GITCORE-TUTORIAL(7) Git Manual GITCORE-TUTORIAL(7)
NAME
gitcore-tutorial - A Git core tutorial for developers
SYNOPSIS
git *
DESCRIPTION
This tutorial explains how to use the "core" Git commands to set up and work with a Git repository.
If you just need to use Git as a revision control system you may prefer to start with "A Tutorial Introduction
to Git" (gittutorial(7)) or 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/
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
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 SHA-1 hash, aka object name, and a reference to an object is always
the 40-byte hex representation of that SHA-1 name. The files in the refs subdirectory are expected to
contain these hex references (usually with a final \n at the end), and you should thus expect to see a
number of 41-byte files containing these references in these refs subdirectories when you actually start
populating your tree.
Note
An advanced user may want to take a look at gitrepository-layout(5) after finishing this tutorial.
You have now created your first Git repository. Of course, since 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 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/??/*
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.
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
$ git diff
diff --git a/hello b/hello
index 557db03..263414f 100644
--- a/hello
+++ b/hello
@@ -1 +1,2 @@
Hello World
+It's a new day for git
COMMITTING GIT STATE
Now, we want to go to the next stage in Git, which is to take the files that Git knows about in the index, and
commit them as a real tree. We do that in two phases: creating a tree object, and committing that tree object
as a commit object together with an explanation of what the tree was all about, along with information of how
we came to that state.
Creating a tree object is trivial, and is done with git write-tree. There are no options or other input: git
write-tree will take the current index state, and write an object that describes that whole index. In other
words, 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
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
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 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 "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).
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-tree
+----+
| |
| |
V V
+-----------+
| Object DB |
| Backing |
| Store |
+-----------+
^ ^
| |
| | diff-index --cached
| |
diff-index | V
| +-----------+
| | Index |
| | "cache" |
| +-----------+
| ^
| |
| | diff-files
| |
V V
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 can explore on your own.
Note
Most likely, you are not directly using the core Git Plumbing commands, but using Porcelain such as git
add, ‘git-rm’ and ‘git-commit’.
TAGGING A VERSION
In Git, there are two kinds of tags, a "light" one, and an "annotated tag".
A "light" tag is technically nothing more than a branch, except we put it in the .git/refs/tags/ subdirectory
instead of calling it a head. So the simplest form of tag involves nothing more than
$ git tag my-first-tag
which just writes the current HEAD into the .git/refs/tags/my-first-tag file, after which point you can then
use this symbolic name for that particular state. You can, for example, do
$ git diff my-first-tag
to diff your current state against that tag which at this point will obviously be an empty diff, but if you
continue to develop and commit stuff, you can use your tag as an "anchor-point" to see what has changed since
you tagged it.
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).
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,
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 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 SHA-1 name of that object into a file under .git/refs/heads/. You can use any filename you want
(and indeed, subdirectories), but the convention is that the "normal" branch is called master. That’s just a
convention, though, and nothing enforces it.
To show that as an example, let’s go back to the git-tutorial repository we used earlier, and create a branch
in it. You do that by simply just saying that you want to check out a new branch:
$ git checkout -b mybranch
will create a new branch based at the current HEAD position, and switch to it.
Note
If you make the decision to start your new branch at some other point in the history than the current
HEAD, you can do so by just telling git checkout what the base of the checkout would be. In other words,
if you have an earlier tag or branch, you’d just do
$ git checkout -b mybranch earlier-commit
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:
$ git checkout master
Here, take a moment to look at the contents of hello, and notice how they don’t contain the work we just did
in mybranch — because that work hasn’t happened in the master branch at all. Then do
$ echo "Play, play, play" >>hello
$ echo "Lots of fun" >>example
$ git commit -m "Some fun." -i hello example
since the master branch is obviously in a much better mood.
Now, you’ve got two branches, and you decide that you want to merge the work done. Before we do that, let’s
introduce a cool graphical tool that helps you view what’s going on:
$ gitk --all
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
Now, let’s pretend you are the one who did all the work in mybranch, and the fruit of your hard work has
finally been merged to the master branch. Let’s go back to mybranch, and run git merge to get the "upstream
changes" back to your branch.
$ git checkout mybranch
$ git merge -m "Merge upstream changes." master
This outputs something like this (the actual commit object names would be different)
Updating from ae3a2da... to a80b4aa....
Fast-forward (no commit created; -m option ignored)
example | 1 +
hello | 1 +
2 files changed, 2 insertions(+)
Because your branch did not contain anything more than what had already been merged into the master branch,
the merge operation did not actually do a merge. Instead, it just updated the top of the tree of your branch
to that of the master branch. This is often called fast-forward merge.
You can run gitk --all again to see how the commit ancestry looks like, or run show-branch, which tells you
this.
$ git show-branch master mybranch
! [master] Merge work in mybranch
* [mybranch] Merge work in mybranch
--
-- [master] Merge work in mybranch
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.
This transport is the same as SSH transport but uses sh to run both ends on the local machine instead of
running other end on the remote machine via ssh.
Git Native
git://remote.machine/path/to/repo.git/
This transport was designed for anonymous downloading. Like SSH transport, it finds out the set of objects
the downstream side lacks and transfers (close to) minimum set of objects.
HTTP(S)
http://remote.machine/path/to/repo.git/
Downloader from http and https URL first obtains the topmost commit object name from the remote site by
looking at the specified refname under repo.git/refs/ 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:
$ git config remote.linus.url http://www.kernel.org/pub/scm/git/git.git/
and use the "linus" keyword with git pull instead of the full URL.
Examples.
1. git pull linus
OK, still with me? To give us an example to look at, let’s go back to the earlier repository with "hello" and
"example" file, and bring ourselves back to the pre-merge state:
$ git show-branch --more=2 master mybranch
! [master] Merge work in mybranch
* [mybranch] Merge work in mybranch
--
-- [master] Merge work in mybranch
+* [master^2] Some work.
+* [master^] Some fun.
Remember, before running git merge, our master head was at "Some fun." commit, while our mybranch head was at
"Some work." commit.
$ git checkout mybranch
$ git reset --hard master^2
$ git checkout master
$ git reset --hard master^
After rewinding, the commit structure should look like this:
$ git show-branch
* [master] Some fun.
! [mybranch] Some work.
--
* [master] Some fun.
+ [mybranch] Some work.
*+ [master^] Initial commit
Now we are ready to experiment with the merge by hand.
git merge command, when merging two branches, uses 3-way merge algorithm. First, it finds the common ancestor
between them. The command it uses is git merge-base:
$ mb=$(git merge-base HEAD mybranch)
The command writes the commit object name of the common ancestor to the standard output, so we captured its
output to a variable, because we will be using it in the next step. By the way, the common ancestor commit is
the "Initial commit" commit in this case. You can tell it by:
$ git name-rev --name-only --tags $mb
my-first-tag
After finding out a common ancestor commit, the second step is this:
$ git read-tree -m -u $mb HEAD mybranch
100644 557db03de997c86a4a028e1ebd3a1ceb225be238 1 hello
100644 ba42a2a96e3027f3333e13ede4ccf4498c3ae942 2 hello
100644 cc44c73eb783565da5831b4d820c962954019b69 3 hello
In our example of only two files, we did not have unchanged files so only example resulted in collapsing. But
in real-life large projects, when only a small number of files change in one commit, this collapsing tends to
trivially merge most of the paths fairly quickly, leaving only a handful of real changes in non-zero stages.
To look at only non-zero stages, use --unmerged flag:
$ git ls-files --unmerged
100644 557db03de997c86a4a028e1ebd3a1ceb225be238 1 hello
100644 ba42a2a96e3027f3333e13ede4ccf4498c3ae942 2 hello
100644 cc44c73eb783565da5831b4d820c962954019b69 3 hello
The next step of merging is to merge these three versions of the file, using 3-way merge. This is done by
giving git merge-one-file command as one of the arguments to git merge-index command:
$ git merge-index git-merge-one-file hello
Auto-merging hello
ERROR: Merge conflict in hello
fatal: merge program failed
git merge-one-file script is called with parameters to describe those three versions, and is responsible to
leave the merge results in the working tree. It is a fairly straightforward shell script, and eventually calls
merge program from RCS suite to perform a file-level 3-way merge. In this case, merge detects conflicts, and
the merge result with conflict marks is left in the working tree.. This can be seen if you run ls-files
--stage again at this point:
$ git ls-files --stage
100644 7f8b141b65fdcee47321e399a2598a235a032422 0 example
100644 557db03de997c86a4a028e1ebd3a1ceb225be238 1 hello
100644 ba42a2a96e3027f3333e13ede4ccf4498c3ae942 2 hello
100644 cc44c73eb783565da5831b4d820c962954019b69 3 hello
This is the state of the index file and the working file after git merge returns control back to you, leaving
the conflicting merge for you to resolve. Notice that the path hello is still unmerged, and what you see with
git diff at this point is differences since stage 2 (i.e. your version).
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
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.
Note
Many installations of sshd do not invoke your shell as the login shell when you directly run programs;
what this means is that if your login shell is bash, only .bashrc is read and not .bash_profile. As a
workaround, make sure .bashrc sets up $PATH so that you can run git-receive-pack program.
Note
If you plan to publish this repository to be accessed over http, you should do mv
my-git.git/hooks/post-update.sample my-git.git/hooks/post-update at this point. This makes sure that every
time you push into this repository, git update-server-info is run.
Your "public repository" is now ready to accept your changes. Come back to the machine you have your private
repository. From there, run this command:
$ git push <public-host>:/path/to/my-git.git master
This synchronizes your public repository to match the named branch head (i.e. master in this case) and objects
reachable from them in your current repository.
As a real example, this is how I update my public Git repository. Kernel.org mirror network takes care of the
propagation to other publicly visible machines:
$ git push master.kernel.org:/pub/scm/git/git.git/
PACKING YOUR REPOSITORY
Earlier, we saw that one file under .git/objects/??/ directory is stored for each Git object you create. This
representation is efficient to create atomically and safely, but not so convenient to transport over the
network. Since Git objects are immutable once they are created, there is a way to optimize the storage by
"packing them together". The command
$ git repack
will do it for you. If you followed the tutorial examples, you would have accumulated about 17 objects in
Once you have packed objects, you do not need to leave the unpacked objects that are contained in the pack
file anymore.
$ git prune-packed
would remove them for you.
You can try running find .git/objects -type f before and after you run git prune-packed if you are curious.
Also git count-objects would tell you how many unpacked objects are in your repository and how much space they
are consuming.
Note
git pull is slightly cumbersome for HTTP transport, as a packed repository may contain relatively few
objects in a relatively large pack. If you expect many HTTP pulls from your public repository you might
want to repack & prune often, or never.
If you run git repack again at this point, it will say "Nothing new to pack.". Once you continue your
development and accumulate the changes, running git repack again will create a new pack, that contains objects
created since you packed your repository the last time. We recommend that you pack your project soon after the
initial import (unless you are starting your project from scratch), and then run git repack every once in a
while, depending on how active your project is.
When a repository is synchronized via git push and git pull objects packed in the source repository are
usually stored 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.
repository" goes like this:
1. Prepare your work repository, by git clone the public repository of the "project lead". The URL used for
the initial cloning is stored in the remote.origin.url configuration variable.
2. Prepare a public repository accessible to others, just like the "project lead" person does.
3. Copy over the packed files from "project lead" public repository to your public repository, unless the
"project lead" repository lives on the same machine as yours. In the latter case, you can use
objects/info/alternates file to point at the repository you are borrowing from.
4. Push into the public repository from your primary repository. Run git repack, and possibly git prune if
the transport used for pulling from your repository supports packed repositories.
5. Keep working in your primary repository. Your changes include modifications of your own, patches you
receive via e-mails, and merges resulting from pulling the "public" repositories of your "project lead"
and possibly your "sub-subsystem maintainers".
You can repack this private repository whenever you feel like.
6. Push your changes to your public repository, and ask your "project lead" and possibly your "sub-subsystem
maintainers" to pull from 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.
WORKING WITH OTHERS, SHARED REPOSITORY STYLE
If you are coming from CVS background, the style of cooperation suggested in the previous section may be new
to you. You do not have to worry. Git supports "shared public repository" style of cooperation you are
probably more familiar with as well.
See gitcvs-migration(7) for the details.
BUNDLING YOUR WORK TOGETHER
It is likely that you will be working on more than one thing at a time. It is easy to manage those
more-or-less independent tasks using branches with Git.
+ [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.
Part of the git(1) suite.
NOTES
1. the Git User Manual
file:///usr/share/doc/git-1.8.3.1/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-1.8.3.1/everyday.html
Git 1.8.3.1 03/23/2016 GITCORE-TUTORIAL(7)
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