Parallel processes.
-j 2
--jobs=2$< -- first prerequisite file
$^ -- all prerequisite files
$@ -- target file
$% -- target name ("file" in "file.json")
$(@D) -- target directory
$(dir $@) -- target directory
$(notdir $@) -- target file name ("file.json" in "/home/user/file.json")
check_virtualenv:
@if [ -z $(VIRTUAL_ENV) ]; then \
echo 'Not in virtual environment. Continuing...'; \
else \
echo 'In virtual environment. Halting.'; \
exit 1; \
fi