learning Git: tracking vs. setting upstream (-u) for remotes?

Question

I am learning Git and am attempting to understand the difference between \"tracking\" a remote and defining an \"upstream\" relationship with it (with the -u tag).

Answer 1

git push origin master explicitly says "push the local branch 'master' to the remote named 'origin'". This does not define a persistent relationship, it just executes a push this one time. Note that the remote branch is assumed to be named "master".

git push -u origin master is the same thing, except it first adds a persistent tracking relationship between your local branch "master" and the remote named "origin". As before, it is assumed the remote branch is named "master".

If you have done a push with -u already, then the relationship is already defined. In the future, you can simply say git push or git pull, and git will automatically use the defined remote tracking branch without being told explicitly.

You can view your tracking relationships with git branch -vv, which will list your local branches along with their current HEAD commit and, if one is set, the remote tracking branch. Here is an example.

$ git branch -vv
* master                   58a0d68 [origin/master] Fix CSS regression bug
  migration_tool           2a24ff7 [origin/migration_tool] [#906] Fix table layout problem
  topic-ajax-timeouts      fe854f2 Adjust timeouts to be more realistic

Here are shown 3 branches along with their current HEAD commits and commit messages. On the first two, there is a tracking branch specified, but the third branch is not tracking a remote branch.