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    Command:

    gitcore-tutorial

    
    
    

    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 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.
    
               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 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/
    
           ?   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.
    
               Note
               An object is identified by its 160-bit SHA1 hash, aka object name,
               and a reference to an object is always the 40-byte hex
               representation of that SHA1 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 it's
           empty, that's not very useful, so let's start populating it with data.
    
    
    

    POPULATING A GIT REPOSITORY

           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
           --remove) flag.
    
           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,
           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
           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
           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.
    
               Note
               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.
    
               Note
               The second example demonstrates that you can abbreviate the object
               name to only the first several hexadecimal digits in most places.
    
           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, 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
    
           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
    
               8988da15d077d4829fc51d8544c097def6644dbb
    
           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.
    
    
    

    MAKING A CHANGE

           Remember how we did the git update-index on file hello and then we
           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 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.
    
               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 "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
           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).
    
    
    

    INSPECTING CHANGES

           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
           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-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. A trivial (but very useful) script called git whatchanged is
           included with git which does exactly this, and shows a log of recent
           activities.
    
           To see the whole history of our pitiful little git-tutorial project,
           you can do
    
               $ git log
    
           which shows just the log messages, or if we want to see the log
           together with the associated patches use the more complex (and much
           more powerful)
    
               $ git whatchanged -p
    
           and you will see exactly what has changed in the repository over its
           short history.
    
               Note
               When using the above two commands, the initial commit will be
               shown. If this is a problem because it is huge, you can hide it by
               setting the log.showroot configuration variable to false. Having
               this, you can still show it for each command just adding the --root
               option, which is a flag for git diff-tree accepted by both
               commands.
    
           With that, you should now be having some inkling of what git does, and
    
               $ 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.
    
    
    

    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 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
               use.
    
           This has two implications:
               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
               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, 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
           repository.
    
    
           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
           files).
    
           Again, this can all be simplified with
    
               $ git clone rsync://rsync.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 else's (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/ 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 SHA1 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.
    
               Note
    
           (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.
    
    
    

    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, 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:
    
           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
           source.
    
           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
           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 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
           lines:
    
               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
    
           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 and
           the first line of the commit log message from 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 log
           messages. 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 git-rev-parse(1) if you want to see more
           complex cases.
    
               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, 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)
           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
    
    
    

    MERGING EXTERNAL WORK

           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
           fetch:
    
               $ git fetch <remote-repository>
    
           One of the following transports can be used to name the repository to
           download from:
    
           Rsync
    
               rsync://remote.machine/path/to/repo.git/
    
               Rsync transport is usable for both uploading and downloading, but
               is completely unaware of what git does, and can produce unexpected
               results when you download from the public repository while the
               repository owner is uploading into it via rsync transport. Most
               notably, it could update the files under refs/ which holds the
               object name of the topmost commits before uploading the files in
               objects/ -- the downloader would obtain head commit object name
               while that object itself is still not available in the repository.
               For this reason, it is considered deprecated.
    
           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
               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/ 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 -- 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
           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 repository's config file like this:
    
    
            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 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)
    
           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 SHA1 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:
    
    
    

    PUBLISHING YOUR WORK

           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
           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 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
           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.
    
           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 how many objects it packed, and stores the
           packed file in .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
    
           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, unless rsync transport is used. 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 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
           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".
               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.
    
            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.
    
           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
    
                +* [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.
    
    
    

    SEE ALSO

           gittutorial(7), gittutorial-2(7), gitcvs-migration(7), git-help(1),
           Everyday git[3], The Git User's Manual[1]
    
    
    

    GIT

           Part of the git(1) suite.
    
    
    

    NOTES

            1. the GIT User Manual
               file:///usr/share/doc/git-1.7.1/user-manual.html
    
            2. Randy Dunlap's presentation
               http://www.xenotime.net/linux/mentor/linux-mentoring-2006.pdf
    
            3. Everyday git
               file:///usr/share/doc/git-1.7.1/everyday.html
    
    
    

    Git 1.7.1 03/04/2013 GITCORE-TUTORIAL(7)

    
    
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