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     NAME
          gitcore-tutorial - A Git core tutorial for developers

     SYNOPSIS
          git *

     DESCRIPTION
          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 m[blue]the Git User Manualm[][1].

          However, an understanding of these low-level tools can be
          helpful if you want to understand Gitcqs 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 when the porcelain isncqt
          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.

              Note

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

     CREATING A GIT REPOSITORY
          Creating a new Git repository couldncqt 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, wecqre going to start a totally new
          repository from scratch, with no pre-existing files, and
          wecqll call it git-tutorial. To start up, create a
          subdirectory for it, change into that subdirectory, and
          initialize the Git infrastructure with git init:

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

          to which Git will reply

              Initialized empty Git repository in .git/

          which is just Gitcqs way of saying that you havencqt 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:

          +o   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.

              Doncqt worry about the fact that the file that the HEAD
              link points to doesncqt even exist yet - you havencqt
              created the commit that will start your HEAD development
              branch yet.

          +o   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.

          +o   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 doesncqt 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 doncqt have to ever even have
          a master branch. A number of the Git tools will assume that

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          .git/HEAD is valid, though.

              Note

              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.

              Note

              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 itcqs empty, thatcqs not very useful, so letcqs start
          populating it with data.

     POPULATING A GIT REPOSITORY
          Wecqll keep this simple and stupid, so wecqll 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. Wecqll 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:

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

          +o   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
          youcqre adding a new entry with the --add flag (or removing
          an entry with the --remove) flag.

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

              $ 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, youcqll 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 do

              $ ls .git/objects/??/*

          and see two files:

              .git/objects/55/7db03de997c86a4a028e1ebd3a1ceb225be238
              .git/objects/f2/4c74a2e500f5ee1332c86b94199f52b1d1d962

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

          If you want to, you can use git cat-file to look at those
          objects, but youcqll 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.

              Note

              Doncqt 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.

              Note

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              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 youcqd 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,
          youcqve 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 status.

          In particular, letcqs not even check in the two files into
          Git yet, wecqll 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 wasncqt 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 @@

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               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. Thatcqs 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

     COMMITTING GIT STATE
          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,
          wecqre now tying together all the different filenames with
          their contents (and their permissions), and wecqre 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 Icqve described) it
          should be

              8988da15d077d4829fc51d8544c097def6644dbb

          which is another incomprehensible object name. Again, if you
          want to, you can use git cat-file -t 8988d... to see that

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          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 youcqll see mainly a binary mess, so
          thatcqs less interesting).

          However - normally youcqd 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, itcqs
          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 thatcqs 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 youcqd 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.

     MAKING A CHANGE
          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

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          difference between the index state, and the state in the
          working tree, and how they doncqt have to match, even when we
          commit things.

          As before, if we do git diff-files -p in our git-tutorial
          project, wecqll still see the same difference we saw last
          time: the index file hasncqt changed by the act of committing
          anything. However, now that we have committed something, we
          can also learn to use a new command: git diff-index.

          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 couldncqt do that, because we didncqt 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 wecqre 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 thatcqs exactly what it does.

              Note

              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

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              "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 thatcqs what the index is there
              for.

          However, our next step is to commit the change we did, and
          again, to understand whatcqs 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 didncqt need the --add flag this time, since Git
          knew about the file already).

          Note what happens to the different git diff-* versions here.
          After wecqve 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 wecqve 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 wecqd have to use
          the -p HEAD flag to tell commit that the HEAD was the parent
          of the new commit, and that this wasncqt an initial commit
          any more), but youcqve done that once already, so letcqs 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

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          doncqt 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.

          Youcqve now made your first real Git commit. And if youcqre
          interested in looking at what git commit really does, feel
          free to investigate: itcqs 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).

     INSPECTING CHANGES
          While creating changes is useful, itcqs 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 wecqve already seen several times, we can now
          do

              $ 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.

              Note

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

                              diff-tree
                               +----+
                               |    |
                               |    |
                               V    V
                            +-----------+
                            | Object DB |
                            |  Backing  |
                            |   Store   |
                            +-----------+
                              ^    ^
                              |    |
                              |    |  diff-index --cached
                              |    |
                  diff-index  |    V
                              |  +-----------+

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                              |  |   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.

     TAGGING A VERSION
          In Git, there are two kinds of tags, a "light" one, and an
          "annotated tag".

          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.


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          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.

     COPYING REPOSITORIES
          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.

              Note

              You can tell Git to split the Git internal information
              from the directory that it tracks, but wecqll ignore that
              for now: itcqs not how normal projects work, and itcqs
              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 itcqs a good model for all normal use.

          This has two implications:

          +o   if you grow bored with the tutorial repository you
              created (or youcqve made a mistake and want to start all
              over), you can just do simple

                  $ rm -rf git-tutorial

              and it will be gone. Therecqs no external repository, and
              therecqs no history outside the project you created.

          +o   if you want to move or duplicate a Git repository, you

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              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 youcqve 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, youcqll want to do

                  $ git update-index --refresh

              in the new repository to make sure that the index file
              is up to date.

          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, youcqll 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 doncqt know what theycqve done and not yet checked in),
          so usually youcqll 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.

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          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 doesncqt even have the .git subdirectory, but has all
          the Git files directly in the repository.

          To create your own local live copy of such a "raw" Git
          repository, youcqd 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, youcqd do the following

              $ mkdir my-git
              $ cd my-git
              $ rsync -rL rsync://rsync.kernel.org/pub/scm/git/git.git/ .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 doncqt actually have any of the working tree
          files to work on. To get those, youcqd 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 doncqt 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 files).

          Again, this can all be simplified with

              $ git clone git://git.kernel.org/pub/scm/git/git.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 elsecqs (mine)
          remote repository, and checked it out.

     CREATING A NEW BRANCH
          Branches in Git are really nothing more than pointers into
          the Git object database from within the .git/refs/

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          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. Thatcqs just a convention,
          though, and nothing enforces it.

          To show that as an example, letcqs 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 switch -c mybranch

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

              Note

              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 switch what the base
              of the checkout would be. In other words, if you have an
              earlier tag or branch, youcqd just do

                  $ git switch -c 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 switch 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 itcqs pointing. To get the list of
          branches you have, you can say

              $ git branch

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          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 switch with the branchname as the
          argument.

     MERGING TWO BRANCHES
          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, letcqs make sure wecqre in that branch, and do
          some work there.

              $ git switch 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, letcqs 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 switch master

          Here, take a moment to look at the contents of hello, and
          notice how they doncqt contain the work we just did in
          mybranch - because that work hasncqt happened in the master
          branch at all. Then do

              $ echo "Play, play, play" >>hello

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              $ 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, youcqve got two branches, and you decide that you want
          to merge the work done. Before we do that, letcqs introduce a
          cool graphical tool that helps you view whatcqs going on:

              $ gitk --all

          will show you graphically both of your branches (thatcqs 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 source.

          Anyway, letcqs 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, therecqs 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 wecqve 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 say:

                      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 youcqve ever
          used CVS, so letcqs just open hello in our editor (whatever
          that may be), and fix it up somehow. Icqd suggest just making
          it so that hello contains all four lines:

              Hello World

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

          and once youcqre happy with your manual merge, just do a

              $ git commit -i hello

          which will very loudly warn you that youcqre 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 youcqre 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 youcqll 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.

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              Note

              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 details.

              Note

              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, letcqs 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. Letcqs go back to mybranch, and
          run git merge to get the "upstream changes" back to your
          branch.

              $ git switch 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

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     MERGING EXTERNAL WORK
          Itcqs usually much more common that you merge with somebody
          else than merging with your own branches, so itcqs worth
          pointing out that Git makes that very easy too, and in fact,
          itcqs 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 fetch:

              $ git fetch <remote-repository>

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

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

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

              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
              /path/to/repo.git/

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

              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.

          HTTP(S)
              http://remote.machine/path/to/repo.git/

              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/

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              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
              downloaders.

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

          However - itcqs such a common thing to fetch and then
          immediately merge, that itcqs 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 argument.

              Note

              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 days.

          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 repositorycqs config
          file like this:

              $ git config remote.linus.url http://www.kernel.org/pub/scm/git/git.git/

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

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          Examples.

           1. git pull linus

           2. git pull linus tag v0.99.1

          the above are equivalent to:

           1. git pull http://www.kernel.org/pub/scm/git/git.git/ HEAD

           2. git pull http://www.kernel.org/pub/scm/git/git.git/ tag
              v0.99.1

     HOW DOES THE MERGE WORK?
          We said this tutorial shows what plumbing does to help you
          cope with the porcelain that isncqt flushing, but we so far
          did not talk about how the merge really works. If you are
          following this tutorial the first time, Icqd 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, letcqs
          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 switch -C mybranch master^2
              $ git switch 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

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          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
              my-first-tag

          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

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          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).

     PUBLISHING YOUR WORK
          So, we can use somebody elsecqs 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

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          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 pushing.

              Note

              This public repository could further be mirrored, and
              that is how Git repositories at kernel.org 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, git-receive-pack.

          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.

              Note

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

              $ 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.

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              Note

              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.

              Note

              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 run.

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

              $ 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. Kernel.org mirror network takes care of the
          propagation to other publicly visible machines:

              $ git push master.kernel.org:/pub/scm/git/git.git/

     PACKING YOUR REPOSITORY
          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

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          how many objects it packed, and stores the packed file in
          the .git/objects/pack directory.

              Note

              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.

              Note

              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

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          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.

     WORKING WITH OTHERS
          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 m[blue]Randy Dunlapcqs presentationm[][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 repository.

          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 public.

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           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 running git clone on
              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 it.

           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.

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           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
              .git/refs/remotes/origin/master.

           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 continue.

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

          See gitcvs-migration(7) for the details.

     BUNDLING YOUR WORK TOGETHER
          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. Letcqs 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:

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              $ 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 letcqs 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 just because you can. An

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          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.

     SEE ALSO
          gittutorial(7), gittutorial-2(7), gitcvs-migration(7), git-
          help(1), giteveryday(7), m[blue]The Git Usercqs Manualm[][1]

     GIT
          Part of the git(1) suite

     NOTES
           1. the Git User Manual
              file:///usr/share/doc/git/html/user-manual.html

           2. Randy Dunlapcqs presentation
              https://web.archive.org/web/20120915203609/http://www.xenotime.net/linux/mentor/linux-mentoring-2006.pdf

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