Build on shell condition with Make - linux

Question

I would like to add to a build list the packages I want to build if it is not installed yet.
The goal is to install some Python packages without pip and from local sources. I don't have access to pip...
So I wrote I Makefile that looks like:
all: natsort foo bar foobar ...
natsort: natsort-4.0.4.tar.gz
tar xvzf $<
cd $(patsubst %.tar.gz,%, $<) && python setup.py install
rm -rf $(patsubst %.tar.gz,%, $<)
python -c 'import natsort'
echo -e "Installation of $< [done]\n" >> install.log
The problem with this implementation is that all the packages will be rebuild and reinstalled each time I run the Make command. I would like to check if the module is already installed. My idea is to do something like this:
ifdef $(shell python -c 'import natsort')
all: natsort
endif
How can I rewrite this to make it works?

Answer 1

You can absolutely do something like that. But it "costs" a shell invocation and an invocation of python every time you run make and that's a relatively high cost.
There are, basically, two ways to do what you want in a cheaper manner.
A stamp file and short-circuiting logic in the recipe.
The stamp file method is basically what you have except you add touch $# to the end of the recipe.
natsort: natsort-4.0.4.tar.gz
tar xvzf $<
cd $(patsubst %.tar.gz,%, $<) && python setup.py install
rm -rf $(patsubst %.tar.gz,%, $<)
python -c 'import natsort'
echo -e "Installation of $< [done]\n" >> install.log
touch $#
That way running the recipe the first time creates the stamp file and until natsort-4.0.4.tar.gz becomes newer than the stamp file or the stamp file gets deleted the recipe will never run again.
Note that second point though. Delete the stamp file and you install again.
That's the thing that the short-circuit logic solution solves.
Instead of your original rule of multiple commands you wrap it all in one command (this is optional but saves repeated checking costs) and check for the module to be installed before doing any work.
natsort: natsort-4.0.4.tar.gz
if ! python -c 'import natsort'; then \
tar xvzf $< || exit 1; \
cd $(patsubst %.tar.gz,%, $<) && python setup.py install || exit 1; \
rm -rf $(patsubst %.tar.gz,%, $<); \
python -c 'import natsort' || exit 1; \
echo -e "Installation of $< [done]\n" >> install.log; \
fi
Note the need to add || exit 1 since we no longer have make handling that for us. Also note that this now always runs the rule (and we should mark natsort and .PHONY) but in the most common case it will stop after the if test.
That all said you can combine these methods to get the best of both worlds.
natsort: natsort-4.0.4.tar.gz
if ! python -c 'import natsort'; then \
tar xvzf $< || exit 1; \
cd $(patsubst %.tar.gz,%, $<) && python setup.py install || exit 1; \
rm -rf $(patsubst %.tar.gz,%, $<); \
python -c 'import natsort' || exit 1; \
echo -e "Installation of $< [done]\n" >> install.log; \
fi
touch $#
and you get the benefits of both methods. The first time you run make the natsort file doesn't exist and the recipe is run. natsort isn't installed so the if test fails and the installation occurs. After that the natsort file it touched. The next time make is installed natsort is newer than natsort-4.0.4.tar.gz so make doesn't think it has anything to do. If, for some reason, you delete the natsort file then the next time make runs it checks for the module to exist, skips the installation and touches the natsort file again to get back into sync.