Automatic initialization and update of submodules in Makefile

Question

I have a git project with several sub-modules (a choice I somewhat regret in retrospect). It uses gnu make

Currently I expect people to issue by hand a git submodule update --init¹ before building the project for the first time and also after pulling any change that updated a submodule reference.

However, I'd like for the Makefile to issue these commands automatically when they are needed. It's OK if they issue the commands sometimes when they aren't needed (spurious update) - but it shouldn't happen regularly.

For the initial init it seems enough to have a rule like (for a submodule that lives in directory module1:

module1/.git:
    git submodule update --init

And here the choice of .git as the file to "represent" the submodule is fairly arbitrary, it could be some other file instead.

However this doesn't work that well to update the submodule when the reference has been updated. I guess I could make the submodule depend on the root .gitmodules file which I guess should change when a submodule reference gets updated, something like:

module1/.git: .gitmodules
    git submodule update --init

Here the use of .git seems wrong though: presumably that directly won't necessarily be updated when update is run (especially if there was no update to this particular submodule), which will leave the update command running every time.

Looking for a cleaner solution here.

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¹ Or possibly use the --recursive argument on the initial clone, which has the same effect.

Answer 1

I've crafted something like this:

.PHONY: check-and-reinit-submodules
check-and-reinit-submodules:
    @if git submodule status | egrep -q '^[-]|^[+]' ; then \
            echo "INFO: Need to reinitialize git submodules"; \
            git submodule update --init; \
    fi

It is using git submodule status, to figure out if submodule is not initialized (- at the beginning) or somehow modified/outdated (+ at the beginning). This make goal will run always, but git submodule update --init will run only when needed.