第四章：探究操作系统探究操作系统

探究操作系统

 

Now that we know how to move around the file system, it’s time for a guided tour of ourLinux system. Before we start however, we’re going to learn some more commands thatwill be useful along the way:

既然我们已经知道了如何在文件系统中跳转，是时候开始 Linux 操作系统之旅了。然而在开始之前，我们先学习一些对研究Linux 系统有帮助的命令。

• ls – List directory contents

• file – Determine file type

• less – View file contents

• ls — 列出目录内容

• file — 确定文件类型

• less — 浏览文件内容

ls 乐趣

The ls command is probably the most used command, and for good reason. With it, wecan see directory contents and determine a variety of important file and directoryattributes. As we have seen, we can simply type ls to see a list of files andsubdirectories contained in the current working directory:

ls 可能是用户最常使用的命令了，这自有它的道理。通过它，我们可以知道目录的内容，以及各种各样重要文件和目录的属性。正如我们已经见到的，只要简单地输入 ls 就能看到在当前目录下所有文件和子目录的列表。

[me@linuxbox ~]$ ls
Desktop Documents Music Pictures Publica Templates Videos

Besides the current working directory, we can specify the directory to list, like so:

除了当前工作目录以外，也可以指定别的目录，就像这样：

me@linuxbox ~]$ ls /usr
bin games
kerberos
libexec
sbin
src
etc include lib
local
share
tmp

Or even specify multiple directories. In this example we will list both the user’s homedirectory (symbolized by the “~” character) and the /usr directory:

甚至可以列出多个指定目录的内容。在这个例子中，将会列出用户家目录（用字符“~”代表）和/usr 目录的内容：

[me@linuxbox ~]$ ls ~ /usr
/home/me:
Desktop
Documents
Music
Pictures
Public
Templates
Videos
/usr:
bin
games
kerberos
libexec
sbin
src
etc
include
lib
local
share
tmp

We can also change the format of the output to reveal more detail:

我们也可以改变输出格式，来得到更多的细节：

[me@linuxbox ~]$ ls -l
total 56
drwxrwxr-x 2
me
me
4096
2007-10-26
17:20
Desktop
drwxrwxr-x 2
me
me
4096
2007-10-26
17:20
Documents
drwxrwxr-x 2
me
me
4096
2007-10-26
17:20
Music
drwxrwxr-x 2
me
me
4096
2007-10-26
17:20
Pictures
drwxrwxr-x 2
me
me
4096
2007-10-26
17:20
Public
drwxrwxr-x 2
me
me
4096
2007-10-26
17:20
Templates
drwxrwxr-x 2
me
me
4096
2007-10-26
17:20
Videos

By adding “-l” to the command, we changed the output to the long format.

使用 ls 命令的“-l”选项，则结果以长模式输出。

选项和参数

This brings us to a very important point about how most commands work. Commands are oftenfollowed by one or more options that modify their behavior, and further, by one or more arguments,the items upon which the command acts. So most commands look kind of like this:

我们将学习一个非常重要的知识点，即大多数命令是如何工作的。命令名经常会带有一个或多个用来更正命令行为的选项，更进一步，选项后面会带有一个或多个参数，这些参数是命令作用的对象。所以大多数命令看起来像这样：

command -options arguments

Most commands use options consisting of a single character preceded by a dash,for example, “-l”, but many commands, including those from the GNU Project, also support long options,consisting of a word preceded by two dashes. Also, many commands allow multiple short options to be strung together. In this example, the ls command is given two options, the “l” optionto produce long format output, and the “t” option to sort the result by the file’s modification time.

大多数命令使用的选项，是由一个中划线加上一个字符组成，例如，“-l”，但是许多命令，包括来自于GNU 项目的命令，也支持长选项，长选项由两个中划线加上一个字组成。当然，许多命令也允许把多个短选项串在一起使用。下面这个例子，ls 命令有两个选项，“l” 选项产生长格式输出，“t”选项按文件修改时间的先后来排序。

[me@linuxbox ~]$ ls -lt

We’ll add the long option “–reverse” to reverse the order of the sort:

加上长选项 “–reverse”，则结果会以相反的顺序输出：

[me@linuxbox ~]$ ls -lt --reverse

The ls command has a large number of possible options. The most common are listed inthe Table 4-1.

ls 命令有大量的选项。表4-1列出了最常使用的选项。

Table 4-1: Common ls Options

Option	Long Option	Description
-a	--all	List all files, even those with names thatbegin with a period, which are normally not listed(i.e.,hidden).
-d	--directory	Ordinaryly,if a directory is specified, lswill list the contents of the directory, not the directoryitself. Use this option in conjunction with the -l optionto see details about the directory rather than its contents.
-F	--classify	This option will append an indicator characterto the end of each listed name. For example, a '/' if the name is a directory.
-h	--human-readable	In long format listings, display file sizes inhuman readable format rather than in bytes.
-l		Display results in long format.
-r	--reverse	Display the results in reverse order. Normally,ls display its results in ascending alphabetical order.
-S		Sort results by file size.
-t		Sort by modification time.

表 4-1: ls 命令选项

选项	长选项	描述
-a	--all	列出所有文件，甚至包括文件名以圆点开头的默认会被隐藏的隐藏文件。
-d	--directory	通常，如果指定了目录名，ls 命令会列出这个目录中的内容，而不是目录本身。把这个选项与 -l 选项结合使用，可以看到所指定目录的详细信息，而不是目录中的内容。
-F	--classify	这个选项会在每个所列出的名字后面加上一个指示符。例如，如果名字是目录名，则会加上一个'/'字符。
-h	--human-readable	当以长格式列出时，以人们可读的格式，而不是以字节数来显示文件的大小。
-l		以长格式显示结果。
-r	--reverse	以相反的顺序来显示结果。通常，ls 命令的输出结果按照字母升序排列。
-S		命令输出结果按照文件大小来排序。
-t		按照修改时间来排序。

深入研究长格式输出

As we saw before, the “-l” option causes ls to display its results in long format. Thisformat contains a great deal of useful information. Here is the Examples directoryfrom an Ubuntu system:

正如我们先前知道的，“-l”选项导致 ls 的输出结果以长格式输出。这种格式包含大量的有用信息。下面的例子目录来自于 Ubuntu 系统：

-rw-r--r-- 1 root root 3576296 2007-04-03 11:05 Experience ubuntu.ogg
-rw-r--r-- 1 root root 1186219 2007-04-03 11:05 kubuntu-leaflet.png
-rw-r--r-- 1 root root
47584 2007-04-03 11:05 logo-Edubuntu.png
-rw-r--r-- 1 root root
44355 2007-04-03 11:05 logo-Kubuntu.png
-rw-r--r-- 1 root root
34391 2007-04-03 11:05 logo-Ubuntu.png
-rw-r--r-- 1 root root
32059 2007-04-03 11:05 oo-cd-cover.odf
-rw-r--r-- 1 root root
159744 2007-04-03 11:05 oo-derivatives.doc
-rw-r--r-- 1 root root
27837 2007-04-03 11:05 oo-maxwell.odt
-rw-r--r-- 1 root root
98816 2007-04-03 11:05 oo-trig.xls
-rw-r--r-- 1 root root
453764 2007-04-03 11:05 oo-welcome.odt
-rw-r--r-- 1 root root
358374 2007-04-03 11:05 ubuntu Sax.ogg

Let’s look at the different fields from one of the files and examine their meanings:

选一个文件，来看一下各个输出字段的含义：

Table 4-2: ls Long Listing Fields

Field	Meaning
-rw-r--r--	Access rights to the file. The first character indicates thetype of file. Among the different types, a leading dash means a regular file,while a “d” indicates a directory. The next three characters are the accessrights for the file's owner, the next three are for members of the file'sgroup, and the final three are for everyone else. The full meaning of thisis discussed in Chapter 10 – Permissions.
1	File's number of hard links. See the discussion of linkslater in this chapter.
root	The user name of the file's owner.
root	The name of the group which owns the file.
32059	Size of the file in bytes.
2007-04-03 11:05	Date and time of the file's last modification.
oo-cd-cover.odf	Name of the file.

表 4-2: ls 长格式列表的字段

字段	含义
-rw-r--r--	对于文件的访问权限。第一个字符指明文件类型。在不同类型之间，开头的“－”说明是一个普通文件，“d”表明是一个目录。其后三个字符是文件所有者的访问权限，再其后的三个字符是文件所属组中成员的访问权限，最后三个字符是其他所有人的访问权限。这个字段的完整含义将在第十章讨论。
1	文件的硬链接数目。参考随后讨论的关于链接的内容。
root	文件所有者的用户名。
root	文件所属用户组的名字。
32059	以字节数表示的文件大小。
2007-04-03 11:05	上次修改文件的时间和日期。
oo-cd-cover.odf	文件名。

确定文件类型

As we explore the system it will be useful to know what files contain. To dothis we will use the file command to determine a file’s type. As we discussed earlier,filenames in Linux are not required to reflect a file’s contents. While a filename like“picture.jpg” would normally be expected to contain a JPEG compressed image, it is notrequired to in Linux. We can invoke the file command this way:

随着探究操作系统的进行，知道文件包含的内容是很有用的。我们将用 file 命令来确定文件的类型。我们之前讨论过，在 Linux 系统中，并不要求文件名来反映文件的内容。然而，一个类似 “picture.jpg” 的文件名，我们会期望它包含JPEG 压缩图像，但 Linux 却不这样要求它。可以这样调用 file 命令：

file filename

When invoked, the file command will print a brief description of the file’scontents. For example:

当调用 file 命令后，file 命令会打印出文件内容的简单描述。例如：

[me@linuxbox ~]$ file picture.jpg
picture.jpg: JPEG image data, JFIF standard 1.01

There are many kinds of files. In fact, one of the common ideas in Unix-like operatingsystems such as Linux is that “everything is a file.” As we proceed with our lessons, wewill see just how true that statement is.

有许多种类型的文件。事实上，在类 Unix 操作系统中比如说 Linux 中，有个普遍的观念就是“一切皆文件”。随着课程的进行，我们将会明白这句话是多么的正确。

While many of the files on your system are familiar, for example MP3 and JPEG, thereare many kinds that are a little less obvious and a few that are quite strange.

虽然系统中许多文件格式是熟悉的，例如 MP3和 JPEG 文件，但也有一些文件格式不太常见，极少数文件相当陌生。

用 less 浏览文件内容

The less command is a program to view text files. Throughout our Linux system, thereare many files that contain human-readable text. The less program provides aconvenient way to examine them.

less 命令是一个用来浏览文本文件的程序。纵观 Linux 系统，有许多人类可读的文本文件。less 程序为我们检查文本文件 提供了方便。

What Is “Text”

什么是“文本”

There are many ways to represent information on a computer. All methodsinvolve defining a relationship between the information and some numbers thatwill be used to represent it. Computers, after all, only understand numbers and alldata is converted to numeric representation.

在计算机中，有许多方法可以表达信息。所有的方法都涉及到，在信息与一些数字之间确立一种关系，而这些数字可以用来代表信息。毕竟，计算机只能理解数字，这样所有的数据都被转换成数值来表示。

Some of these representation systems are very complex (such as compressedvideo files), while others are rather simple. One of the earliest and simplest iscalled ASCII text. ASCII (pronounced “As-Key”) is short for American StandardCode for Information Interchange. This is a simple encoding scheme that was firstused on Teletype machines to map keyboard characters to numbers.

有些数值表达法非常复杂（例如压缩的视频文件），而其它的就相当简单。最早也是最简单的一种表达法，叫做ASCII 文本。ASCII（发音是”As-Key”）是美国信息交换标准码的简称。这是一个简单的编码方法，它首先被用在电传打字机上，用来实现键盘字符到数字的映射。

Text is a simple one-to-one mapping of characters to numbers. It is verycompact. Fifty characters of text translates to fifty bytes of data. It isimportant to understand that text only contains a simple mapping of charactersto numbers. It is not the same as a word processor document such as onecreated by Microsoft Word or OpenOffice.org Writer. Those files, in contrastto simple ASCII text, contain many non-text elements that are used to describeits structure and formatting. Plain ASCII text files contain only thecharacters themselves and a few rudimentary control codes like tabs, carriagereturns and line feeds. Throughout a Linux system, many files are stored intext format and there are many Linux tools that work with text files. EvenWindows recognizes the importance of this format. The well-known NOTEPAD.EXEprogram is an editor for plain ASCII text files.

文本是简单的字符与数字之间的一对一映射。它非常紧凑。五十个字符的文本翻译成五十个字节的数据。文本只是包含简单的字符到数字的映射，理解这点很重要。它和一些文字处理器文档不一样，比如说由微软或OpenOffice.org 文档 编辑器创建的文件。这些文件，和简单的 ASCII文件形成鲜明对比，它们包含许多非文本元素，来描述它的结构和格式。 纯 ASCII文件只包含字符本身，和一些基本的控制符，像制表符、回车符及换行符。纵观 Linux系统，许多文件以文本格式存储，也有许多 Linux 工具来处理文本文件。甚至 Windows也承认这种文件格式的重要性。著名的 NOTEPAD.EXE 程序就是一个纯 ASCII文本文件编辑器。

Why would we want to examine text files? Because many of the files that contain systemsettings (called configuration files) are stored in this format, and being able to read themgives us insight about how the system works. In addition, many of the actual programsthat the system uses (called scripts) are stored in this format. In later chapters, we willlearn how to edit text files in order to modify systems settings and write our own scripts,but for now we will just look at their contents.

为什么我们要查看文本文件呢？ 因为许多包含系统设置的文件（叫做配置文件），是以文本格式存储的，阅读它们可以更深入的了解系统是如何工作的。另外，许多系统所用到的实际程序（叫做脚本）也是以这种格式存储的。在随后的章节里，我们将要学习怎样编辑文本文件以修改系统设置，还要学习编写自己的脚本文件，但现在我们只是看看它们的内容而已。

The less command is used like this:

less 命令是这样使用的：

less filename

Once started, the less program allows you to scroll forward and backward through atext file. For example, to examine the file that defines all the system’s user accounts,enter the following command:

一旦运行起来，less 程序允许你前后滚动文件。例如，要查看一个定义了系统中全部用户身份的文件，输入以下命令：

[me@linuxbox ~]$ less /etc/passwd

Once the less program starts, we may view the contents of the file. If the file is longerthan one page, we can scroll up and down. To exit less, press the “q” key.The table below lists the most common keyboard commands used by less.

一旦 less 程序运行起来，我们就能浏览文件内容了。如果文件内容多于一页，那么我们可以上下滚动文件。按下“q”键，退出 less 程序。

下表列出了 less 程序最常使用的键盘命令。

Table 4-3: less Commands

Command	Action
Page UP or b	Scroll back one page
Page Down or space	Scroll forward one page
UP Arrow	Scroll Up one line
Down Arrow	Scrow Down one line
G	Move to the end of the text file
1G or g	Move to the beginning of the text file
/charaters	Search forward for the next occurrence of characters
n	Search forward for the next occurrence of the previous search
h	Display help screen
q	Quit less

表 4-3: less 命令

命令	行为
Page UP or b	向上翻滚一页
Page Down or space	向下翻滚一页
UP Arrow	向上翻滚一行
Down Arrow	向下翻滚一行
G	移动到最后一行
1G or g	移动到开头一行
/charaters	向前查找指定的字符串
n	向前查找下一个出现的字符串，这个字符串是之前所指定查找的
h	显示帮助屏幕
q	退出 less 程序

less 就是 more（禅语：色即是空）

The less program was designed as an improved replacement of an earlier Unixprogram called more. The name “less” is a play on the phrase “less is more”—amotto of modernist architects and designers.

less 程序是早期 Unix 程序 more 的改进版。“less” 这个名字，对习语 “less is more” 开了个玩笑，这个习语是现代主义建筑师和设计者的座右铭。

less falls into the class of programs called “pagers,” programs that allow theeasy viewing of long text documents in a page by page manner. Whereas themore program could only page forward, the less program allows paging bothforward and backward and has many other features as well.

less 属于”页面调度器”类程序，这些程序允许以逐页方式轻松浏览长文本文档。 more程序只能向前翻页，而 less 程序允许前后翻页，此外还有很多其它的特性。

旅行指南

The file system layout on your Linux system is much like that found on other Unix-likesystems. The design is actually specified in a published standard called the LinuxFilesystem Hierarchy Standard. Not all Linux distributions conform to the standardexactly but most come pretty close.

Linux 系统中，文件系统布局与类 Unix 系统的文件布局很相似。实际上，一个已经发布的标准，叫做 Linux 文件系统层次标准，详细说明了这种设计模式。不是所有Linux发行版都根据这个标准，但大多数都是。

Next, we are going to wander around the file system ourselves to see what makes ourLinux system tick. This will give you a chance to practice your navigation skills. One ofthe things we will discover is that many of the interesting files are in plain human-readable text. As we go about our tour, try the following:

下一步，我们将在文件系统中漫游，来了解 Linux 系统的工作原理。这会给你一个温习跳转命令的机会。我们会发现很多有趣的文件都是纯人类可读文本。下面旅行开始，做做以下练习：

1. cd into a given directory
2. List the directory contents with ls -l
3. If you see an interesting file, determine its contents with file

4. If it looks like it might be text, try viewing it with less

1. cd 到给定目录
2. 列出目录内容 ls -l
3. 如果看到一个有趣的文件，用 file 命令确定文件内容
4. 如果文件看起来像文本，试着用 less 命令浏览它

---

Remember the copy and paste trick! If you are using a mouse, you can doubleclick on a filename to copy it and middle click to paste it into commands.

记得复制和粘贴技巧！如果你正在使用鼠标，双击文件名，来复制它，然后按下鼠标中键，粘贴文件名到命令行中。

---

As we wander around, don’t be afraid to look at stuff. Regular users are largelyprohibited from messing things up. That’s the system administrators job! If a commandcomplains about something, just move on to something else. Spend some time lookingaround. The system is ours to explore. Remember, in Linux, there are no secrets!Table 4-4 lists just a few of the directories we can explore. Feel free to try more!

在系统中漫游时，不要害怕四处看看。普通用户是很难把东西弄乱的。那是系统管理员的工作！如果一个命令抱怨一些事情，不要管它，尝试一下别的东西。花一些时间四处看看。系统是我们自己的，尽情地探究吧。记住在 Linux 中，没有秘密存在！表4-4仅仅列出了一些我们可以浏览的目录。随意尝试更多！

Table 4-4: Directories Found On Linux Systems

Drectory	Comments
/	The root directory.Where everything begins.
/bin	Contains binaries (programs) that must be present forthe system to boot and run.
/boot	Contains the linux kernel, intial RAM disk image (fordrivers needed at boot time), and the boot loader. Interesting files: /boot/grub/grub.conf or menu.lst, which are used to configure the bootloader. /boot/vmlinuz,the linux kernel.
/dev	This is a special directory which contains device nodes."Everything is a file" also applies to devices. Here is where the kernelmaintains a list of all the devices it understands.
/etc	The /etc directory contains all of the system-wideconfiguration files. It also contains a collection of shell scripts which starteach of the system services at boot time. Everything in this directory shouldbe readable text. Interesting files:While everything in /etc is interesting, here are some of myall-time favorites: /etc/crontab, a file that defines when automated jobs will run. /etc/fstab, a table of storage devices and their associated mountpoints. /etc/passwd, a list of the user accounts.
/home	In normal configurations, each user is given a directory in/home. Ordinary users can only write files in their homedirectories. This limitation protects the system from errantuser activity.
/lib	Contains shared library files used by the core systemprograms. These are similar to DLLs in Windows.
/lost+found	Each formatted partition or device using a Linux file system,such as ext3, will have this directory. It is used in the caseof a partial recovery from a file system corruption event.Unless something really bad has happened to your system,this directory will remain empty.
/media	On modern Linux systems the /media directory willcontain the mount points for removable media such USB drives, CD-ROMs, etc.that are mounted automatically at insertion.
/mnt	On older Linux systems, the /mnt directory contains mountpoints for removable devices that have been mounted manually.
/opt	The /opt directory is used to install “optional” software.This is mainly used to hold commercial software productsthat may be installed on your system.
/proc	The /proc directory is special. It's not a real file system inthe sense of files stored on your hard drive. Rather, it is a virtual filesystem maintained by the Linux kernel. The “files” it contains are peepholesinto the kernel itself. The files are readable and will give you a picture of how thekernel sees your computer.
/root	This is the home directory for the root account.
/sbin	This directory contains “system” binaries. These are programsthat perform vital system tasks that are generally reserved for the superuser.
/tmp	The /tmp directory is intended for storage of temporary,transient files created by various programs. Some configurations cause thisdirectory to be emptied each time the system is rebooted.
/usr	The /usr directory tree is likely the largest one on a Linuxsystem. It contains all the programs and support files used by regular users.
/usr/bin	/usr/bin contains the executable programs installed by yourLinux distribution. It is not uncommon for this directory to hold thousands ofprograms.
/usr/lib	The shared libraries for the programs in /usr/bin.
/usr/local	The /usr/local tree is where programs that are notincluded with your distribution but are intended for system-wide use are installed. Programs compiled from source codeare normally installed in /usr/local/bin. On a newlyinstalled Linux system, this tree exists, but it will be emptyuntil the system administrator puts something in it.
/usr/sbin	Contains more system administration programs.
/usr/share	/usr/share contains all the shared data used byprograms in /usr/bin. This includes things like defaultconfiguration files, icons, screen backgrounds, sound files, etc.
/usr/share/doc	Most packages installed on the system will include somekind of documentation. In /usr/share/doc, we willfind documentation files organized by package.
/var	With the exception of /tmp and /home, the directories wehave looked at so far remain relatively static, that is, theircontents don't change. The /var directory tree is wheredata that is likely to change is stored. Various databases,spool files, user mail, etc. are located here.
/var/log	/var/log contains log files, records of various systemactivity. These are very important and should be monitoredfrom time to time. The most useful one is/var/log/messages. Note that for security reasons onsome systems, you must be the superuser to view log files.

表 4-4: Linux 系统中的目录

目录	评论
/	根目录，万物起源。
/bin	包含系统启动和运行所必须的二进制程序。
/boot	包含 Linux 内核、初始 RAM 磁盘映像（用于启动时所需的驱动）和启动加载程序。 有趣的文件： /boot/grub/grub.conf or menu.lst， 被用来配置启动加载程序。 /boot/vmlinuz，Linux 内核。
/dev	这是一个包含设备结点的特殊目录。“一切都是文件”，也适用于设备。在这个目录里，内核维护着所有设备的列表。
/etc	这个目录包含所有系统层面的配置文件。它也包含一系列的 shell 脚本，在系统启动时，这些脚本会开启每个系统服务。这个目录中的任何文件应该是可读的文本文件。 有趣的文件：虽然/etc 目录中的任何文件都有趣，但这里只列出了一些我一直喜欢的文件： /etc/crontab， 定义自动运行的任务。 /etc/fstab，包含存储设备的列表，以及与他们相关的挂载点。 /etc/passwd，包含用户帐号列表。
/home	在通常的配置环境下，系统会在/home 下，给每个用户分配一个目录。普通用户只能在自己的目录下写文件。这个限制保护系统免受错误的用户活动破坏。
/lib	包含核心系统程序所使用的共享库文件。这些文件与 Windows 中的动态链接库相似。
/lost+found	每个使用 Linux 文件系统的格式化分区或设备，例如 ext3文件系统，都会有这个目录。当部分恢复一个损坏的文件系统时，会用到这个目录。这个目录应该是空的，除非文件系统真正的损坏了。
/media	在现在的 Linux 系统中，/media 目录会包含可移动介质的挂载点，例如 USB 驱动器，CD-ROMs 等等。这些介质连接到计算机之后，会自动地挂载到这个目录结点下。
/mnt	在早些的 Linux 系统中，/mnt 目录包含可移动介质的挂载点。
/opt	这个/opt 目录被用来安装“可选的”软件。这个主要用来存储可能安装在系统中的商业软件产品。
/proc	这个/proc 目录很特殊。从存储在硬盘上的文件的意义上说，它不是真正的文件系统。相反，它是一个由 Linux 内核维护的虚拟文件系统。它所包含的文件是内核的窥视孔。这些文件是可读的，它们会告诉你内核是怎样监管计算机的。
/root	root 帐户的家目录。
/sbin	这个目录包含“系统”二进制文件。它们是完成重大系统任务的程序，通常为超级用户保留。
/tmp	这个/tmp 目录，是用来存储由各种程序创建的临时文件的地方。一些配置导致系统每次重新启动时，都会清空这个目录。
/usr	在 Linux 系统中，/usr 目录可能是最大的一个。它包含普通用户所需要的所有程序和文件。
/usr/bin	/usr/bin 目录包含系统安装的可执行程序。通常，这个目录会包含许多程序。
/usr/lib	包含由/usr/bin 目录中的程序所用的共享库。
/usr/local	这个/usr/local 目录，是非系统发行版自带程序的安装目录。通常，由源码编译的程序会安装在/usr/local/bin 目录下。新安装的 Linux 系统中会存在这个目录，并且在管理员安装程序之前，这个目录是空的。
/usr/sbin	包含许多系统管理程序。
/usr/share	/usr/share 目录包含许多由/usr/bin 目录中的程序使用的共享数据。其中包括像默认的配置文件、图标、桌面背景、音频文件等等。
/usr/share/doc	大多数安装在系统中的软件包会包含一些文档。在/usr/share/doc 目录下，我们可以找到按照软件包分类的文档。
/var	除了/tmp 和/home 目录之外，相对来说，目前我们看到的目录是静态的，这是说，它们的内容不会改变。/var 目录存放的是动态文件。各种数据库，假脱机文件，用户邮件等等，都位于在这里。
/var/log	这个/var/log 目录包含日志文件、各种系统活动的记录。这些文件非常重要，并且应该时时监测它们。其中最重要的一个文件是/var/log/messages。注意，为了系统安全，在一些系统中，你必须是超级用户才能查看这些日志文件。

符号链接

As we look around, we are likely to see a directory listing with an entry like this:

在我们到处查看时，我们可能会看到一个目录，列出像这样的一条信息：

lrwxrwxrwx 1 root root 11 2007-08-11 07:34 libc.so.6 -> libc-2.6.so

Notice how the first letter of the listing is “l” and the entry seems to have two filenames?This is a special kind of a file called a symbolic link (also known as a soft link orsymlink.) In most Unix-like systems it is possible to have a file referenced by multiplenames. While the value of this may not be obvious, it is really a useful feature.

注意看，为何这条信息第一个字符是“l”，并且有两个文件名呢？这是一个特殊文件，叫做符号链接（也称为软链接或者 symlink ）。 在大多数“类 Unix” 系统中，有可能一个文件被多个文件名所指向。虽然这种特性的意义并不明显，但它真的很有用。

Picture this scenario: a program requires the use of a shared resource of some kindcontained in a file named “foo,” but “foo” has frequent version changes. It would begood to include the version number in the filename so the administrator or otherinterested party could see what version of “foo” is installed. This presents a problem. Ifwe change the name of the shared resource, we have to track down every program thatmight use it and change it to look for a new resource name every time a new version ofthe resource is installed. That doesn’t sound like fun at all.

描绘一下这样的情景：一个程序要求使用某个包含在名为“foo”文件中的共享资源，但是“foo”经常改变版本号。这样，在文件名中包含版本号，会是一个好主意，因此管理员或者其它相关方，会知道安装了哪个“foo”版本。这会导致另一个问题。如果我们更改了共享资源的名字，那么我们必须跟踪每个可能使用了这个共享资源的程序，当每次这个资源的新版本被安装后，都要让使用了它的程序去寻找新的资源名。这听起来很没趣。

Here is where symbolic links save the day. Let’s say we install version 2.6 of “foo,”which has the filename “foo-2.6” and then create a symbolic link simply called “foo” thatpoints to “foo-2.6.” This means that when a program opens the file “foo”, it is actuallyopening the file “foo-2.6”. Now everybody is happy. The programs that rely on “foo”can find it and we can still see what actual version is installed. When it is time toupgrade to “foo-2.7,” we just add the file to our system, delete the symbolic link “foo”and create a new one that points to the new version. Not only does this solve the problemof the version upgrade, but it also allows us to keep both versions on our machine.Imagine that “foo-2.7” has a bug (damn those developers!) and we need to revert to theold version. Again, we just delete the symbolic link pointing to the new version andcreate a new symbolic link pointing to the old version.

这就是符号链接存在至今的原因。比方说，我们安装了文件 “foo” 的 2.6 版本，它的文件名是 “foo-2.6”，然后创建了叫做 “foo” 的符号链接，这个符号链接指向 “foo-2.6”。这意味着，当一个程序打开文件 “foo” 时，它实际上是打开文件 “foo-2.6”。现在，每个人都很高兴。依赖于 “foo” 文件的程序能找到这个文件，并且我们能知道安装了哪个文件版本。当升级到 “foo-2.7” 版本的时候，仅添加这个文件到文件系统中，删除符号链接 “foo”，创建一个指向新版本的符号链接。这不仅解决了版本升级问题，而且还允许在系统中保存两个不同的文件版本。假想 “foo-2.7” 有个错误（该死的开发者！），那我们得回到原来的版本。一样的操作，我们只需要删除指向新版本的符号链接，然后创建指向旧版本的符号链接就可以了。

The directory listing above (from the /lib directory of a Fedora system) shows asymbolic link called “libc.so.6” that points to a shared library file called “libc-2.6.so.”This means that programs looking for “libc.so.6” will actually get the file “libc-2.6.so.”We will learn how to create symbolic links in the next chapter.

在上面列出的目录（来自于 Fedora 的 /lib 目录）展示了一个叫做 “libc.so.6” 的符号链接，这个符号链接指向一个叫做 “libc-2.6.so” 的共享库文件。这意味着，寻找文件 “libc.so.6” 的程序，实际上得到是文件 “libc-2.6.so”。在下一章节，我们将学习如何建立符号链接。

硬链接

While we are on the subject of links, we need to mention that there is a second type oflink called a hard link. Hard links also allow files to have multiple names, but they do itin a different way. We’ll talk more about the differences between symbolic and hardlinks in the next chapter.

讨论到链接问题，我们需要提一下，还有一种链接类型，叫做硬链接。硬链接同样允许文件有多个名字，但是硬链接以不同的方法来创建多个文件名。在下一章中，我们会谈到更多符号链接与硬链接之间的差异问题。

拓展阅读

• The full version of the Linux Filesystem Hierarchy Standard can be found here:

• 完整的 Linux 文件系统层次体系标准可通过以下链接找到：

  http://www.pathname.com/fhs/

 

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