What does FETCH_HEAD in Git mean?
As mentioned in Jonathan's answer, FETCH_HEAD corresponds to the file .git/FETCH_HEAD
. Typically, the file will look like this:
71f026561ddb57063681109aadd0de5bac26ada9 branch 'some-branch' of <remote URL>
669980e32769626587c5f3c45334fb81e5f44c34 not-for-merge branch 'some-other-branch' of <remote URL>
b858c89278ab1469c71340eef8cf38cc4ef03fed not-for-merge branch 'yet-some-other-branch' of <remote URL>
Note how all branches but one are marked not-for-merge
. The odd one out is the branch that was checked out before the fetch. In summary: FETCH_HEAD essentially corresponds to the remote version of the branch that's currently checked out.
The FETCH_HEAD is a reference to the tip of the last fetch, whether that fetch was initiated directly using the fetch command or as part of a pull. The current value of FETCH_HEAD is stored in the .git
folder in a file named, you guessed it, FETCH_HEAD
.
So if I issue:
git fetch https://github.com/ryanmaxwell/Fragaria
FETCH_HEAD may contain
3cfda7cfdcf9fb78b44d991f8470df56723658d3 https://github.com/ryanmaxwell/Fragaria
If I have the remote repo configured as a remote tracking branch then I can follow my fetch with a merge of the tracking branch. If I don't I can merge the tip of the last fetch directly using FETCH_HEAD.
git merge FETCH_HEAD
FETCH_HEAD
is a short-lived ref, to keep track of what has just been fetched from the remote repository. git pull
first invokes git fetch
, in normal cases fetching a branch from the remote; FETCH_HEAD
points to the tip of this branch (it stores the SHA1 of the commit, just as branches do). git pull
then invokes git merge
, merging FETCH_HEAD
into the current branch.
The result is exactly what you'd expect: the commit at the tip of the appropriate remote branch is merged into the commit at the tip of your current branch.
This is a bit like doing git fetch
without arguments (or git remote update
), updating all your remote branches, then running git merge origin/<branch>
, but using FETCH_HEAD
internally instead to refer to whatever single ref was fetched, instead of needing to name things.