Update Git submodule to latest commit on origin

Question

I have a project with a Git submodule. It is from an ssh://... URL, and is on commit A. Commit B has been pushed to that URL, and I want the submodule to retrieve the commit, and change to it.

Now, my understanding is that git submodule update should do this, but it doesn't. It doesn't do anything (no output, success exit code). Here's an example:

$ mkdir foo
$ cd foo
$ git init .
Initialized empty Git repository in /.../foo/.git/
$ git submodule add ssh://user@host/git/mod mod
Cloning into mod...
user@host's password: hunter2
remote: Counting objects: 131, done.
remote: Compressing objects: 100% (115/115), done.
remote: Total 131 (delta 54), reused 0 (delta 0)
Receiving objects: 100% (131/131), 16.16 KiB, done.
Resolving deltas: 100% (54/54), done.
$ git commit -m "Hello world."
[master (root-commit) 565b235] Hello world.
 2 files changed, 4 insertions(+), 0 deletions(-)
 create mode 100644 .gitmodules
 create mode 160000 mod
# At this point, ssh://user@host/git/mod changes; submodule needs to change too.
$ git submodule init
Submodule 'mod' (ssh://user@host/git/mod) registered for path 'mod'
$ git submodule update
$ git submodule sync
Synchronizing submodule url for 'mod'
$ git submodule update
$ man git-submodule 
$ git submodule update --rebase
$ git submodule update
$ echo $?
0
$ git status
# On branch master
nothing to commit (working directory clean)
$ git submodule update mod
$ ...

I've also tried git fetch mod, which appears to do a fetch (but can't possibly, because it's not prompting for a password!), but git log and git show deny the existence of new commits. Thus far I've just been rm-ing the module and re-adding it, but this is both wrong in principle and tedious in practice.

Answer 1

The git submodule update command actually tells Git that you want your submodules to each check out the commit already specified in the index of the superproject. If you want to update your submodules to the latest commit available from their remote, you will need to do this directly in the submodules.

So in summary:

# Get the submodule initially
git submodule add ssh://bla submodule_dir
git submodule init

# Time passes, submodule upstream is updated
# and you now want to update

# Change to the submodule directory
cd submodule_dir

# Checkout desired branch
git checkout master

# Update
git pull

# Get back to your project root
cd ..

# Now the submodules are in the state you want, so
git commit -am "Pulled down update to submodule_dir"

Or, if you're a busy person:

git submodule foreach git pull origin master