Using git, you'll sometimes find that you're not on any branch. This usually happens when you're using a submodule inside another project. Sometimes you'll make some changes to this submodule, commit them and then try to push them up to a remote repository:

ed$ git commit -m "My excellent commit"
[detached HEAD d2bdb98] My excellent commit
 3 files changed, 3 insertions(+), 3 deletions(-)
ed$ git push origin master
Everything up-to-date

Er, what? Everything is not up to date - I just made changes! The clue is in the first part of the commit response - [detached HEAD d2bdb98]. This just means that we've made a commit without actually being on any branch.

Luckily, this is easy to solve - all we need to do is checkout the branch we should have been on and merge in that commit SHA:

ed$ git checkout master
Previous HEAD position was d2bdb98... My excellent commit
Switched to branch 'master'
ed$ git merge d2bdb98
Updating 88f218b..d2bdb98
Fast forward
 ext-mvc-all-min.js |    2 +-
 ext-mvc-all.js     |    2 +-
 view/FormWindow.js |    2 +-
 3 files changed, 3 insertions(+), 3 deletions(-)

I received an unhelpful error while trying to push to a repository on Github today:

git push
To git@github.com:user/repo.git
! [rejected] branchname -> branchname (non-fast forward)
error: failed to push some refs to 'git@github.com:user/repo.git'

In case you ever have the same problem, all you have to do is a quick git pull first, then you can carry on as normal. Easy when you know how...

Having recently made the switch from svn to git, I wanted to achieve what svn externals did (and what Piston did better). Turns out this is pretty simple, for example to get rails on edge:

cd your_git_dir
git submodule add git://github.com/rails/rails.git vendor/rails

A couple of other default submodules you'll want:

git submodule add git://github.com/dchelimsky/rspec.git vendorpluginsrspec
git submodule add git://github.com/dchelimsky/rspec-rails.git vendorpluginsrspec-rails

What submodule does is to check out the submodules as their own repositories, so they are tracked independently of the repository you made them submodules of. The submodules you have are tracked in the .gitmodules file, which might look something like this: