Subversion implements the concept of a version control repository much as any other modern version control system would. Unlike a working copy, a Subversion repository is an abstract entity, able to be operated upon almost exclusively by Subversion's own libraries and tools. As most of a user's Subversion interactions involve the use of the Subversion client and occur in the context of a working copy, we spend the majority of this book discussing the Subversion working copy and how to manipulate it. For the finer details of the repository, though, check out 第 5 章 版本库管理.

A Subversion client commits (that is, communicates the changes made to) any number of files and directories as a single atomic transaction. By atomic transaction, we mean simply this: either all of the changes are accepted into the repository, or none of them is. Subversion tries to retain this atomicity in the face of program crashes, system crashes, network problems, and other users' actions.

Each time the repository accepts a commit, this creates a new state of the filesystem tree, called a revision. Each revision is assigned a unique natural number, one greater than the number assigned to the previous revision. The initial revision of a freshly created repository is numbered 0 and consists of nothing but an empty root directory.

图 1.6 “Tree changes over time”可以更形象的描述版本库，想象有一组修订号，从 0 开始，从左到右，每一个修订号有一个目录树挂在它下面，每一个树好像是一次提交后的版本库“快照”。

图 1.6. Tree changes over time

Subversion client programs use URLs to identify versioned files and directories in Subversion repositories. For the most part, these URLs use the standard syntax, allowing for server names and port numbers to be specified as part of the URL.

Subversion repository URLs aren't limited to only the http:// variety. Because Subversion offers several different ways for its clients to communicate with its servers, the URLs used to address the repository differ subtly depending on which repository access mechanism is employed. 表 1.1 “版本库访问 URL” describes how different URL schemes map to the available repository access methods. For more details about Subversion's server options, see 第 6 章 服务配置.

Subversion's handling of URLs has some notable nuances. For example, URLs containing the file:// access method (used for local repositories) must, in accordance with convention, have either a server name of localhost or no server name at all:

同样，在 Windows 平台下使用 file:// 模式时需要使用一个非正式的“标准”语法来访问本机上不在同一个磁盘分区中的版本库。下面的任意一个 URL 路径语法都可以工作，其中的 X 表示版本库所在的磁盘分区：

Note that a URL uses forward slashes even though the native (non-URL) form of a path on Windows uses backslashes. Also note that when using the file:///X|/ form at the command line, you need to quote the URL (wrap it in quotation marks) so that the vertical bar character is not interpreted as a pipe.

	注意
	也必须意识到 Subversion 的 file:// URL 不能在普通的 web 服务器中工作。当你尝试在 web 服务器查看一个 file:// URL 时，它会通过直接检测文件系统读取和显示那个位置的文件内容，但是 Subversion 的资源存在于虚拟文件系统(见第 1.1 节 “版本库层”)中，你的浏览器不会理解怎样读取这个文件系统。

The Subversion client will automatically encode URLs as necessary, just like a web browser does. For example, the URL http://host/path with space/project/españa — which contains both spaces and upper-ASCII characters — will be automatically interpreted by Subversion as if you'd provided http://host/path%20with%20space/project/espa%C3%B1a. If the URL contains spaces, be sure to place it within quotation marks at the command line so that your shell treats the whole thing as a single argument to the program.

There is one notable exception to Subversion's handling of URLs which also applies to its handling of local paths in many contexts, too. If the final path component of your URL or local path contains an at sign (@), you need to use a special syntax—described in 第 2 节 “Peg 和实施修订版本”—in order to make Subversion properly address that resource.

In Subversion 1.6, a new caret (^) notation was introduced as a shorthand for “the URL of the repository's root directory”. For example, you can use the ^/tags/bigsandwich/ to refer to the URL of the /tags/bigsandwich directory in the root of the repository. Note that this URL syntax works only when your current working directory is a working copy—the command-line client knows the repository's root URL by looking at the working copy's metadata. Also note that when you wish to refer precisely to the root directory of the repository, you must do so using ^/ (with the trailing slash character), not merely ^.

一个 Subversion工 作副本是你本地机器上的一个普通目录，保存着一些文件，你可以任意的编辑文件，而且如果是源代码文件，你可以像平常一样编译，你的工作副本是你的私有工作区，在你明确的做了特定操作之前，Subversion 不会把你的修改与其他人的合并，也不会把你的修改展示给别人，你甚至可以拥有同一个项目的多个工作副本。

当你在工作副本作了一些修改并且确认它们工作正常之后，Subversion 提供了一个命令可以“发布”你的修改给项目中的其他人(通过写到版本库)，如果别人发布了各自的修改，Subversion 提供了手段可以把这些修改与你的工作目录进行合并(通过读取版本库)。

工作副本也包括一些由 Subversion 创建并维护的额外文件，用来协助执行命令。通常情况下，你的工作副本的每个文件夹都有一个以 .svn 为名的文件夹，也被叫做工作副本的管理目录，这个目录里的文件能够帮助 Subversion 识别哪些文件做过修改，哪些文件相对于别人的工作已经过期。

2.4.2. 工作副本基本交互操作

一个典型的 Subversion 版本库经常包含许多项目的文件(或者说源代码)，通常每一个项目都是版本库的子目录，在这种布局下，一个用户的工作副本往往对应版本库的的一个子目录。

举一个例子，你的版本库包含两个软件项目，paint 和 calc。每个项目在它们各自的顶级子目录下，见图 1.7 “版本库的文件系统”。

图 1.7. 版本库的文件系统

To get a working copy, you must check out some subtree of the repository. (The term check out may sound like it has something to do with locking or reserving resources, but it doesn't; it simply creates a working copy of the project for you.) For example, if you check out /calc, you will get a working copy like this:

$ svn checkout http://svn.example.com/repos/calc
A    calc/Makefile
A    calc/integer.c
A    calc/button.c
Checked out revision 56.
$ ls -A calc
Makefile  button.c integer.c .svn/
$

列表中的 A 表示 Subversion 增加了一些条目到工作副本，你现在有了一个 /calc 的个人拷贝，有一个附加的目录—.svn—保存着前面提及的 Subversion 需要的额外信息。

假定你修改了 button.c，因为 .svn 目录记录着文件的修改日期和原始内容，Subversion 可以告诉你已经修改了文件，然而，在你明确告诉它之前，Subversion 不会将你的改变公开，将改变公开的操作被叫做提交(committing，或者是检入)修改到版本库。

将你的修改发布给别人，你可以使用 Subversion 的 commit 命令：

$ svn commit button.c -m "Fixed a typo in button.c."
Sending        button.c
Transmitting file data .
Committed revision 57.
$

这时你对 button.c 的修改已经提交到了版本库，其中包含了关于此次提交的日志信息(例如是修改了拼写错误)。如果其他人取出了 /calc 的一个工作副本，他们会看到这个文件最新的版本。

假设你有个合作者 Sally，她和你同时取出了 /calc 的一个工作拷贝，你提交了对 button.c 的修改，Sally 的工作副本并没有改变，Subversion 只在用户要求的时候才改变工作副本。

要使项目最新，Sally 可以通过使用 svn update 命令，要求 Subversion 更新她的工作副本。这将结合你和所有其他人在她上次更新之后的改变到她的工作副本。

$ pwd
/home/sally/calc
$ ls -A
Makefile button.c integer.c .svn/
$ svn update
U    button.c
Updated to revision 57.
$

svn update 命令的输出表明 Subversion 更新了 button.c 的内容，注意，Sally 不必指定要更新的文件，subversion 利用 .svn 以及版本库的进一步信息决定哪些文件需要更新。