Having a hard time understanding git-fetch

Question

Hi,

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, then will it 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 How would I use the refspec to fetch contents form my remote master into it's remote tracking branch? I believe this may be possible if I current HEAD is on master and I run

git fetch origin master

However, can I use the <+?src:dest> refpec 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.

Thanks in advance for the help.

--
Regards
Parag

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.

Answer 2

felipec have answered most of issues in question in his answer.

A few remaining (most taken from git fetch manpage; which is a bit dated in some places, unfortunately):

• If remote-tracking branch (branch which tracks some branch in some remote repository) does not exists, it would be created.

• The branch you fetch into (the <dst> in [+]<src>:<dst>) doesn't need to reside in remotes/<remote>/ namespace. For example for mirroring repositories ("git clone --mirror") refspec is 1 to 1. In old days before separate remotes layout (before remotes/<remote>/ namespace for remote-tracking refs) 'master' branch was fetched into branch called 'origin'. Even currently tags are fetched directly into tags/ namespace in mirroring fashion.

• If branch you are fetching into (the right hand side of refspec <src>:<dst> does exist, Git would check if download would result in fast-forward, i.e. if current state in <dst> is ancestor of state in <src> in given remote repository. If it isn't, and you don't use -f / --force option to git-fetch, or prefix refspec with '+' (use +<src>:<dst> refspec) fetch would refuse to update that branch.

• "git fetch origin master" is equivalent to "git fetch origin master:", not to "git fetch origin master:master"; it stores fetched value of 'master' branch (of remote ''origin') in FETCH_HEAD, and not in 'master' branch or remote-tracking 'remotes/origin/master' branch. It can be followed by "git merge FETCH_HEAD". Usually not used directly, but as part of one-time pull without setting remote-tracking branch: "git pull <URL> <branch>".

• +refs/heads/*:refs/remotes/origin/* as value for "remote.origin.fetch" configuration variable means that each branch (ref in refs/heads/ namespace) in remote 'origin' is fetched into respectively named remote-tracking branch in refs/remotes/origin/ namespace, e.g. 'master' branch in 'origin' (i.e. 'refs/heads/master' ref) would be fetched into 'origin/master' remote-tracking branch (i.e. 'refs/remotes/origin/master' ref). The '+' prefix means that fetch would succeed even in non fast-forward case, which means when branch on remote is rebased, or rewound (reset to some state in past) or otherwise amended.

Sidenote: You would probably want to use higher level git remote command to manage remote repositories and get updates.