git fetch, FETCH_HEAD and origin/master

Question

I'm very new to git and I'm having trouble with a simple fetch operation.

I'm trying to fetch a coworker's progress from his repository. At first I did git fetch HEAD which prompted git to download about 350MB of data so I was sure it had done something. However, origin/master ended up still pointing to the same old commit (actually it's under the name dev but I'll call it master - he doesn't have a master).

After that I tried git fetch origin master but it didn't seem to do anything, it only updated FETCH_HEAD. I tagged the FETCH_HEAD commit so I wouldn't lose it, but I'd still like to have an updated remote branch.

What was it that went wrong? I do not have access to the remote repository. Can I still fix it at home?

Answer 1

I'm a bit confused by the commands you use. HEAD is usually a label git uses to track the commit that is currently in the working directory. The git fetch command expects a remote or a remote commit configuration to know what you want fetched. Using git fetch HEAD would indicate HEAD is a remote in your repository. That the command worked without error is curious.

For example: git fetch HEAD in the repository I'm currently working results in the following error

fatal: 'HEAD' does not appear to be a git repository
fatal: Could not read from remote repository.

Please make sure you have the correct access rights
and the repository exists.

The command git remote will list all remotes, while git remote --verbose will include the address of the remote. Could you use this to see if you have a remote defined as HEAD and what remote addresses your friends repository?

However, my questions aside and to help clear up your confusion. The git fetch ... command only updates remote refs -- not your local ones.

To make this clear, look inside the .git folder in your repository (it is hidden by default so you may need to unhide it). You will find a folder structure similar to the following

working directory
|=>.git
|  |=>objects           <= contains data for each commit
|  |=>refs
|     |=>heads
|        |-master       <= file containing current commit of local master branch
|     |=>remotes
|        |=>origin
|           |-master    <= file containing current commit of remote origin's master branch
|-FETCH_HEAD            <= file updated by `git fetch`, contains info of what was fetched

Say you checkout the master branch, git checkout master -- git will change your working directory to match the commit data in the 'objects' folder that matches the commit value in the '.git/refs/heads/master' file.

If you then git fetch origin master, the '.git/refs/remotes/origin/master' file is updated to the commit of the master branch on the remote origin -- and all commit data needed for that commit is downloaded and placed in the 'objects' folder.

The important point here is git fetch does not update your working directory reflects the local branch checked out and git fetch never updates a local branch.

Using either git merge ... or git rebase ... is needed to update the local master branch with the changes in origin/master. git pull ... does both git fetch ... and either git merge ... or git rebase ..., depending on options and configuration (git merge ... is the default).

After all that explanation, you want to be able to see what -- if anything -- was fetched from your friends repository. The git branch -avv command will list all local and remote branches, with commit numbers and in the case of local branches, what remote branch it is tracking.

To see how the branches relate to each other I find it helpful to use a tool to graph the repository tree. There are several to choose from but I find the git log command sufficient; such as git log --all --graph --oneline --decorate. Fair warning, this can be quite long and convoluted for a large repository. A shorter output can be obtained with by adding the --simplify-by-decoration argument.

To summarize: if you can fix it at home depends on the information in your repository. The above mentioned commands; git remote --verbose, git branch -avv and git log ... should be give you an understanding of the current state of your repository. From there you can determine if you need to do something more to get the data in your local branch(es) using git merge or git rebase.

As always, if you run into trouble, post back with what you learn.