Adding git submodules automatically (.gitmodules)

Question

I have a repo with several submodules. I want to add some others, and the fastest way for me is to use the .gitmodules (which in my opinion should clearly allow any kind of submodule management).

However, when editing this file and adding submodules, after a git submodule init nothing is added (except the submodules that were already present before the modification).

Is there any solution to add a submodule without going through git submodule add (ie, just by editing the .gitmodules file and then git submodule update --init) ?

That is, the following workflow should automatically add the submodule "foo/bar":

Add the following to .gitmodules:
    [submodule "foo/bar"]
        path = foo/bar
        url = https://example.com/foo.git

Run the following command after saving:
    git submodule init
    git submodule update

Expected result:
    submodule 'foo/bar' automatically gets added
    it is also updated (the update command)

Answer 1

When you add a Git submodule, Git creates .gitmodules file and for a submodule named git-submodule will add something like this:

[submodule "git-submodule"]
    path = git-submodule
    url = https://github.com/TomasHubelbauer/git-submodule

The same is added to .git/config after the existing content in that file.

A folder for the submodule named after the submodule is created in .git/modules. This folder is nearly identical to the the .git directory of the actual submodule repository, but it doesn't contain the actual objects (instead the submodule data is checked out to its directory and its metadata are here).

This means that theoretically, you might be able to add a submodule by hand without using git submodule add, but you would have to recreate all these config files. But one can still imagine cloning the submodule repository to a separate directory and copying its .git over to this one. That might work.

However, adding a submodule also changes the index, .git/index, so you would have to manually update this hash as well, and at this point, you're reimplementing Git, but manually.

As a result, I don't believe it is anywhere near practical to add a Git submodule by hand.