What s the difference between HEAD and HEAD in Git

Question

When I specify an ancestor commit object in Git  I m confused between HEAD  and HEAD    Both have a  numbered  version like HEAD 3 and HEAD 2   They seem very similar or the same to me  but are there any differences between the tilde and the caret

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The   lt n gt  format allows you to select the nth parent of the commit  relevant in merges    The   lt n gt  format allows you to select the nth ancestor commit  always following the first parent   See git-rev-parse s documentation for some examples

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My two                  cents

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Simply put  for the first level of parentage  ancestry  inheritance  lineage  etc   HEAD  and HEAD  both point to the same commit  which is  located  one parent above the HEAD  commit     Furthermore  HEAD    HEAD 1   HEAD    HEAD 1  But HEAD      HEAD 2    HEAD 2   Yet HEAD     HEAD 2   Read on   Beyond the first level of parentage  things get trickier  especially if the working branch master branch has had merges  from other branches    There is also the matter of syntax with the caret  HEAD     HEAD 2  they re equivalent  BUT HEAD      HEAD 2  they re two different things entirely     Each the caret refers to the HEAD s first parent  which is why carets stringed together are equivalent to tilde expressions  because they refer to the first parent s  first parent s  first parents  etc   etc  based strictly on the number on connected carets or on the number following the tilde  either way  they both mean the same thing   i e  stay with the first parent and go up x generations   HEAD 2  or HEAD    refers to the commit that is two levels of ancestry up above the current commit  the HEAD  in the hierarchy  meaning the HEAD s grandparent commit    HEAD 2  on the other hand  refers NOT to the first parent s second parent s commit  but simply to the second parent s commit  That is because the caret means the parent of the commit  and the number following signifies which what parent commit is referred to  the first parent  in the case when the caret is not followed by a number  because it is shorthand for the number being 1  meaning the first parent    Unlike the caret  the number that follows afterwards does not imply another level of hierarchy upwards  but rather it implies how many levels sideways  into the hierarchy  one needs to go find the correct parent  commit   Unlike the number in a tilde expression  it is only one parent up in the hierarchy  regardless of the number  immediately  proceeding the caret  Instead of upward  the caret s trailing number counts sideways for parents across the hierarchy  at a level of parents upwards that is equivalent to the number of consecutive carets    So HEAD 3 is equal to the third parent of the HEAD commit  NOT the great-grandparent  which is what HEAD    AND HEAD 3 would be

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The difference between HEAD  and HEAD  is well described by the illustration  by Jon Loeliger  found on http   www kernel org pub software scm git docs git-rev-parse html    This documentation can be a bit obscure to beginners so I ve reproduced that illustration below   G   H   I   J                D   E   F                                                B     C                                   A A          A 0 B   A      A 1       A 1 C   A 2 D   A      A 1 1     A 2 E   B 2    A  2 F   B 3    A  3 G   A      A 1 1 1   A 3 H   D 2    B  2      A   2    A 2 2 I   F      B 3       A  3  J   F 2    B 3 2     A  3 2

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If you re wondering whether to type HEAD  or HEAD  in your command  just use either  They re both names for the same commit - the first parent of the current commit  Likewise with master  and master  - both names for the first parent of master  In the same way as 2   2 and 2 x 2 are both 4 - they re different ways of getting there  but the answer is the same  This answers the question  What s the difference between HEAD  and HEAD  in Git  If you just did a merge  so your current commit has more than one parent   or you re still interested in how the caret and tilde work  see the other answers  which I won t duplicate here  for an in-depth explanation  as well as how to use them repeatedly  e g HEAD      or with numbers  e g HEAD 2   Otherwise  I hope this answer saves you some time

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actual example of the difference between HEAD  and HEAD

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BRANCH Selector git checkout HEAD 2 Selects the 2nd branch of a  merge  commit by moving onto the selected branch  one step backwards on the commit-tree    COMMIT Selector git checkout HEAD 2 Moves 2 commits backwards on the default selected branch  Defining both   and   relative refs as PARENT selectors is far the dominant definition published everywhere on the internet I have come across so far - including the official Git Book  Yes they are PARENT selectors  but the problem with this  quot explanation quot  is that it is completely against our goal  which is how to distinguish the two       The other problem is when we are encouraged to use the   BRANCH selector for COMMIT selection  aka HEAD      HEAD    Again  yes  you can use it this way  but this is not its designed purpose  The   BRANCH selector s backwards move behaviour is a side effect not its purpose  At merged commits only  can a number be assigned to the   BRANCH selector  Thus its full capacity can only be utilised where there is a need for selecting among branches  And the most straightforward way to express a selection in a fork is by stepping onto the selected path   branch - that s for the one step backwards on the commit-tree  It is a side effect only  not its main purpose

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TLDR    is what you want most of the time  it references past commits to the current branch    references parents  git-merge creates a 2nd parent or more   A  is always the same as A   A   is always the same as A    and so on  A 2 is not the same as A 2 however   because  2 is shorthand for     while  2 is not shorthand for anything  it means the 2nd parent

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Both   and   on their own refer to the parent of the commit     and    both refer to the grandparent commit  etc   But they differ in meaning when they are used with numbers     2 means up two levels in the hierarchy  via the first parent if a commit has more than one parent  2 means the second parent where a commit has more than one parent  i e  because it s a merge    These can be combined  so HEAD 2 3 means HEAD s grandparent commit s third parent commit

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Here s a very good explanation taken verbatim from http   www paulboxley com blog 2011 06 git-caret-and-tilde    ref  is shorthand for ref 1 and means the commit s first parent  ref 2 means the commit s first parent s first parent  ref 3 means the commit s first parent s first parent s first parent  And so on  ref  is shorthand for ref 1 and means the commit s first parent  But where the two differ is that ref 2 means the commit s second parent  remember  commits can have two parents when they are a merge   The   and   operators can be combined

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It is worth noting that git also has a syntax for tracking  from-where-you-came   want-to-go-back-now  - for example  HEAD  1  will reference the place from where you jumped to new commit location   Basically HEAD    variables capture the history of HEAD movement  and you can decide to use a particular head by looking into reflogs of git using the command git reflog   Example   0aee51f HEAD  0   reset  moving to HEAD  5  290e035 HEAD  1   reset  moving to HEAD  7  0aee51f HEAD  2   reset  moving to HEAD  3  290e035 HEAD  3   reset  moving to HEAD  3  9e77426 HEAD  4   reset  moving to HEAD  3  290e035 HEAD  5   reset  moving to HEAD  3  0aee51f HEAD  6   reset  moving to HEAD  3  290e035 HEAD  7   reset  moving to HEAD  3  9e77426 HEAD  8   reset  moving to HEAD  3  290e035 HEAD  9   reset  moving to HEAD  1  0aee51f HEAD  10   reset  moving to HEAD  4  290e035 HEAD  11   reset  moving to HEAD  9e77426 HEAD  12   reset  moving to HEAD  eb48179 HEAD  13   reset  moving to HEAD  f916d93 HEAD  14   reset  moving to HEAD  0aee51f HEAD  15   reset  moving to HEAD  5  f19fd9b HEAD  16   reset  moving to HEAD 1 290e035 HEAD  17   reset  moving to HEAD 2 eb48179 HEAD  18   reset  moving to HEAD 2 0aee51f HEAD  19   reset  moving to HEAD  5  eb48179 HEAD  20   reset  moving to HEAD 2 0aee51f HEAD  21   reset  moving to HEAD  1  f916d93 HEAD  22   reset  moving to HEAD  1  0aee51f HEAD  23   reset  moving to HEAD  1  f916d93 HEAD  24   reset  moving to HEAD  0aee51f HEAD  25   commit  amend   3rd commmit 35a7332 HEAD  26   checkout  moving from temp2 new br to temp2 new br 35a7332 HEAD  27   commit  amend   3rd commmit 72c0be8 HEAD  28   commit  amend   3rd commmit   An example could be that I did local-commits a- b- c- d and then I went back discarding 2 commits to check my code - git reset HEAD 2 - and then after that I want to move my HEAD back to d - git reset HEAD  1

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HEAD  specifies the first parent on a  branch  HEAD  allows you to select a specific parent of the commit   An Example   If you want to follow a side branch  you have to specify something like  master 209 2 15

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this means parent     if it has parents of two or more  like merge commit  we can select second of parent or another   so if just one thing like  HEAD  or HEAD    it has same results

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HEAD    is the same as HEAD 3  selecting the third commit before HEAD  HEAD 2 specifies the second head in a merge commit

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Rules of thumb  Use   most of the time     to go back a number of generations  usually what you want Use   on merge commits     because they have two or more  immediate  parents  Mnemonics   Tilde   is almost linear in appearance and wants to go backward in a straight line Caret   suggests an interesting segment of a tree or a fork in the road  Tilde The    Specifying Revisions    section of the git rev-parse documentation defines   as   lt rev gt   lt n gt   e g  master 3 A suffix   lt n gt  to a revision parameter means the commit object that is the nth generation ancestor of the named commit object  following only the first parents  For example   lt rev gt  3 is equivalent to  lt rev gt     which is equivalent to  lt rev gt  1 1 1      You can get to parents of any commit  not just HEAD  You can also move back through generations  for example  master 2 means the grandparent of the tip of the master branch  favoring the first parent on merge commits  Caret Git history is nonlinear  a directed acyclic graph  DAG  or tree  For a commit with only one parent  rev  and rev  mean the same thing  The caret selector becomes useful with merge commits because each one is the child of two or more parents     and strains language borrowed from biology  HEAD  means the first immediate parent of the tip of the current branch  HEAD  is short for HEAD 1  and you can also address HEAD 2 and so on as appropriate  The same section of the git rev-parse documentation defines it as   lt rev gt    e g  HEAD   v1 5 1 0 A suffix   to a revision parameter means the first parent of that commit object    lt n gt  means the nth parent   e g    lt rev gt   is equivalent to  lt rev gt  1   As a special rule   lt rev gt  0 means the commit itself and is used when  lt rev gt  is the object name of a tag object that refers to a commit object   Examples These specifiers or selectors can be chained arbitrarily  e g   topic 3 2 in English is the second parent of the merge commit that is the great-grandparent  three generations back  of the current tip of the branch topic  The aforementioned section of the git rev-parse documentation traces many paths through a notional git history  Time flows generally downward  Commits D  F  B  and A are merge commits   Here is an illustration  by Jon Loeliger  Both commit nodes B and C are parents of commit node A  Parent commits are ordered left-to-right   N B  The git log --graph command displays history in the opposite order   G   H   I   J                D   E   F                                                B     C                                   A  A          A 0 B   A      A 1       A 1 C   A 2 D   A      A 1 1     A 2 E   B 2    A  2 F   B 3    A  3 G   A      A 1 1 1   A 3 H   D 2    B  2      A   2    A 2 2 I   F      B 3       A  3  J   F 2    B 3 2     A  3 2   Run the code below to create a git repository whose history matches the quoted illustration      usr bin env perl  use strict  use warnings  use subs qw  postorder    use File  Temp qw  mkdtemp     my  sha1  my  parents       A   gt    qw  B C                      B   gt    qw      D E F                C   gt    qw          F                D   gt    qw            G H            F   gt    qw                I J          sub postorder     my  root  hash          my  parents       parents  root             postorder      hash  for  parents    return if  sha1  root      parents   map  quot -p  sha1     quot    parents    chomp  sha1  root     git commit-tree  parents -m  quot  root quot   hash      die  quot  0  git commit-tree failed quot  if       system  quot git tag -a -m   sha1  root     root    sha1  root   quot      0 or die  quot  0  git tag failed quot       0    s              fix Stack Overflow highlighting my  repo   mkdtemp  quot repoXXXXXXXX quot   chdir  repo or die  quot  0  chdir     quot   system  quot git init quot      0               or die  quot  0  git init failed quot   chomp my  tree    git write-tree         die  quot  0  git write-tree failed quot  if      postorder  A    tree  system  quot git update-ref HEAD    sha1 A  quot   die  quot  0  git update-ref failed quot  if     system  quot git update-ref master  sha1 A  quot   die  quot  0  git update-ref failed quot  if        for browsing history - http   blog kfish org 2010 04 git-lola html system  quot git config alias lol   log --graph --decorate --pretty oneline --abbrev-commit  quot   system  quot git config alias lola  log --graph --decorate --pretty oneline --abbrev-commit --all  quot    It adds aliases in the new throwaway repo only for git lol and git lola so you can view history as in   git lol     29392c8  HEAD - gt  master  tag  A  A        a1ef6fd  tag  C  C           -      8ae20e9  tag  B  B                       03160db  tag  F  F                9df28cb  tag  J  J       2afd329  tag  I  I     a77cb1f  tag  E  E     cd75703  tag  D  D        3043d25  tag  H  H   4ab0473  tag  G  G  Note that on your machine the SHA-1 object names will differ from those above  but the tags allow you to address commits by name and check your understanding    git log -1 --format  f   git rev-parse A   B   git log -1 --format  f   git rev-parse A  3   I   git log -1 --format  f   git rev-parse A 2   F  The    Specifying Revisions    in the git rev-parse documentation is full of great information and is worth an in-depth read  See also Git Tools - Revision Selection from the book Pro Git  Order of Parent Commits The commit 89e4fcb0dd from git   s own history is a merge commit  as git show 89e4fcb0dd indicates with the Merge header line that displays the immediate ancestors    object names   commit 89e4fcb0dd01b42e82b8f27f9a575111a26844df Merge  c670b1f876 649bf3a42f b67d40adbb Author  Junio C Hamano  lt gitster pobox com gt  Date    Mon Oct 29 10 15 31 2018  0900      Merge branches  bp reset-quiet  and  js mingw-http-ssl  into nd config-split         We can confirm the ordering by asking git rev-parse to show 89e4fcb0dd   s immediate parents in sequence    git rev-parse 89e4fcb0dd 1 89e4fcb0dd 2 89e4fcb0dd 3 c670b1f876521c9f7cd40184bf7ed05aad843433 649bf3a42f344e71b1b5a7f562576f911a1f7423 b67d40adbbaf4f5c4898001bf062a9fd67e43368  Querying the non-existent fourth parent results in an error    git rev-parse 89e4fcb0dd 4 89e4fcb0dd 4 fatal  ambiguous argument  89e4fcb0dd 4   unknown revision or path not in the working tree  Use  --  to separate paths from revisions  like this   git  lt command gt    lt revision gt      --   lt file gt        If you want to extract the parents only  use pretty format  P for the full hashes   git log -1 --pretty  P 89e4fcb0dd c670b1f876521c9f7cd40184bf7ed05aad843433 649bf3a42f344e71b1b5a7f562576f911a1f7423 b67d40adbbaf4f5c4898001bf062a9fd67e43368  or  p for abbreviated parents    git log -1 --pretty  p 89e4fcb0dd c670b1f876 649bf3a42f b67d40adbb

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Simplistically      specifies ancestors   specifies parents   You can specify one or more branches when merging  Then a commit has two or more parents and then   is useful to indicate parents   Suppose you are on branch A and you have two more branches  B and C   On each branch the three last commits are    A  A1  A2  A3 B  B1  B2  B3 C  C1  C3  C3   If now on branch A you execute the command   git merge B C   then you are combining three branches together  here your merge commit has three parents   and    indicates the n th ancestor in the first branch  so   HEAD  indicates A3 HEAD 2 indicates A2 HEAD 3 indicates A1     indicates the n th parent  so   HEAD  indicates A3 HEAD 2 indicates B3 HEAD 3 indicates C3   The next use of   or   next to each other is in the context of the commit designated by previous characters   Notice 1    HEAD 3 is always equal to  HEAD    and to  HEAD     every indicates A1      nbsp  nbsp  nbsp  nbsp  nbsp  nbsp  nbsp  nbsp and generally    HEAD n is always equal to  HEAD       n times     and to  HEAD       n times       Notice 2    HEAD 3 is not the same as HEAD     the first indicates C3 and the second indicates A1      nbsp  nbsp  nbsp  nbsp  nbsp  nbsp  nbsp  nbsp and generally    HEAD 1 is the same as HEAD   but for n   1  HEAD n is always not the same as HEAD       n times

[git] What's the difference between HEAD^ and HEAD~ in Git?

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