git submodule foreach checkout supermodule branch

Question

Consider a git repository Foo/, which has submodules bar1/ and bar2/.

Each of these has the same branches: 1 & 2.

I enter the supermodule, and I want to

Answer 1

A submodule is always by default in detached HEAD mode.

You can make each submodule follow a branch.
See "How to make an existing submodule track a branch".

cd /path/to/your/parent/repo/Foo
git config -f .gitmodules submodule.bar1.branch branch1
git config -f .gitmodules submodule.bar2.branch branch2

Then all you need to do is:

git submodule update --remote

That will update each submodule to the latest of their respective upstream branch (fetch + checkout).