Git Internals
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1. $9
Git
Internals
Source code control and beyond
by Scott Chacon
2. Git Internalsother peepcode products
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2
3. contents
4
54
Using Git
111
Commands Overview
About this book54 Setting Up Your Profile111 Basic Git
Installing Git55 Getting a Git Repository115 Inspecting Repositories
5Installing on Linux57 Normal Workflow Examples117 Extra Tools
6Installing on Mac63 Log – the Commit History7Windows65 Browsing Git119 Web Documentation
8A Short History of Git71 Searching Git120 Screencasts
Understanding Git72 Git Diff
5
10
10 What is Git?75 Branching
1177 Simple Merging
Focus and Design
13 Git Object Types80 Rebasing
22 The Git Data Model86 Stashing
28 Branching and Merging89 Tagging
41 The Treeish91 Exporting Git
44 The Git Directory92 The Care and Feeding of Git
48 Working Directory94 Distributed Workflow Examples
49 The Index105 Sharing Repositories
49 Non-SCM Uses of Git107 Hosted Repositories
119
References and Endnotes
4. About this book
It took me a pretty long time to really get Git. As I’ve continued to
use Git more and more where I work, I’ve found myself trying to
teach people what it is and why we use it over and over again, and
the reality is that Git generally has a pretty steep learning curve
compared to many other systems. I’ve seen case after case of devel-
opers who love Git after they finally understand it, but getting to that
point is often somewhat painstaking.
This book is aimed at the developer who does not particularly like
Subversion, Perforce or whatever SCM system they are currently
using, has heard good things about Git, but doesn’t know where to
start or why it’s so wonderful. It is meant to explain Git as simply as
possible in a clean, concise, easily readable volume. My goal is to
help you understand Git internals as well as usage at a fundamental
level by the time you finish this book.
To accomplish this, I’m starting the book out (after the introduction)
with a section about what Git actually does, rather than how to use
it. I found that I didn’t really understand Git and had many problems
using it until I understood what it was actually doing at a low level,
rather than thinking of it as a different, weird SVN-like system.
4
5. Installing Git
chapter 1
Before we can start playing with Git, we’ll have to install it. I’ll quickly
cover installing Git on Linux, Mac and Windows. I will not get into
really fine detail, because others have done that much better, but I
will give you an overview and links as to where to find more detailed
instructions on each platform.
For any of these examples, you can find a link to the most current Git
source code at git.or.cz (http://git.or.cz).
I would recommend compiling from source if possible, simply
because Git is lately making big strides in usability, so more current
versions may be a bit easier to use.
Installing on Linux
If you are installing from source, it will go something like the stan-
dard:
$ wget http://kernel.org/pub/software/scm/git/git-
1.5.4.4.tar.bz2
$ tar jxpvf git-1.5.4.4.tar.bz2
$ cd git-1.5.4.4
$ make prefix=/usr all doc info
$ sudo make prefix=/usr install install-doc install-info
If you are running Ubuntu or another Debian based system, you can
run
$ apt-get git-core
5
6. or on yum based systems, you can often run:
$ yum install git-core
Installing on Mac
You are likely going to want to install Git without the asciidoc depen-
dency because it is a pain to install. Other than that, what you
basically need is Curl and Expat. With the exception of the Leopard
binary OS X installer, you will also need the Developer Tools installed.
If you don’t have the OS X install discs anymore, you can get the
tools from the Apple Website (http://developer.apple.com/tools)
Mac 10.4 – Tiger
There are some requirements you’ll have to install before you can
compile Git. Expat can be installed roughly like this:
curl -O http://surfnet.dl.sourceforge.net/sourceforge/expat/
expat-2.0.1.tar.gz
tar zxvf expat-2.0.1.tar.gz
cd expat-2.0.1
./configure --prefix=/usr/local
make
make check
sudo make install
cd ..
Then download and compile Git as per the Linux instructions.
However, if you want an easier path, you can use the excellent Mac-
Ports software. To install MacPorts, simply follow the instructions on
the MacPorts Homepage (http://www.macports.org), and then just run :
6
7. $ sudo port install git-core
For an in depth tutorial on installing on 10.4, see this article (http://blog.
kineticweb.com/articles/2007/08/26/compiling-git-for-mac-os-x-10-4-10-intel)
If you hope to use Git with an existing Subversion repository, pass
the +svn flag to port. This will build Subversion and the necessary
Perl interfaces for Subversion as well.
Mac 10.5 – Leopard
The easiest way to install is most likely the “Git OSX Installer”:http://
code.google.com/p/git-osx-installer/, which you can get from Google
Code, and has just recently been linked to as the official Mac ver-
sion on the Git homepage. Just download and run the DMG from the
website.
If you want to compile from source, all the requirements are installed
with the developer CD. You can just download the Git source and
compile pretty easily if the developer tools are installed.
Finally, MacPorts is also an easy option if you have that installed.
For an in-depth tutorial on installations under Leopard, see this
article (http://blog.kineticweb.com/articles/2007/10/30/compiling-git-for-mac-os-x-
leopard-10-5)
Windows
There are two options on Windows currently, but the popular one is
“MSysGit”:http://code.google.com/p/msysgit/, which installs easily
7
8. and can be run on the Windows command line. Simply download the
exe file from the “downloads list”:http://code.google.com/p/msysgit/
downloads/list, execute it and follow the on-screen instructions.
A Short History of Git
The Git project started with Linus Torvalds scratching the very serious
itch of needing a fast, efficient and massively distributed source code
management system for Linux kernel development.
The kernel team had moved from a patch emailing system to the
proprietary BitKeeper SCM in 2002. That ended in April 2005 when
BitMover stopped providing a free version of its tool to the open
source community because they felt some developers had reverse
engineered it in violation of the license.
Since Linus had (and still has) a passionate dislike of just about all
existing source code management systems, he decided to write his
own. Thus, in April of 2005, Git was born. A few months later, in July,
maintenance was turned over to Junio Hamano, who has maintained
the project ever since.
“I’m an egotistical bastard, and I name all my projects after myself.
First Linux, now git.” – Linus
Git started out as a collection of lower level functions used in various
combinations by shell and perl scripts. Recently (since 1.0), more and
more of the scripts have been re-written in C (referred to as built-ins),
increasing portability and speed.
Though originally used for just the Linux kernel, the Git project
spread rapidly, and quickly became used to manage a number of
other Linux projects, such as the X.org, Mesa3D, Wine, Fedora and
8
9. Samba projects. Recently it has begun to spread outside the Linux
world to manage projects such as Rubinius, Merb, Ruby on Rails, Nu,
Io and many more major open source projects.
9
10. Understanding Git
chapter 2
In this section, we will go over what Git was built for and how it works,
hopefully laying the groundwork to properly understand what it is
doing when we run the commands.
The first commit message for the Git project was ‘initial version of
“git”, the information manager from hell’ – Linus, 4/7/05
When I learned Git, as many people do, I learned it in the context of
other SCMs I had used – Subversion or CVS. I have come to believe
that this is a horrible way to learn Git. I felt far more comfortable with
it when I stopped thinking that ‘git add’ was sort of like ‘svn add’, but
instead understood what it was actually doing. Then I found I could
find new and interesting ways to use what is really a very powerful
and cool toolset.
Git
Subversion Magic!
So, let’s see what it’s doing behind the scenes first.
What is Git?
Git is a stupid content tracker. That is probably the best description
of it – don’t think of it in a ‘like (insert favorite SCM system), but…’
context, but more like a really interesting file system.
Git tracks content – files and directories. It is at its heart a collection
of simple tools that implement a tree history storage and directory
10
11. content management system. It is simply used as an SCM, not really
designed as one.
“In many ways you can just see git as a filesystem — it’s content-
addressable, and it has a notion of versioning, but I really really
designed it coming at the problem from the viewpoint of a
filesystem person (hey, kernels is what I do), and I actually have
absolutely zero interest in creating a traditional SCM system.” –
Linus (http://marc.info/?l=linux-kernel&m=111314792424707)
When most SCMs store a new version of a project, they store the
code delta or diff. When Git stores a new version of a project, it stores
a new tree – a bunch of blobs of content and a collection of point-
ers that can be expanded back out into a full directory of files and
subdirectories. If you want a diff between two versions, it doesn’t add
up all the deltas, it simply looks at the two trees and runs a new diff
on them.
This is what fundamentally allows the system to be easily distributed
– it doesn’t have issues figuring out how to apply a complex series of
deltas, it simply transfers all the directories and content that one user
has and another does not have but is requesting. It is efficient about
it – it only stores identical files and directories once and it can com-
press and transfer its content using delta-compressed packfiles – but
in concept, it is a very simple beast. Git is at it’s heart very stupid-
simple.
Focus and Design
There are a number of areas that the developers of Git, including
and especially Linus, have focused on in conceiving and building Git.
There may be a lot of things that Git is not good at, but these things
are what Git is very good at.
11
12. Non-Linear Development
Git is optimized for cheap and efficient branching and merging. It is
built to be worked on simultaneously by many people, having mul-
tiple branches developed by individual developers, being merged,
branched and re-merged constantly. Because of this, branching is
incredibly cheap and merging is incredibly easy.
Distributed Development
Git is built to make distributed development simple. No repository
is special or central in Git – each clone is basically equal and could
generally replace any other one at any time. It works completely
offline or with hundreds of remote repositories that can push to and/
or fetch from each other over several simple and standard protocols.
Efficiency
Git is very efficient. Compared to many popular SCM systems, it
seems downright unbelievably fast. Most operations are local, which
reduces unnecessary network overhead. Repositories are generally
packed very efficiently, which often leads to surprisingly small repo
sizes.
The Ruby on Rails Git repository download, which includes the
full history of the project – every version of every file, weighs in at
around 13M, which is not even twice the size of a single checkout of
the project (~9M). The Subversion server repository for the same
project is about 115M.
Git also is efficient in its network operations – the common Git trans-
fer protocols transfer only packed versions of only the objects that
have changed. It also won’t try to transfer content twice, so if you
12
13. have the same file under two different names, it will only transfer the
content once.
A Toolkit Design
Git is not really a single binary, but a collection of dozens of small
specialized programs, which is sometimes annoying to people trying
to learn Git, but is pretty cool when you want to do anything non-
standard with it. Git is less a program and more a toolkit that can be
combined and chained to do new and interesting things.
For a long time, Git was just the raw toolkit and the project to wrap
those into a user friendly SCM was called Cogito. That project has
since been abandoned as Git itself became easier to use.
The tools can be more or less divided into two major camps, often
referred to as the porcelain and the plumbing. The plumbing is not
really meant to be used by people on the command line, but rather
to do simple things flexibly and are combined by programs and
scripts into porcelain programs. The porcelain programs are largely
what we will be focusing on in this book—the user-oriented interfaces
to do SCM type things—hiding the low-level fun.
Git Object Types
Git objects are the actual data of Git, the main thing that the reposi-
tory is made up of. There are four main object types in Git, the first
three being the most important to really understand the main func-
tions of Git.
All of these types of objects are stored in the Git Object Database,
which is kept in the Git Directory. Each object is compressed (with
13
14. Zlib) and referenced by the SHA-1 value of its contents plus a small
header. In the examples, I will use the first 6 characters of the SHA-1
for simplicity, but the actual value is 40 characters long.
SHA stands for Secure Hash Algorithm. A SHA creates an identifier
of fixed length that uniquely identifies a specific piece of content.
SHA-1 succeeded SHA-0 and is the most commonly used
algorithm. Wikipedia (http://en.wikipedia.org/wiki/SHA1) has more on the
topic.
To demonstrate these examples, we will develop a small ruby library
that provides very simple bindings to Git, keeping the project in a Git
repository. The basic layout of the project is this:
Working Directory
./
Rakefile
README
lib/
simplegit.rb
Fig. A Sample project with files and directories
Let’s take a look at what Git does when this is committed to a reposi-
tory.
14
15. The Blob
In Git, the contents of files are stored as blobs.
Working Directory
Git Directory
./
Rakefileblob : a874b7
READMEblob : a906cb
lib/
simplegit.rb
blob : a0a60a
Fig. B Files are stored as blobs
It is important to note that it is the contents that are stored, not the
files. The names and modes of the files are not stored with the blob,
just the contents.
This means that if you have two files anywhere in your project that
are exactly the same, even if they have different names, Git will only
store the blob once. This also means that during repository transfers,
such as clones or fetches, Git will only transfer the blob once, then
expand it out into multiple files upon checkout.
15
16.
SimpleGit Ruby Library
======================
blob : a906cb
Zlib::Deflate
This library calls git commands and
returns the output.
Author : Scott Chacon
Fig. C The contents of a blob, uncompressed
16
17. The Tree
Directories in Git basically correspond to trees.
Working Directory
./
Git Directory
tree : 1a738d
Rakefileblob : a874b7
READMEblob : a906cb
lib/tree : fe8971
simplegit.rb
blob : a0a60a
Fig. D Trees are pointers to blobs and other trees
A tree is a simple list of trees and blobs that the tree contains, along
with the names and modes of those trees and blobs. The contents
section of a tree object consists of a very simple text file that lists the
mode, type, name and sha of each entry.
100644 blob a906cb
100644 blob a874b7
040000 tree fe8971
README
Rakefile
lib
tree : 1a738d
Zlib::Deflate
Fig. E An uncompressed tree
17
18. The Commit
So, now that we can store arbitrary trees of content in Git, where
does the ‘history’ part of ‘tree history storage system’ come in? The
answer is the commit object.
Working Directory
./
Git Directory
tree : 1a738d
Rakefileblob : a874b7
READMEblob : a906cb
lib/tree : fe8971
simplegit.rb
blob : a0a60a
commit : a11bef
Fig. F A commit references a tree
The commit is very simple, much like the tree. It simply points to a
tree and keeps an author, committer, message and any parent com-
mits that directly preceded it.
18
19. commit : a11bef
tree 1a738d
author Scott Chacon
<schacon@gmail.com> 1205602288
committer Scott Chacon
<schacon@gmail.com> 1205602288
first commit
Fig. G Uncompressed initial commit
Since this was my first commit, there are no parents. If I commit a
second time, the commit object will look more like this:
commit : a7d991
tree e1b3ec
parent a11bef
author Scott Chacon
<schacon@gmail.com> 1205624433
committer Scott Chacon
<schacon@gmail.com> 1205624433
my second commit, which is better than the first
Fig. H A commit with a parent
Notice how the parent in that commit is the same SHA-1 value of
the last commit we did? Most times a commit will only have a single
parent like that, but if you merge two branches, the next commit will
point to both of them.
19
20. The current record for number of commit parents in the Linux kernel
is 12 branches merged in a single commit!
20
21. The Tag
The final type of object you will find in a Git database is the tag. This
is an object that provides a permanent shorthand name for a par-
ticular commit. It contains an object, type, tag, tagger and a mes-
sage. Normally the type is commit and the object is the SHA-1 of the
commit you’re tagging. The tag can also be GPG signed, providing
cryptographic integrity to a release or version.
tag : 0c819c
object 0576fa
type commit
tag v0.1
tagger Scott Chacon
<schacon@gmail.com> 1205624655
this is my v0.1 tag
Fig. I Uncompressed tag
We’ll talk a little bit more about tags and how they differ from
branches (which also point to commits, but are not stored as objects)
in the next section, where we’ll pull all of this together into how all
these objects relate to each other conceptually.
21
22. The Git Data Model
In computer science speak, the Git object data is a directed acyclic
graph. That is, starting at any commit you can traverse its parents
in one direction and there is no chain that begins and ends with the
same object.
All commit objects point to a tree and optionally to previous com-
mits. All trees point to one or many blobs and/or trees. Given this
simple model, we can store and retrieve vast histories of complex
trees of arbitrarily changing content quickly and efficiently.
This section is meant to demonstrate how that model looks.
References
In addition to the Git objects, which are immutable – that is, they
cannot ever be changed, there are references also stored in Git.
Unlike the objects, references can constantly change. They are
simple pointers to a particular commit, something like a tag, but eas-
ily moveable.
Examples of references are branches and remotes. A branch in Git is
nothing more than a file in the .git/refs/heads/ directory that con-
tains the SHA-1 of the most recent commit of that branch. To branch
that line of development, all Git does is create a new file in that direc-
tory that points to the same SHA-1. As you continue to commit, one
of the branches will keep changing to point to the new commit SHA-
1s, while the other one can stay where it was. If this is confusing, don’t
worry. We’ll go over it again later.
22
23. The Model
The basic data model I’ve been explaining looks something like this:
HEAD
remote
branch
tag
commit
tree
blob
The cheap references I’ve represented as the grey boxes, the immu-
table objects are the colored round cornered boxes.
An Example
Lets look at an example of simple usage of Git and which objects are
stored in the Git object database as we go.
To begin with, we commit an initial tree of three files and two sub-
directories, each directory with one file in it. Possibly something like
this:
.
|-- init.rb
`-- lib
|-- base
|
`-- base_include.rb
23
24. `-- my_plugin.rb
When we first commit this tree, our Git model may look something
like this:
We have three trees, three blobs and a
single commit that points to the top of
the tree. The current branch points to our
last commit and the HEAD file points to
the branch we’re currently on. This lets
Git know which commit will be the parent
for the next commit.
HEAD
branch
commit
tree
treeblob
treeblob
blob
Now let’s assume that we change the
lib/base/base_include.rb file and com-
mit again. At this point, a new blob is
added, which changes the tree that
points to it, which changes the tree that
points to that tree and so on to the top of
the entire directory. Then a new commit
object is added which points to its parent
and the new tree, then the branch refer-
ence is moved forward.
Let’s also say at this point we tag this
commit as a release, which adds a new
tag object.
At this point, we’ll have the following in Git:
24
25. HEAD
tag
branch
commitcommit
treetree
treeblobtree
treeblobtree
blob
blob
Notice how the other two blobs that were not changed were not
added again. The new trees that were added point to the same blobs
in the data store that the previous trees pointed to.
Now let’s say we modify the init.rb file at the base of the project.
The new blob will have to be added, which will add a new top tree,
but all the subtrees will not be modified, so Git will reuse those refer-
ences. Again, the branch reference will move forward and the new
commit will point to its parent.
25
26. HEAD
tagbranch
commitcommitcommit
treetreetree
blob
treeblobtree
treeblobtree
blob
blob
At this point, let’s stop to look at the objects we now have in our
repository. From this, we can easily recreate any of the three directo-
ries we committed by following the graph from the most recent com-
mit object, and having Git expand the trees that are pointed to.
For instance, if we wanted the first tree, we could look for the parent
of the parent of the HEAD, or the parent of the tag. If we wanted the
second tree, we could ask for the commit pointed to by the tag, and
so on.
26
27. HEAD
tagbranch
commitcommitcommit
treetreetree
blob
treeblobtree
treeblobtree
blob
blob
So, to keep all the information and history on the three versions of
this tree, Git stores 16 immutable, signed, compressed objects.
Traversal
So, what do all the arrows in these illustrations really mean? How
does Git actually retrieve these objects in practice? Well, it gets the
initial SHA-1 of the starting commit object by looking in the .git/
refs directory for the branch, tag or remote you specify. Then it tra-
verses the objects by walking the trees one by one, checking out the
27
28. blobs under the names listed.
1$ git checkout v0.1
2.git/refs/tags/v0.1
3tag : 0c819c
"0c819c"
commit : a11bef
tree : 1a738d
Rakefileblob : a874b7
READMEblob : a906cb
libtree : fe8971
simplegit.rb
blob : a0a60a
Branching and Merging
Here we come to one of the real strengths of Git, cheap inline
branching. This is a feature that truly sets it apart and will likely
change the way you think about developing code once you get used
to it.
28
29. When you are working on code in Git, storing trees in any state and
keeping pointers to them is very simple, as we’ve seen. In fact, in Git
the act of creating a new branch is simply writing a file in the .git/
refs/heads directory that has the SHA-1 of the last commit for that
branch.
Creating a branch is nothing more than just writing 40 characters to
a file.
Switching to that branch simply means having Git make your work-
ing directory look like the tree that SHA-1 points to and updating
the HEAD file so each commit from that point on moves that branch
pointer forward (in other words, it changes the 40 characters in .git/
refs/heads/[current_branch_name] be the SHA-1 of your last com-
mit).
29
30. Simple Case
branch
branch
}
commit
tree
Now, let’s see how Git handles
branching, fetching and merging
operations abstractly. For the
following illustrations, we will
represent the entire tree and the
commit it points to as a single
object.
C1
Suppose that we work on a project
for a while, then we get an idea for
something that may not work out,
blob
but we want to do a quick proof-
of-concept. We create a new
branch called experiment off of our main branch, which is by conven-
tion called master. We then switch to the new branch and create a
few commits.
master
C0
experiment
C1
C2
C3
Then, our boss comes in and says we need a hot fix to production.
So we switch back to our master branch, make the change, push the
release and then tag the new commit with the release number. Then
we go back to our experiment branch, continue working and commit
again.
30
31. T1
C0
C1
master
C4
C2
C3
C5
experiment
At this point, we show the new branch code to our co-workers and
everyone likes the new changes. We decide we want to merge them
back into our main branch, so we merge the changes and delete our
experiment branch.
Our history of commit objects now looks like this:
C0
C1
C2
C3
T1master
C4C6
C5
experiment
31
32. Remotes
Now lets take a look at remotes. Remotes are basically pointers to
branches in other peoples copies of the same repository, often on
other computers. If you got your repository by cloning it, rather than
initializing it, you should have a remote branch of where you copied
it from automatically added as origin by default. Which means the
tree that was checked out during your initial clone would be refer-
enced as origin/master, which means “the master branch of the
origin remote.”
Lets say you clone someone’s repository and make a few changes.
You would have two references, one to origin/master which points to
where the master branch was on the person’s repository you cloned
from when you did so, and a master branch that points the most
recent local commit.
origin/master
C0
master
C1
C2
Now let’s say you run a fetch. A fetch pulls all the refs and objects
that you don’t already have from the remote repository you specify.
By default, it is origin, but you can name your remotes anything, and
you can have more than one. Suppose we fetch from the repository
that we originally cloned from and they had been doing some work.
They have now committed a few times on their master branch, but
they also branched off at one point to try an idea, and they named
the branch idea locally, then pushed that branch. We now have
access to those changes as origin/idea.
32
33. origin/idea
CR2
C0
origin/master
CR1CR3
CL1CL2
CR4
master
We look at the idea branch and like where they’re going with it, but
we also want the changes they’ve made on their master branch, so
we do a 3-way merge of their two branches and our master. We don’t
know how well this is going to work, so we make a tryidea branch first
and then do the merge there.
33
34. origin/idea
origin/master
CR2
C0
CR1CR3
CR4
CL1CL2CL3
mastertryidea
Now we can run our tests and merge everything back into our master
branch if we want. Then we can tell our friend we cloned from to fetch
from our repository, where we’ve merged their two branches for them
and integrated some of our changes as well. They can choose to
accept or reject that patch.
Rebasing
Let’s say you and another developer, Jen, are working on the same
project simultaneously. She clones from you, and works for a while
and commits. You have committed in the meantime and want to get
your work in sync, so you add her repository as the remote jen, do
a fetch and merge her changes in, creating a new merge commit.
(All commits that are simply merges are given a darker color in this
example)
34
35.
R1
C0
C1
C2
At this point, you both do work and commit changes and then you
fetch and merge from her again. Then she does another commit and
you fetch and merge once more. At this point, you’ll have a commit
history that looks something like this:
jen/master
R1
C0
C1
R2
C2
R3
C3
C4
C5
master
Perfectly fine, but it can get a little confusing when you litter the
history with all those commits that do nothing but merge unshared
changes. The longer you keep out of sync, the worse this can get.
This is where the rebasing command comes in. With rebase, Git will
checkout the upstream branch, in this case, Jen’s master branch, and
35
36. then replay all the changes you’ve done since you forked on top of
those files, as if you had forked your work off at that point and done
all your changes, rather than earlier.
Rebase will literally produce a series of patch files of your work and
start applying them to the upstream branch, automatically making
new commits with the same messages as before and orphaning your
older ones. These objects can then be removed, since nothing points
to them, when you run the garbage collection tools (see “The Care
and Feeding of Git” section).
jen/master
R1
C2
C0
C0:C1
C1
master
So let’s see what happens if
we rebase rather than merge
in the same scenario. Here we
have our first merge and we
can see that it orphans Com-
mit 1 and applies the changes
between Commit 0 and
Commit 1 to the files in
Remote Commit 1, creating a
new Commit 2.
Then, as you’ll remember, you
and Jen both commit again. You’ll notice that now it looks like she
cloned you and committed and then you changed that code, rather
than you both working at the same time and merging.
36
37. jen/master
R1
R2
C2
C0
C3
C1
master
At this point, instead of merging two more times like we did originally,
we rebase the next two commits she makes.
jen/master
R1
R2
C0
R1:C2
C1
C2
C3
C2:C3
C4
C5
master
37
38. jen/master
R1
R2
R3
C0
C4:C5
R2:C4
C1
C2
C3
C4
C6
C7
C5
master
And finally, we are left with a commit history that looks like Figure 1,
rather than Figure 2, which is what we would have if we had merged
instead.
R1
1
R2
R3
C0
C6
R1
2
C0
C1
R2
C2
C7
R3
C3
C4
C5
You should remember to only do this on local branches before you
push or on repositories that nobody has fetch access to – if anyone
pulls down the objects that will become abandoned during a rebase,
it gets a bit frustrating.
38
39. Use Cases
So why is this helpful, exactly? It means that you can keep your
development cycles loosely coupled. Here is an example of a com-
mon workflow with cheap branches.
You have a master branch that is always stable – you never merge
anything into it that you wouldn’t put into production. Then you have
a development branch that you merge any experimental code into
before you imagine pulling it into the master branch.
It’s a common error to think of the master branch as being
equivalent to Subversion’s trunk. However, a custom development
branch is much closer in practice to the Subversion trunk, where
experimental work is done.
You create a new branch each time you begin to work on a story or
feature, branching it off your current development branch each time,
so if you get blocked and need to put it on hold, it doesn’t effect
anything else. When you do get back to them, you rebase them
to the current development and it is just like you started from there.
Often times you merge the branch back into development and delete
it the same day that you created it.
If you get a huge project or idea – say refactoring the entire code
base to the newest version of your framework or switching database
vendors or something, you create a long-term branch, continuously
rebase it to keep it in line with other development, and once every-
thing is tested and ready, merge it in with your master.
Working with others is unbelievably easy. You ask in an IRC room if
someone has implemented a feature in a library you are using. Turns
out that someone has and you are sent the URL of their public Git
repo for that project. You add it as a remote, fetch it, create a new
merge-feature branch off your development branch, merge in the new
39
40. changes and you’re done. There’s no awkward emailing of patches –
you can just add contributors as a remote and try out their branches
before deciding to merge them in. If it breaks things or is not a good
patch, you simply delete the merge-feature branch and that’s it.
Fig. J Project collaboration using multiple branch forks and merges
A common problem with open source projects managed with
Subversion is that a patch is sent which was made against and
older version of the code base. With Git, it’s as easy as applying
the outdated patch in a new branch, then rebasing from master to
bring it up to date with the current code base.
You branch and rebase or merge several times a day in and out of
several different branches, some of which last for hours and some are
continually there. Once you get used to this pattern, it completely
changes the way you approach your development and the way you
40
41. contribute and collaborate.
The Treeish
Besides branch heads, there are a number of shorthand ways to
refer to particular objects in the Git data store. These are often
referred to as a treeish. Any Git command that takes an object – be
it a commit, tree or blob – as an argument can take one of these
shorthand versions as well.
I will list here the most common, but please read the rev-parse com-
mand (http://www.kernel.org/pub/software/scm/git/docs/git-rev-parse.html) for full
descriptions of all the available syntaxes.
full sha-1
dae86e1950b1277e545cee180551750029cfe735
You can always list out the entire SHA-1 value of the object to refer-
ence it. This is sometimes easy if you’re copying and pasting values
from a tree listing or some other command.
partial sha-1
dae86e
Just about anything you can reference with the full SHA-1 can be
referenced fine with the first 6 or 7 characters. Even though the SHA-1
is always 40 characters long, it’s very uncommon for more than the
first few to actually be the same. Git is smart enough to figure out a
partial SHA-1 as long as it’s unique.
41
42. branch or tag name
master
Anything in .git/refs/heads or .git/refs/tags can be used to refer to
the commit it points to.
date spec
master@{yesterday}
master@{1 month ago}
This example would refer to the value of that branch yesterday.
Importantly, this is the value of that branch in your repository yester-
day. This value is relative to your repo – your ‘master@{yesterday}’
will likely be different than someone else’s, even on the same project,
whereas the SHA-1 values will always point to the same commit in
every copy of the repository.
ordinal spec
master@{5}
This indicates the 5th prior value of the master branch. Like the Date
Spec, this depends on special files in the .git/log directory that are
written during commits, and is specific to your repository.
carrot parent
e65s46^2
master^2
This refers to the Nth parent of that commit. This is only really help-
ful for commits that merged two or more commits – it is how you
42
43. can refer to a commit other than the first parent.
tilde spec
e65s46~5
The tilde character, followed by a number, refers to the Nth genera-
tion grandparent of that commit. To clarify from the carrot, this is the
equivalent commit in caret syntax:
e65s46^^^^^
master^^2^
master^2
R1
C0C1
master^^^^^master~4
R2
C2
R3
C3
C4C5
master^master
tree pointer
e65s46^{tree}
This points to the tree of that commit. Any time you add a ^{tree}
to any commit-ish, it resolves to its tree.
blob spec
master:/path/to/file
43
44. This is very helpful for referring to a blob under a particular commit
or tree.
The Git Directory
When you initialize a Git repository, either by cloning an existing
one or creating a new one, the first thing Git does is create a Git
directory. This is the directory that stores all the object data, tags,
branches, hooks and more. Everything that Git permanently stores
goes in this single directory. When you clone someone else’s reposi-
tory, it basically just copies the contents of this directory to your
computer. Without a checkout (called a working directory) this is
called a bare Git repo and moving it to another computer backs up
your entire project history. It is the soul of Git.
When you run git init to initialize your repository, the Git directory
is by default installed in the directory you are currently in as .git.
This can be overridden with the GIT_DIR environment variable at any
time. In fact, the Git directory does not need to be in your source
tree at all. It’s perfectly acceptable to keep all your Git directories in
a central place (ex: /opt/git/myproject.git) and just make sure to
set the GIT_DIR variable when you switch projects you are working on
(ex: /home/username/projects/myproject).
The Git directory for our little project looks something like this:
.
|-- HEAD
|-- config
|-- hooks
|
|-- post-commit
|
`-- pre-commit
|-- index
|-- info
44
45. |
`-- exclude
|-- objects
|
|-- 05
|
|
`-- 76fac355dd17e39fd2671b010e36299f713b4d
|
|-- 0c
|
|
`-- 819c497e4eca8e08422e61adec781cc91d125d
|
|-- 1a
|
|
`-- 738da87a85f2b1c49c1421041cf41d1d90d434
|
|-- 47
|
|
`-- c6340d6459e05787f644c2447d2595f5d3a54b
|
|-- 99
|
|
`-- f1a6d12cb4b6f19c8655fca46c3ecf317074e0
|
|-- a0
|
|
`-- a60ae62dd2244a68d78151331067c5fb5d6b3e
|
|-- a1
|
|
`-- 1bef06a3f659402fe7563abf99ad00de2209e6
|
|-- a8
|
|
`-- 74b732e12a5c04b5a73d7f1123c249997b0b2d
|
|-- a9
|
|
`-- 06cb2a4a904a152e80877d4088654daad0c859
|
|-- e1
|
|
`-- b3ececb0cbaf2320ca3eebb8aa2beb1bb45c66
|
|-- fe
|
|
`-- 897108953cc224f417551031beacc396b11fb0
|
|-- info
|
`-- pack
`-- refs
|-- heads
|
`-- master
`-- tags
`-- v0.1
For more in-depth information on the Git directory layout, see the
git repository layout docs. (http://www.kernel.org/pub/software/scm/git/docs/
repository-layout.html)
For now, let’s go over some of the more important contents of this
directory.
45
46. .git/config
This is the main Git configuration file. It keeps your project specific
Git options, such as your remotes, push configurations, tracking
branches and more. Your configuration will be loaded first from this
file, then from a ~/.gitconfig file and then from an /etc/gitconfig
file, if they exist.
Here is an example of what a config file might look like:
[core]
repositoryformatversion = 0
filemode = true
bare = false
logallrefupdates = true
[remote “origin”]
url = git@github.com:username/myproject.git
fetch = +refs/heads/*:refs/remotes/origin/*
[branch “master”]
remote = origin
merge = refs/heads/master
See the git-config (http://www.kernel.org/pub/software/scm/git/docs/git-config.
html) docs for more information on available configuration options.
.git/index
This is the default location of the index file for your Git project. This
location can be overridden with the GIT_INDEX environment variable,
which is sometimes useful for temporary tree operations. See the
chapter on the “Git Index” for more information on what this file is
used for.
.git/objects/
46
47. This is the main directory that holds the data of your Git objects –
that is, all the contents of the files you have ever checked in, plus
your commit, tree and tag objects.
The files are stored by their SHA-1 values. The first two characters
make up the subdirectory and the last 38 is the filename. For exam-
ple, if the SHA-1 for a blob we’ve checked in was
a576fac355dd17e39fd2671b010e36299f713b4d
the file we would find the Zlib compressed contents in is:
[GIT_DIR]/objects/a5/76fac355dd17e39fd2671b010e36299f713b4d
.git/refs/
This directory normally has three subdirectories in it – heads,
remotes and tags. Each of these directories will hold files that cor-
respond to your local branches, remote branches and tags, respec-
tively.
For example, if you create a development branch, the file .git/refs/
heads/development will be created and will contain the SHA-1 of the
commit that is the latest commit of that branch.
.git/HEAD
This file holds a reference to the branch you are currently on. This
basically tells Git what to use as the parent of your next commit. The
contents of it will generally look like this:
ref: refs/heads/master
47
48. .git/hooks
This directory contains shell scripts that are invoked after the Git
commands they are named after. For example, after you run a com-
mit, Git will try to execute the post-commit script, if it has executable
permissions.
See the online hooks documentation (http://www.kernel.org/pub/software/
scm/git/docs/hooks.html) for more information on what you can do with
hooks.
Working Directory
The working directory is the checkout of the current branch you are
working on. What is really important to note here is that this code is
a working copy – it is not really important.
This is something that developers from most other SCMs have a
hard time understanding and tends to scare them mightily. If you
check out a different branch, Git will make your working directory
look like that branch, removing any checked in content that is cur-
rently in your working directory that is not in the new tree. This is why
Git will only let you checkout another branch if everything is checked
in – there are no uncommitted modified files. The reason for this is
that Git will remove files that are not necessary in the branch you are
checking out – it needs to make sure that you can get them back
again.
Most users don’t like to see content automatically removed from their
directories, but that’s one of the mental shifts you’ll need to make.
Your working directory is temporary – everything is stored perma-
48
49. nently in your Git repository. Your working directory is just a copy of
a tree so you can edit it and commit changes.
The Index
Git has two places that content states are stored. The working direc-
tory stores one state at a time in a human-editable format. When
committed, that state becomes permanent and repeatable by being
stored in the object database. However, how do you determine what
changes in your working directory will go into your next commit? Per-
haps you have edited three files and you want two to go into the first
commit (because they are related changes) and the other file into a
second commit. This is where the index file comes in.
The index was called the cache for a while, because that’s largely
what it does. It is a staging area for changes that are made to files or
trees that are not committed to your repository yet. It acts as sort of
a middle ground between your working directory and your repository.
When you run git commit, the resulting tree and commit object will
be built based on the contents of the index.
Early on, the index was called the cache or the current directory
cache.
It is also used to speed up some operations. It keeps track of the
state of all the files in your working directory so you can quickly see
what has been modified since the last commit.
Non-SCM Uses of Git
I keep saying that Git is primarily a content tracking toolkit with SCM
tools built on top of it. So, if it’s not built specifically to be an SCM,
49
50. perhaps it would be useful to see some other examples of things it
might be good for.
This is a simple listing of some other tools that have been built on Git
internals to demonstrate that an SCM is the bundled application, but
Git can also be a useful toolkit for any application needing to track
and manage slowly changing distributed trees of content.
Peer to Peer Content Distribution Network
Imagine you are a retail chain or university campus and have a
network of digital signage displays that play flash content advertise-
ments or show campus news, etc. You have to get new content out
to them every day or two, which may consist of any combination of
XML files, images, animations, text and sound.
You need to build a content distribution framework that will easily
and efficiently transfer all the necessary content to the machines on
your network. You need to constantly determine what content each
machine has and what it needs to have and transfer the difference
as efficiently as possible, because networking to these units may be
spotty.
It turns out that Git is an excellent solution to this problem. You can
simply check all the needed content into Git, create a branch for
each unit and point that branch to the exact subtree of content it
needs. Then at some interval, you have the unit fetch its branch. If
nothing has changed, nothing happens – if content has changed
somehow, it gets only the files it does not already have in a delta
compressed package and then expands them locally. Log and status
files could even be transferred back by a push.
An example of a media company actually using this approach is
Reactrix (http://reactrix.com), which also happens to be where I work.
50
51. Somewhat interestingly, Git being a good solution to this problem
is what exposed me to the tool in the first place. We were using Git
for content distribution on our network since the 1.0 release back in
2005, but using Perforce for version control internally. It wasn’t until
nearly a year later that we switched to actually using it to manage
our source code.
Distributed Document Oriented Database
Using Git as a backend for a document oriented database could
have some interesting applications. Git provides a number of features
such as replication, searching with grep, and full versioning history for
free.
distributed wiki
Suppose we wanted to have a wiki for documentation on a project.
If we create a wiki that works on files, we can simply write those files
into a Git repository and run a commit after every change. This gives
us good performance, since it’s just reading the content off the disk.
We also get full file version history and easy ‘recently changed’ data.
Searching is built in and we can edit the wiki offline.
The other cool feature we could use is the distributed nature of Git.
We could add other people on the project as remote repositories and
push to and fetch from them, merging changes to write a book or
documentation collaboratively. We could branch to try out a re-write
and then either merge it in or throw it away if we don’t like it. We
could send a pull request to our colleagues for them to try out the
branch to preview it before we decide whether to merge it in or not.
It’s possible the entire wiki project could even live in a bare branch
(that is, a branch with no common ancestors with any existing
branch) in the same repository as our project, so clones can get the
51
52. documentation as well, without it muddying up our code tree.
See the git-wiki (http://github.com/al3x/git-wiki/tree/master) project for an
example of this.
distributed issue tracker
Another similar project might be a distributed ticketing system, where
all the tickets (bugs and features) for a project could be stored in a
Git repository, worked on offline and transferred with a project.
Examples of projects trying to do this are Ditz (http://ditz.rubyforge.org),
Kipling (http://gitorious.org/projects/kipling) and my own TicGit (http://github.
com/schacon/ticgit/wikis).
Backup Tool
Let’s suppose that you want to build something like a distributed
Time Machine™ backup system (Apple all rights reserved) that
efficiently packs up its backups and transfers them to multiple
machines. I’m hoping by now that you could see the benefits of
using the Git toolkit to accomplish this, but this particular problem is
interesting because of something that Git doesn’t do, which is per-
missions. Git stores the mode of its content in the tree, but it doesn’t
store any permissions data, which means it’s not good for backing
up directories in which permissions are important. A good example
would be the /etc directory on a Unix machine.
One project that has tackled this is Gibak (http://eigenclass.org/hiki/gibak-
backup-system-introduction). It implements a metastore in OCaml and it’s
worth a look if this topic interests you.
If you are interested in using Git in a non-standard way and
plan to use Ruby, you might be interested in my git gem (http://
52
53. jointheconversation.org/rubygit), which provides an object oriented
interface in Ruby to the Git tools, including several of the lower level
functions.
53
54. Using Git
chapter 3
Now that you hopefully understand what Git is designed to do at a
fundamental level – how it tracks and stores content, how it stores
branches and merges and tracks remote copies of the repository,
let’s see how to actually use it. This next section presents some of
the basic commands that you will need to know in order to use Git
effectively.
At the end of each chapter, there will be a link to the official Git doc-
umentation for each of the commands used in that section, in case
you want to learn more or see all the options for that command.
Setting Up Your Profile
For every commit you do, Git will try to associate a name and email
address. One of the first things you’ll want to do in Git is to set these
values. You can set them as global configuration values with the git
config command:
$ git config --global user.name “Scott Chacon”
$ git config --global user.email “schacon@gmail.com”
This will create a new ~/.gitconfig file that will look like this:
$ cat ~/.gitconfig
[user]
name = Scott Chacon
email = schacon@gmail.com
You can change those variables at any time either by editing that
setup, init and cloning screencast
I have produced a series of short screencasts
demonstrating the topics of several of the
chapters in this book so you can see them in
practice on a real command line. You should be
able to download these movies with this book
from the PeepCode website.
The first in this series is Git Setup, Initializa-
tion and Cloning and shows you how to setup
your Git configuration, how to initialize a new
repository and how to clone an existing reposi-
tory over both the Git transport and the HTTP
transport.
54
55. file, or running the git config commands again.
If you want to set different values for a specific project, just leave out
the —global and it will write the same snippet into your .git/config
file in that repository, which will overwrite your global values.
• git config (http://www.kernel.org/pub/software/scm/git/docs/git-config.html)
Getting a Git Repository
There are two major ways you will get a Git repository – you will
either clone an existing project, or you will initialize a new one.
New Repositories
To create a new Git repository somewhere, simply go to the directory
you want to add version control to and type:
git init
This will create a .git directory in your current working directory that
is entirely empty. If you have existing files you want to add to your
new repository, type:
git add .
git commit -m ‘my first commit’
This will add all of your current files into your new repository and
index and then create your first commit object, pointing your new
master branch to it. Congratulations, you have now added your
source code to Git.
55
56. • git init (http://www.kernel.org/pub/software/scm/git/docs/git-init.html)
• git commit (http://www.kernel.org/pub/software/scm/git/docs/git-commit.html)
• git add (http://www.kernel.org/pub/software/scm/git/docs/git-add.html)
Cloning a Repository
Many times you will be cloning a repository, however. This means
that you are creating a complete copy of another repo, including all
of its history and published branches.
A clone is, for all intents and purposes, a full backup. If the server
that you cloned from has a hard disk failure or something equally
catastrophic, you can basically take any of the clones and stick it
back up there when the server is restored without anyone really the
worse for wear.
In order to do this, you simply need a URL that has a Git repository
hosted there, which can be over HTTP, HTTPS, SSH or the special git
protocol. We will use the public hosted repository of the simple library
I mentioned at the beginning of the book.
git clone git://github.com/schacon/simplegit.git
This will by default create a new directory called simplegit and do
an initial checkout of the master branch. If you want to put it in a
different directory than the name of the project, you can specify that
on the command line, too.
git clone git://github.com/schacon/simplegit.git my_directory
• git clone (http://www.kernel.org/pub/software/scm/git/docs/git-clone.html)
56
57. Normal Workflow Examples
Now that we have our repository, let’s go through some normal work-
flow examples of a single person developing.
Ignoring
First off, we will often want Git to automatically ignore certain files –
often ones that are automatically generated during our development.
For example, in Rails development we often want to ignore the log
files, the production specific configuration files, etc. To do this, we can
add patterns into the .gitignore file to tell Git that we don’t want it to
track them.
normal workflow screencast
The second screencast we’ve provided is Nor-
mal Workflow in Git, which demonstrates how
to setup your .gitignore file, how to use and
interpret git status output, how to add and
remove files from your index, how to commit
and what git does in the object database dur-
ing these operations.
Here is an example .gitignore file.
tmp/*
log/*
config/database.yml
config/environments/production.rb
• .gitignore (http://www.kernel.org/pub/software/scm/git/docs/gitignore.html)
Adding and Committing
Now we’ll do some development and periodically commit our
changes. We have a few options here – we can commit individual
files or we can tell the commit command to automatically add all
modified files in our working directory to the index, then commit it.
A good way to find out what you’re about to commit (that is, what is
in your index) is to use the status command.
$ git status
57
58. # On branch master
# Changed but not updated:
#
(use “git add <file>...” to update what will be
committed)
#
#
modified:
README
#
modified:
Rakefile
#
modified:
lib/simplegit.rb
#
no changes added to commit (use “git add” and/or “git commit
-a”)
In this example, I can see that I’ve modified three files in my working
directory, but none of them have been added to the index yet – they
are not staged and ready to be committed. If I want to make these
changes in two separate commits, or I have completed work on some
of them and would like to push that out, I can specify which files to
add individually and then commit.
$ git add Rakefile
$ git status
# On branch master
# Changes to be committed:
#
(use “git reset HEAD <file>...” to unstage)
#
#
modified:
Rakefile
#
# Changed but not updated:
#
(use “git add <file>...” to update what will be
committed)
#
#
modified:
README
#
modified:
lib/simplegit.rb
#
You can see that if we commit at this point, only the Rakefile will
show up as changed in the commit.
58
59. working directory
git add
index
git commit
repository
If we want to commit all our changes, we can use this shorthand,
which will automatically run a git add on every modified file to our
index, then commit the whole thing:
$ git commit -a -m ‘committing all changes’
working directory
index
git commit -a
repository
If you would like to give a more useful commit message, you can
leave out the -m option. That will fire up your $EDITOR to add your
commit message.
Give special care to the first line of your commit message – it will
often be the only thing people see when they are looking through
your commit history.
Now we can continue this loop – modifying, adding and committing
– during our development.
• git status (http://www.kernel.org/pub/software/scm/git/docs/git-status.html)
59
60. Interactive Adding
interactive add screencast
Although that will work for all of your development needs – many
developers simply use -a nearly every time they commit to just
automatically add everything to the index – there is another way of
adding files that makes for a more controlled and thematic set of
commits. This is called interactive adding, and it is a very powerful
tool to controlling what goes into each commit.
The next screencast is Interactive Add in
Git, which demonstrates how to use the git
add—interactive command. It covers all of the
major features of interactive adding, including
status, update, revert, add untracked, patch
and diff.
Let’s say we add a new function to our lib/simplegit.rb file, add a
new task to our Rakefile and then add a new TODO file to our proj-
ect. Later we come back and want to commit, but we don’t remem-
ber which files had to do with each other and we don’t just want to
commit them all together because that’s confusing for collaborators
trying to review our code. Interactive mode lets us modify our index
interactively before committing. To fire it up, type git add -i:
$ git add -i
staged
1:
unchanged
2:
unchanged
*** Commands ***
1: status
untracked
5: patch
What now>
unstaged path
+5/-0 Rakefile
+4/-0 lib/simplegit.rb
2: update3: revert4: add
6: diff7: quit8: help
We can see that we have two files that are being tracked (have been
added at some point in the past) that have been modified. We can-
not yet see our new TODO file, though. To add that, type 4 for the add
untracked option and hit Enter.
What now> 4
1: TODO
Add untracked>> 1
* 1: TODO
60
61. Add untracked>>
added one path
*** Commands ***
1: status
untracked
5: patch
What now>
2: update3: revert4: add
6: diff7: quit8: help
You will see all the untracked files in your working directory. Type the
numbers of the files you want to add, or a range (i.e.: 1-5), and hit
enter twice when you’re done. This will drop you back to the main
menu. You can then type 1 to see what your index looks like now.
What now> 1
staged
1:
unchanged
2:
+5/-0
3:
unchanged
unstaged path
+5/-0 Rakefile
nothing TODO
+4/-0 lib/simplegit.rb
You can see that the TODO file is now staged (in the index), but the
other two are not. Let’s add the Rakefile, but not the lib/simplegit.
rb file and commit it. To do that, we hit 2, which lists the files we can
update, type 1 and enter to add the Rakefile, then hit enter again
to go back to the main menu. Then we hit 7 to exit and run the git
commit command
What now> 2
staged
1:
unchanged
2:
unchanged
Update>> 1
staged
* 1:
unchanged
2:
unchanged
Update>>
updated one path
unstaged path
+5/-0 Rakefile
+4/-0 lib/simplegit.rb
unstaged path
+5/-0 Rakefile
+4/-0 lib/simplegit.rb
61
62. *** Commands ***
1: status
untracked
5: patch
What now> 7
Bye.
2: update3: revert4: add
6: diff7: quit8: help
$ git commit -m ‘rakefile and todo file added’
Created commit 4b0780c: rakefile and todo file added
2 files changed, 9 insertions(+), 0 deletions(-)
create mode 100644 TODO
The interactive shell is pretty simple and very powerful – playing with
it instead of running git add commands directly may help in under-
standing what’s happening, since you can see the status of your
files in the index versus the working directory more clearly. It helps
visualize that what is in your index (the staged column) is what will be
committed when you run git commit.
You can also do more complicated things, like going through all of
your change patches hunk by hunk, deciding if each hunk should be
applied to the next commit or not. This means that if you’ve made a
bunch of changes to one file, you can commit part of those changes
in one commit, and the rest in a second. Try the patch menu option
in the Interactive Adding menu to try this out.
Beware of using interactive adding if you are already used to
running git commit -a. If you run through the whole interactive
add process and then run git commit -a, it will basically ignore
everything you just did and just add all modified files.
removing
For removing files from your tree, you can simply run:
git rm <filename>
62
63. This will remove that file from the index (and thus from the next com-
mit) as well as from your working directory. On your next commit, the
tree that commit points to will simply not contain that file anymore.
Log – the Commit History
So, now we have all this history in our Git repository. So what? What
can we do with it? How can we see this history?
The answer is the very powerful git log command. The log com-
mand can show you nearly anything you want to know about your
commit history. Also, since the entire history is stored locally, it’s
really fast compared with most other SCM systems, especially if your
repository is packed (see the “Care and Feeding” section).
git log options screencast
The next screencast is on git log, which
demonstrates most of the major features and
options to the git log command. It includes
showing the stat, short-stat and name-stat
options, the —pretty options, the since and
until limiters, the path limiter and author field
searching.
If you just run git log, you will get output like this:
$ git log
commit cf25cc3bfb0ece7dc3609b8dc0cc4a1e19ffbcd4
Author: Scott Chacon <schacon@gmail.com>
Date:
Mon Mar 17 21:52:20 2008 -0700
committing all changes
commit 0c8a9ec46029a4e92a428cb98c9693f09f69a3ff
Author: Scott Chacon <schacon@gmail.com>
Date:
Mon Mar 17 21:52:11 2008 -0700
changed the verison number
commit 0576fac355dd17e39fd2671b010e36299f713b4d
Author: Scott Chacon <schacon@gmail.com>
Date:
Sat Mar 15 16:40:33 2008 -0700
my second commit, which is better than the first
63
64. commit a11bef06a3f659402fe7563abf99ad00de2209e6
Author: Scott Chacon <schacon@gmail.com>
Date:
Sat Mar 15 10:31:28 2008 -0700
first commit
This will show you the SHA-1 of each commit, the committer and
date of the commit, and the full message, starting from the last com-
mit on your current branch and going backward in reverse chrono-
logical order (so if there are multiple parents, it just squishes them
together, interleaving the commits ordered by date)
Formatting Log Output
The default format takes up a lot of space though, so there are ways
to limit and format this output differently. —pretty is a useful option
for formatting the output in different ways.
For example, we can list the commit SHA-1s and the first line of the
message with —pretty=oneline:
$ git log --pretty=oneline
cf25cc3bfb0ece7dc3609b8dc0c committing all changes
0c8a9ec46029a4e92a428cb98c9 changed the verison number
0576fac355dd17e39fd2671b010 my second commit, which is..
a11bef06a3f659402fe7563abf9 first commit
With —pretty, you can choose between oneline, short, medium, full,
fuller, email, raw and format:(string), where (string) is a format you
specify with variables (ex: —format:”%an added %h on %ar” will give
you a bunch of lines like “Scott Chacon added f1cc9df 4 days ago”).
64
65. Filtering Log Output
There are also a number of options for filtering the log output. You
can specify the maximum number of commits you want to see with
-n, you can limit the range of dates you want to see commits for with
—since and —until, you can filter it on the author or committer, text in
the commit message and more. Here is an example showing at most
30 commits between yesterday and a month ago by me :
git log -n 30 --since=”1 month ago” --until=yesterday
--author=”schacon”
• git log (http://www.kernel.org/pub/software/scm/git/docs/git-log.html)
Browsing Git
Git also gives you access to a number of lower level tools that can be
used to browse the repository, inspect the status and contents of any
of the objects, and are generally helpful for inspection and debug-
ging.
Showing Objects
The git show command is really useful for presenting any of the
objects in a very human readable format. Running this command on
a file will simply output the contents of the file. Running it on a tree
will just give you the filenames of the contents of that tree, but none
of its subtrees. Where it’s most useful is using it to look at commits.
git object browsing screencast
In this screencast, we show how to browse and
inspect raw Git objects. The major tools covered
are the git cat-file and git ls-tree com-
mands to inspect the object contents, and then
we cover some of the included graphical brows-
ers, gitk and gitweb.
showing commits
If you call it on a tree-ish that is a commit object, you will get simple
information about the commit (the author, message, date, etc) and a
diff of what changed between that commit and its parents.
65
66. $ git show master^
commit 0c8a9ec46029a4e92a428cb98c9693f09f69a3ff
Author: Scott Chacon <schacon@gmail.com>
Date:
Mon Mar 17 21:52:11 2008 -0700
changed the verison number
diff --git a/Rakefile b/Rakefile
index a874b73..8f94139 100644
--- a/Rakefile
+++ b/Rakefile
@@ -5,7 +5,7 @@ require ‘rake/gempackagetask’
spec = Gem::Specification.new do |s|
s.platform =
Gem::Platform::RUBY
s.name
=
“simplegit”
-
s.version
=
“0.1.0”
+
s.version
=
“0.1.1”
s.author
=
“Scott Chacon”
s.email
=
“schacon@gmail.com”
s.summary
=
“A simple gem for using Git in Ruby
code.”
showing trees
Instead of the git show command, it’s generally more useful to use
the lower level git ls-tree command to view trees, because it gives
you the SHA-1s of all the blobs and trees that it points to.
$ git ls-tree master^{tree}
100644 blob 569b350811e7bfcb2cc781956641c3
100644 blob 8f94139338f9404f26296befa88755
040000 tree ae850bd698b2b5dfbac1ab5fd95a48
README
Rakefile
lib
You can also run this command recursively, so you can see all the
subtrees as well. This is a great way to get the SHA-1 of any blob
anywhere in the tree without having to walk it one node at a time.
$ git ls-tree -r -t master^{tree}
66
67. 100644 blob 569b350811e7bfcb2cc781956641c
100644 blob 8f94139338f9404f26296befa8875
040000 tree ae850bd698b2b5dfbac1ab5fd95a4
100644 blob 7e92ed361869246dc76f0cd0e526e
rb
README
Rakefile
lib
lib/simplegit.
The -t makes it also show the SHA-1s of the subtrees themselves,
rather than just all the blobs.
showing blobs
Lastly, you may want to extract the contents of individual blobs. The
cat-file command is an easy way to do that, and can also serve to
let you know what type of object a SHA-1 is, if you don’t know. It is
sort of a catch-all command that you can use to inspect objects.
$ git cat-file -t ae850bd698b2b5dfbac
tree
$ git cat-file -p ae850bd698b2b5dfbac
100644 blob 7e92ed361869246dc76
simplegit.rb
$ git cat-file -t 569b350811
blob
$ git cat-file -p 569b350811
SimpleGit Ruby Library
======================
This library calls git commands and returns the output.
It is an example for the Git Peepcode book.
Author : Scott Chacon
With those basic commands, you should be able to explore and
inspect any object in any git repository relatively easily.
67
68. Graphical Interfaces
There are two major graphical interfaces that come with Git as tools
to browse the repository.
gitk
A very popular choice for browsing Git repositories is the Tcl/Tk
based browser called gitk. If you want to see a simple visualization
of your repository, gitk is a great tool.
Gitk will also take most of the same arguments that git log will take,
including —since, —until, —max-count, revision ranges and path limit-
ers. One of the most interesting visualizations that I regularly use
is gitk—all, which will show all of your branches next to each other
68
69. (rather than just the one you are currently on).
instaweb
If you don’t want to fire up Tk, you can also browse your repository
quickly via the git instaweb command. This will basically fire up a
web server running the gitweb (http://git.or.cz/gitwiki/Gitweb) CGI script
using lighttpd, apache or webrick. It then tries to automatically fire up
your default web browser and points it at the new server.
$ git instaweb --httpd=webrick
[2008-04-08 20:32:29] INFO WEBrick 1.3.1
[2008-04-08 20:32:29] INFO ruby 1.8.4 (2005-12-24) [i686-
darwin8.8.2]
69
70. When you are done, you can run the following to shut down the
server:
$ git instaweb --stop
This is a quick way to throw up a web interface on your git repository
70
71. for sharing with others or simply browsing it in a different way.
For a more long term web interface to your repository, you can put
the gitweb Perl files that come with Git into your cgi-bin directory.
• git show (http://www.kernel.org/pub/software/scm/git/docs/git-show.html)
• git ls-tree (http://www.kernel.org/pub/software/scm/git/docs/git-ls-tree.html)
• git cat-file (http://www.kernel.org/pub/software/scm/git/docs/git-cat-fileß.html)
• gitk (http://www.kernel.org/pub/software/scm/git/docs/gitk.html)
• git instaweb (http://www.kernel.org/pub/software/scm/git/docs/git-instaweb.html)
Searching Git
Git has an easy way for searching through trees in your repository
without having to check them out into your working directory to do
it manually. It is called ‘git-grep’ and works very much like the tradi-
tional UNIX ‘grep’ command, with the difference that instead of listing
the files you want to search as an argument, you list the trees you
want to search.
For example, if we wanted to search for the string ‘log_syslog’ in ver-
sions 1.0 and 1.5.3.8 of the Git source code in the C files only, we can
find that very easily.
$ git grep -n ‘log_syslog’ v1.5.3.8 v1.0.0 -- *.c
v1.5.3.8:daemon.c:16:static int log_syslog;
v1.5.3.8:daemon.c:92:
if (log_syslog) {
v1.5.3.8:daemon.c:768:
if (log_syslog)
v1.5.3.8:daemon.c:1055:
log_syslog = 1;
v1.5.3.8:daemon.c:1063:
log_syslog = 1;
v1.5.3.8:daemon.c:1112:
log_syslog = 1;
v1.5.3.8:daemon.c:1177: if (log_syslog) {
v1.0.0:daemon.c:13:static int log_syslog;
v1.0.0:daemon.c:45:
if (log_syslog) {
v1.0.0:daemon.c:423:
if (log_syslog)
71
72. v1.0.0:daemon.c:615:
v1.0.0:daemon.c:623:
v1.0.0:daemon.c:653:
log_syslog = 1;
log_syslog = 1;
if (log_syslog)
$ git grep -n -c ‘log_syslog’ v1.5.3.8 v1.0.0 -- *.c
v1.5.3.8:daemon.c:7
v1.0.0:daemon.c:6
You can see that you can view the number of lines that match, or the
actual lines, and you can list as many trees (in this case, I used tags
to reference them) as you want to search.
Another interesting example is to see which files in these versions do
not contain the string ‘git’ anywhere in them:
$ git grep -L -v git v1.5.3.8 v1.0.0
v1.5.3.8:contrib/fast-import/git-p4.bat
v1.5.3.8:contrib/p4import/README
v1.5.3.8:t/t5100/patch0007
v1.5.3.8:t/t5100/patch0008
v1.5.3.8:templates/this--description
v1.0.0:debian/git-arch.files
v1.0.0:debian/git-cvs.files
v1.0.0:debian/git-doc.files
v1.0.0:debian/git-email.files
v1.0.0:debian/git-svn.files
v1.0.0:debian/git-tk.files
v1.0.0:templates/this--description
• git grep (http://www.kernel.org/pub/software/scm/git/docs/git-grep.html)
Git Diff
Git has a great diff utility built in that can give you statistics or a
patch file given any combination of tree objects, working directory
and index.
72
73. Two common uses of this include seeing what you’ve worked on but
not committed yet, and creating a patch file to send to someone
over email (though there is a much preferred way to share changes
which we will learn about in the “distributed workflow” section a bit
later).
What has changed?
If you simply run ‘git diff’ with no arguments, it will show you the dif-
ferences between your current working directory and your index, that
is, the last time you ran ‘git add’ on your files.
For example, if I add my email to the README file and run it, I will see
this:
$ git diff
diff --git a/README b/README
index 569b350..26c6ac8 100644
--- a/README
+++ b/README
@@ -5,4 +5,4 @@ This library calls git commands and returns
the output.
It is an example for the Git Peepcode book.
-Author : Scott Chacon
+Author : Scott Chacon (schacon@gmail.com)
You can also use ‘git diff’ to show you some spiffy stats for a diff,
rather than a patch file, if you want to see a wider overview of what
changed, then drill down into specific files later. Here are some
examples getting stats, the first for the differences between two com-
mits and the second a summary between a commit and the current
HEAD.
$ git diff --numstat a11bef06a3f65..cf25cc3bfb0
73
74. 3
1
4
1
1
5
README
Rakefile
lib/simplegit.rb
$ git diff --stat 0576fac35..
README
|
4 +++-
Rakefile
|
2 +-
lib/simplegit.rb |
4 ++++
3 files changed, 8 insertions(+), 2 deletions(-)
If you want to see what the specific difference is in one of those files,
you can just add a path limiter to the diff command.
$ git diff a11bef06a3f65..cf25cc3bfb0 -- Rakefile
diff --git a/Rakefile b/Rakefile
index a874b73..8f94139 100644
--- a/Rakefile
+++ b/Rakefile
@@ -5,7 +5,7 @@ require ‘rake/gempackagetask’
spec = Gem::Specification.new do |s|
s.platform =
Gem::Platform::RUBY
s.name
=
“simplegit”
-
s.version
=
“0.1.0”
+
s.version
=
“0.1.1”
s.author
=
“Scott Chacon”
s.email
=
“schacon@gmail.com”
s.summary
=
“A simple gem for using Git in Ruby
code.”
You can use this command to detect changes between your index
and any tree, or your working directory and any tree, your working
directory and your index, etc.
Generating Patch Files
The default output of the ‘git diff’ command is a valid patch file. If
you pipe the output into a file and email it to someone, they can
74
75. apply it with the ‘patch’ command. If you’ve done some work off of a
project in an ‘experiment’ branch, you could create a patch file this
way:
$ git diff master..experiment > experiment.patch
You can then email that file to anyone, who could apply it with the
‘-p1’ argument:
$ patch -p1 < ~/experiment.patch
patching file lib/simplegit.rb
• git diff (http://www.kernel.org/pub/software/scm/git/docs/git-diff.html)
Branching
This is the fun part of Git that you’ll come to love like a child. When
you first initialize a git repository, or clone one, you’ll get a ‘master’
branch by default if you don’t specify something else. This is really
just a git suggestion and you don’t have to use it – like just about
everything in Git, it can be overridden.
Switching Branches
However, let’s say we’re working on our project and we want to add a
new function to our library, so we’ll make a new branch called ‘new-
func’ and switch to it. There are two ways we can do this, one is to
create the branch and then switch to it:
$ git branch newfunc; git checkout newfunc
The other way is to checkout a branch that doesn’t exist yet and tell
branching and merging screencast
In this screencast, we take you through a work-
flow where we branch, stash and merge several
times. It demonstrates the branch and show-
branch commands, how to switch branches,
how to stash changes, how to list and apply
stashes, how to resolve conflicts, how to create
and delete topic branches, and what fast-for-
ward merges are.
75
76. git you want to create it by passing the ‘-b’ flag:
$ git checkout -b newfunc
Now, to check which branch we are on, we just type ‘git branch’:
$ git branch
master
* newfunc
We can see we are now on our new branch. This means that if we
modify a file and commit it, this branch will include that change, but
the ‘master’ branch will not have it yet. So, we add a new method to
our library and commit it.
$ vim lib/simplegit.rb; git commit -a -m ‘added lstree
function’
Created commit 1a8c32e: added lstree function
1 files changed, 4 insertions(+), 0 deletions(-)
Now we want to change something in the README in the ‘master’
branch, but we haven’t tested this function yet so we don’t want to
merge our new branch in yet. That’s fine, we just switch back and
make the change.
$ git checkout master
$
vim README
# add some text
$ git commit -a -m ‘added more description’
Created commit d6fad7d: added more description
1 files changed, 1 insertions(+), 1 deletions(-)
Now lets see what the differences in our branches are.
76
77. $ git diff --stat master newfunc
README
|
4 ++--
lib/simplegit.rb |
4 ++++
2 files changed, 6 insertions(+), 2 deletions(-)
We could also get a patch file for one to apply to the other, but what
we really want to do next is merge the two.
• git branch (http://www.kernel.org/pub/software/scm/git/docs/git-branch.html)
• git checkout (http://www.kernel.org/pub/software/scm/git/docs/git-checkout.html)
Simple Merging
So now we want to move the changes in our newfunc branch back
into our master branch and remove it. This will require us merging
one branch into another. Since we’re already in our master branch,
we’ll merge in the newfunc branch like this:
$ git merge newfunc
Easy peasy. We can see that the simplegit.rb file now has four new
lines and the README file was auto-merged.
Now we can get rid of our ‘newfunc’ branch with a simple:
$ git branch -d newfunc
Deleted branch newfunc.
Resolving Conflicts
That was a fairly simple problem, but what if we branch our code and
then edit the same place in a file in different ways in each branch?
77
78. In that case, we’ll get a conflict when we try to merge them back
together. Git is not too aggressive in trying to resolve conflicts, since
you don’t want it to make assumptions that are not necessarily cor-
rect, so bugs aren’t introduced without your knowledge.
Let’s say that we created a versioning branch and then modified the
version in the Rakefile to different versions in both the new branch
and the master branch, then tried to merge them together.
$ git merge versioning
Auto-merged Rakefile
CONFLICT (content): Merge conflict in Rakefile
Automatic merge failed; fix conflicts and then commit the
result.
It tells us that there was a conflict and so the new commit object was
not created. We will have to merge the conflicted file manually and
then commit it again. The output tells us the files that had conflicts,
in this case it was the Rakefile.
rakefile.rb
spec = Gem::Specification.new do |s|
s.platform =
Gem::Platform::RUBY
s.name
=
“simplegit”
<<<<<<< HEAD:Rakefile
s.version
=
“0.1.2”
=======
s.version
=
“0.2.0”
>>>>>>> versioning:Rakefile
s.author
=
“Scott Chacon”
s.email
=
“schacon@gmail.com”
s.summary
=
“A simple gem for using Git in Ruby
code.”
s.files
=
FileList[‘lib/**/*’].to_a
s.require_path =
“lib”
end
78
79. We can see that in the master branch, the version was changed to
0.1.2 and in the versioning branch, the same line was changed to
0.2.0. All we have to do is choose which one is correct and remove
the rest of the lines, like so:
rakefile-post.rb
spec = Gem::Specification.new do |s|
s.platform =
Gem::Platform::RUBY
s.name
=
“simplegit”
s.version
=
“0.2.0”
s.author
=
“Scott Chacon”
s.email
=
“schacon@gmail.com”
s.summary
=
“A simple gem for using Git in Ruby
code.”
s.files
=
FileList[‘lib/**/*’].to_a
s.require_path =
“lib”
end
Now we add and commit that file, and we’re good.
$ git add Rakefile
$ git commit -m ‘fixed conflict’
Created commit 47c668a: fixed conflict
Undoing a Merge
Assume we have gone through some massive merge because some-
one on your team hasn’t committed in a while, or you have a branch
that was created some time ago but hasn’t been rebasing and you
want to pull it in. So you try to git merge old_branch it and you get
conflict after conflict and it is just too much trouble to deal with and
you just want to undo it all.
This is where git reset comes in. To reset your working directory and
index back to what it was before you tried the merge, simply run:
79
80. $ git reset --hard HEAD
The —hard makes sure both your index file and working directory
are changed to match what it used it be. By default it will only reset
your index, leaving the partially merged files in your working direc-
tory. If you happen to have worked through it all and committed,
then decided that it was a mistake because all of your tests break or
something, you can still go back (and throw away that commit) by
running:
$ git reset --hard ORIG_HEAD
This is only helpful if you want to undo the latest change or changes.
If you happen to commit again then decide that you want to keep
the latest commit, but undo a commit that was added sometime
before that, you’ll need to use git revert, which is a bit too danger-
ous to cover here.
• git branch (http://www.kernel.org/pub/software/scm/git/docs/git-branch.html)
• git merge (http://www.kernel.org/pub/software/scm/git/docs/git-merge.html)
• git reset (http://www.kernel.org/pub/software/scm/git/docs/git-reset.html)
• git revert (http://www.kernel.org/pub/software/scm/git/docs/git-revert.html)
Rebasing
To review, rebasing is an alternative to merging that takes all the
changes you’ve done since you branched off and applies those
changes as patches to where the branch you are rebasing to is now,
abandoning your original commit objects. For clean merges, this is
a relatively simple process. Say we have been working in a branch
called ‘story84’ and it’s completed and we want to merge it into the
master branch.
80
81. rebasing screencast
If we do a simple merge, our history will look like this:
This screencast follows roughly the same
course as the previous one on branching and
merging, only we replace merging with rebas-
ing. This screencast also demonstrates the
interactive rebase command git rebase -i.
We also demonstrate some slightly more com-
plex branching, by using both interactive and
normal rebasing techniques simultaneously
on separate branches, then choosing one and
deleting the other.
But we don’t want to mess up our history with a bunch of branches
and merges when it can be clearer. Instead of running ‘git merge
story84’ from the master branch, we can stay in the ‘story84’ branch
and run ‘git rebase master’
$ git rebase master
First, rewinding head to replay your work on top of it...
HEAD is now at 2c0d4d7... added limit to log function
Applying -added todo options
Adds trailing whitespace.
.dotest/patch:12:* add
warning: 1 line adds whitespace errors.
Wrote tree 2d0bd54dc9e4c398769cdcb59256ca03bb482ccb
Committed: b669c78acffaafd5ba34449e7faf88217394864a
Applying limiting log to 30
error: patch failed: lib/simplegit.rb:14
error: lib/simplegit.rb: patch does not apply
Using index info to reconstruct a base tree...
81
82. Falling back to patching base and 3-way merge...
Auto-merged lib/simplegit.rb
CONFLICT (content): Merge conflict in lib/simplegit.rb
Failed to merge in the changes.
Patch failed at 0002.
When you have resolved this problem run “git rebase
--continue”.
If you would prefer to skip this patch, instead run “git
rebase --skip”.
To restore the original branch and stop rebasing run “git
rebase --abort”.
Many times this goes very smoothly and you can see all the new
commits and trees written in place of the old ones. In this case, I
had edited the ‘lib/simplegit.rb’ file differently in each branch which
caused a conflict. I will have to resolve this conflict before I can con-
tinue the rebase.
This gives us some options, since the rebase can do this at any point
– say you have 8 commits to move onto the new branch – each one
could cause a conflict and you will have to resolve them each manu-
ally. The ‘rebase’ command will stop at each patch if it needs to and
let you do this.
You have three things you can do here, you can either fix the file, run
a ‘git add’ on it and then run a ‘git rebase—continue’, which will move
on to the next patch until it’s done. Our second option is to run ‘git
rebase—abort’, which will reset us to what our repo looked like before
we tried the rebase. Or, we can run ‘git rebase—skip’, which will leave
this one patch out, abandoning the change forever.
Git rebase options for a conflict : —continue : trys to keep going
once you’ve resolved it, —abort : gives up altogether and returns to
the state before the rebase, —skip : skips this patch, abandoning it
forever
82
83. In this case we will simply fix the conflict, run ‘git add’ on the file and
then run ‘git rebase—continue’ which then makes our history look like
this:
Then all we have to do is switch to the master branch and merge
in ‘story84’ (which is called a ‘fast-forward’, since ‘master’ is now a
direct ancestor of ‘story84’) to get this:
Interactive Rebasing
Much like Git provides a nicer way to work with your index before
committing with ‘git add—interactive’, there is an interactive rebasing
option that can only be fairly described as the “bee’s knees”.
Assume we have started working on a story to add the ‘git add’
functionality to our library and so we’ve started a new branch called
‘story92’ and done the work there. Then we decide that the ‘ls-tree’
function needs to be recursive and make that change, then we tweak
the library again, committing each time. Meanwhile we’ve pulled in
83
84. a change that implements the same ‘ls-tree’ change differently into
our ‘master’ branch.
We can see before we try the merge that the same change is in each
branch, and I can see that the master branch version is better, so I
don’t even really want to merge it, I just want to throw my change
away. Also, I don’t really need the other two commits to be two com-
mits, because the second one is just a tweak and should be included
in the first one. Lets use ‘git rebase -i’ to rebase this branch and
make those changes. When we run the command, our editor comes
up, showing this:
# Rebasing c110d7f..c4f10f2 onto c110d7f
#
# Commands:
# pick = use commit
# edit = use commit, but stop for amending
# squash = use commit, but meld into previous commit
#
# If you remove a line here THAT COMMIT WILL BE LOST.
#
pick 2b6ae91 added git.add() function
pick bdfd292 made ls-tree recursive
pick c4f10f2 added verbose option to add()
Now we can see all of the commits that we are going to rebase.
If we remove the ‘made ls-tree recursive’ line, it effectively ditches
that commit so we’ll avoid a conflict and not have to worry about it.
Changing the action on the last line to ‘squash’ tells git to just make
this and the previous commit into a single commit. So if we exit the
editor with this as the new text:
84
85. pick 2b6ae91 added git.add() function
squash c4f10f2 added verbose option to add() function
Then git sees we have squashed two commits and wants us to pick a
commit message for it, giving us the commit messages of both for us
to create a new one for.
# This is a combination of two commits.
# The first commit’s message is:
added git.add() function
# This is the 2nd commit message:
added verbose option to add() function
# Please enter the commit message for your changes.
# (Comment lines starting with ‘#’ will not be included)
# Not currently on any branch.
# Changes to be committed:
#
(use “git reset HEAD <file>...” to unstage)
#
#
modified:
lib/simplegit.rb
#
So we stick with the first message, save and exit the editor.
$ git rebase -i master
Created commit 8341085: added git.add() function
1 files changed, 4 insertions(+), 0 deletions(-)
Successfully rebased and updated refs/heads/story92.
Now we’ve rebased and instead of three commits on top of our
master and having to reconcile a useless conflict, we’ve just added a
single commit with no resolving neccesary:
85
86. The rebase command is one of the most useful and unique in the git
workflow. To learn more about some spiffy things you can do with it,
check out the [History Manipulation] and [Advanced Merging] sec-
tions.
• git rebase (http://www.kernel.org/pub/software/scm/git/docs/git-rebase.html)
• git reset (http://www.kernel.org/pub/software/scm/git/docs/git-reset.html)
Stashing
Stashing is a pretty simple concept that is incredibly useful and very
easy to use. If you are working on your code and you need to switch
to another branch for some reason and don’t want to commit your
current state because it is only partially completed, you can run ‘git
stash’, which will basically take the changes from your last commit to
the current state of your working directory and store it temporarily.
In the following example, I have a change to my ‘lib/simplegit.rb’ file,
but it’s not complete.
$ git status
# On branch master
# Changed but not updated:
#
(use “git add <file>...” to update what will be
committed)
#
#
modified:
lib/simplegit.rb
#
no changes added to commit (use “git add” and/or “git commit
-a”)
$ git stash
86
87. Saved working directory and index state “WIP on master:
c110d7f... made the ls-tree function recursive and list
trees”
(To restore them type “git stash apply”)
HEAD is now at c110d7f made the ls-tree function recursive
and list trees
$ git status
# On branch master
nothing to commit (working directory clean)
Now I can see that my working directory is clean, as if I had commit-
ted, but I did not. Now I can switch branches, work for a while some-
where else, then switch back. So where did that change go? How do I
get it back? Well, I can see my stashes by running ‘git stash list’.
$ git stash list
stash@{0}: WIP on experiment: 89e6d12... trying git archive
stash@{1}: WIP on master: c110d7f... made the ls-tree
function recursive
stash@{2}: WIP on master: c110d7f... made the ls-tree
function recursive
I see I have two stashes on the ‘master’ branch, both saved off of
working from the same commit, and I have one stashed change off
the ‘experiment’ branch. However, I can’t remember which stash was
the one I want, so I can use ‘git stash show’ to figure it out.
$ git stash show stash@{1}
lib/simplegit.rb |
4 ++++
1 files changed, 4 insertions(+), 0 deletions(-)
$ git stash show stash@{2}
lib/simplegit.rb |
8 ++++++++
1 files changed, 8 insertions(+), 0 deletions(-)
87
88. I can also use any normal git tools that will take a tree on it, for
instance, ‘git diff’:
$ git diff stash@{1}
diff --git a/lib/simplegit.rb b/lib/simplegit.rb
index e939f77..b03bc9c 100644
--- a/lib/simplegit.rb
+++ b/lib/simplegit.rb
@@ -21,10 +21,6 @@ class SimpleGit
command(“git ls-tree -r -t #{treeish}”)
end
-
-
-
-
def commit(message = ‘commit message’)
command(“git commit -m #{message}”)
end
private
def command(git_cmd)
And finally, I can apply it:
$ git stash apply stash@{1}
# On branch master
# Changed but not updated:
#
(use “git add <file>...” to update what will be
committed)
#
#
modified:
lib/simplegit.rb
#
no changes added to commit (use “git add” and/or “git commit
-a”)
Now we can see that our working directory is back to where it was,
with one file in an unstaged state. Now I would have to ‘git add’ and
‘git commit’ it if I wanted to keep the change.
Normally it’s not even this complicated. If you run ‘git stash apply’
88
89. without the actual stash reference, it will just apply the last stash
you saved on that branch. Normally I will just use ‘git stash’ to save
something, go work elsewhere, then come back and run ‘git stash
apply’ to get back to where I was.
• git stash (http://www.kernel.org/pub/software/scm/git/docs/git-stash.html)
Tagging
As we previously covered, creating a tag in Git is much like making
a branch. A tag in Git serves is basically a signed branch that never
moves – it is simply an arbitrary string that points to a specific com-
mit.
For example, if you wanted to tag your code base every time you
released to production or created a new binary to release, you would
run something like this:
$ git tag -a v0.1 -m ‘this is my v0.1 tag’
As we recall from section one, that command will create a git object
that looks something like this:
89
90. tag : 0c819c
object 0576fa
type commit
tag v0.1
tagger Scott Chacon
<schacon@gmail.com> 1205624655
this is my v0.1 tag
and will store that in the ‘.git/objects/’ directory and then will create
a permanent reference to it in ‘.git/refs/tags/v0.1’ that contains the
SHA-1 of that tag.
Then you can use that as a reference to that commit at any time in
commands like ‘diff’ or ‘archive’ (see next chapter).
Lightweight Tags
You can also create a tag that doesn’t actually add a Tag object to
the database, but just creates a reference to it in the ‘.git/refs/tags’
directory. If you run the following command:
$ git tag v0.1
Git will create the same file as before, ‘.git/refs/tags/v0.1’, but it will
contain the SHA-1 of the current HEAD commit itself, not the SHA-1 of
a Tag object pointing to that commit. Unlike object Tags, these can
be moved around easily, which is generally undesirable.
90
91. Signed Tags
At the other end of the tagging spectrum, you can sign Tag object
with a GPG key to ensure its cryptographic integrity. Replacing ‘-a’
with ‘-s’ in the command will create a Tag object and sign it with the
current users email address GPG key. If you want to specify a key,
you can run it with ‘-u’ instead:
$ git tag -u <key-id> v0.1 -m ‘the 0.1 release’
Then, you or others can later verify that signed tag with a ‘-v’
$ git tag -v v0.1
• git tag (http://www.kernel.org/pub/software/scm/git/docs/git-tag.html)
Exporting Git
If you want to create a release of your code, or provide some poor
non-git user with a snapshot of just a specific tree, you can use the
git-archive command.
‘git-archive’ used to be called ‘git-tar-tree’, in case you ever see that
command around in older articles
You can create the archive in either ‘tar’ or ‘zip’ formats, the default
being ‘tar’. You can use the ‘—prefix’ argument to determine what
directory, if any, the files are expanded into. To create a gzipped tar-
ball, you’ll have to pipe the output through ‘gzip’ first.
$ git-archive --prefix=simplegit/ v0.1 | gzip > simple-git-
0.1.tgz
91
92. Then, if you email that tarball to someone, they would get this when
they opened it:
$ tar zxpvf simple-git-0.1.tgz
simplegit/README
simplegit/Rakefile
simplegit/TODO
simplegit/lib/
simplegit/lib/simplegit.rb
You can also archive parts of your project. This command will create
a zip file of just the ‘lib’ directory of the first parent of your master
branch that will expand out into the current directory:
$ git-archive --format=zip master^ lib/ > simple-git-lib.zip
Which will unzip like this:
$ unzip simple-git-lib.zip
Archive: simple-git-lib.zip
ce9b0d5551762048735dd67917046b44176317e0
creating: lib/
inflating: lib/simplegit.rb
• git archive (http://www.kernel.org/pub/software/scm/git/docs/git-archive.html)
The Care and Feeding of Git
Git requires a bit of tender loving care from time to time. It may
seem a bit odd, but occasionally you should run a few commands on
your repositories to make sure they’re healthy and running as quickly
as possible.
92
93. garbage collection
The ‘git gc’ command is an important one to remember. It will pack
up your objects into the delta-compressed format, saving you a lot
of space and seriously speeding up several commands.
$ git gc
Generating pack...
Done counting 91 objects.
Deltifying 91 objects...
100% (91/91) done
Writing 91 objects...
100% (91/91) done
Total 91 (delta 33), reused 0 (delta 0)
Pack pack-9ca918b18abca509b2de71439522a62178415ebd created.
Removing unused objects 100%...
Done.
If can turn gc’ing automatically on and off by setting a configuration
setting to ‘1’ or ‘0’:
$ git config --global gc.auto 1
This will make git automatically gc itself occasionally. You may want
to setup a cron to do this at night, however, as it can take a while
sometimes on really large repositories.
fsck and prune
If you want to check the health of your repository, you can run ‘git-
fsck’, which will tell you if you have any unreachable or corrupted
objects in your database and help you fix them.
$ git fsck
dangling tree 8276318347b8e971733ca5fab77c8f5018c75261
dangling blob 2302a5a4baec369fb631bb89cfe287cc002dc049
93
94. dangling blob cb54512d0a989dcfb2d78a7f3c8909f76ad2326a
dangling tree 8e1088e1cc1bc67e0ef01e018707dcb07a2a562b
dangling blob 5e069ed35afae29015b6622fe715c0aee10112ad
Which you can then remove with ‘git-prune’ (you can run it with ‘-n’
first to see what it will do)
$ git prune -n
2302a5a4baec369fb631bb89cfe287cc002dc049 blob
5e069ed35afae29015b6622fe715c0aee10112ad blob
8276318347b8e971733ca5fab77c8f5018c75261 tree
8e1088e1cc1bc67e0ef01e018707dcb07a2a562b tree
cb54512d0a989dcfb2d78a7f3c8909f76ad2326a blob
$ git prune
$ git fsck
$
• git gc (http://www.kernel.org/pub/software/scm/git/docs/git-gc.html)
• git fsck (http://www.kernel.org/pub/software/scm/git/docs/git-fsck.html)
• git prune (http://www.kernel.org/pub/software/scm/git/docs/git-prune.html)
Distributed Workflow Examples
Now we’ve gone over most of the basic commands that you’ll use
on a day to day basis as a single developer. This chapter covers
some examples of what you will use in order to collaborate with other
developers on a code base.
Cloning
If you want to begin working on an existing project, you will need to
get an initial version of it – copy its repository of objects and refer-
ences to your machine. This is done with a clone. Git can clone a
94
95. repository over several transports, including local, HTTP, HTTPS, SSH,
its own git protocol, and rsync. The git protocol and SSH are pre-
ferred because they are more efficient and not difficult to set up.
When you clone a repository, it in essence copies all the git objects
to a new directory, checks you out a single local branch named
the same as the HEAD branch on the cloned repo (normally ‘mas-
ter’), and stores all the other branches under a remote reference by
default named ‘origin’.
That means that if we cloned the repo in the previous examples,
instead of ‘story84’ being a local branch you can switch to, it
becomes ‘origin/story84’ that you have to create a local branch to
pull into in order to work on (eg: ‘git checkout—track story84 origin/
story84’) The ‘—track’ indicates that you may want to pull from or
push to the origin of this branch later, so remember where it came
from.
distributed workflow screencast
This screencast demonstrates a distributed
workflow. It takes two personas, creating a
project in GitHub and pushing to it in the first
persona, then cloning that project in the sec-
ond. The second sets up a public, read-only
HTTP repository on his own server. The first
then fetches from that, merges changes and
pushes back to GitHub. It also demonstrates
an instance in which the Author and Committer
fields can differ for a commit.
local clones
Local clones are the simplest types of clones – it is basically the
equivalent of copying the .git directory and doing a checkout. The
only major difference is that it adds all the original branches in as
origin branches. Often you will do this when creating a bare reposi-
tory (that is, a repository without a working directory) for putting on a
public server, or if you’re working with people using a shared reposi-
tory over NFS or something similar.
$ git clone --bare simplegit/.git simplegit-bare.git
ssh and git transports
Cloning over SSH requires that you have user credentials on the
machine you are cloning from. The git transport does not have this
authentication and so is normally used for fetching only.
95
96. $ git clone git@github.com:schacon/ticgit.git ticgit_
directory
http and https transports
Another popular way to clone a repository is over HTTP, just because
it is so simple. You don’t need to setup any special service or give out
user credentials (made easier by services like gitosis), you simply scp
your bare repository into any web server’s static content directory. It
is not as efficient as the other protocols – it will transfer loose objects
and packfiles over a number of calls instead of packing them up, but
it is simple.
$ git clone http://git.gitorious.org/piston/mainline.git
piston
Once you have run one of these commands, you will have a copy of
the git repository, full of all the history – basically every blob and tree
and commit that project has ever had. This is really a full backup of
the repository – if the main server ever goes down or gets corrupted,
everyone who has ever cloned it has a fully capable backup that can
replace it. With Git, there is really no single point of failure.
• git clone (http://www.kernel.org/pub/software/scm/git/docs/git-clone.html)
Fetching and Pulling
So let’s say that we’re going to hack on TicGit, our git based ticket
tracking system project. After we clone it, we look through the source
code but don’t do anything right away. After some time passes we
come back to the project but it may not still be up to date – changes
may have occurred in the meantime. So we fetch an update.
$ git fetch origin
96
97. This will contact the server over the same protocol we used to clone
it and grab all of the objects and references that have been added
since our clone and update our ‘origin/[branch]’ branches to point to
what the server is pointing at now.
So, if we did create a tracking branch on ‘story84’ and it was
changed on the server (someone pushed an update), before we fetch,
our local ‘story84’ branch and our remote ‘origin/story84’ branch will
be the same. After we fetch, they will be different. ‘origin/story84’ will
now point to one of the new commit objects we downloaded during
the fetch.
At this point, we may want to merge ‘origin/story84’ into our local
‘story84’ branch. That’s easy enough, but if we want to do it auto-
matically every time we fetch, we can use ‘git pull’, which is just a
‘fetch’ and then a ‘merge’ command.
So, these commands are functionally equivalent:
$ git pull origin/story84
$ git fetch origin/story84; git merge origin/story84
• git fetch (http://www.kernel.org/pub/software/scm/git/docs/git-fetch.html)
• git pull (http://www.kernel.org/pub/software/scm/git/docs/git-pull.html)
Pushing
Now we can get updates from other repositories, but how can we
push changes to them? If we have commit rights on the repository
(normally over SSH), we can simply run git push.
97
98. $ git push origin master
The ‘origin’ in that case will be inferred if you leave it out, but if
you’ve used a different name for your remote or you are trying to
push one of your other branches, you can do that, too.
$ git push scott-public experimental
If you don’t specify a branch, it will infer that you want to push every
branch that you and the server have in common. So, if you have
pushed your ‘master’ branch and your ‘experimental’ branch to the
‘scott-public’ server at any point, running this will update the server to
have the newest versions of both of them:
$ git push scott-public
Whereas this will only update the ‘master’ branch:
$ git push scott-public master
In Git, the opposite of ‘push’ is not ‘pull’, but ‘fetch’. A ‘pull’ is a
‘fetch’ and then a ‘merge’.
• git push (http://www.kernel.org/pub/software/scm/git/docs/git-push.html)
Multiple Remotes
Although a bit different in syntax maybe, most of that should seem
familiar to any users of other SCM systems. However, this is where
the ‘decentralized’ part comes in. In Git, there is really no special
repository. You can add as many remote repositories that are related
98
99. to your codebase in some way as you want. You can add each of
your co-workers repositories as read-only repositories, you can have
a centralized one you all share, one out on your production servers
outside the firewall, a public one for stable or sanitized pushes on
your personal webserver, one on your build server, etc, etc.
Pushing to and pulling from multiple sources is easy and straightfor-
ward. You simply add remotes :
$ git remote add mycap git@github.com:schacon/capistrano.git
$ git remote add official git://github.com/jamis/capistrano.
git
Then, if the the project is updated, I can pull in the changes from one
remote, merge them locally, and then push to another remote.
$ git fetch official
$ git merge official/master
$ git push mycap master
I can also add several remotes to pull and merge from, in this case,
one for every developer with a public fork of that project that might
push changes I care to try.
99
100. nick's computer
private repo
push public
fetch origin
ssh://nickh@hengeveld.com:git/ticgit.gitgit://github.com/schacon/ticgit.git
http://hengeveld.com/ticgit.gitssh://git@github.com:schacon/ticgit.git
fetch nick
push github
private repo
add & commit
working directory
my computer
100
101. You can also remove remotes at any time, which simply removes the
lines that contain the URL in your .git/config file and the references
to their remote branches in .git/refs/[remote_name] directory. It will
not remove any of the git objects, so if you decide to add it again
and fetch, very little will be transferred.
You can also view useful information about a remote branch by using
the remote show command. For example, if I run this on a checkout of
the Git source code itself, I will see this:
$ git remote show origin
* remote origin
URL: git://git.kernel.org/pub/scm/git/git.git
Remote branch(es) merged with ‘git pull’ while on branch
master
master
Stale tracking branches in remotes/origin (use ‘git remote
prune’)
old-next
Tracked remote branches
html maint man master next pu todo
• git remote (http://www.kernel.org/pub/software/scm/git/docs/git-remote.html)
Possible Workflows
The idea of having multiple remote repositories that you can push
to and/or pull from is probably new to you, and many people have a
hard time figuring out what their workflow should look like, especially
if they are moving from a centralized SCM system. I will present a
couple of possible workflows that I have seen, so you can determine
what will work best for you and your team.
Most of these are simply a matter of convention, not even configu-
ration. Each of the models can pretty easily change to another with
minimal configuration changes – maybe some permissions tweaked
101
102. here or there.
central repository model
There is a single repository that all developers push to and pull from.
developer
developer
developer
developer
shared repository
developer
developer
developer
developer
Fig. K All developers push to a single server
This model works just like a centralized SCM and Git can work that
way just fine. If you setup a repository for your team on a server that
everyone has SSH or NFS access to, Git can very easily function as
a centralized repository. This may be common on small teams with
non-public projects where you don’t want to worry about a hierarchy
– the strength of this model is that it forces everyone to stay up to
date with each other and it doesn’t depend on a single role.
Even large teams could use this, but in general there are a lot of
gains to be made in larger teams with a different or hybrid model.
102
103. dictator and lieutenant model
This is a highly hierarchical model where one individual has com-
mit rights to a blessed repository that everyone else fetches from.
Changes are fetched from developers by lieutenants responsible for
specific subsystems and merged and tested. Lieutenant branches
are then fetched by the dictator and merged and pushed into the
blessed repository, where the cycle starts over again.
dictator
blessed repository
lieutenant
lieutenant
developer
developer
developer
developer
Fig. L Approved features gradually make their way up the ladder
This is a model something like the Linux kernel uses, Linus being the
benevolent dictator. This model is much better for large teams, and
can be implemented with multiple and varied levels of lieutenants
and sub-lieutenants in charge of various subsystems. At any stage
in this process, patches or commits can be rejected – not merged in
and sent up the chain.
integration manager model
This is where each developer has a public repository, but one is
considered the ‘official’ repository – it is used to create the pack-
103
104. ages and binaries. A person or core team has commit rights to it, but
many other developers have public forks of that repository. When
they have changes, they issue a pull request to an integration man-
ager, who adds them as a remote if they haven’t already – then
merges, tests, accepts and pushes.
blessed
repositorydeveloper
publicdeveloper
publicdeveloper
public
integration
managerdeveloper
privatedeveloper
privatedeveloper
private
Fig. M Private and public repositories driven by read-only pull requests
This is largely how community-based git repositories like GitHub
were built to work and how many smaller open source projects oper-
ate.
In the end, there is really no single right way to do it – being a
decentralized system, you can have a model with all of these aspects
to it, or any combination you can think of. You can also have sub-
groups using different models on the same codebase. For example, if
your company might have an internal fork of the Linux kernel that is
managed by the Integration Manager model in addition to pulling in
changes occasionally from Linus’s branch. In the end, you and your
team will have to think about what will work best for you.
104
105. Sharing Repositories
Over Git
Git provides its own special protocol, which is basically just a really
thin wrapper over the ‘git-upload-pack’ command that will tell you
what is available, then you tell it what you have and it gives you a
packfile of the difference. To start it up manually, run something like
the following command:
$ git-daemon --detach --export-all --base-path=/opt/git /
opt/git/ambition
Though for long term running, you’ll likely want to add this to your
inet.d configuration. See the git-daemon (http://www.kernel.org/pub/soft-
ware/scm/git/docs/git-daemon.html) docs for specific information on how to
set that up.
The git protocol has no built in authentication, so generally you
cannot push over it (although some people have open push policies
and so allow that – I would not recommend setting that up, so you’ll
have to look up how to do that). If you want your users to have push
access, it’s recommended to use the SSH protocol. Many reposito-
ries, like GitHub, have SSH enabled for account owners to push over,
and git-daemon enabled for the public to pull over – which is often
the most efficient combination.
• git-daemon (http://www.kernel.org/pub/software/scm/git/docs/git-daemon.html)
Over SSH
Git can work entirely over SSH – it actually does much the same
thing that happens over the git protocol, except it implicitly has
authentication built in. If a user has SSH and write access to the repo
105
106. and its subdirectories, then that user can push over SSH as well.
$ git clone --bare
scp -r project.bare user@host:/repos/project.git
git clone user@host:/repos/project.git
Git ships with the git-shell, a slightly more secure shell that restricts
the user to performing only Git-related actions. If you use SSH
public keys, you won’t need to give out any passwords of create
user accounts for other developers who need to have write-access to
the repository. However, the restricted git user needs to have write
access to the files and directories in the repository, which can be
slightly awkward to setup and maintain.
Over HTTP
This is pretty easy to setup because there is no requirement other
than a static web server – it is not like SVN which requires a DAV
server to run. If you want to push over http, however, you will need
DAV setup – it’s generally a much better idea to use SSH to do so,
however.
The only caveat is that you need to run git update-server-info
each time you commit to the repository.
$ git update-server-info
It is generally a good idea to put this in the post-commit hook file on
any server that you want to be able to fetch from over HTTP.
Actually, you can run this without update-server-info if you
never pack objects and you have a pre-defined list of branches you
are always fetching. All update-server-info does is put a list of
106
107. branches into one file (.git/info/refs) and a list of pack files into
another (.git/objects/info/pack) to get around the fact that you
cannot reliably list contents of a directory over HTTP – it’s not built
into the protocol. So for Git to know what packfiles and branches are
available, it needs to have one URL it can get those from.
Hosted Repositories
If you don’t want to deal with setting up and maintaining your own
server for your git repository, you can use one of the growing number
of public Git hosted servers.
I will focus on some interesting features of a commercial service
called GitHub (http://github.com) here, but there is also an open source
project called Gitorious (http://gitorious.com) that has many of the same
features.
One of the compelling features of GitHub is the ability to create a
private repository and share it with only a few developers. However,
the complete source to Gitorious is available as a Ruby on Rails
application. It could be modified to run on your company’s server if
your project needs to remain private.
GitHub hosts many popular projects featured in the PeepCode
series, including Ruby on Rails (http://github.com/rails/rails/tree/master),
Merb (http://github.com/wycats/merb-core/tree/master), RSpec (http://github.com/
dchelimsky/rspec/tree/master), and Capistrano (http://github.com/jamis/capist-
rano/tree/master).
GitHub
GitHub is interesting as a source code hosting service because it
107
108. includes some social networking features, which is not what most
people imagine when they think of hosting source code.
You can follow friends, other developers, or just individual projects.
You then subscribe to a single Atom feed and are kept up to date on
what all those projects and people are doing, code-wise.
Fig. N Custom GitHub home page with your projects
More interestingly, you can publicly fork a project to get your own
copy of it. GitHub is unique in concept in that it is really centered
around individuals rather than projects. For instance, if you want to
follow or work on Merb, you would follow or fork wycats’s Merb. There
is really no official Merb page in GitHub. It’s simply that wycats is
known to be the blessed repository by the Merb project.
In fact, Rails itself has recently been forked (http://github.com/ezmobius/
108
109. rails/tree/master) and significantly enhanced by Ezra Zygmuntowicz. In
other contexts this would be a bold divisive gesture, but with Git it’s
common practice.
You could just as easily follow and fork someone else’s repository of
the project. An interesting example of this is the git-wiki (http://github.
com/sr/git-wiki) project, which was started by a user named sr, then was
forked and greatly modified by al3x. sr wanted to keep the project
simple, so now there are two major versions of the project. I have a
checkout of the project and remotes added for each one, so I could
work on and contribute to either.
Fig. O Project network visualization
109
110. Let’s say there is a popular feature request that is either not com-
pleted or not accepted by the main project maintainer. You can very
easily fork the project and keep your patch or patches up to date
with the head on a regular basis and people can pull from yours
instead. Perhaps enough demand builds up or enough testing is
done from all these users that the patch is accepted. You can then
delete your fork and revert to the original project head.
This will also really change patch submission for large projects.
Instead of emailing patches around, you can fork the project, add
your patch and submit a pull request for your branch with the fix to
one of the core members or through the ticketing system. A mem-
ber of the core team can add you as a remote easily, create a new
testing branch, merge in or rebase your branch, test and accept or
reject. If your patch is ignored or the team doesn’t have time to deal
with it yet, it’s easy to keep up to date by continually rebasing and
re-sending the pull requests until it’s either rejected or accepted. It
doesn’t just go stale until it’s difficult to apply the patch anymore.
Services like GitHub and an increased adoption of distributed SCM
systems will dramatically change open source development work-
flows on teams of all sizes, in addition to changing the way individual
developers work.
110
111. Commands Overview
chapter 4
This section is meant to be a really quick reference to the commands
we have reviewed in Git and a quick description of what they do,
where we have talked about them and where to find out more infor-
mation on them.
Basic Git
git config (http://www.kernel.org/pub/
software/scm/git/docs/git-config.html)
Sets configuration values for things like your user name, email, and
gpg key, your preferred diff algorithm, file formats to use, proxies,
remotes and tons of other stuff. For a full list, see the git-config docs
(http://www.kernel.org/pub/software/scm/git/docs/git-config.html)
git init (http://www.kernel.org/pub/
software/scm/git/docs/git-init.html)
Initializes a git repository – creates the initial ‘.git’ directory in a new
or existing project.
git clone (http://www.kernel.org/pub/
software/scm/git/docs/git-clone.html)
Copies a Git repository from another place and adds the original
location as a remote you can fetch from again and possibly push to
111
112. if you have permission.
git add (http://www.kernel.org/pub/
software/scm/git/docs/git-add.html)
Adds changes in files in your working directory to your index, or
staging area.
git rm (http://www.kernel.org/pub/
software/scm/git/docs/git-rm.html)
Removes files from your index and your working directory so they will
stopped being tracked.
git commit (http://www.kernel.org/pub/
software/scm/git/docs/git-commit.html)
Takes all of the changes staged in the index (that have been ‘git
add’ed), creates a new commit object pointing to it, and advances
the branch to point to that new commit.
git status (http://www.kernel.org/pub/
software/scm/git/docs/git-status.html)
Shows you the status of files in your index versus your working
directory. It will list out files that are untracked (only in your working
directory), modified (tracked but not yet updated in your index), and
staged (added to your index and ready for committing).
112
113. git branch (http://www.kernel.org/pub/
software/scm/git/docs/git-branch.html)
Lists existing branches, including remote branches if ‘-a’ is provided.
Creates a new branch if a branch name is provided. Branches can
also be created with ‘-b’ option to ‘git checkout’.
git checkout (http://www.kernel.org/pub/
software/scm/git/docs/git-checkout.html)
Checks out a different branch – makes your working directory look
like the tree of the commit that branch points to and updates your
HEAD to point to this branch now, so your next commit will modify it.
git merge (http://www.kernel.org/pub/
software/scm/git/docs/git-merge.html)
Merges one or more branches into your current branch and auto-
matically creates a new commit if there are no conflicts.
git reset (http://www.kernel.org/pub/
software/scm/git/docs/git-reset.html)
Resets your index and working directory to the state of your last
commit, in the event that something screwed up and you just want to
go back.
git rebase (http://www.kernel.org/pub/
software/scm/git/docs/git-rebase.html)
113
114. An alternative to merge that rewrites your commit history to move
commits since you branched off to apply to the current head
instead. A bit dangerous as it discards existing commit objects.
git stash (http://www.kernel.org/pub/
software/scm/git/docs/git-stash.html)
Temporarily saves changes that you don’t want to commit immedi-
ately for later. Can re-apply the saved changes at any time.
git tag (http://www.kernel.org/pub/
software/scm/git/docs/git-tag.html)
Tags a specific commit with a simple, human readable handle that
never moves.
git fetch (http://www.kernel.org/pub/
software/scm/git/docs/git-fetch.html)
Fetches all the objects that a remote version of your repository has
that you do not yet so you can merge them into yours or simply
inspect them.
git pull (http://www.kernel.org/pub/
software/scm/git/docs/git-pull.html)
Runs a ‘git fetch’ then a ‘git merge’.
git push (http://www.kernel.org/pub/
114
115. software/scm/git/docs/git-push.html)
Pushes all the objects that you have that a remote version does not
yet have to that repository and advances its branches.
git remote (http://www.kernel.org/pub/
software/scm/git/docs/git-remote.html)
Lists all the remote versions of your repository, or can be used to add
and delete them.
Inspecting Repositories
git log (http://www.kernel.org/pub/
software/scm/git/docs/git-log.html)
Shows a listing of commits on a branch or involving a specific file
and optionally details about what changed between it and its par-
ents.
git show (http://www.kernel.org/pub/
software/scm/git/docs/git-show.html)
Shows information about a git object, normally used to view commit
information.
git ls-tree (http://www.kernel.org/pub/
software/scm/git/docs/git-ls-tree.html)
115
116. Shows a tree object, including the mode and name of each node and
the SHA-1 value of the blob or tree that it points to. Can also be run
recursively to see all subtrees as well.
git cat-file (http://www.kernel.org/pub/
software/scm/git/docs/git-cat-file.html)
Used to view the type of an object if you only have the SHA-1 value,
or used to redirect contents of files or view raw information about any
object.
git grep (http://www.kernel.org/pub/
software/scm/git/docs/git-grep.html)
Lets you search through your trees of content for words and phrases
without having to actually check them out.
git diff (http://www.kernel.org/pub/
software/scm/git/docs/git-diff.html)
Generates patch files or statistics of differences between paths or
files in your git repository, or your index or your working directory.
gitk (http://www.kernel.org/pub/
software/scm/git/docs/gitk.html)
Graphical Tcl/Tk based interface to a local Git repository.
git instaweb (http://www.kernel.org/pub/
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117. software/scm/git/docs/git-instaweb.html)
Wrapper script to quickly run a web server with an interface into your
repository and automatically directs a web browser to it.
Extra Tools
git archive (http://www.kernel.org/pub/
software/scm/git/docs/git-archive.html)
Creates a tar or zip file of the contents of a single tree from your
repository. Easiest way to export a snapshot of content from your
repository.
git gc (http://www.kernel.org/pub/
software/scm/git/docs/git-gc.html)
Garbage collector for your repository. Packs all your loose objects
for space and speed efficiency and optionally removes unreachable
objects as well. Should be run occasionally on each of your repos.
git fsck (http://www.kernel.org/pub/
software/scm/git/docs/git-fsck.html)
Does an integrity check of the Git “filesystem”, identifying dangling
pointers and corrupted objects.
git prune (http://www.kernel.org/pub/
software/scm/git/docs/git-prune.html)
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118. Removes objects that are no longer pointed to by any object in any
reachable branch.
git-daemon (http://www.kernel.org/pub/
software/scm/git/docs/git-daemon.html)
Runs a simple, unauthenticated wrapper on the git-upload-pack pro-
gram, used to provide efficient, anonymous and unencrypted fetch
access to a Git repository.
118
119. References and Endnotes
chapter 5
Here are some references that I used or that you may use to find out
more about Git.
The example git repository that I was working with throughout this
book can be cloned from its GitHub repository (http://github.com/schacon/
simplegit)
For anything you cannot find in this book or these references, be sure
to ask the fantastic people hanging out at the ‘#git’ channel on irc.
freenode.net
Web Documentation
Main Git Documentation (http://www.kernel.org/pub/software/scm/git/docs) –
fantastic reference for all the command line programs
Git User’s Manual (http://www.kernel.org/pub/software/scm/git/docs/user-manual.
html)
Git for Computer Scientists (http://eagain.net/articles/git-for-computer-scien-
tists) – good detail about the DAG object model
A Tutorial Introduction to Git (http://www.kernel.org/pub/software/scm/git/docs/
tutorial.html)
Git Rebase Explained (http://wincent.com/knowledge-base/Git_rebase_explained)
A Tour of Git, the Basics (http://cworth.org/hgbook-git/tour)
Junio Hamano New Git Maintainer (http://kerneltrap.org/node/5496) –
119
120. some history on git and Junio becoming the new maintainer
Screencasts
Git Peepcode Screencast (http://peepcode.com/products/git)
RailsCasts Git Screencast (http://railscasts.com/episodes/96)
Using Git to Manage and Deploy Rails Apps (http://www.jointheconversa-
tion.org/railsgit)
120