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 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.
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
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:
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
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:
and this will just output the name of the resulting tree, in this case (if you
have done exactly as I’ve described) it should be
8988da15d077d4829fc51d8544c097def6644dbb
which is another incomprehensible object name. Again, if you want to, you can
use
git cat-file -t 8988d... to see that this time the object is not a
"blob" object, but a "tree" object (you can also use
git cat-file to actually output the raw object contents, but
you’ll see mainly a binary mess, so that’s less interesting).
However — normally you’d never use
git write-tree on its
own, because normally you always commit a tree into a commit object using the
git commit-tree command. In fact, it’s easier to not actually
use
git write-tree on its own at all, but to just pass its result in as
an argument to
git commit-tree.
git commit-tree normally takes several arguments — it wants to
know what the
parent of a commit was, but since this is the first
commit ever in this new repository, and it has no parents, we only need to
pass in the object name of the tree. However,
git commit-tree also
wants to get a commit message on its standard input, and it will write out the
resulting object name for the commit to its standard output.
And this is where we create the
.git/refs/heads/master file which is
pointed at by
HEAD. This file is supposed to contain the reference to
the top-of-tree of the master branch, and since that’s exactly what
git commit-tree spits out, we can do this all with a sequence of simple
shell commands:
$ tree=$(git write-tree)
$ commit=$(echo 'Initial commit' | git commit-tree $tree)
$ git update-ref HEAD $commit
In this case this creates a totally new commit that is not related to anything
else. Normally you do this only
once for a project ever, and all later
commits will be parented on top of an earlier commit.
Again, normally you’d never actually do this by hand. There is a helpful
script called
git commit that will do all of this for you. So you could
have just written
git commit instead, and it would have done the above
magic scripting for you.
MAKING A CHANGE¶
Remember how we did the
git update-index on file
hello and then we
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
(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
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:
(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:
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
(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.
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
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
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:
which will sign the current
HEAD (but you can also give it another
argument that specifies the thing to tag, e.g., you could have tagged the
current
mybranch point by using
git tag <tagname>
mybranch).
You normally only do signed tags for major releases or things like that, while
the light-weight tags are useful for any marking you want to do — any
time you decide that you want to remember a certain point, just create a
private tag for it, and you have a nice symbolic name for the state at that
point.
COPYING REPOSITORIES¶
Git repositories are normally totally self-sufficient and relocatable. Unlike
CVS, for example, there is no separate notion of "repository" and
"working tree". A Git repository normally
is the working
tree, with the local Git information hidden in the
.git subdirectory.
There is nothing else. What you see is what you got.
Note
You can tell Git to split the Git internal information from the directory that
it tracks, but we’ll ignore that for now: it’s not how normal
projects work, and it’s really only meant for special uses. So the
mental model of "the Git information is always tied directly to the
working tree that it describes" may not be technically 100% accurate, but
it’s a good model for all normal use.
This has two implications:
•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
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
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
followed by
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
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
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
(or any other branch-name, for that matter) and if you forget which branch you
happen to be on, a simple
will tell you where it’s pointing. To get the list of branches you have,
you can say
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:
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:
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
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 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:
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:
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:
HOW DOES THE MERGE WORK?¶
We said this tutorial shows what plumbing does to help you cope with the
porcelain that isn’t flushing, but we so far did not talk about how the
merge really works. If you are following this tutorial the first time,
I’d suggest to skip to "Publishing your work" section and
come back here later.
OK, still with me? To give us an example to look at, let’s go back to the
earlier repository with "hello" and "example" file, and
bring ourselves back to the pre-merge state:
$ git show-branch --more=2 master mybranch
! [master] Merge work in mybranch
* [mybranch] Merge work in mybranch
--
-- [master] Merge work in mybranch
+* [master^2] Some work.
+* [master^] Some fun.
Remember, before running
git merge, our
master head was at
"Some fun." commit, while our
mybranch head was at "Some
work." commit.
$ git checkout mybranch
$ git reset --hard master^2
$ git checkout master
$ git reset --hard master^
After rewinding, the commit structure should look like this:
$ git show-branch
* [master] Some fun.
! [mybranch] Some work.
--
* [master] Some fun.
+ [mybranch] Some work.
*+ [master^] Initial commit
Now we are ready to experiment with the merge by hand.
git merge command, when merging two branches, uses 3-way merge algorithm.
First, it finds the common ancestor between them. The command it uses is
git merge-base:
$ mb=$(git merge-base HEAD mybranch)
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).
PUBLISHING YOUR WORK¶
So, we can use somebody else’s work from a remote repository, but how can
you prepare a repository to let other people pull from it?
You do your real work in your working tree that has your primary repository
hanging under it as its
.git subdirectory. You
could make that
repository accessible remotely and ask people to pull from it, but in practice
that is not the way things are usually done. A recommended way is to have a
public repository, make it reachable by other people, and when the changes you
made in your primary working tree are in good shape, update the public
repository from it. This is often called
pushing.
Note
This public repository could further be mirrored, and that is how Git
repositories at
kernel.org are managed.
Publishing the changes from your local (private) repository to your remote
(public) repository requires a write privilege on the remote machine. You need
to have an SSH account there to run a single command,
git-receive-pack.
First, you need to create an empty repository on the remote machine that will
house your public repository. This empty repository will be populated and be
kept up-to-date by pushing into it later. Obviously, this repository creation
needs to be done only once.
Note
git push uses a pair of commands,
git send-pack on your local
machine, and
git-receive-pack on the remote machine. The communication
between the two over the network internally uses an SSH connection.
Your private repository’s Git directory is usually
.git, but your
public repository is often named after the project name, i.e.
<project>.git. Let’s create such a public repository for
project
my-git. After logging into the remote machine, create an empty
directory:
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
will do it for you. If you followed the tutorial examples, you would have
accumulated about 17 objects in
.git/objects/??/ directories by now.
git repack tells you how many objects it packed, and stores the packed
file in
.git/objects/pack directory.
Note
You will see two files,
pack-*.pack and
pack-*.idx, in
.git/objects/pack directory. They are closely related to each other,
and if you ever copy them by hand to a different repository for whatever
reason, you should make sure you copy them together. The former holds all the
data from the objects in the pack, and the latter holds the index for random
access.
If you are paranoid, running
git verify-pack command would detect if you
have a corrupt pack, but do not worry too much. Our programs are always
perfect ;-).
Once you have packed objects, you do not need to leave the unpacked objects that
are contained in the pack file anymore.
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.
WORKING WITH OTHERS¶
Although Git is a truly distributed system, it is often convenient to organize
your project with an informal hierarchy of developers. Linux kernel
development is run this way. There is a nice illustration (page 17,
"Merges to Mainline") in
Randy Dunlap’s
presentation[2].
It should be stressed that this hierarchy is purely
informal. There is
nothing fundamental in Git that enforces the "chain of patch flow"
this hierarchy implies. You do not have to pull from only one remote
repository.
A recommended workflow for a "project lead" goes like this:
1.Prepare your primary repository on your local machine.
Your work is done there.
2.Prepare a public repository accessible to others.
If other people are pulling from your repository over dumb transport protocols
(HTTP), you need to keep this repository
dumb transport friendly. After
git init,
$GIT_DIR/hooks/post-update.sample copied from the
standard templates would contain a call to
git update-server-info but
you need to manually enable the hook with
mv post-update.sample
post-update. This makes sure
git update-server-info keeps the
necessary files up-to-date.
3.Push into the public repository from your primary
repository.
4.git repack the public repository. This
establishes a big pack that contains the initial set of objects as the
baseline, and possibly git prune if the transport used for pulling from
your repository supports packed repositories.
5.Keep working in your primary repository. Your changes
include modifications of your own, patches you receive via e-mails, and merges
resulting from pulling the "public" repositories of your
"subsystem maintainers".
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 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.
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 because you can. An octopus is a valid thing
to do and often makes it easier to view the commit history if you are merging
more than two independent changes at the same time. However, if you have merge
conflicts with any of the branches you are merging in and need to hand
resolve, that is an indication that the development happened in those branches
were not independent after all, and you should merge two at a time,
documenting how you resolved the conflicts, and the reason why you preferred
changes made in one side over the other. Otherwise it would make the project
history harder to follow, not easier.
SEE ALSO¶
gittutorial(7),
gittutorial-2(7),
gitcvs-migration(7),
git-help(1),
giteveryday(7),
The Git User’s
Manual[1]
GIT¶
Part of the
git(1) suite.
NOTES¶
- 1.
- the Git User Manual
- 2.
- Randy Dunlap’s presentation
