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GITCORE-TUTORIAL(7) Git Manual GITCORE-TUTORIAL(7)
gitcore-tutorial - A Git core tutorial for developers
git *
This tutorial explains how to use the "core" Git commands to set
up and work with a Git repository.
If you just need to use Git as a revision control system you may
prefer to start with "A Tutorial Introduction to Git" (‐
gittutorial(7)) or the Git User Manual[1].
However, an understanding of these low-level tools can be helpful
if you want to understand Git’s internals.
The core Git is often called "plumbing", with the prettier user
interfaces on top of it called "porcelain". You may not want to
use the plumbing directly very often, but it can be good to know
what the plumbing does 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 new Git repository couldn’t be easier: all Git
repositories start out empty, and the only thing you need to do
is find yourself a subdirectory that you want to use as a working
tree - either an empty one for a totally new project, or an
existing working tree that you want to import into Git.
For our first example, we’re going to start a totally new
repository from scratch, with no pre-existing files, and we’ll
call it git-tutorial. To start up, create a subdirectory for it,
change into that subdirectory, and initialize the Git
infrastructure with git init:
$ mkdir git-tutorial
$ cd git-tutorial
$ git init
to which Git will reply
Initialized empty Git repository in .git/
which is just Git’s way of saying that you haven’t been doing
anything strange, and that it will have created a local .git
directory setup for your new project. You will now have a .git
directory, and you can inspect that with ls. For your new empty
project, it should show you three entries, among other things:
• a file called HEAD, that has ref: refs/heads/master in it.
This is similar to a symbolic link and points at
refs/heads/master relative to the HEAD file.
Don’t worry about the fact that the file that the HEAD link
points to doesn’t even exist yet — you haven’t created the
commit that will start your HEAD development branch yet.
• a subdirectory called objects, which will contain all the
objects of your project. You should never have any real
reason to look at the objects directly, but you might want to
know that these objects are what contains all the real data
in your repository.
• a subdirectory called refs, which contains references to
objects.
In particular, the refs subdirectory will contain two other
subdirectories, named heads and tags respectively. They do
exactly what their names imply: they contain references to any
number of different heads of development (aka branches), and to
any tags that you have created to name specific versions in your
repository.
One note: the special master head is the default branch, which is
why the .git/HEAD file was created points to it even if it
doesn’t yet exist. Basically, the HEAD link is supposed to always
point to the branch you are working on right now, and you always
start out expecting to work on the master branch.
However, this is only a convention, and you can name your
branches anything you want, and don’t have to ever even have a
master branch. A number of the Git tools will assume that
.git/HEAD is valid, though.
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.
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:
.git/objects/55/7db03de997c86a4a028e1ebd3a1ceb225be238
.git/objects/f2/4c74a2e500f5ee1332c86b94199f52b1d1d962
which correspond with the objects with names of 557db... and
f24c7... respectively.
If you want to, you can use git cat-file to look at those
objects, but you’ll have to use the object name, not the filename
of the object:
$ git cat-file -t 557db03de997c86a4a028e1ebd3a1ceb225be238
where the -t tells git cat-file to tell you what the "type" of
the object is. Git will tell you that you have a "blob" object
(i.e., just a regular file), and you can see the contents with
$ git cat-file blob 557db03
which will print out "Hello World". The object 557db03 is nothing
more than the contents of your file hello.
Note
Don’t confuse that object with the file hello itself. The
object is literally just those specific contents of the file,
and however much you later change the contents in file hello,
the object we just looked at will never change. Objects are
immutable.
Note
The second example demonstrates that you can abbreviate the
object name to only the first several hexadecimal digits in
most places.
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
hello.
In other words, git diff-files always shows us the difference
between what is recorded in the index, and what is currently in
the working tree. That’s very useful.
A common shorthand for git diff-files -p is to just write git
diff, which will do the same thing.
$ git diff
diff --git a/hello b/hello
index 557db03..263414f 100644
--- a/hello
+++ b/hello
@@ -1 +1,2 @@
Hello World
+It's a new day for git
Now, we want to go to the next stage in Git, which is to take the
files that Git knows about in the index, and commit them as a
real tree. We do that in two phases: creating a tree object, and
committing that tree object as a commit object together with an
explanation of what the tree was all about, along with
information of how we came to that state.
Creating a tree object is trivial, and is done with git
write-tree. There are no options or other input: git write-tree
will take the current index state, and write an object that
describes that whole index. In other words, we’re now tying
together all the different filenames with their contents (and
their permissions), and we’re creating the equivalent of a Git
"directory" object:
$ git write-tree
and this will just output the name of the resulting tree, in this
case (if you have done exactly as I’ve described) it should be
8988da15d077d4829fc51d8544c097def6644dbb
which is another incomprehensible object name. Again, if you want
to, you can use git cat-file -t 8988d... to see that this time
the object is not a "blob" object, but a "tree" object (you can
also use git cat-file to actually output the raw object contents,
but you’ll see mainly a binary mess, so that’s less interesting).
However — normally you’d never use git write-tree on its own,
because normally you always commit a tree into a commit object
using the git commit-tree command. In fact, it’s easier to not
actually use git write-tree on its own at all, but to just pass
its result in as an argument to git commit-tree.
git commit-tree normally takes several arguments — it wants to
know what the parent of a commit was, but since this is the first
commit ever in this new repository, and it has no parents, we
only need to pass in the object name of the tree. However, git
commit-tree also wants to get a commit message on its standard
input, and it will write out the resulting object name for the
commit to its standard output.
And this is where we create the .git/refs/heads/master file which
is pointed at by HEAD. This file is supposed to contain the
reference to the top-of-tree of the master branch, and since
that’s exactly what git commit-tree spits out, we can do this all
with a sequence of simple shell commands:
$ tree=$(git write-tree)
$ commit=$(echo 'Initial commit' | git commit-tree $tree)
$ git update-ref HEAD $commit
In this case this creates a totally new commit that is not
related to anything else. Normally you do this only once for a
project ever, and all later commits will be parented on top of an
earlier commit.
Again, normally you’d never actually do this by hand. There is a
helpful script called git commit that will do all of this for
you. So you could have just written git commit instead, and it
would have done the above magic scripting for you.
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
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).
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
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
+-----------+
| Working |
| Directory |
+-----------+
More interestingly, you can also give git diff-tree the --pretty
flag, which tells it to also show the commit message and author
and date of the commit, and you can tell it to show a whole
series of diffs. Alternatively, you can tell it to be "silent",
and not show the diffs at all, but just show the actual commit
message.
In fact, together with the git rev-list program (which generates
a list of revisions), git diff-tree ends up being a veritable
fount of changes. You can emulate git log, git log -p, etc. with
a trivial script that pipes the output of git rev-list to git
diff-tree --stdin, which was exactly how early versions of git
log were implemented.
In Git, there are two kinds of tags, a "light" one, and an
"annotated 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).
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.
Note
You can tell Git to split the Git internal information from
the directory that it tracks, but we’ll ignore that for now:
it’s not how normal projects work, and it’s really only meant
for special uses. So the mental model of "the Git information
is always tied directly to the working tree that it
describes" may not be technically 100% accurate, but it’s a
good model for all normal use.
This has two implications:
• 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://rsync.kernel.org/pub/scm/git/git.git/ .git
followed by
$ git read-tree HEAD
to populate the index. However, now you have populated the index,
and you have all the Git internal files, but you will notice that
you 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://git.kernel.org/pub/scm/git/git.git/ my-git
$ cd my-git
$ git checkout
which will end up doing all of the above for you.
You have now successfully copied somebody else’s (mine) remote
repository, and checked it out.
Branches in Git are really nothing more than pointers into the
Git object database from within the .git/refs/ subdirectory, and
as we already discussed, the HEAD branch is nothing but a symlink
to one of these object pointers.
You can at any time create a new branch by just picking an
arbitrary point in the project history, and just writing the
SHA-1 name of that object into a file under .git/refs/heads/. You
can use any filename you want (and indeed, subdirectories), but
the convention is that the "normal" branch is called master.
That’s just a convention, though, and nothing enforces it.
To show that as an example, let’s go back to the git-tutorial
repository we used earlier, and create a branch in it. You do
that by simply just saying that you want to check out a new
branch:
$ git switch -c mybranch
will create a new branch based at the current HEAD position, and
switch to it.
Note
If you make the decision to start your new branch at some
other point in the history than the current HEAD, you can do
so by just telling git switch what the base of the checkout
would be. In other words, if you have an earlier tag or
branch, 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
doing
$ git switch master
(or any other branch-name, for that matter) and if you forget
which branch you happen to be on, a simple
$ cat .git/HEAD
will tell you where 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.
Note
Without the --more=1 option, git show-branch would not output
the [master^] commit, as [mybranch] commit is a common
ancestor of both master and mybranch tips. Please see
git-show-branch(1) for details.
Note
If there were more commits on the master branch after the
merge, the merge commit itself would not be shown by git
show-branch by default. You would need to provide --sparse
option to make the merge commit visible in this case.
Now, let’s pretend you are the one who did all the work in
mybranch, and the fruit of your hard work has finally been merged
to the master branch. Let’s go back to mybranch, and run git
merge to get the "upstream changes" back to your branch.
$ git 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
fetch:
$ git fetch <remote-repository>
One of the following transports can be used to name the
repository to download from:
SSH
remote.machine:/path/to/repo.git/ or
ssh://remote.machine/path/to/repo.git/
This transport can be used for both uploading and
downloading, and requires you to have a log-in privilege over
ssh to the remote machine. It finds out the set of objects
the other side lacks by exchanging the head commits both ends
have and transfers (close to) minimum set of objects. It is
by far the most efficient way to exchange Git objects between
repositories.
Local directory
/path/to/repo.git/
This transport is the same as SSH transport but uses sh to
run both ends on the local machine instead of running other
end on the remote machine via ssh.
Git Native
git://remote.machine/path/to/repo.git/
This transport was designed for anonymous downloading. Like
SSH transport, it finds out the set of objects the downstream
side lacks and transfers (close to) minimum set of objects.
HTTP(S)
http://remote.machine/path/to/repo.git/
Downloader from http and https URL first obtains the topmost
commit object name from the remote site by looking at the
specified refname under repo.git/refs/ 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 https://git.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
2. git pull linus tag v0.99.1
the above are equivalent to:
1. git pull http://www.kernel.org/pub/scm/git/git.git/ HEAD
2. git pull http://www.kernel.org/pub/scm/git/git.git/ tag
v0.99.1
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
my-first-tag
After finding out a common ancestor commit, the second step is
this:
$ git read-tree -m -u $mb HEAD mybranch
This is the same git read-tree command we have already seen, but
it takes three trees, unlike previous examples. This reads the
contents of each tree into different stage in the index file (the
first tree goes to stage 1, the second to stage 2, etc.). After
reading three trees into three stages, the paths that are the
same in all three stages are collapsed into stage 0. Also paths
that are the same in two of three stages are collapsed into stage
0, taking the SHA-1 from either stage 2 or stage 3, whichever is
different from stage 1 (i.e. only one side changed from the
common ancestor).
After collapsing operation, paths that are different in three
trees are left in non-zero stages. At this point, you can inspect
the index file with this command:
$ git ls-files --stage
100644 7f8b141b65fdcee47321e399a2598a235a032422 0 example
100644 557db03de997c86a4a028e1ebd3a1ceb225be238 1 hello
100644 ba42a2a96e3027f3333e13ede4ccf4498c3ae942 2 hello
100644 cc44c73eb783565da5831b4d820c962954019b69 3 hello
In our example of only two files, we did not have unchanged files
so only example resulted in collapsing. But in real-life large
projects, when only a small number of files change in one commit,
this collapsing tends to trivially merge most of the paths fairly
quickly, leaving only a handful of real changes in non-zero
stages.
To look at only non-zero stages, use --unmerged flag:
$ git ls-files --unmerged
100644 557db03de997c86a4a028e1ebd3a1ceb225be238 1 hello
100644 ba42a2a96e3027f3333e13ede4ccf4498c3ae942 2 hello
100644 cc44c73eb783565da5831b4d820c962954019b69 3 hello
The next step of merging is to merge these three versions of the
file, using 3-way merge. This is done by giving git
merge-one-file command as one of the arguments to git merge-index
command:
$ git merge-index git-merge-one-file hello
Auto-merging hello
ERROR: Merge conflict in hello
fatal: merge program failed
git merge-one-file script is called with parameters to describe
those three versions, and is responsible to leave the merge
results in the working tree. It is a fairly straightforward shell
script, and eventually calls merge program from RCS suite to
perform a file-level 3-way merge. In this case, merge detects
conflicts, and the merge result with conflict marks is left in
the working tree.. This can be seen if you run ls-files --stage
again at this point:
$ git ls-files --stage
100644 7f8b141b65fdcee47321e399a2598a235a032422 0 example
100644 557db03de997c86a4a028e1ebd3a1ceb225be238 1 hello
100644 ba42a2a96e3027f3333e13ede4ccf4498c3ae942 2 hello
100644 cc44c73eb783565da5831b4d820c962954019b69 3 hello
This is the state of the index file and the working file after
git merge returns control back to you, leaving the conflicting
merge for you to resolve. Notice that the path hello is still
unmerged, and what you see with git diff at this point is
differences since stage 2 (i.e. your version).
So, we can use somebody else’s work from a remote repository, but
how can you prepare a repository to let other people pull from
it?
You do your real work in your working tree that has your primary
repository hanging under it as its .git subdirectory. You could
make that repository accessible remotely and ask people to pull
from it, but in practice that is not the way things are usually
done. A recommended way is to have a public repository, make it
reachable by other people, and when the changes you made in your
primary working tree are in good shape, update the public
repository from it. This is often called pushing.
Note
This public repository could further be mirrored, and that is
how Git repositories at kernel.org are managed.
Publishing the changes from your local (private) repository to
your remote (public) repository requires a write privilege on the
remote machine. You need to have an SSH account there to run a
single command, git-receive-pack.
First, you need to create an empty repository on the remote
machine that will house your public repository. This empty
repository will be populated and be kept up to date by pushing
into it later. Obviously, this repository creation needs to be
done only once.
Note
git push uses a pair of commands, git send-pack on your local
machine, and git-receive-pack on the remote machine. The
communication between the two over the network internally
uses an SSH connection.
Your private repository’s Git directory is usually .git, but your
public repository is often named after the project name, i.e.
<project>.git. Let’s create such a public repository for project
my-git. After logging into the remote machine, create an empty
directory:
$ mkdir my-git.git
Then, make that directory into a Git repository by running git
init, but this time, since its name is not the usual .git, we do
things slightly differently:
$ GIT_DIR=my-git.git git init
Make sure this directory is available for others you want your
changes to be pulled via the transport of your choice. Also you
need to make sure that you have the git-receive-pack program on
the $PATH.
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/
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
directory.
Note
You will see two files, pack-*.pack and pack-*.idx, in
.git/objects/pack directory. They are closely related to each
other, and if you ever copy them by hand to a different
repository for whatever reason, you should make sure you copy
them together. The former holds all the data from the objects
in the pack, and the latter holds the index for random
access.
If you are paranoid, running git verify-pack command would detect
if you have a corrupt pack, but do not worry too much. Our
programs are always perfect ;-).
Once you have packed objects, you do not need to leave the
unpacked objects that are contained in the pack file anymore.
$ git prune-packed
would remove them for you.
You can try running find .git/objects -type f before and after
you run git prune-packed if you are curious. Also git
count-objects would tell you how many unpacked objects are in
your repository and how much space they are consuming.
Note
git pull is slightly cumbersome for HTTP transport, as a
packed repository may contain relatively few objects in a
relatively large pack. If you expect many HTTP pulls from
your public repository you might want to repack & prune
often, or never.
If you run git repack again at this point, it will say "Nothing
new to pack.". Once you continue your development and accumulate
the changes, running git repack again will create a new pack,
that contains objects created since you packed your repository
the last time. We recommend that you pack your project soon after
the initial import (unless you are starting your project from
scratch), and then run git repack every once in a while,
depending on how active your project is.
When a repository is synchronized via git push and git pull
objects packed in the source repository are usually stored
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
like.
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
this:
1. Prepare your work repository, by running git clone on the
public repository of the "project lead". The URL used for the
initial cloning is stored in the remote.origin.url
configuration variable.
2. Prepare a public repository accessible to others, just like
the "project lead" person does.
3. Copy over the packed files from "project lead" public
repository to your public repository, unless the "project
lead" repository lives on the same machine as yours. In the
latter case, you can use objects/info/alternates file to
point at the repository you are borrowing from.
4. Push into the public repository from your primary repository.
Run git repack, and possibly git prune if the transport used
for pulling from your repository supports packed
repositories.
5. Keep working in your primary repository. Your changes include
modifications of your own, patches you receive via e-mails,
and merges resulting from pulling the "public" repositories
of your "project lead" and possibly your "sub-subsystem
maintainers".
You can repack this private repository whenever you feel
like.
6. Push your changes to your public repository, and ask your
"project lead" and possibly your "sub-subsystem maintainers"
to pull from it.
7. Every once in a while, git repack the public repository. Go
back to step 5. and continue working.
A recommended work cycle for an "individual developer" who does
not have a "public" repository is somewhat different. It goes
like this:
1. Prepare your work repository, by git clone the public
repository of the "project lead" (or a "subsystem
maintainer", if you work on a subsystem). The URL used for
the initial cloning is stored in the remote.origin.url
configuration variable.
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"
branches:
$ git show-branch
! [commit-fix] Fix commit message normalization.
! [diff-fix] Fix rename detection.
* [master] Release candidate #1
---
+ [diff-fix] Fix rename detection.
+ [diff-fix~1] Better common substring algorithm.
+ [commit-fix] Fix commit message normalization.
* [master] Release candidate #1
++* [diff-fix~2] Pretty-print messages.
Both fixes are tested well, and at this point, you want to merge
in both of them. You could merge in diff-fix first and then
commit-fix next, like this:
$ git merge -m "Merge fix in diff-fix" diff-fix
$ git merge -m "Merge fix in commit-fix" commit-fix
Which would result in:
$ git show-branch
! [commit-fix] Fix commit message normalization.
! [diff-fix] Fix rename detection.
* [master] Merge fix in commit-fix
---
- [master] Merge fix in commit-fix
+ * [commit-fix] Fix commit message normalization.
- [master~1] Merge fix in diff-fix
+* [diff-fix] Fix rename detection.
+* [diff-fix~1] Better common substring algorithm.
* [master~2] Release candidate #1
++* [master~3] Pretty-print messages.
However, there is no particular reason to merge in one branch
first and the other next, when what you have are a set of truly
independent changes (if the order mattered, then they are not
independent by definition). You could instead merge those two
branches into the current branch at once. First let’s undo what
we just did and start over. We would want to get the master
branch before these two merges by resetting it to master~2:
$ git reset --hard master~2
You can make sure git show-branch matches the state before those
two git merge you just did. Then, instead of running two git
merge commands in a row, you would merge these two branch heads
(this is known as making an Octopus):
$ git merge commit-fix diff-fix
$ git show-branch
! [commit-fix] Fix commit message normalization.
! [diff-fix] Fix rename detection.
* [master] Octopus merge of branches 'diff-fix' and 'commit-fix'
---
- [master] Octopus merge of branches 'diff-fix' and 'commit-fix'
+ * [commit-fix] Fix commit message normalization.
+* [diff-fix] Fix rename detection.
+* [diff-fix~1] Better common substring algorithm.
* [master~1] Release candidate #1
++* [master~2] Pretty-print messages.
Note that you should not do Octopus 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.
gittutorial(7), gittutorial-2(7), gitcvs-migration(7),
git-help(1), giteveryday(7), The Git User’s Manual[1]
Part of the git(1) suite
1. the Git User Manual
file:///home/mtk/share/doc/git-doc/user-manual.html
2. Randy Dunlap’s presentation
https://web.archive.org/web/20120915203609/http://www.xenotime.net/linux/mentor/linux-mentoring-2006.pdf
This page is part of the git (Git distributed version control
system) project. Information about the project can be found at
⟨http://git-scm.com/⟩. If you have a bug report for this manual
page, see ⟨http://git-scm.com/community⟩. This page was obtained
from the project's upstream Git repository
⟨https://github.com/git/git.git⟩ on 2024-06-14. (At that time,
the date of the most recent commit that was found in the
repository was 2024-06-12.) If you discover any rendering
problems in this HTML version of the page, or you believe there
is a better or more up-to-date source for the page, or you have
corrections or improvements to the information in this COLOPHON
(which is not part of the original manual page), send a mail to
[email protected]
Git 2.45.2.492.gd63586 2024-06-12 GITCORE-TUTORIAL(7)
Pages that refer to this page: git(1), gitcvs-migration(7), gittutorial-2(7), gittutorial(7)
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