Is it possible to checkout branches of main repository in sync with submodules?

Question

What I want to achieve is whenever I use git checkout on my main repository for example:

git checkout my-branch

my submodules will follow my-branch instead of a detached head.

Is it possible, and if yes, how?

Answer 1

If those submodule repositories have a my-branch of their own, they can be declared to follow that branch

cd /path/to/your/parent/repo/Foo
git config -f .gitmodules submodule.bar1.branch my-branch
git config -f .gitmodules submodule.bar2.branch my-branch

git submodule update --remote

But that involves repeating that each time you checkout a branch in the parent repo.

torek points out in the comments that those submodules might have submodules of their own, hence the need for the --recursive option.

You might also want to add --recursive and/or --no-fetch to your git submodule update --remote command.
Rather than individual git config -f operations, you might want git submodule foreach, again maybe with --recursive.

git submodule foreach -q --recursive 'git config -f $toplevel/.gitmodules submodule.$name.branch my_branch'

In multiple lines for readability:

git submodule foreach -q --recursive \
  'git config -f $toplevel/.gitmodules submodule.$name.branch my_branch'

Then you can checkout each submodule to that branch:

git submodule foreach -q --recursive 'branch="$(git config -f $toplevel/.gitmodules submodule.$name.branch)"; git checkout $branch'

In multiple lines for readability:

git submodule foreach -q --recursive \
  'branch="$(git config -f $toplevel/.gitmodules submodule.$name.branch)"; \
   git checkout $branch'