Having a hard time understanding git-fetch

Question

I am having a hard time understanding the nuances of git-fetch. I understand that doing a fetch, fetches the remote refs into a local tracking branch.

I have a few questions though:

1. Can it be possible that a local tracking branch does not exist? If so, will it then be created automatically?

2. What will happen if I do a fetch and specify a non tracking branch as the destination?

3. The man page for git-fetch specifies:

   git-fetch <options> <repository> <refspec>
   

How would I use the refspec to fetch contents from my remote master into its remote tracking branch? I believe this may be possible if my current HEAD is on master and I run

git fetch origin master

However, can I use the <+?src:dest> refspec to achieve the same thing? I think this will help me understand the concepts better.

And one more question:

My .git/config file has the following line for fetching (showing only relevant lines):

fetch = +refs/heads/*:refs/remotes/origin/*

Can someone please explain what this line exactly means?

Answer 1

First, there's no such concept of local tracking branches, only remote tracking branches. So origin/master is a remote tracking branch for master in the origin repo.

Typically you do git fetch $remote which updates all your remote tracking branches, and creates new ones if needed.

However, you can also specify a refspec, but that will not touch your remote tracking branches, instead, it will fetch the branch you specified and save it on FETCH_HEAD, unless you specify a destination. In general you don't want to mess with this.

Finally,

fetch = +refs/heads/*:refs/remotes/origin/*

That means if you do

git fetch origin

It will actually do:

git fetch origin +refs/heads/*:refs/remotes/origin/*

Which means a remote heads/foobar will be local remotes/origin/foobar, and the plus sign means they'll be updated even if they are not fast-forward.

Perhaps what you think as a tracking branch is something related to git pull and the merge config.