この章で Git への最初の一歩を踏み出しましょう. まずはじめに少しバージョン管理ツールについて説明します. そして Git を各 OS で走らせ, 適切に設定するところまでご案内します. この章の最後までに, Git が支持される所以, ご案内する利用方法の利点を理解しましょう.
バージョン管理とは, なぜ必要なのか. バージョン管理はファイルへの変更を時系列に記録するシステムで、後から特定の時系列バージョンを呼び出すことができます.例えば本書では, バージョン管理されたソフトウェア・ソースコードを扱いますが, 実際にはソースコードだけがバージョン管理の対象ではありません.
もしあなたがグラフィックや Web のデザイナーで, 画像やレイアウトの中に全てのバージョンを保存しておきたいものがある場合, バージョン管理システム (以降 VCS と呼ぶ) はとても賢い選択です. VCS は, 特定のファイルでもプロジェクト全体でも, ある過去の時点に戻せます. 時系列の変更を比較し, 誰がバグの原因となる修正を施し, またバグを解消し, いつそれが行われたかを確認できます. 一般的に VCS はファイルの損傷や消失を簡単に元に戻せます. 加えてこれら全てにかかる手間はごくわずかなものです.
バージョン管理の手法として, ファイルを別ディレクトリにコピーするというのがあります(賢い方はディレクトリを時系列に分けているかも知れません). この方法は非常に一般的で, その単純明快さが選ばれる理由であり, 同時に途方もないエラーを生む傾向にあります. どのディレクトリが対象か簡単に忘れてしまいますし, 意図しないファイルに書き込んでしまったり, 間違ったファイルを上書きしてしまうこともあります.
この問題を解決するため, プログラマーたちはローカルVCSを開発しました. 当時のそれは単純なデータベースによりファイルへの変更全てをリビジョン管理下に置くものでした(図1-1).
Insert 18333fig0101.png 図1-1. ローカル・バージョン管理の図
もっとも有名な VCS ツールの一つに rcs と呼ばれるシステムがありました. これは今日でもまだ多くのコンピュータで使用されています. 有名な Mac OS X でさえも, 開発ツールをインストールすると rcs コマンドが含まれています. このツールは基本的に, 変更から変更を特別なディスク・フォーマットのパッチ・セット(これはファイルとは異なります)に記録して動きます. これはパッチに追加された全ての時点のファイルを再生成することができます.
次に大きな課題として人々が直面したのが, 他システムを使う開発者たちとのコラボレーションです. これを解決するため, 中央集権型バージョン管理システム(以降 CVCS と呼ぶ)が開発されました. これら CVS や Subversion, Perforce といった CVCS は, 一つのサーバでファイル全てを管理し, 複数のクライアントがサーバからファイルをチェックアウトします. 長年これはバージョン管理のスタンダードでした(図1-2).
Insert 18333fig0102.png 図1-2. 中央集権型バージョン管理の図
This setup offers many advantages, especially over local VCSs. For example, everyone knows to a certain degree what everyone else on the project is doing. Administrators have fine-grained control over who can do what; and it’s far easier to administer a CVCS than it is to deal with local databases on every client.
However, this setup also has some serious downsides. The most obvious is the single point of failure that the centralized server represents. If that server goes down for an hour, then during that hour nobody can collaborate at all or save versioned changes to anything they’re working on. If the hard disk the central database is on becomes corrupted, and proper backups haven’t been kept, you lose absolutely everything—the entire history of the project except whatever single snapshots people happen to have on their local machines. Local VCS systems suffer from this same problem—whenever you have the entire history of the project in a single place, you risk losing everything.
This is where Distributed Version Control Systems (DVCSs) step in. In a DVCS (such as Git, Mercurial, Bazaar or Darcs), clients don’t just check out the latest snapshot of the files: they fully mirror the repository. Thus if any server dies, and these systems were collaborating via it, any of the client repositories can be copied back up to the server to restore it. Every checkout is really a full backup of all the data (see Figure 1-3).
Insert 18333fig0103.png Figure 1-3. Distributed version control diagram
Furthermore, many of these systems deal pretty well with having several remote repositories they can work with, so you can collaborate with different groups of people in different ways simultaneously within the same project. This allows you to set up several types of workflows that aren’t possible in centralized systems, such as hierarchical models.
As with many great things in life, Git began with a bit of creative destruction and fiery controversy. The Linux kernel is an open source software project of fairly large scope. For most of the lifetime of the Linux kernel maintenance (1991–2002), changes to the software were passed around as patches and archived files. In 2002, the Linux kernel project began using a proprietary DVCS system called BitKeeper.
In 2005, the relationship between the community that developed the Linux kernel and the commercial company that developed BitKeeper broke down, and the tool’s free-of-charge status was revoked. This prompted the Linux development community (and in particular Linus Torvalds, the creator of Linux) to develop their own tool based on some of the lessons they learned while using BitKeeper. Some of the goals of the new system were as follows:
- Speed
- Simple design
- Strong support for non-linear development (thousands of parallel branches)
- Fully distributed
- Able to handle large projects like the Linux kernel efficiently (speed and data size)
Since its birth in 2005, Git has evolved and matured to be easy to use and yet retain these initial qualities. It’s incredibly fast, it’s very efficient with large projects, and it has an incredible branching system for non-linear development (See Chapter 3).
So, what is Git in a nutshell? This is an important section to absorb, because if you understand what Git is and the fundamentals of how it works, then using Git effectively will probably be much easier for you. As you learn Git, try to clear your mind of the things you may know about other VCSs, such as Subversion and Perforce; doing so will help you avoid subtle confusion when using the tool. Git stores and thinks about information much differently than these other systems, even though the user interface is fairly similar; understanding those differences will help prevent you from becoming confused while using it.
The major difference between Git and any other VCS (Subversion and friends included) is the way Git thinks about its data. Conceptually, most other systems store information as a list of file-based changes. These systems (CVS, Subversion, Perforce, Bazaar, and so on) think of the information they keep as a set of files and the changes made to each file over time, as illustrated in Figure 1-4.
Insert 18333fig0104.png Figure 1-4. Other systems tend to store data as changes to a base version of each file.
Git doesn’t think of or store its data this way. Instead, Git thinks of its data more like a set of snapshots of a mini filesystem. Every time you commit, or save the state of your project in Git, it basically takes a picture of what all your files look like at that moment and stores a reference to that snapshot. To be efficient, if files have not changed, Git doesn’t store the file again—just a link to the previous identical file it has already stored. Git thinks about its data more like Figure 1-5.
Insert 18333fig0105.png Figure 1-5. Git stores data as snapshots of the project over time.
This is an important distinction between Git and nearly all other VCSs. It makes Git reconsider almost every aspect of version control that most other systems copied from the previous generation. This makes Git more like a mini filesystem with some incredibly powerful tools built on top of it, rather than simply a VCS. We’ll explore some of the benefits you gain by thinking of your data this way when we cover Git branching in Chapter 3.
Most operations in Git only need local files and resources to operate – generally no information is needed from another computer on your network. If you’re used to a CVCS where most operations have that network latency overhead, this aspect of Git will make you think that the gods of speed have blessed Git with unworldly powers. Because you have the entire history of the project right there on your local disk, most operations seem almost instantaneous.
For example, to browse the history of the project, Git doesn’t need to go out to the server to get the history and display it for you—it simply reads it directly from your local database. This means you see the project history almost instantly. If you want to see the changes introduced between the current version of a file and the file a month ago, Git can look up the file a month ago and do a local difference calculation, instead of having to either ask a remote server to do it or pull an older version of the file from the remote server to do it locally.
This also means that there is very little you can’t do if you’re offline or off VPN. If you get on an airplane or a train and want to do a little work, you can commit happily until you get to a network connection to upload. If you go home and can’t get your VPN client working properly, you can still work. In many other systems, doing so is either impossible or painful. In Perforce, for example, you can’t do much when you aren’t connected to the server; and in Subversion and CVS, you can edit files, but you can’t commit changes to your database (because your database is offline). This may not seem like a huge deal, but you may be surprised what a big difference it can make.
Everything in Git is check-summed before it is stored and is then referred to by that checksum. This means it’s impossible to change the contents of any file or directory without Git knowing about it. This functionality is built into Git at the lowest levels and is integral to its philosophy. You can’t lose information in transit or get file corruption without Git being able to detect it.
The mechanism that Git uses for this checksumming is called a SHA-1 hash. This is a 40-character string composed of hexadecimal characters (0–9 and a–f) and calculated based on the contents of a file or directory structure in Git. A SHA-1 hash looks something like this:
24b9da6552252987aa493b52f8696cd6d3b00373
You will see these hash values all over the place in Git because it uses them so much. In fact, Git stores everything not by file name but in the Git database addressable by the hash value of its contents.
When you do actions in Git, nearly all of them only add data to the Git database. It is very difficult to get the system to do anything that is not undoable or to make it erase data in any way. As in any VCS, you can lose or mess up changes you haven’t committed yet; but after you commit a snapshot into Git, it is very difficult to lose, especially if you regularly push your database to another repository.
This makes using Git a joy because we know we can experiment without the danger of severely screwing things up. For a more in-depth look at how Git stores its data and how you can recover data that seems lost, see “Under the Covers” in Chapter 9.
Now, pay attention. This is the main thing to remember about Git if you want the rest of your learning process to go smoothly. Git has three main states that your files can reside in: committed, modified, and staged. Committed means that the data is safely stored in your local database. Modified means that you have changed the file but have not committed it to your database yet. Staged means that you have marked a modified file in its current version to go into your next commit snapshot.
This leads us to the three main sections of a Git project: the Git directory, the working directory, and the staging area.
Insert 18333fig0106.png Figure 1-6. Working directory, staging area, and git directory
The Git directory is where Git stores the metadata and object database for your project. This is the most important part of Git, and it is what is copied when you clone a repository from another computer.
The working directory is a single checkout of one version of the project. These files are pulled out of the compressed database in the Git directory and placed on disk for you to use or modify.
The staging area is a simple file, generally contained in your Git directory, that stores information about what will go into your next commit. It’s sometimes referred to as the index, but it’s becoming standard to refer to it as the staging area.
The basic Git workflow goes something like this:
- You modify files in your working directory.
- You stage the files, adding snapshots of them to your staging area.
- You do a commit, which takes the files as they are in the staging area and stores that snapshot permanently to your Git directory.
If a particular version of a file is in the git directory, it’s considered committed. If it’s modified but has been added to the staging area, it is staged. And if it was changed since it was checked out but has not been staged, it is modified. In Chapter 2, you’ll learn more about these states and how you can either take advantage of them or skip the staged part entirely.
Let’s get into using some Git. First things first—you have to install it. You can get it a number of ways; the two major ones are to install it from source or to install an existing package for your platform.
If you can, it’s generally useful to install Git from source, because you’ll get the most recent version. Each version of Git tends to include useful UI enhancements, so getting the latest version is often the best route if you feel comfortable compiling software from source. It is also the case that many Linux distributions contain very old packages; so unless you’re on a very up-to-date distro or are using backports, installing from source may be the best bet.
To install Git, you need to have the following libraries that Git depends on: curl, zlib, openssl, expat, and libiconv. For example, if you’re on a system that has yum (such as Fedora) or apt-get (such as a Debian based system), you can use one of these commands to install all of the dependencies:
$ yum install curl-devel expat-devel gettext-devel \
openssl-devel zlib-devel
$ apt-get install curl-devel expat-devel gettext-devel \
openssl-devel zlib-devel
When you have all the necessary dependencies, you can go ahead and grab the latest snapshot from the Git web site:
http://git-scm.com/download
Then, compile and install:
$ tar -zxf git-1.6.0.5.tar.gz
$ cd git-1.6.0.5
$ make prefix=/usr/local all
$ sudo make prefix=/usr/local install
After this is done, you can also get Git via Git itself for updates:
$ git clone git://git.kernel.org/pub/scm/git/git.git
If you want to install Git on Linux via a binary installer, you can generally do so through the basic package-management tool that comes with your distribution. If you’re on Fedora, you can use yum:
$ yum install git-core
Or if you’re on a Debian-based distribution like Ubuntu, try apt-get:
$ apt-get install git-core
There are two easy ways to install Git on a Mac. The easiest is to use the graphical Git installer, which you can download from the Google Code page (see Figure 1-7):
http://code.google.com/p/git-osx-installer
Insert 18333fig0107.png Figure 1-7. Git OS X installer
The other major way is to install Git via MacPorts (http://www.macports.org). If you have MacPorts installed, install Git via
$ sudo port install git-core +svn +doc +bash_completion +gitweb
You don’t have to add all the extras, but you’ll probably want to include +svn in case you ever have to use Git with Subversion repositories (see Chapter 8).
Installing Git on Windows is very easy. The msysGit project has one of the easier installation procedures. Simply download the installer exe file from the Google Code page, and run it:
http://code.google.com/p/msysgit
After it’s installed, you have both a command-line version (including an SSH client that will come in handy later) and the standard GUI.
Now that you have Git on your system, you’ll want to do a few things to customize your Git environment. You should have to do these things only once; they’ll stick around between upgrades. You can also change them at any time by running through the commands again.
Git comes with a tool called git config that lets you get and set configuration variables that control all aspects of how Git looks and operates. These variables can be stored in three different places:
/etc/gitconfigfile: Contains values for every user on the system and all their repositories. If you pass the option--systemtogit config, it reads and writes from this file specifically.~/.gitconfigfile: Specific to your user. You can make Git read and write to this file specifically by passing the--globaloption.- config file in the git directory (that is,
.git/config) of whatever repository you’re currently using: Specific to that single repository. Each level overrides values in the previous level, so values in.git/configtrump those in/etc/gitconfig.
On Windows systems, Git looks for the .gitconfig file in the $HOME directory (C:\Documents and Settings\$USER for most people). It also still looks for /etc/gitconfig, although it’s relative to the MSys root, which is wherever you decide to install Git on your Windows system when you run the installer.
The first thing you should do when you install Git is to set your user name and e-mail address. This is important because every Git commit uses this information, and it’s immutably baked into the commits you pass around:
$ git config --global user.name "John Doe"
$ git config --global user.email johndoe@example.com
Again, you need to do this only once if you pass the --global option, because then Git will always use that information for anything you do on that system. If you want to override this with a different name or e-mail address for specific projects, you can run the command without the --global option when you’re in that project.
Now that your identity is set up, you can configure the default text editor that will be used when Git needs you to type in a message. By default, Git uses your system’s default editor, which is generally Vi or Vim. If you want to use a different text editor, such as Emacs, you can do the following:
$ git config --global core.editor emacs
Another useful option you may want to configure is the default diff tool to use to resolve merge conflicts. Say you want to use vimdiff:
$ git config --global merge.tool vimdiff
Git accepts kdiff3, tkdiff, meld, xxdiff, emerge, vimdiff, gvimdiff, ecmerge, and opendiff as valid merge tools. You can also set up a custom tool; see Chapter 7 for more information about doing that.
If you want to check your settings, you can use the git config --list command to list all the settings Git can find at that point:
$ git config --list
user.name=Scott Chacon
user.email=schacon@gmail.com
color.status=auto
color.branch=auto
color.interactive=auto
color.diff=auto
...
You may see keys more than once, because Git reads the same key from different files (/etc/gitconfig and ~/.gitconfig, for example). In this case, Git uses the last value for each unique key it sees.
You can also check what Git thinks a specific key’s value is by typing git config {key}:
$ git config user.name
Scott Chacon
If you ever need help while using Git, there are three ways to get the manual page (manpage) help for any of the Git commands:
$ git help <verb>
$ git <verb> --help
$ man git-<verb>
For example, you can get the manpage help for the config command by running
$ git help config
These commands are nice because you can access them anywhere, even offline.
If the manpages and this book aren’t enough and you need in-person help, you can try the #git or #github channel on the Freenode IRC server (irc.freenode.net). These channels are regularly filled with hundreds of people who are all very knowledgeable about Git and are often willing to help.
You should have a basic understanding of what Git is and how it’s different from the CVCS you may have been using. You should also now have a working version of Git on your system that’s set up with your personal identity. It’s now time to learn some Git basics.