useful for make clean.)

You can do it like so:

filenames := a b c

files := $(strip $(foreach f,$(filenames),$(wildcard $(f))))

all: $(filenames)

$(filenames):
    touch $@

clean:
ifneq ($(files),)
    rm -f $(files)
endif

Example session:

$ make
touch a
touch b
touch c
$ make clean
rm -f a b c
$ make clean
make: Nothing to be done for 'clean'.

Useful perhaps for some purposes, but it strikes me as a strained refinement for make clean.

Answer 2

This can be easily remedied:

clean:
    for file in *.log *.synctex.gz *.aux *.out *.toc; do \
        if [ -e "$file" ]; then \
            rm "$$file" || exit 1; \
        else \
            printf 'No such file: %s\n' "$file" \
        fi \
    done

The if statement is necessary unless your shell supports and has enabled nullglob or something similar.

If your printf supports %q you should use that instead of %s to avoid possible corruptions of your terminal when printing weird filenames.