Git submodules are a convenient way to create a dependency on another git project. This can be useful if you depend on another project and want to make it easy for others to get your system up and running.
I won't cover creating submodules here as that is pretty well convered in the git manual, but updating them is slightly less obvious. When creating submodule dependencies, git actually saves a reference to the version of the tree at that point in time. That's a good thing, it prevents things from breaking due to your dependency changing out from under you. But there are times when you'll want to update that reference. The trick is to realize that git is keeping a reference to whatever version of the git tree is currently checked out. So all you need to do is go and pull the submodule, then commit the project as a whole.
- Make sure that your submodule is already checked out, ie, do 'git submodule init', 'git submodule update' if necessary
- CD into the root directory for your submodule.
- Change the branch for the submodule to master: 'git checkout master'
- Pull the latest version: 'git pull'
- That's it, now you can go back to your project's root directory, do a 'git status' to see the state of things, add the changed submodule with 'git add' then commit your changes with 'git commit'
That should do it. I'm still coming to terms whether I like this particular style of dependencies, for those of us with limited bandwidth it does cause rather large repositories to be sent over the wire just to get things running. Having dependencies be managed via PiPi in Python is certainly the better option if possible.