What does FETCH HEAD in Git mean

Question

git pull --help says      In its default mode  git pull is shorthand for git fetch followed by git merge FETCH HEAD    What is this FETCH HEAD and what is actually merged during git pull

User · Answer

FETCH HEAD is a short-lived ref  to keep track of what has just been fetched from the remote repository   Actually      not always considering that  with Git 2 29  Q4 2020    quot git fetch quot  man  learned --no-write-fetch-head option to avoid writing the FETCH HEAD file  See commit 887952b  18 Aug 2020  by Junio C Hamano  gitster    Merged by Junio C Hamano -- gitster -- in commit b556050  24 Aug 2020   fetch  optionally allow disabling FETCH HEAD update Signed-off-by  Derrick Stolee   If you run fetch but record the result in remote-tracking branches  and either if you do nothing with the fetched refs  e g  you are merely mirroring  or if you always work from the remote-tracking refs  e g  you fetch and then merge origin branchname separately   you can get away with having no FETCH HEAD at all  Teach  quot git fetch quot  man  a command line option  quot -- no- write-fetch-head quot    The default is to write FETCH HEAD  and the option is primarily meant to be used with the  quot --no- quot  prefix to override this default  because there is no matching fetch writeFetchHEAD configuration variable to flip the default to off  in which case  the positive form may become necessary to defeat it    Note that under  quot --dry-run quot  mode  FETCH HEAD is never written  otherwise you d see list of objects in the file that you do not actually have  Passing --write-fetch-head does not force  git fetch  https   github com git git blob 887952b8c680626f4721cb5fa57704478801aca4 Documentation git-fetch txt  lt sup gt   man  https   git-scm com docs git-fetch   lt  sup gt  to write the file   fetch-options now includes in its man page   -- no- write-fetch-head Write the list of remote refs fetched in the FETCH HEAD file directly under  GIT DIR  This is the default  Passing --no-write-fetch-head from the command line tells Git not to write the file  Under --dry-run option  the file is never written    Consider also  still with Git 2 29  Q4 2020   the FETCH HEAD is now always read from the filesystem regardless of the ref backend in use  as its format is much richer than the normal refs  and written directly by  quot git fetch quot  man  as a plain file   See commit e811530  commit 5085aef  commit 4877c6c  commit e39620f  19 Aug 2020  by Han-Wen Nienhuys  hanwen    Merged by Junio C Hamano -- gitster -- in commit 98df75b  27 Aug 2020   refs  read FETCH HEAD and MERGE HEAD generically Signed-off-by  Han-Wen Nienhuys   The FETCH HEAD and MERGE HEAD refs must be stored in a file  regardless of the type of ref backend  This is because they can hold more than just a single ref  To accomodate them for alternate ref backends  read them from a file generically in refs read raw ref      With Git 2 29  Q4 2020   Updates to on-demand fetching code in lazily cloned repositories  See commit db3c293  02 Sep 2020   and commit 9dfa8db  commit 7ca3c0a  commit 5c3b801  commit abcb7ee  commit e5b9421  commit 2b713c2  commit cbe566a  17 Aug 2020  by Jonathan Tan  jhowtan    Merged by Junio C Hamano -- gitster -- in commit b4100f3  03 Sep 2020   fetch  no FETCH HEAD display if --no-write-fetch-head Signed-off-by  Jonathan Tan   887952b8c6   quot fetch  optionally allow disabling FETCH HEAD update quot   2020-08-18  Git v2 29 0 -- merge listed in batch  10  introduced the ability to disable writing to FETCH HEAD during fetch  but did not suppress the  quot  lt source gt  - gt  FETCH HEAD quot  message when this ability is used  This message is misleading in this case  because FETCH HEAD is not written  Also  because  quot fetch quot  is used to lazy-fetch missing objects in a partial clone  this significantly clutters up the output in that case since the objects to be fetched are potentially numerous  Therefore  suppress this message when --no-write-fetch-head is passed  but not when --dry-run is set

User · Answer

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  lt remote URL gt  669980e32769626587c5f3c45334fb81e5f44c34        not-for-merge   branch  some-other-branch  of  lt remote URL gt  b858c89278ab1469c71340eef8cf38cc4ef03fed        not-for-merge   branch  yet-some-other-branch  of  lt remote URL gt    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

User · Answer

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  lt branch gt   but using FETCH HEAD internally instead to refer to whatever single ref was fetched  instead of needing to name things

User · Answer

I have just discovered and used FETCH HEAD  I wanted a local copy of some software from a server and I did  git fetch gitserver release 1   gitserver is the name of my machine that stores git repositories  release 1 is a tag for a version of the software  To my surprise  release 1 was then nowhere to be found on my local machine  I had to type   git tag release 1 FETCH HEAD    to complete the copy of the tagged chain of commits  release 1  from the remote repository to the local one  Fetch had found the remote tag  copied the commit to my local machine  had not created a local tag  but had set FETCH HEAD to the value of the commit  so that I could find and use it  I then used FETCH HEAD to create a local tag which matched the tag on the remote  That is a practical illustration of what FETCH HEAD is and how it can be used  and might be useful to someone else wondering why git fetch doesn t do what you would naively expect   In my opinion it is best avoided for that purpose and a better way to achieve what I was trying to do is  git fetch gitserver release 1 release 1   i e  to fetch release 1 and call it release 1 locally   It is source dest  see https   git-scm com book en v2 Git-Internals-The-Refspec  just in case you d like to give it a different name    You might want to use FETCH HEAD at times though -   git fetch gitserver bugfix1234 git cherry-pick FETCH HEAD   might be a nice way of using bug fix number 1234 from your Git server  and leaving Git s garbage collection to dispose of the copy from the server once the fix has been cherry-picked onto your current branch   I am assuming that there is a nice clean tagged commit containing the whole of the bug fix on the server

User · Answer

git pull is combination of a fetch followed by a merge  When git fetch happens it notes the head commit of what it fetched in FETCH HEAD  just a file by that name in  git  And these commits are then merged into your working directory

User · Answer

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

[git] What does FETCH_HEAD in Git mean?

`fetch`: optionally allow disabling `FETCH_HEAD` update

`--[no-]write-fetch-head`

`refs`: read `FETCH_HEAD` and `MERGE_HEAD` generically

`fetch`: no `FETCH_HEAD` display if --no-write-fetch-head

Examples related to git

Examples related to git-pull

Examples related to git-fetch

[git] What does FETCH_HEAD in Git mean?

fetch: optionally allow disabling FETCH_HEAD update

--[no-]write-fetch-head

refs: read FETCH_HEAD and MERGE_HEAD generically

fetch: no FETCH_HEAD display if --no-write-fetch-head