What s the difference between HEAD working tree and index in Git

Question

Can someone tell me the difference between HEAD  working tree and index  in Git   From what I understand  they are all names for different branches  Is my assumption correct     Edit  I found this      A single git repository can track an arbitrary number of branches  but your working tree is associated with just one of them  the  current  or  checked out  branch   and HEAD points to that branch    Does this mean that HEAD and working tree are always the same

User · Answer

This is an inevitably long yet easy to follow explanation from ProGit book  Note  For reference you can read Chapter 7 7 of the book  Reset Demystified Git as a system manages and manipulates three trees in its normal operation   HEAD  Last commit snapshot  next parent Index  Proposed next commit snapshot Working Directory  Sandbox  The HEAD HEAD is the pointer to the current branch reference  which is in turn a pointer to the last commit made on that branch  That means HEAD will be the parent of the next commit that is created  It   s generally simplest to think of HEAD as the snapshot of your last commit on that branch  What does it contain  To see what that snapshot looks like run the following in root directory of your repository                                   git ls-tree -r HEAD  it would result in something like this                           git ls-tree -r HEAD                          100644 blob a906cb2a4a904a152    README                          100644 blob 8f94139338f9404f2    Rakefile                          040000 tree 99f1a6d12cb4b6f19    lib    The Index Git populates this index with a list of all the file contents that were last checked out into your working directory and what they looked like when they were originally checked out  You then replace some of those files with new versions of them  and git commit converts that into the tree for a new commit  What does it contain  Use  git ls-files -s  to see what it looks like  You should see something like this                   100644 a906cb2a4a904a152e80877d4088654daad0c859 0 README                     100644 8f94139338f9404f26296befa88755fc2598c289 0 Rakefile                    100644 47c6340d6459e05787f644c2447d2595f5d3a54b 0 lib simplegit rb    The Working Directory This is where your files reside and where you can try changes out before committing them to your staging area  index  and then into history  Visualized Sample Let s see how do these three trees  As the ProGit book refers to them  work together  Git   s typical workflow is to record snapshots of your project in successively better states  by manipulating these three trees  Take a look at this picture   To get a good visualized understanding consider this scenario  Say you go into a new directory with a single file in it  Call this v1 of the file  It is indicated in blue  Running git init will create a Git repository with a HEAD reference which points to the unborn master branch  At this point  only the working directory tree has any content  Now we want to commit this file  so we use git add to take content in the working directory and copy it to the index   Then we run git commit  which takes the contents of the index and saves it as a permanent snapshot  creates a commit object which points to that snapshot  and updates master to point to that commit   If we run git status  we   ll see no changes  because all three trees are the same  The beautiful point git status shows the difference between these trees in the following manner   If the Working Tree is different from index  then git status will show there are some changes not staged for commit If the Working Tree is the same as index  but they are different from HEAD  then git status will show some files under changes to be committed section in its result If the Working Tree is different from the index  and index is different from HEAD  then git status will show some files under changes not staged for commit section and some other files under changes to be committed section in its result   For the more curious Note about git reset command Hopefully  knowing how reset command works will further brighten the reason behind the existence of these three trees  reset command is your Time Machine in git which can easily take you back in time and bring some old snapshots for you to work on  In this manner  HEAD is the wormhole through which you can travel in time  Let s see how it works with an example from the book  Consider the following repository which has a single file and 3 commits which are shown in different colours and different version numbers   The state of trees is like the next picture   Step 1  Moving HEAD  --soft   The first thing reset will do is move what HEAD points to  This isn   t the same as changing HEAD itself  which is what checkout does   reset moves the branch that HEAD is pointing to  This means if HEAD is set to the master branch  running git reset 9e5e6a4 will start by making master point to 9e5e6a4  If you call reset  with --soft option it will stop here  without changing index and working directory  Our repo will look like this now  Notice  HEAD  is the parent of HEAD  Looking a second time at the image  we can see that the command essentially undid the last commit  As the working tree and the index are the same but different from HEAD  git status will now show changes in green ready to be committed  Step 2  Updating the index  --mixed   This is the default option of the command Running reset with --mixed option updates the index with the contents of whatever snapshot HEAD points to currently  leaving Working Directory intact  Doing so  your repository will look like when you had done some work that is not staged and git status will show that as changes not staged for commit in red  This option will also undo the last commit and also unstage all the changes  It s like you made changes but have not called git add command yet  Our repo would look like this now   Step 3  Updating the Working Directory  --hard  If you call reset with --hard option it will copy contents of the snapshot HEAD is pointing to into HEAD  index and Working Directory  After executing reset --hard command  it would mean like you got back to a previous point in time and haven t done anything after that at all  see the picture below   Conclusion I hope now you have a better understanding of these trees and have a great idea of the power they bring to you by enabling you to change your files in your repository to undo or redo things you have done mistakenly

User · Answer

Your working tree is what is actually in the files that you are currently working on     HEAD is a pointer to the branch or commit that you last checked out  and which will be the parent of a new commit if you make it  For instance  if you re on the master branch  then HEAD will point to master  and when you commit  that new commit will be a descendent of the revision that master pointed to  and master will be updated to point to the new commit   The index is a staging area where the new commit is prepared  Essentially  the contents of the index are what will go into the new commit  though if you do git commit -a  this will automatically add all changes to files that Git knows about to the index before committing  so it will commit the current contents of your working tree   git add will add or update files from the working tree into your index

User · Answer

A few other good references on those topics   My Git Workflow    I use the index as a checkpoint    When I m about to make a change that might go awry     when I want to explore some direction that I m not sure if I can follow through on or even whether it s a good idea  such as a conceptually demanding refactoring or changing a representation type     I checkpoint my work into the index  If this is the first change I ve made since my last commit  then I can use the local repository as a checkpoint  but often I ve got one conceptual change that I m implementing as a set of little steps  I want to checkpoint after each step  but save the commit until I ve gotten back to working  tested code    Notes   the workspace is the directory tree of  source  files that you see and edit   The index is a single  large  binary file in  lt baseOfRepo gt   git index  which lists all files in the current branch  their sha1 checksums  time stamps and the file name -- it is not another directory with a copy of files in it   The local repository is a hidden directory   git  including an objects directory containing all versions of every file in the repo  local branches and copies of remote branches  as a compressed  quot blob quot  file    Don t think of the four  disks  represented in the image above as separate copies of the repo files    Why Git is better than X    Git Is Your Friend not a Foe Vol  3  Refs and Index   They are basically named references for Git commits  There are two major types of refs  tags and heads   Tags are fixed references that mark a specific point in history  for example v2 6 29  On the contrary  heads are always moved to reflect the current position of project development      note  as commented  by Timo Huovinen  those arrows are not what the commits point to  it s the workflow order  basically showing arrows as 1 - gt  2 - gt  3 - gt  4 where 1 is the first commit and 4 is the last   Now we know what is happening in the project  But to know what is happening right here  right now there is a special reference called HEAD  It serves two major purposes   it tells Git which commit to take files from when you checkout  and it tells Git where to put new commits when you commit   When you run git checkout ref it points HEAD to the ref you   ve designated and extracts files from it  When you run git commit it creates a new commit object  which becomes a child of current HEAD  Normally HEAD points to one of the heads  so everything works out just fine

User · Answer

Working tree  Your working tree are the files that you are currently working on    Git index   The git  index  is where you place files you want commit to the git repository  The index is also known as cache  directory cache  current directory cache  staging area  staged files  Before you  commit   checkin  files to the git repository  you need to first place the files in the git  index   The index is not the working directory  you can type a command such as git status  and git will tell you what files in your working directory have been added to the git index  for example  by using the git add filename command   The index is not the git repository  files in the git index are files that git would commit to the git repository if you used the git commit command

User · Answer

The difference between HEAD  current branch or last committed state on current branch   index  aka  staging area  and working tree  the state of files in checkout  is described in  The Three States  section of the  1 3 Git Basics  chapter of Pro Git book by Scott Chacon  Creative Commons licensed    Here is the image illustrating it from this chapter       In the above image  working directory  is the same as  working tree   the  staging area  is an alternate name for git  index   and HEAD points to currently checked out branch  which tip points to last commit in the  git directory  repository    Note that git commit -a would stage changes and commit in one step

[git] What's the difference between HEAD, working tree and index, in Git?

This is an inevitably long yet easy to follow explanation from ProGit book:

Note: For reference you can read Chapter 7.7 of the book, Reset Demystified

The HEAD

The Index

The Working Directory

Visualized Sample

For the more curious

Step 1: Moving HEAD (--soft):

Step 2: Updating the index (--mixed):

Step 3: Updating the Working Directory (--hard)

Conclusion

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