Is it possible to generate the list of prerequisites for a class of targets
from an external command?
For example Python requirements.txt files (of the type compiled by pip-compile
[1]) can import other requirements files with lines like "-r other.txt" or "-c
other.txt". I'm no UNIX wizard but it seems easy to get a list of the other
requirements files that a given requirements file (foo.txt in this example)
depends on with a command like:
grep --color=never '^[[:blank:]]*-r\|^[[:blank:]]*-c' foo.in | sed
's/^[[:blank:]]*\(-r\|-c\)[[:blank:]]*//'
I want to get my Makefile to use this command to automatically figure out the
prerequisites of any requirements/*.txt file. This is what I tried:
requirements/%.txt: requirements/$(shell grep --color=never
'^[[:blank:]]*-r\|^[[:blank:]]*-c' requirements/%.in | sed
's/^[[:blank:]]*\(-r\|-c\)[[:blank:]]*//')
But it doesn't work:
grep: %.in: No such file or directory
It seems that make didn't expand the % within the $(shell ...) command.
I couldn't find a way to get this to work. Is it possible?
Thanks
[1]: https://pip-tools.readthedocs.io/
<https://pip-tools.readthedocs.io/en/latest/>
