Is it possible to add a submodule to a Git repository by hand? That is, just by editing files in a text editor, not touching the Git CLI at all?
When a submodule is added, Git creates .submodules file and for a submodule
named git-submodule, it will contain 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 of that file.
A directory 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.