gitcore-tutorial - A Git core tutorial for developers


   git *


   This tutorial explains how to use the "core" Git commands to set up and
   work with a Git repository.

   If you just need to use Git as a revision control system you may prefer
   to start with "A Tutorial Introduction to Git" (gittutorial(7)) or the
   Git User Manual[1].

   However, an understanding of these low-level tools can be helpful if
   you want to understand Git's internals.

   The core Git is often called "plumbing", with the prettier user
   interfaces on top of it called "porcelain". You may not want to use the
   plumbing directly very often, but it can be good to know what the
   plumbing does for when the porcelain isn't flushing.

   Back when this document was originally written, many porcelain commands
   were shell scripts. For simplicity, it still uses them as examples to
   illustrate how plumbing is fit together to form the porcelain commands.
   The source tree includes some of these scripts in contrib/examples/ for
   reference. Although these are not implemented as shell scripts anymore,
   the description of what the plumbing layer commands do is still valid.

       Deeper technical details are often marked as Notes, which you can
       skip on your first reading.


   Creating a new Git repository couldn't be easier: all Git repositories
   start out empty, and the only thing you need to do is find yourself a
   subdirectory that you want to use as a working tree - either an empty
   one for a totally new project, or an existing working tree that you
   want to import into Git.

   For our first example, we're going to start a totally new repository
   from scratch, with no pre-existing files, and we'll call it
   git-tutorial. To start up, create a subdirectory for it, change into
   that subdirectory, and initialize the Git infrastructure with git init:

       $ mkdir git-tutorial
       $ cd git-tutorial
       $ git init

   to which Git will reply

       Initialized empty Git repository in .git/

   which is just Git's way of saying that you haven't been doing anything
   strange, and that it will have created a local .git directory setup for
   your new project. You will now have a .git directory, and you can
   inspect that with ls. For your new empty project, it should show you
   three entries, among other things:

   *   a file called HEAD, that has ref: refs/heads/master in it. This is
       similar to a symbolic link and points at refs/heads/master relative
       to the HEAD file.

       Don't worry about the fact that the file that the HEAD link points
       to doesn't even exist yet --- you haven't created the commit that
       will start your HEAD development branch yet.

   *   a subdirectory called objects, which will contain all the objects
       of your project. You should never have any real reason to look at
       the objects directly, but you might want to know that these objects
       are what contains all the real data in your repository.

   *   a subdirectory called refs, which contains references to objects.

   In particular, the refs subdirectory will contain two other
   subdirectories, named heads and tags respectively. They do exactly what
   their names imply: they contain references to any number of different
   heads of development (aka branches), and to any tags that you have
   created to name specific versions in your repository.

   One note: the special master head is the default branch, which is why
   the .git/HEAD file was created points to it even if it doesn't yet
   exist. Basically, the HEAD link is supposed to always point to the
   branch you are working on right now, and you always start out expecting
   to work on the master branch.

   However, this is only a convention, and you can name your branches
   anything you want, and don't have to ever even have a master branch. A
   number of the Git tools will assume that .git/HEAD is valid, though.

       An object is identified by its 160-bit SHA-1 hash, aka object name,
       and a reference to an object is always the 40-byte hex
       representation of that SHA-1 name. The files in the refs
       subdirectory are expected to contain these hex references (usually
       with a final \n at the end), and you should thus expect to see a
       number of 41-byte files containing these references in these refs
       subdirectories when you actually start populating your tree.

       An advanced user may want to take a look at gitrepository-layout(5)
       after finishing this tutorial.

   You have now created your first Git repository. Of course, since it's
   empty, that's not very useful, so let's start populating it with data.


   We'll keep this simple and stupid, so we'll start off with populating a
   few trivial files just to get a feel for it.

   Start off with just creating any random files that you want to maintain
   in your Git repository. We'll start off with a few bad examples, just
   to get a feel for how this works:

       $ echo "Hello World" >hello
       $ echo "Silly example" >example

   you have now created two files in your working tree (aka working
   directory), but to actually check in your hard work, you will have to
   go through two steps:

   *   fill in the index file (aka cache) with the information about your
       working tree state.

   *   commit that index file as an object.

   The first step is trivial: when you want to tell Git about any changes
   to your working tree, you use the git update-index program. That
   program normally just takes a list of filenames you want to update, but
   to avoid trivial mistakes, it refuses to add new entries to the index
   (or remove existing ones) unless you explicitly tell it that you're
   adding a new entry with the --add flag (or removing an entry with the
   --remove) flag.

   So to populate the index with the two files you just created, you can

       $ git update-index --add hello example

   and you have now told Git to track those two files.

   In fact, as you did that, if you now look into your object directory,
   you'll notice that Git will have added two new objects to the object
   database. If you did exactly the steps above, you should now be able to

       $ ls .git/objects/??/*

   and see two files:


   which correspond with the objects with names of 557db... and f24c7...

   If you want to, you can use git cat-file to look at those objects, but
   you'll have to use the object name, not the filename of the object:

       $ git cat-file -t 557db03de997c86a4a028e1ebd3a1ceb225be238

   where the -t tells git cat-file to tell you what the "type" of the
   object is. Git will tell you that you have a "blob" object (i.e., just
   a regular file), and you can see the contents with

       $ git cat-file blob 557db03

   which will print out "Hello World". The object 557db03 is nothing more
   than the contents of your file hello.

       Don't confuse that object with the file hello itself. The object is
       literally just those specific contents of the file, and however
       much you later change the contents in file hello, the object we
       just looked at will never change. Objects are immutable.

       The second example demonstrates that you can abbreviate the object
       name to only the first several hexadecimal digits in most places.

   Anyway, as we mentioned previously, you normally never actually take a
   look at the objects themselves, and typing long 40-character hex names
   is not something you'd normally want to do. The above digression was
   just to show that git update-index did something magical, and actually
   saved away the contents of your files into the Git object database.

   Updating the index did something else too: it created a .git/index
   file. This is the index that describes your current working tree, and
   something you should be very aware of. Again, you normally never worry
   about the index file itself, but you should be aware of the fact that
   you have not actually really "checked in" your files into Git so far,
   you've only told Git about them.

   However, since Git knows about them, you can now start using some of
   the most basic Git commands to manipulate the files or look at their

   In particular, let's not even check in the two files into Git yet,
   we'll start off by adding another line to hello first:

       $ echo "It's a new day for git" >>hello

   and you can now, since you told Git about the previous state of hello,
   ask Git what has changed in the tree compared to your old index, using
   the git diff-files command:

       $ git diff-files

   Oops. That wasn't very readable. It just spit out its own internal
   version of a diff, but that internal version really just tells you that
   it has noticed that "hello" has been modified, and that the old object
   contents it had have been replaced with something else.

   To make it readable, we can tell git diff-files to output the
   differences as a patch, using the -p flag:

       $ git diff-files -p
       diff --git a/hello b/hello
       index 557db03..263414f 100644
       --- a/hello
       +++ b/hello
       @@ -1 +1,2 @@
        Hello World
       +It's a new day for git

   i.e. the diff of the change we caused by adding another line to hello.

   In other words, git diff-files always shows us the difference between
   what is recorded in the index, and what is currently in the working
   tree. That's very useful.

   A common shorthand for git diff-files -p is to just write git diff,
   which will do the same thing.

       $ git diff
       diff --git a/hello b/hello
       index 557db03..263414f 100644
       --- a/hello
       +++ b/hello
       @@ -1 +1,2 @@
        Hello World
       +It's a new day for git


   Now, we want to go to the next stage in Git, which is to take the files
   that Git knows about in the index, and commit them as a real tree. We
   do that in two phases: creating a tree object, and committing that tree
   object as a commit object together with an explanation of what the tree
   was all about, along with information of how we came to that state.

   Creating a tree object is trivial, and is done with git write-tree.
   There are no options or other input: git write-tree will take the
   current index state, and write an object that describes that whole
   index. In other words, we're now tying together all the different
   filenames with their contents (and their permissions), and we're
   creating the equivalent of a Git "directory" object:

       $ git write-tree

   and this will just output the name of the resulting tree, in this case
   (if you have done exactly as I've described) it should be


   which is another incomprehensible object name. Again, if you want to,
   you can use git cat-file -t 8988d... to see that this time the object
   is not a "blob" object, but a "tree" object (you can also use git
   cat-file to actually output the raw object contents, but you'll see
   mainly a binary mess, so that's less interesting).

   However --- normally you'd never use git write-tree on its own, because
   normally you always commit a tree into a commit object using the git
   commit-tree command. In fact, it's easier to not actually use git
   write-tree on its own at all, but to just pass its result in as an
   argument to git commit-tree.

   git commit-tree normally takes several arguments --- it wants to know
   what the parent of a commit was, but since this is the first commit
   ever in this new repository, and it has no parents, we only need to
   pass in the object name of the tree. However, git commit-tree also
   wants to get a commit message on its standard input, and it will write
   out the resulting object name for the commit to its standard output.

   And this is where we create the .git/refs/heads/master file which is
   pointed at by HEAD. This file is supposed to contain the reference to
   the top-of-tree of the master branch, and since that's exactly what git
   commit-tree spits out, we can do this all with a sequence of simple
   shell commands:

       $ tree=$(git write-tree)
       $ commit=$(echo 'Initial commit' | git commit-tree $tree)
       $ git update-ref HEAD $commit

   In this case this creates a totally new commit that is not related to
   anything else. Normally you do this only once for a project ever, and
   all later commits will be parented on top of an earlier commit.

   Again, normally you'd never actually do this by hand. There is a
   helpful script called git commit that will do all of this for you. So
   you could have just written git commit instead, and it would have done
   the above magic scripting for you.


   Remember how we did the git update-index on file hello and then we
   changed hello afterward, and could compare the new state of hello with
   the state we saved in the index file?

   Further, remember how I said that git write-tree writes the contents of
   the index file to the tree, and thus what we just committed was in fact
   the original contents of the file hello, not the new ones. We did that
   on purpose, to show the difference between the index state, and the
   state in the working tree, and how they don't have to match, even when
   we commit things.

   As before, if we do git diff-files -p in our git-tutorial project,
   we'll still see the same difference we saw last time: the index file
   hasn't changed by the act of committing anything. However, now that we
   have committed something, we can also learn to use a new command: git

   Unlike git diff-files, which showed the difference between the index
   file and the working tree, git diff-index shows the differences between
   a committed tree and either the index file or the working tree. In
   other words, git diff-index wants a tree to be diffed against, and
   before we did the commit, we couldn't do that, because we didn't have
   anything to diff against.

   But now we can do

       $ git diff-index -p HEAD

   (where -p has the same meaning as it did in git diff-files), and it
   will show us the same difference, but for a totally different reason.
   Now we're comparing the working tree not against the index file, but
   against the tree we just wrote. It just so happens that those two are
   obviously the same, so we get the same result.

   Again, because this is a common operation, you can also just shorthand
   it with

       $ git diff HEAD

   which ends up doing the above for you.

   In other words, git diff-index normally compares a tree against the
   working tree, but when given the --cached flag, it is told to instead
   compare against just the index cache contents, and ignore the current
   working tree state entirely. Since we just wrote the index file to
   HEAD, doing git diff-index --cached -p HEAD should thus return an empty
   set of differences, and that's exactly what it does.

       git diff-index really always uses the index for its comparisons,
       and saying that it compares a tree against the working tree is thus
       not strictly accurate. In particular, the list of files to compare
       (the "meta-data") always comes from the index file, regardless of
       whether the --cached flag is used or not. The --cached flag really
       only determines whether the file contents to be compared come from
       the working tree or not.

       This is not hard to understand, as soon as you realize that Git
       simply never knows (or cares) about files that it is not told about
       explicitly. Git will never go looking for files to compare, it
       expects you to tell it what the files are, and that's what the
       index is there for.

   However, our next step is to commit the change we did, and again, to
   understand what's going on, keep in mind the difference between
   "working tree contents", "index file" and "committed tree". We have
   changes in the working tree that we want to commit, and we always have
   to work through the index file, so the first thing we need to do is to
   update the index cache:

       $ git update-index hello

   (note how we didn't need the --add flag this time, since Git knew about
   the file already).

   Note what happens to the different git diff-* versions here. After
   we've updated hello in the index, git diff-files -p now shows no
   differences, but git diff-index -p HEAD still does show that the
   current state is different from the state we committed. In fact, now
   git diff-index shows the same difference whether we use the --cached
   flag or not, since now the index is coherent with the working tree.

   Now, since we've updated hello in the index, we can commit the new
   version. We could do it by writing the tree by hand again, and
   committing the tree (this time we'd have to use the -p HEAD flag to
   tell commit that the HEAD was the parent of the new commit, and that
   this wasn't an initial commit any more), but you've done that once
   already, so let's just use the helpful script this time:

       $ git commit

   which starts an editor for you to write the commit message and tells
   you a bit about what you have done.

   Write whatever message you want, and all the lines that start with #
   will be pruned out, and the rest will be used as the commit message for
   the change. If you decide you don't want to commit anything after all
   at this point (you can continue to edit things and update the index),
   you can just leave an empty message. Otherwise git commit will commit
   the change for you.

   You've now made your first real Git commit. And if you're interested in
   looking at what git commit really does, feel free to investigate: it's
   a few very simple shell scripts to generate the helpful (?) commit
   message headers, and a few one-liners that actually do the commit
   itself (git commit).


   While creating changes is useful, it's even more useful if you can tell
   later what changed. The most useful command for this is another of the
   diff family, namely git diff-tree.

   git diff-tree can be given two arbitrary trees, and it will tell you
   the differences between them. Perhaps even more commonly, though, you
   can give it just a single commit object, and it will figure out the
   parent of that commit itself, and show the difference directly. Thus,
   to get the same diff that we've already seen several times, we can now

       $ git diff-tree -p HEAD

   (again, -p means to show the difference as a human-readable patch), and
   it will show what the last commit (in HEAD) actually changed.

       Here is an ASCII art by Jon Loeliger that illustrates how various
       diff-* commands compare things.

                        |    |
                        |    |
                        V    V
                     | Object DB |
                     |  Backing  |
                     |   Store   |
                       ^    ^
                       |    |
                       |    |  diff-index --cached
                       |    |
           diff-index  |    V
                       |  +-----------+
                       |  |   Index   |
                       |  |  "cache"  |
                       |  +-----------+
                       |    ^
                       |    |
                       |    |  diff-files
                       |    |
                       V    V
                     |  Working  |
                     | Directory |

   More interestingly, you can also give git diff-tree the --pretty flag,
   which tells it to also show the commit message and author and date of
   the commit, and you can tell it to show a whole series of diffs.
   Alternatively, you can tell it to be "silent", and not show the diffs
   at all, but just show the actual commit message.

   In fact, together with the git rev-list program (which generates a list
   of revisions), git diff-tree ends up being a veritable fount of
   changes. You can emulate git log, git log -p, etc. with a trivial
   script that pipes the output of git rev-list to git diff-tree --stdin,
   which was exactly how early versions of git log were implemented.


   In Git, there are two kinds of tags, a "light" one, and an "annotated

   A "light" tag is technically nothing more than a branch, except we put
   it in the .git/refs/tags/ subdirectory instead of calling it a head. So
   the simplest form of tag involves nothing more than

       $ git tag my-first-tag

   which just writes the current HEAD into the .git/refs/tags/my-first-tag
   file, after which point you can then use this symbolic name for that
   particular state. You can, for example, do

       $ git diff my-first-tag

   to diff your current state against that tag which at this point will
   obviously be an empty diff, but if you continue to develop and commit
   stuff, you can use your tag as an "anchor-point" to see what has
   changed since you tagged it.

   An "annotated tag" is actually a real Git object, and contains not only
   a pointer to the state you want to tag, but also a small tag name and
   message, along with optionally a PGP signature that says that yes, you
   really did that tag. You create these annotated tags with either the -a
   or -s flag to git tag:

       $ git tag -s <tagname>

   which will sign the current HEAD (but you can also give it another
   argument that specifies the thing to tag, e.g., you could have tagged
   the current mybranch point by using git tag <tagname> mybranch).

   You normally only do signed tags for major releases or things like
   that, while the light-weight tags are useful for any marking you want
   to do --- any time you decide that you want to remember a certain point,
   just create a private tag for it, and you have a nice symbolic name for
   the state at that point.


   Git repositories are normally totally self-sufficient and relocatable.
   Unlike CVS, for example, there is no separate notion of "repository"
   and "working tree". A Git repository normally is the working tree, with
   the local Git information hidden in the .git subdirectory. There is
   nothing else. What you see is what you got.

       You can tell Git to split the Git internal information from the
       directory that it tracks, but we'll ignore that for now: it's not
       how normal projects work, and it's really only meant for special
       uses. So the mental model of "the Git information is always tied
       directly to the working tree that it describes" may not be
       technically 100% accurate, but it's a good model for all normal

   This has two implications:

   *   if you grow bored with the tutorial repository you created (or
       you've made a mistake and want to start all over), you can just do

           $ rm -rf git-tutorial

       and it will be gone. There's no external repository, and there's no
       history outside the project you created.

   *   if you want to move or duplicate a Git repository, you can do so.
       There is git clone command, but if all you want to do is just to
       create a copy of your repository (with all the full history that
       went along with it), you can do so with a regular cp -a
       git-tutorial new-git-tutorial.

       Note that when you've moved or copied a Git repository, your Git
       index file (which caches various information, notably some of the
       "stat" information for the files involved) will likely need to be
       refreshed. So after you do a cp -a to create a new copy, you'll
       want to do

           $ git update-index --refresh

       in the new repository to make sure that the index file is

   Note that the second point is true even across machines. You can
   duplicate a remote Git repository with any regular copy mechanism, be
   it scp, rsync or wget.

   When copying a remote repository, you'll want to at a minimum update
   the index cache when you do this, and especially with other peoples'
   repositories you often want to make sure that the index cache is in
   some known state (you don't know what they've done and not yet checked
   in), so usually you'll precede the git update-index with a

       $ git read-tree --reset HEAD
       $ git update-index --refresh

   which will force a total index re-build from the tree pointed to by
   HEAD. It resets the index contents to HEAD, and then the git
   update-index makes sure to match up all index entries with the
   checked-out files. If the original repository had uncommitted changes
   in its working tree, git update-index --refresh notices them and tells
   you they need to be updated.

   The above can also be written as simply

       $ git reset

   and in fact a lot of the common Git command combinations can be
   scripted with the git xyz interfaces. You can learn things by just
   looking at what the various git scripts do. For example, git reset used
   to be the above two lines implemented in git reset, but some things
   like git status and git commit are slightly more complex scripts around
   the basic Git commands.

   Many (most?) public remote repositories will not contain any of the
   checked out files or even an index file, and will only contain the
   actual core Git files. Such a repository usually doesn't even have the
   .git subdirectory, but has all the Git files directly in the

   To create your own local live copy of such a "raw" Git repository,
   you'd first create your own subdirectory for the project, and then copy
   the raw repository contents into the .git directory. For example, to
   create your own copy of the Git repository, you'd do the following

       $ mkdir my-git
       $ cd my-git
       $ rsync -rL rsync:// .git

   followed by

       $ git read-tree HEAD

   to populate the index. However, now you have populated the index, and
   you have all the Git internal files, but you will notice that you don't
   actually have any of the working tree files to work on. To get those,
   you'd check them out with

       $ git checkout-index -u -a

   where the -u flag means that you want the checkout to keep the index
   up-to-date (so that you don't have to refresh it afterward), and the -a
   flag means "check out all files" (if you have a stale copy or an older
   version of a checked out tree you may also need to add the -f flag
   first, to tell git checkout-index to force overwriting of any old

   Again, this can all be simplified with

       $ git clone git:// my-git
       $ cd my-git
       $ git checkout

   which will end up doing all of the above for you.

   You have now successfully copied somebody else's (mine) remote
   repository, and checked it out.


   Branches in Git are really nothing more than pointers into the Git
   object database from within the .git/refs/ subdirectory, and as we
   already discussed, the HEAD branch is nothing but a symlink to one of
   these object pointers.

   You can at any time create a new branch by just picking an arbitrary
   point in the project history, and just writing the SHA-1 name of that
   object into a file under .git/refs/heads/. You can use any filename you
   want (and indeed, subdirectories), but the convention is that the
   "normal" branch is called master. That's just a convention, though, and
   nothing enforces it.

   To show that as an example, let's go back to the git-tutorial
   repository we used earlier, and create a branch in it. You do that by
   simply just saying that you want to check out a new branch:

       $ git checkout -b mybranch

   will create a new branch based at the current HEAD position, and switch
   to it.

       If you make the decision to start your new branch at some other
       point in the history than the current HEAD, you can do so by just
       telling git checkout what the base of the checkout would be. In
       other words, if you have an earlier tag or branch, you'd just do

           $ git checkout -b mybranch earlier-commit

       and it would create the new branch mybranch at the earlier commit,
       and check out the state at that time.

   You can always just jump back to your original master branch by doing

       $ git checkout master

   (or any other branch-name, for that matter) and if you forget which
   branch you happen to be on, a simple

       $ cat .git/HEAD

   will tell you where it's pointing. To get the list of branches you
   have, you can say

       $ git branch

   which used to be nothing more than a simple script around ls
   .git/refs/heads. There will be an asterisk in front of the branch you
   are currently on.

   Sometimes you may wish to create a new branch without actually checking
   it out and switching to it. If so, just use the command

       $ git branch <branchname> [startingpoint]

   which will simply create the branch, but will not do anything further.
   You can then later --- once you decide that you want to actually develop
   on that branch --- switch to that branch with a regular git checkout with
   the branchname as the argument.


   One of the ideas of having a branch is that you do some (possibly
   experimental) work in it, and eventually merge it back to the main
   branch. So assuming you created the above mybranch that started out
   being the same as the original master branch, let's make sure we're in
   that branch, and do some work there.

       $ git checkout mybranch
       $ echo "Work, work, work" >>hello
       $ git commit -m "Some work." -i hello

   Here, we just added another line to hello, and we used a shorthand for
   doing both git update-index hello and git commit by just giving the
   filename directly to git commit, with an -i flag (it tells Git to
   include that file in addition to what you have done to the index file
   so far when making the commit). The -m flag is to give the commit log
   message from the command line.

   Now, to make it a bit more interesting, let's assume that somebody else
   does some work in the original branch, and simulate that by going back
   to the master branch, and editing the same file differently there:

       $ git checkout master

   Here, take a moment to look at the contents of hello, and notice how
   they don't contain the work we just did in mybranch --- because that work
   hasn't happened in the master branch at all. Then do

       $ echo "Play, play, play" >>hello
       $ echo "Lots of fun" >>example
       $ git commit -m "Some fun." -i hello example

   since the master branch is obviously in a much better mood.

   Now, you've got two branches, and you decide that you want to merge the
   work done. Before we do that, let's introduce a cool graphical tool
   that helps you view what's going on:

       $ gitk --all

   will show you graphically both of your branches (that's what the --all
   means: normally it will just show you your current HEAD) and their
   histories. You can also see exactly how they came to be from a common

   Anyway, let's exit gitk (^Q or the File menu), and decide that we want
   to merge the work we did on the mybranch branch into the master branch
   (which is currently our HEAD too). To do that, there's a nice script
   called git merge, which wants to know which branches you want to
   resolve and what the merge is all about:

       $ git merge -m "Merge work in mybranch" mybranch

   where the first argument is going to be used as the commit message if
   the merge can be resolved automatically.

   Now, in this case we've intentionally created a situation where the
   merge will need to be fixed up by hand, though, so Git will do as much
   of it as it can automatically (which in this case is just merge the
   example file, which had no differences in the mybranch branch), and

               Auto-merging hello
               CONFLICT (content): Merge conflict in hello
               Automatic merge failed; fix conflicts and then commit the result.

   It tells you that it did an "Automatic merge", which failed due to
   conflicts in hello.

   Not to worry. It left the (trivial) conflict in hello in the same form
   you should already be well used to if you've ever used CVS, so let's
   just open hello in our editor (whatever that may be), and fix it up
   somehow. I'd suggest just making it so that hello contains all four

       Hello World
       It's a new day for git
       Play, play, play
       Work, work, work

   and once you're happy with your manual merge, just do a

       $ git commit -i hello

   which will very loudly warn you that you're now committing a merge
   (which is correct, so never mind), and you can write a small merge
   message about your adventures in git merge-land.

   After you're done, start up gitk --all to see graphically what the
   history looks like. Notice that mybranch still exists, and you can
   switch to it, and continue to work with it if you want to. The mybranch
   branch will not contain the merge, but next time you merge it from the
   master branch, Git will know how you merged it, so you'll not have to
   do that merge again.

   Another useful tool, especially if you do not always work in X-Window
   environment, is git show-branch.

       $ git show-branch --topo-order --more=1 master mybranch
       * [master] Merge work in mybranch
        ! [mybranch] Some work.
       -  [master] Merge work in mybranch
       *+ [mybranch] Some work.
       *  [master^] Some fun.

   The first two lines indicate that it is showing the two branches with
   the titles of their top-of-the-tree commits, you are currently on
   master branch (notice the asterisk * character), and the first column
   for the later output lines is used to show commits contained in the
   master branch, and the second column for the mybranch branch. Three
   commits are shown along with their titles. All of them have non blank
   characters in the first column (* shows an ordinary commit on the
   current branch, - is a merge commit), which means they are now part of
   the master branch. Only the "Some work" commit has the plus + character
   in the second column, because mybranch has not been merged to
   incorporate these commits from the master branch. The string inside
   brackets before the commit log message is a short name you can use to
   name the commit. In the above example, master and mybranch are branch
   heads. master^ is the first parent of master branch head. Please see
   gitrevisions(7) if you want to see more complex cases.

       Without the --more=1 option, git show-branch would not output the
       [master^] commit, as [mybranch] commit is a common ancestor of both
       master and mybranch tips. Please see git-show-branch(1) for

       If there were more commits on the master branch after the merge,
       the merge commit itself would not be shown by git show-branch by
       default. You would need to provide --sparse option to make the
       merge commit visible in this case.

   Now, let's pretend you are the one who did all the work in mybranch,
   and the fruit of your hard work has finally been merged to the master
   branch. Let's go back to mybranch, and run git merge to get the
   "upstream changes" back to your branch.

       $ git checkout mybranch
       $ git merge -m "Merge upstream changes." master

   This outputs something like this (the actual commit object names would
   be different)

       Updating from ae3a2da... to a80b4aa....
       Fast-forward (no commit created; -m option ignored)
        example | 1 +
        hello   | 1 +
        2 files changed, 2 insertions(+)

   Because your branch did not contain anything more than what had already
   been merged into the master branch, the merge operation did not
   actually do a merge. Instead, it just updated the top of the tree of
   your branch to that of the master branch. This is often called
   fast-forward merge.

   You can run gitk --all again to see how the commit ancestry looks like,
   or run show-branch, which tells you this.

       $ git show-branch master mybranch
       ! [master] Merge work in mybranch
        * [mybranch] Merge work in mybranch
       -- [master] Merge work in mybranch


   It's usually much more common that you merge with somebody else than
   merging with your own branches, so it's worth pointing out that Git
   makes that very easy too, and in fact, it's not that different from
   doing a git merge. In fact, a remote merge ends up being nothing more
   than "fetch the work from a remote repository into a temporary tag"
   followed by a git merge.

   Fetching from a remote repository is done by, unsurprisingly, git

       $ git fetch <remote-repository>

   One of the following transports can be used to name the repository to
   download from:

       remote.machine:/path/to/repo.git/ or


       This transport can be used for both uploading and downloading, and
       requires you to have a log-in privilege over ssh to the remote
       machine. It finds out the set of objects the other side lacks by
       exchanging the head commits both ends have and transfers (close to)
       minimum set of objects. It is by far the most efficient way to
       exchange Git objects between repositories.

   Local directory

       This transport is the same as SSH transport but uses sh to run both
       ends on the local machine instead of running other end on the
       remote machine via ssh.

   Git Native

       This transport was designed for anonymous downloading. Like SSH
       transport, it finds out the set of objects the downstream side
       lacks and transfers (close to) minimum set of objects.


       Downloader from http and https URL first obtains the topmost commit
       object name from the remote site by looking at the specified
       refname under repo.git/refs/ directory, and then tries to obtain
       the commit object by downloading from repo.git/objects/xx/xxx...
       using the object name of that commit object. Then it reads the
       commit object to find out its parent commits and the associate tree
       object; it repeats this process until it gets all the necessary
       objects. Because of this behavior, they are sometimes also called
       commit walkers.

       The commit walkers are sometimes also called dumb transports,
       because they do not require any Git aware smart server like Git
       Native transport does. Any stock HTTP server that does not even
       support directory index would suffice. But you must prepare your
       repository with git update-server-info to help dumb transport

   Once you fetch from the remote repository, you merge that with your
   current branch.

   However --- it's such a common thing to fetch and then immediately merge,
   that it's called git pull, and you can simply do

       $ git pull <remote-repository>

   and optionally give a branch-name for the remote end as a second

       You could do without using any branches at all, by keeping as many
       local repositories as you would like to have branches, and merging
       between them with git pull, just like you merge between branches.
       The advantage of this approach is that it lets you keep a set of
       files for each branch checked out and you may find it easier to
       switch back and forth if you juggle multiple lines of development
       simultaneously. Of course, you will pay the price of more disk
       usage to hold multiple working trees, but disk space is cheap these

   It is likely that you will be pulling from the same remote repository
   from time to time. As a short hand, you can store the remote repository
   URL in the local repository's config file like this:

       $ git config remote.linus.url

   and use the "linus" keyword with git pull instead of the full URL.


    1. git pull linus

    2. git pull linus tag v0.99.1

   the above are equivalent to:

    1. git pull HEAD

    2. git pull tag v0.99.1


   We said this tutorial shows what plumbing does to help you cope with
   the porcelain that isn't flushing, but we so far did not talk about how
   the merge really works. If you are following this tutorial the first
   time, I'd suggest to skip to "Publishing your work" section and come
   back here later.

   OK, still with me? To give us an example to look at, let's go back to
   the earlier repository with "hello" and "example" file, and bring
   ourselves back to the pre-merge state:

       $ git show-branch --more=2 master mybranch
       ! [master] Merge work in mybranch
        * [mybranch] Merge work in mybranch
       -- [master] Merge work in mybranch
       +* [master^2] Some work.
       +* [master^] Some fun.

   Remember, before running git merge, our master head was at "Some fun."
   commit, while our mybranch head was at "Some work." commit.

       $ git checkout mybranch
       $ git reset --hard master^2
       $ git checkout master
       $ git reset --hard master^

   After rewinding, the commit structure should look like this:

       $ git show-branch
       * [master] Some fun.
        ! [mybranch] Some work.
       *  [master] Some fun.
        + [mybranch] Some work.
       *+ [master^] Initial commit

   Now we are ready to experiment with the merge by hand.

   git merge command, when merging two branches, uses 3-way merge
   algorithm. First, it finds the common ancestor between them. The
   command it uses is git merge-base:

       $ mb=$(git merge-base HEAD mybranch)

   The command writes the commit object name of the common ancestor to the
   standard output, so we captured its output to a variable, because we
   will be using it in the next step. By the way, the common ancestor
   commit is the "Initial commit" commit in this case. You can tell it by:

       $ git name-rev --name-only --tags $mb

   After finding out a common ancestor commit, the second step is this:

       $ git read-tree -m -u $mb HEAD mybranch

   This is the same git read-tree command we have already seen, but it
   takes three trees, unlike previous examples. This reads the contents of
   each tree into different stage in the index file (the first tree goes
   to stage 1, the second to stage 2, etc.). After reading three trees
   into three stages, the paths that are the same in all three stages are
   collapsed into stage 0. Also paths that are the same in two of three
   stages are collapsed into stage 0, taking the SHA-1 from either stage 2
   or stage 3, whichever is different from stage 1 (i.e. only one side
   changed from the common ancestor).

   After collapsing operation, paths that are different in three trees are
   left in non-zero stages. At this point, you can inspect the index file
   with this command:

       $ git ls-files --stage
       100644 7f8b141b65fdcee47321e399a2598a235a032422 0       example
       100644 557db03de997c86a4a028e1ebd3a1ceb225be238 1       hello
       100644 ba42a2a96e3027f3333e13ede4ccf4498c3ae942 2       hello
       100644 cc44c73eb783565da5831b4d820c962954019b69 3       hello

   In our example of only two files, we did not have unchanged files so
   only example resulted in collapsing. But in real-life large projects,
   when only a small number of files change in one commit, this collapsing
   tends to trivially merge most of the paths fairly quickly, leaving only
   a handful of real changes in non-zero stages.

   To look at only non-zero stages, use --unmerged flag:

       $ git ls-files --unmerged
       100644 557db03de997c86a4a028e1ebd3a1ceb225be238 1       hello
       100644 ba42a2a96e3027f3333e13ede4ccf4498c3ae942 2       hello
       100644 cc44c73eb783565da5831b4d820c962954019b69 3       hello

   The next step of merging is to merge these three versions of the file,
   using 3-way merge. This is done by giving git merge-one-file command as
   one of the arguments to git merge-index command:

       $ git merge-index git-merge-one-file hello
       Auto-merging hello
       ERROR: Merge conflict in hello
       fatal: merge program failed

   git merge-one-file script is called with parameters to describe those
   three versions, and is responsible to leave the merge results in the
   working tree. It is a fairly straightforward shell script, and
   eventually calls merge program from RCS suite to perform a file-level
   3-way merge. In this case, merge detects conflicts, and the merge
   result with conflict marks is left in the working tree.. This can be
   seen if you run ls-files --stage again at this point:

       $ git ls-files --stage
       100644 7f8b141b65fdcee47321e399a2598a235a032422 0       example
       100644 557db03de997c86a4a028e1ebd3a1ceb225be238 1       hello
       100644 ba42a2a96e3027f3333e13ede4ccf4498c3ae942 2       hello
       100644 cc44c73eb783565da5831b4d820c962954019b69 3       hello

   This is the state of the index file and the working file after git
   merge returns control back to you, leaving the conflicting merge for
   you to resolve. Notice that the path hello is still unmerged, and what
   you see with git diff at this point is differences since stage 2 (i.e.
   your version).


   So, we can use somebody else's work from a remote repository, but how
   can you prepare a repository to let other people pull from it?

   You do your real work in your working tree that has your primary
   repository hanging under it as its .git subdirectory. You could make
   that repository accessible remotely and ask people to pull from it, but
   in practice that is not the way things are usually done. A recommended
   way is to have a public repository, make it reachable by other people,
   and when the changes you made in your primary working tree are in good
   shape, update the public repository from it. This is often called

       This public repository could further be mirrored, and that is how
       Git repositories at are managed.

   Publishing the changes from your local (private) repository to your
   remote (public) repository requires a write privilege on the remote
   machine. You need to have an SSH account there to run a single command,

   First, you need to create an empty repository on the remote machine
   that will house your public repository. This empty repository will be
   populated and be kept up-to-date by pushing into it later. Obviously,
   this repository creation needs to be done only once.

       git push uses a pair of commands, git send-pack on your local
       machine, and git-receive-pack on the remote machine. The
       communication between the two over the network internally uses an
       SSH connection.

   Your private repository's Git directory is usually .git, but your
   public repository is often named after the project name, i.e.
   <project>.git. Let's create such a public repository for project
   my-git. After logging into the remote machine, create an empty

       $ mkdir my-git.git

   Then, make that directory into a Git repository by running git init,
   but this time, since its name is not the usual .git, we do things
   slightly differently:

       $ GIT_DIR=my-git.git git init

   Make sure this directory is available for others you want your changes
   to be pulled via the transport of your choice. Also you need to make
   sure that you have the git-receive-pack program on the $PATH.

       Many installations of sshd do not invoke your shell as the login
       shell when you directly run programs; what this means is that if
       your login shell is bash, only .bashrc is read and not
       .bash_profile. As a workaround, make sure .bashrc sets up $PATH so
       that you can run git-receive-pack program.

       If you plan to publish this repository to be accessed over http,
       you should do mv my-git.git/hooks/post-update.sample
       my-git.git/hooks/post-update at this point. This makes sure that
       every time you push into this repository, git update-server-info is

   Your "public repository" is now ready to accept your changes. Come back
   to the machine you have your private repository. From there, run this

       $ git push <public-host>:/path/to/my-git.git master

   This synchronizes your public repository to match the named branch head
   (i.e. master in this case) and objects reachable from them in your
   current repository.

   As a real example, this is how I update my public Git repository. mirror network takes care of the propagation to other
   publicly visible machines:

       $ git push


   Earlier, we saw that one file under .git/objects/??/ directory is
   stored for each Git object you create. This representation is efficient
   to create atomically and safely, but not so convenient to transport
   over the network. Since Git objects are immutable once they are
   created, there is a way to optimize the storage by "packing them
   together". The command

       $ git repack

   will do it for you. If you followed the tutorial examples, you would
   have accumulated about 17 objects in .git/objects/??/ directories by
   now. git repack tells you how many objects it packed, and stores the
   packed file in .git/objects/pack directory.

       You will see two files, pack-*.pack and pack-*.idx, in
       .git/objects/pack directory. They are closely related to each
       other, and if you ever copy them by hand to a different repository
       for whatever reason, you should make sure you copy them together.
       The former holds all the data from the objects in the pack, and the
       latter holds the index for random access.

   If you are paranoid, running git verify-pack command would detect if
   you have a corrupt pack, but do not worry too much. Our programs are
   always perfect ;-).

   Once you have packed objects, you do not need to leave the unpacked
   objects that are contained in the pack file anymore.

       $ git prune-packed

   would remove them for you.

   You can try running find .git/objects -type f before and after you run
   git prune-packed if you are curious. Also git count-objects would tell
   you how many unpacked objects are in your repository and how much space
   they are consuming.

       git pull is slightly cumbersome for HTTP transport, as a packed
       repository may contain relatively few objects in a relatively large
       pack. If you expect many HTTP pulls from your public repository you
       might want to repack & prune often, or never.

   If you run git repack again at this point, it will say "Nothing new to
   pack.". Once you continue your development and accumulate the changes,
   running git repack again will create a new pack, that contains objects
   created since you packed your repository the last time. We recommend
   that you pack your project soon after the initial import (unless you
   are starting your project from scratch), and then run git repack every
   once in a while, depending on how active your project is.

   When a repository is synchronized via git push and git pull objects
   packed in the source repository are usually stored unpacked in the
   destination. While this allows you to use different packing strategies
   on both ends, it also means you may need to repack both repositories
   every once in a while.


   Although Git is a truly distributed system, it is often convenient to
   organize your project with an informal hierarchy of developers. Linux
   kernel development is run this way. There is a nice illustration (page
   17, "Merges to Mainline") in Randy Dunlap's presentation[2].

   It should be stressed that this hierarchy is purely informal. There is
   nothing fundamental in Git that enforces the "chain of patch flow" this
   hierarchy implies. You do not have to pull from only one remote

   A recommended workflow for a "project lead" goes like this:

    1. Prepare your primary repository on your local machine. Your work is
       done there.

    2. Prepare a public repository accessible to others.

       If other people are pulling from your repository over dumb
       transport protocols (HTTP), you need to keep this repository dumb
       transport friendly. After git init,
       $GIT_DIR/hooks/post-update.sample copied from the standard
       templates would contain a call to git update-server-info but you
       need to manually enable the hook with mv post-update.sample
       post-update. This makes sure git update-server-info keeps the
       necessary files up-to-date.

    3. Push into the public repository from your primary repository.

    4. git repack the public repository. This establishes a big pack that
       contains the initial set of objects as the baseline, and possibly
       git prune if the transport used for pulling from your repository
       supports packed repositories.

    5. Keep working in your primary repository. Your changes include
       modifications of your own, patches you receive via e-mails, and
       merges resulting from pulling the "public" repositories of your
       "subsystem maintainers".

       You can repack this private repository whenever you feel like.

    6. Push your changes to the public repository, and announce it to the

    7. Every once in a while, git repack the public repository. Go back to
       step 5. and continue working.

   A recommended work cycle for a "subsystem maintainer" who works on that
   project and has an own "public repository" goes like this:

    1. Prepare your work repository, by git clone the public repository of
       the "project lead". The URL used for the initial cloning is stored
       in the remote.origin.url configuration variable.

    2. Prepare a public repository accessible to others, just like the
       "project lead" person does.

    3. Copy over the packed files from "project lead" public repository to
       your public repository, unless the "project lead" repository lives
       on the same machine as yours. In the latter case, you can use
       objects/info/alternates file to point at the repository you are
       borrowing from.

    4. Push into the public repository from your primary repository. Run
       git repack, and possibly git prune if the transport used for
       pulling from your repository supports packed repositories.

    5. Keep working in your primary repository. Your changes include
       modifications of your own, patches you receive via e-mails, and
       merges resulting from pulling the "public" repositories of your
       "project lead" and possibly your "sub-subsystem maintainers".

       You can repack this private repository whenever you feel like.

    6. Push your changes to your public repository, and ask your "project
       lead" and possibly your "sub-subsystem maintainers" to pull from

    7. Every once in a while, git repack the public repository. Go back to
       step 5. and continue working.

   A recommended work cycle for an "individual developer" who does not
   have a "public" repository is somewhat different. It goes like this:

    1. Prepare your work repository, by git clone the public repository of
       the "project lead" (or a "subsystem maintainer", if you work on a
       subsystem). The URL used for the initial cloning is stored in the
       remote.origin.url configuration variable.

    2. Do your work in your repository on master branch.

    3. Run git fetch origin from the public repository of your upstream
       every once in a while. This does only the first half of git pull
       but does not merge. The head of the public repository is stored in

    4. Use git cherry origin to see which ones of your patches were
       accepted, and/or use git rebase origin to port your unmerged
       changes forward to the updated upstream.

    5. Use git format-patch origin to prepare patches for e-mail
       submission to your upstream and send it out. Go back to step 2. and


   If you are coming from CVS background, the style of cooperation
   suggested in the previous section may be new to you. You do not have to
   worry. Git supports "shared public repository" style of cooperation you
   are probably more familiar with as well.

   See gitcvs-migration(7) for the details.


   It is likely that you will be working on more than one thing at a time.
   It is easy to manage those more-or-less independent tasks using
   branches with Git.

   We have already seen how branches work previously, with "fun and work"
   example using two branches. The idea is the same if there are more than
   two branches. Let's say you started out from "master" head, and have
   some new code in the "master" branch, and two independent fixes in the
   "commit-fix" and "diff-fix" branches:

       $ git show-branch
       ! [commit-fix] Fix commit message normalization.
        ! [diff-fix] Fix rename detection.
         * [master] Release candidate #1
        +  [diff-fix] Fix rename detection.
        +  [diff-fix~1] Better common substring algorithm.
       +   [commit-fix] Fix commit message normalization.
         * [master] Release candidate #1
       ++* [diff-fix~2] Pretty-print messages.

   Both fixes are tested well, and at this point, you want to merge in
   both of them. You could merge in diff-fix first and then commit-fix
   next, like this:

       $ git merge -m "Merge fix in diff-fix" diff-fix
       $ git merge -m "Merge fix in commit-fix" commit-fix

   Which would result in:

       $ git show-branch
       ! [commit-fix] Fix commit message normalization.
        ! [diff-fix] Fix rename detection.
         * [master] Merge fix in commit-fix
         - [master] Merge fix in commit-fix
       + * [commit-fix] Fix commit message normalization.
         - [master~1] Merge fix in diff-fix
        +* [diff-fix] Fix rename detection.
        +* [diff-fix~1] Better common substring algorithm.
         * [master~2] Release candidate #1
       ++* [master~3] Pretty-print messages.

   However, there is no particular reason to merge in one branch first and
   the other next, when what you have are a set of truly independent
   changes (if the order mattered, then they are not independent by
   definition). You could instead merge those two branches into the
   current branch at once. First let's undo what we just did and start
   over. We would want to get the master branch before these two merges by
   resetting it to master~2:

       $ git reset --hard master~2

   You can make sure git show-branch matches the state before those two
   git merge you just did. Then, instead of running two git merge commands
   in a row, you would merge these two branch heads (this is known as
   making an Octopus):

       $ git merge commit-fix diff-fix
       $ git show-branch
       ! [commit-fix] Fix commit message normalization.
        ! [diff-fix] Fix rename detection.
         * [master] Octopus merge of branches 'diff-fix' and 'commit-fix'
         - [master] Octopus merge of branches 'diff-fix' and 'commit-fix'
       + * [commit-fix] Fix commit message normalization.
        +* [diff-fix] Fix rename detection.
        +* [diff-fix~1] Better common substring algorithm.
         * [master~1] Release candidate #1
       ++* [master~2] Pretty-print messages.

   Note that you should not do Octopus because you can. An octopus is a
   valid thing to do and often makes it easier to view the commit history
   if you are merging more than two independent changes at the same time.
   However, if you have merge conflicts with any of the branches you are
   merging in and need to hand resolve, that is an indication that the
   development happened in those branches were not independent after all,
   and you should merge two at a time, documenting how you resolved the
   conflicts, and the reason why you preferred changes made in one side
   over the other. Otherwise it would make the project history harder to
   follow, not easier.


   gittutorial(7), gittutorial-2(7), gitcvs-migration(7), git-help(1),
   giteveryday(7), The Git User's Manual[1]


   Part of the git(1) suite.


    1. the Git User Manual

    2. Randy Dunlap's presentation


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