gitcore-tutorial(7) — Linux manual page


GITCORE-TUTORIAL(7)            Git Manual            GITCORE-TUTORIAL(7)

NAME         top

       gitcore-tutorial - A Git core tutorial for developers

SYNOPSIS         top

       git *

DESCRIPTION         top

       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 when the porcelain isn’t flushing.

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


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


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

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

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

       to which Git will reply

           Initialized empty Git repository in .git/

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

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

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

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

       •   a subdirectory called refs, which contains references to

       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

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

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


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


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

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


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

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

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

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

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

       •   commit that index file as an object.

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

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


       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.


           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


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

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

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

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

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

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

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


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

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

           $ git write-tree

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


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

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

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

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

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

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

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

MAKING A CHANGE         top

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

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

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

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

           $ git diff HEAD

       which ends up doing the above for you.

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


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

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

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

           $ git update-index hello

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

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

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

           $ git commit

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

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

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


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

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


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

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

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

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


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

       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>

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


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


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

       This has two implications:

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

               $ rm -rf git-tutorial

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

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

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

               $ git update-index --refresh

           in the new repository to make sure that the index file is 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 create your own local live copy of such a "raw" Git
       repository, you’d first create your own subdirectory for the
       project, and then copy the raw repository contents into the .git
       directory. For example, to create your own copy of the Git
       repository, you’d do the following

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

       followed by

           $ git read-tree HEAD

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

           $ git checkout-index -u -a

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

       Again, this can all be simplified with

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

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

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


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

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

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

           $ git switch -c mybranch

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


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

           $ 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 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 switch with the branchname as the argument.


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

           $ git 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, let’s assume that
       somebody else does some work in the original branch, and simulate
       that by going back to the master branch, and editing the same
       file differently there:

           $ git switch master

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

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

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

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

           $ gitk --all

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

           $ git commit -i hello

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

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

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

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

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


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


           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 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


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

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

           $ git fetch <remote-repository>

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

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


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

       Local directory

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

       Git Native

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


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

           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


           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

           $ git config remote.linus.url

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


        1. git pull linus

        2. git pull linus tag v0.99.1

       the above are equivalent to:

        1. git pull HEAD

        2. git pull tag


       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 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

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

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

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

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

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

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

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

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

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

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

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

       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

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

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

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

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


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

       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.


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

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


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

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

           $ mkdir my-git.git

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

           $ GIT_DIR=my-git.git git init

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


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


           If you plan to publish this repository to be accessed over
           http, you should do mv my-git.git/hooks/post-update.sample
           my-git.git/hooks/post-update at this point. This makes sure
           that every time you push into this repository, git
           update-server-info is 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. mirror network takes care of the propagation to other
       publicly visible machines:

           $ git push


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

           $ git repack

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


           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

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

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

           $ git prune-packed

       would remove them for you.

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


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

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

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


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

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

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

        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

        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

        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

           You can repack this private repository whenever you feel

        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.


       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.


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

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

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

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

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

       Which would result in:

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

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

           $ git reset --hard master~2

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

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

       Note that you should not do Octopus just 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         top

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

GIT         top

       Part of the git(1) suite

NOTES         top

        1. the Git User Manual

        2. Randy Dunlap’s presentation

COLOPHON         top

       This page is part of the git (Git distributed version control
       system) project.  Information about the project can be found at
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       page, see ⟨⟩.  This page was obtained
       from the project's upstream Git repository
       ⟨⟩ on 2023-12-22.  (At that time,
       the date of the most recent commit that was found in the
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Git         2023-12-20            GITCORE-TUTORIAL(7)

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