What is HEAD in Git

Question

You see the Git documentation saying things like     The branch must be fully merged in HEAD    But what is Git HEAD exactly

User · Answer

A great way to drive home the point made in the correct answers is to run git reflog HEAD  you get a history of all of the places HEAD has pointed

User · Answer

I think  HEAD  is current check out commit  In other words  HEAD  points to the commit that is currently checked out   If you have just cloned and not checked out I don t know what it points to  probably some invalid location

User · Answer

As a concept  the head is the latest revision in a branch  If you have more than one head per named branch you probably created it when doing local commits without merging  effectively creating an unnamed branch   To have a  clean  repository  you should have one head per named branch and always merge to a named branch after you worked locally   This is also true for Mercurial

User · Answer

Assuming it is not a special case called  detached HEAD   then  as stated in the O Reilly Git book  2nd edtion  p 69  HEAD means      HEAD always refers to the most recent commit on the current   branch  When you change branches  HEAD is updated to refer to the new   branch   s latest commit    so      HEAD is the  tip  of the current branch    Note that we can use HEAD to refer to the most recent commit  and use HEAD  as the commit before the tip  and HEAD   or HEAD 2 as the commit even earlier  and so forth

User · Answer

To quote other people       A head is simply a reference to a   commit object  Each head has a name    branch name or tag name  etc   By   default  there is a head in every   repository called master  A repository   can contain any number of heads  At   any given time  one head is selected   as the    current head     This head is   aliased to HEAD  always in capitals        Note this difference  a    head       lowercase  refers to any one of the   named heads in the repository     HEAD       uppercase  refers exclusively to the   currently active head  This   distinction is used frequently in Git   documentation    Another good source that quickly covers the inner workings of git  and therefor a better understanding of heads HEAD  can be found here   References  ref   or heads or branches can be considered like post-it notes stuck onto commits in the commit history   Usually they point to the tip of series of commits  but they can be moved around with git checkout or git reset etc

User · Answer

I d just like to detail a few things in Greg Hewgil s accepted answer  According to the Git Pocket Guide  Branch      the branch itself is defined as all points reachable in the commit   graph from the named commit  the    tip    of the branch     HEAD  A special type of Ref     The special ref HEAD determines what branch you are on      Refs     Git defines two kinds of references  or named pointers  which it calls      refs             A simple ref  which points directly to an object ID  usually a commit or tag    A symbolic ref  or symref   which points to another ref  either simple or symbolic       As Greg mentioned  HEAD can be in a  detached state   So HEAD can be either a simple ref  for a detached HEAD  or a symref       if HEAD is a symbolic ref for an existing branch  then you are    on      that branch  If  on the other hand  HEAD is a simple ref directly   naming a commit by its SHA-1 ID  then you are not    on    any branch  but   rather in    detached HEAD    mode  which happens when you check out some   earlier commit to examine

User · Answer

Take a look at Creating and playing with branches  HEAD is actually a file whose contents determines where the HEAD variable refers     cat  git HEAD ref  refs heads master   cat  git refs heads master 35ede5c916f88d8ba5a9dd6afd69fcaf773f70ed   In this repository  the contents of the HEAD file refers to a second file named refs heads master  The file refs heads master contains the hash of the most recent commit on the master branch   The result is HEAD points to the master branch commit from the  git refs heads master file

User · Answer

There is a  perhaps subtle  but important misconception in a number these answers  I thought I d add my answer to clear it up   What is HEAD   HEAD is YOU HEADis a symbolic reference pointing to wherever you are in your commit history  It follows you wherever you go  whatever you do  like a shadow  If you make a commit  HEAD will move  If you checkout something  HEAD will move  Whatever you do  if you have moved somewhere new in your commit history  HEAD has moved along with you  To address one common misconception  you cannot detach yourself from HEAD  That is not what a detached HEAD state is  If you ever find yourself thinking   quot oh no  i m in detached HEAD state  I ve lost my HEAD  quot  Remember  it s your HEAD  HEAD is you  You haven t detached from the HEAD  you and your HEAD have detached from something else  What can HEAD attach to  HEAD can point to a commit  yes  but typically it does not  Let me say that again  Typically HEAD does not point to a commit  It points to a branch reference  It is attached to that branch  and when you do certain things  e g   commit or reset   the attached branch will move along with HEAD  You can see what it is pointing to by looking under the hood  cat  git HEAD  Normally you ll get something like this  ref  refs heads master  Sometimes you ll get something like this  a3c485d9688e3c6bc14b06ca1529f0e78edd3f86  That s what happens when HEAD points directly to a commit  This is called a detached HEAD  because HEAD is pointing to something other than a branch reference  If you make a commit in this state  master  no longer being attached to HEAD  will no longer move along with you  It does not matter where that commit is  You could be on the same commit as your master branch   but if HEAD is pointing to the commit rather than the branch  it is detached and a new commit will not be associated with a branch reference  You can look at this graphically if you try the following exercise  From a git repository  run this  You ll get something slightly different  but they key bits will be there  When it is time to checkout the commit directly  just use whatever abbreviated hash you get from the first output  here it is a3c485d   git checkout master git log --pretty format  quot  h    d quot  -1   a3c485d     HEAD - gt  master   git checkout a3c485d -q    -q is for dramatic effect  git log --pretty format  quot  h    d quot  -1      a3c485d     HEAD  master   OK  so there is a small difference in the output here  Checking out the commit directly  instead of the branch  gives us a comma instead of an arrow  What do you think  are we in a detached HEAD state  HEAD is still referring to a specific revision that is associated with a branch name  We re still on the master branch  aren t we  Now try  git status   HEAD detached at a3c485d  Nope  We re in  detached HEAD  state  You can see the same representation of  HEAD - gt  branch  vs   HEAD  branch  with git log -1  In conclusion HEAD is you  It points to whatever you checked out  wherever you are  Typically that is not a commit  it is a branch  If HEAD does point to a commit  or tag   even if it s the same commit  or tag  that a branch also points to  you  and HEAD  have been detached from that branch  Since you don t have a branch attached to you  the branch won t follow along with you as you make new commits  HEAD  however  will

User · Answer

HEAD actually is just a file for storing current branch info and if you use HEAD in your git commands you are pointing to your current branch you can see the data of this file by cat  git HEAD

User · Answer

These two may confusing you   head  Pointing to named references a branch recently submitted  Unless you use the package reference   heads typically stored in   GIT DIR refs heads    HEAD  Current branch  or your working tree is usually generated from the tree HEAD is pointing to  HEAD must point to a head  except you are using a detached HEAD

User · Answer

Take a look at http   git-scm com book en Git-Branching-What-a-Branch-Is     Figure 3-5  HEAD file pointing to the branch you   re on

User · Answer

Head points to the tip of the currently checked out branch     In your repository  there is a  git folder  Open the file in this location   git refs heads  The  sha-1 hash  code in that file  master in most cases  will be the most recent commit  i e the one seen in the output of the command git log  More info on the  git folder  http   gitready com advanced 2009 03 23 whats-inside-your-git-directory html

User · Answer

HEAD is just a special pointer that points to the local branch you   re currently on   From the Pro Git book  chapter 3 1 Git Branching - Branches in a Nutshell  in the section Creating a New Branch      What happens if you create a new branch  Well  doing so creates a new   pointer for you to move around  Let   s say you create a new branch   called testing  You do this with the git branch command      git branch testing        This creates a new pointer at the same commit you   re currently on             How does Git know what branch you   re currently on  It keeps a special pointer called HEAD  Note that this is a lot different than   the concept of HEAD in other VCSs you may be used to  such as   Subversion or CVS  In Git  this is a pointer to the local branch   you   re currently on  In this case  you   re still on master  The git   branch command only created a new branch     it didn   t switch to that   branch

User · Answer

I am also still figuring out the internals of git  and have figured out this so far  Let s say the current branch is master   HEAD is a file in your  git  directory that normally looks something like this     cat  git HEAD ref  refs heads master   refs heads master is itself a file that normally has the hash value of the latest commit of master     cat  git refs heads master  f342e66eb1158247a98d74152a1b91543ece31b4   If you do a git log  you will see that this is the latest commit for master     git log --oneline  f342e66  HEAD - gt  master      latest commit fa99692 parent of latest commit  So my thinking is the HEAD file is a convenient way to track the latest commit  instead of remembering long hash values

User · Answer

A branch is actually a pointer that holds a commit ID such as 17a5  HEAD is a pointer to a branch the user is currently working on    HEAD has a reference filw which looks like this   ref   You can check these files by accessing   git HEAD  git refs that are in the repository you are working in

User · Answer

It feels like that HEAD is just a tag for the last commit that you checked out   This can be the tip of a specific branch  such as  master   or some in-between commit of a branch   detached head

User · Answer

Git is all about commits   And Head points to the commit which you currently checked out     git cat-file -t HEAD commit   Whenever you checkout a branch  the HEAD points to the latest commit on that branch  Contents of HEAD can checked as below  for master branch      cat  git refs heads master   b089141cc8a7d89d606b2f7c15bfdc48640a8e25

User · Answer

I recommend this definition from github developer Scott Chacon  video reference       Head is your current branch   It is a symbolic reference   It is a reference to a branch   You always have HEAD  but HEAD will be pointing to one of these other pointers  to one of the branches that you re on   It is the parent of your next commit   It is what should be what was last checked-out into your working directory    This is the last known state of what your working directory was    The whole video will give a fair introduction to the whole git system so I also recommend you to watch it all if have the time to

User · Answer

In addition to all definitions  the thing that stuck in my mind was  when you make a commit  GIT creates a commit object within the repository  Commit objects should have a parent   or multiple parents if it is a merge commit   Now  how does git know the parent of the current commit  So HEAD is a pointer to the  reference of the  last commit which will become the parent of the current commit

User · Answer

You can think of the HEAD as the  current branch   When you switch branches with git checkout  the HEAD revision changes to point to the tip of the new branch   You can see what HEAD points to by doing   cat  git HEAD   In my case  the output is     cat  git HEAD ref  refs heads master   It is possible for HEAD to refer to a specific revision that is not associated with a branch name  This situation is called a detached HEAD

User · Answer

After reading all of the previous answers  I still wanted more clarity  This blog at the official git website http   git-scm com blog gave me what I was looking for   The HEAD  Pointer to last commit snapshot  next parent     The HEAD in Git is the pointer to the current branch reference  which is in turn a pointer to the last commit you made or the last commit that was checked out into your working directory  That also means it will be the parent of the next commit you do  It s generally simplest to think of it as HEAD is the snapshot of your last commit

User · Answer

HEAD refers to the current commit that your working copy points to  i e  the commit you currently have checked-out   From the official Linux Kernel documentation on specifying Git revisions      HEAD names the commit on which you based the changes in the working tree    Note  however  that in the upcoming version 1 8 4 of Git    can also be used as a shorthand for HEAD  as noted by Git contributor Junio C Hamano in his Git Blame blog      Instead of typing  HEAD   you can say     instead  e g   git log       Stack Overflow user VonC also found some interesting information on why   was chosen as a shorthand in his answer to another question   Also of interest  in some environments it s not necessary to capitalize HEAD  specifically in operating systems that use case-insensitive file systems  specifically Windows and OS X

[git] What is HEAD in Git?

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