Git Submodule .gitmodules

 https://stackoverflow.com/questions/11358082/empty-git-submodule-folder-when-repo-cloned

.gitmodules

[submodule "external/common"]
   path = external/common
   url = ../common.git
[submodule "external/project"]
   path = external/project
   url = ../project.git
[submodule "external/tools"]
   path = external/tools
   url = ../tools.git

git submodule update --init

How to git add submodule with error "already exists in the index"?

https://stackoverflow.com/questions/12898278/issue-with-adding-common-code-as-git-submodule-already-exists-in-the-index

You need to remove your submodule git repository (projectfolder in this case) first for git path.

rm -rf projectfolder

git rm -r projectfolder

git submodule add <ssh/https> projectfolder

git submodule add git@gitlab.com:user/project.git external/project

Git submodule with branch

https://stackoverflow.com/questions/1777854/how-can-i-specify-a-branch-tag-when-adding-a-git-submodule

Existing answers have the second step missing and overloaded with details.

To switch an existing submodule to track a new remote url and/or a new branch:

1. Edit the source of truth in .gitmodules.

For example, from

[submodule "api"]
    path = api
    url = https://github.com/<original_repo>/api.git

to

[submodule "api"]
    path = api
    url = https://github.com/<another_repo>/api.git
    branch = work-in-progress

2. git submodule sync: Updates the description of submodules cached by git in .git/modules from the just-edited source of truth specified in .gitmodules.

3. git submodule update --init --recursive --remote: Updates the checked-out submodules in the working copy.

4. Commit the changes.