中英对照

• 1: 1 引言
• 2: 2 什么是 shell
• 3: 3 文件系统中跳转
• 4: 4 探究操作系统
• 5: 5 操作文件和目录
• 6: 6 使用命令
• 7: 7 重定向
• 8: 8 从 shell 眼中看世界
• 9: 9 键盘高级操作技巧
• 10: 10 权限
• 11: 11 进程
• 12: 12 shell 环境
• 13: 13 vi 简介
• 14: 14 自定制 shell 提示符
• 15: 15 软件包管理
• 16: 16 存储媒介
• 17: 17 网络系统
• 18: 18 查找文件
• 19: 19 归档和备份
• 20: 20 正则表达式
• 21: 21 文本处理
• 22: 22 格式化输出
• 23: 23 打印
• 24: 24 编译程序
• 25: 25 编写第一个 Shell 脚本
• 26: 26 启动一个项目
• 27: 27 自顶向下设计
• 28: 28 流程控制：if 分支结构
• 29: 29 读取键盘输入
• 30: 30 流程控制：while/until 循环
• 31: 31 疑难排解
• 32: 32 流程控制：case 分支
• 33: 33 位置参数
• 34: 34 流程控制：for 循环
• 35: 35 字符串和数字
• 36: 36 数组
• 37: 37 奇珍异宝

1 - 1 引言

引言

http://billie66.github.io/TLCL/book/chap01.html

I want to tell you a story.

​ 我想给大家讲个故事。

No, not the story of how, in 1991, Linus Torvalds wrote the first version of the Linux kernel. You can read that story in lots of Linux books. Nor am I going to tell you the story of how, some years earlier, Richard Stallman began the GNU Project to create a free Unix-like operating system. That’s an important story too, but most other Linux books have that one, as well.

​ 故事内容不是 Linus Torvalds 在1991年怎样写了 Linux 内核的第一个版本， 因为这些内容你可以在许多 Linux 书籍中读到。我也不是来告诉你，更早之前，Richard Stallman 是如何开始 GNU 项目，设计了一个免费的类 Unix 的操作系统。那也是一个很有意义的故事， 但大多数 Linux 书籍也讲到了它。

No, I want to tell you the story of how you can take back control of your computer.

​ 我想告诉大家一个你如何才能夺回计算机管理权的故事。

When I began working with computers as a college student in the late 1970s, there was a revolution going on. The invention of the microprocessor had made it possible for ordinary people like you and me to actually own a computer. It’s hard for many people today to imagine what the world was like when only big business and big government ran all the computers. Let’s just say, you couldn’t get much done.

​ 在20世纪70年代末，我刚开始和计算机打交道时，正进行着一场革命，那时的我还是一名大学生。 微处理器的发明，使普通老百姓（就如你和我）真正拥有一台计算机成为可能。今天， 人们难以想象，只有大企业和强大的政府才能够拥有计算机的世界，是怎样的一个世界。 简单说，你做不了多少事情。

Today, the world is very different. Computers are everywhere, from tiny wristwatches to giant data centers to everything in between. In addition to ubiquitous computers, we also have a ubiquitous network connecting them together. This has created a wondrous new age of personal empowerment and creative freedom, but over the last couple of decades something else has been happening. A single giant corporation has been imposing its control over most of the world’s computers and deciding what you can and cannot do with them. Fortunately, people from all over the world are doing something about it. They are fighting to maintain control of their computers by writing their own software. They are building Linux.

​ 今天，世界已经截然不同了。计算机遍布各个领域，从小手表到大型数据中心，及大小介于它们之间的每件东西。 除了随处可见的计算机之外，我们还有一个无处不在的连接所有计算机的网络。这已经开创了一个奇妙的， 个人授权和创作自由的新时代，但是在过去的二三十年里，正在发生另一些事情。一个大公司不断地把它的 管理权强加到世界上绝大多数的计算机上，并且决定你对计算机的操作权力。幸运地是，来自世界各地的人们， 正积极努力地做些事情来改变这种境况。通过编写自己的软件，他们一直在为维护电脑的管理权而战斗着。 他们建设着 Linux。

Many people speak of “freedom” with regard to Linux, but I don’t think most people know what this freedom really means. Freedom is the power to decide what your computer does, and the only way to have this freedom is to know what your computer is doing. Freedom is a computer that is without secrets, one where everything can be known if you care enough to find out.

​ 一提到 Linux，许多人都会说到“自由”，但我不认为他们都知道“自由”的真正涵义。“自由”是一种权力， 它决定你的计算机能做什么，同时能够拥有这种“自由”的唯一方式就是知道计算机正在做什么。 “自由”是指一台没有任何秘密的计算机，你可以从它那里了解一切，只要你用心的去寻找。

为什么使用命令行

Have you ever noticed in the movies when the “super hacker,”— you know, the guy who can break into the ultra-secure military computer in under thirty seconds —sits down at the computer, he never touches a mouse? It’s because movie makers realize that we, as human beings, instinctively know the only way to really get anything done on a computer is by typing on a keyboard.

​ 你是否注意到，在电影中一个“超级黑客”坐在电脑前，从不摸一下鼠标， 就能够在30秒内侵入到超安全的军用计算机中。这是因为电影制片人意识到， 作为人类，本能地知道让计算机圆满完成工作的唯一途径，是用键盘来操纵计算机。

Most computer users today are only familiar with the graphical user interface (GUI) and have been taught by vendors and pundits that the command line interface (CLI) is a terrifying thing of the past. This is unfortunate, because a good command line interface is a marvelously expressive way of communicating with a computer in much the same way the written word is for human beings. It’s been said that “graphical user interfaces make easy tasks easy, while command line interfaces make difficult tasks possible” and this is still very true today.

​ 现在，大多数的计算机用户只是熟悉图形用户界面（GUI），并且产品供应商和此领域的学者会灌输给用户这样的思想， 命令行界面（CLI）是过去使用的一种很恐怖的东西。这就很不幸，因为一个好的命令行界面， 是用来和计算机进行交流沟通的非常有效的方式，正像人类社会使用文字互通信息一样。人们说，“图形用户界面让简单的任务更容易完成， 而命令行界面使完成复杂的任务成为可能”，到现在这句话仍然很正确。

Since Linux is modeled after the Unix family of operating systems, it shares the same rich heritage of command line tools as Unix. Unix came into prominence during the early 1980s (although it was first developed a decade earlier), before the widespread adoption of the graphical user interface and, as a result, developed an extensive command line interface instead. In fact, one of the strongest reasons early adopters of Linux chose it over, say, Windows NT was the powerful command line interface which made the “difficult tasks possible.”

​ 因为 Linux 是以 Unix 家族的操作系统为模型写成的，所以它分享了 Unix 丰富的命令行工具。 Unix 在20世纪80年代初显赫一时(虽然，开发它在更早之前），这使得图形界面的流行之前，命令行界面就已被广泛应用。 事实上，很多人选择 Linux（而不是其他的系统，比如说 Windows NT）是因为其可以使“完成复杂的任务成为可能”的强大的命令行界面。

这本书讲什么

This book is a broad overview of “living” on the Linux command line. Unlike some books that concentrate on just a single program, such as the shell program, bash, this book will try to convey how to get along with the command line interface in a larger sense. How does it all work? What can it do? What’s the best way to use it?

​ 这本书介绍如何生存在 Linux 命令行的世界。不像一些书籍仅仅涉及一个程序，比如像 shell 程序，bash。 本书着眼于更宏大的视角，试着向你传授如何与命令行界面友好相处。 它是怎样工作的？ 它能做什么？ 使用它的最好方法是什么？

This is not a book about Linux system administration. While any serious discussion of the command line will invariably lead to system administration topics, this book only touches on a few administration issues. It will, however, prepare the reader for additional study by providing a solid foundation in the use of the command line, an essential tool for any serious system administration task.

​ 这不是一本关于 Linux 系统管理的书。然而任何一个关于命令行的深入讨论，都一定会牵涉到 系统管理方面的内容，这本书仅仅提到一点儿管理方面的知识。但是这本书为读者准备好了学习更多内容的坚实基础， 毕竟要胜任系统管理工作也需要良好的命令行使用基本功。

This book is very Linux-centric. Many other books try to broaden their appeal by including other platforms such as generic Unix and MacOS X. In doing so, they “water down” their content to feature only general topics. This book, on the other hand, only covers contemporary Linux distributions. Ninety-five percent of the content is useful for users of other Unix-like systems, but this book is highly targeted at the modern Linux command line user.

​ 这本书是围绕 Linux 而写的。许多书籍，为了扩大自身的影响力，会包含一些其它平台的知识， 比如 Unix, MacOS X 等。这样做，很多内容只能比较空泛的去讲了。另一方面， 这本书只研究了当代 Linux 发行版。虽然，对于使用其它类 Unix 系统的用户来说， 书中95％的内容是有用的，但这本书主要面向的对象是现代 Linux 命令行用户。

谁应该读这本书

This book is for new Linux users who have migrated from other platforms. Most likely you are a “power user” of some version of Microsoft Windows. Perhaps your boss has told you to administer a Linux server, or maybe you’re just a desktop user who is tired of all the security problems and want to give Linux a try. That’s fine.here. All are welcome.

​ 这本书是为从其它平台迁移到 Linux 系统的新手而写的。可能你是使用某个版本 Windows 系统的高手, 或许是老板让你去管理一个 Linux 服务器，或许你只是一个桌面用户，厌倦了系统出现的各种 安全问题而想要体验一下 Linux。很好，这里欢迎你们！

That being said, there is no shortcut to Linux enlightenment. Learning the command line is challenging and takes real effort. It’s not that it’s so hard, but rather it’s so vast. The average Linux system has literally thousands of programs you can employ on the command line. Consider yourself warned; learning the command line is not a casual endeavor.

​ 不过一般来说，对于 Linux 的启蒙教育，没有捷径可言。学习命令行富于挑战性，而且很费气力。 这并不是说 Linux 命令行很难学，而是它的知识量很大，不容易掌握。Linux 操作系统 差不多有数以千计的命令可供用户操作。有必要给你提个醒，命令行可不是轻轻松松就能学好的。

On the other hand, learning the Linux command line is extremely rewarding. If you think you’re a “power user” now, just wait. You don’t know what real power is — yet. And, unlike many other computer skills, knowledge of the command line is long lasting. The skills learned today will still be useful ten years from now. The command line has survived the test of time.

​ 另一方面，学习 Linux 命令行会让你受益匪浅，给你极大的回报。如果你认为 现在你已经是高手了。别急，其实你还不知道什么才是真正的高手。不像其他一些计算机技能， 一段时间之后可能就被淘汰了，命令行知识却不会落伍，你今天所学到的，在十年以后 都会有用处。命令行通过了时间的考验。

It is also assumed that you have no programming experience, but not to worry, we’ll start you down that path as well.

​ 如果你没有编程经验，也不要担心，我会带你入门。

这本书的内容

This material is presented in a carefully chosen sequence, much like a tutor sitting next to you guiding you along. Many authors treat this material in a “systematic” fashion, which makes sense from a writer’s perspective, but can be very confusing to new users.

​ 这些材料是经过精心安排的，很像一位老师坐在你身旁，耐心地指导你。 许多作者用系统化的方式讲解这些材料，虽然从一个作者的角度考虑很有道理，但对于 Linux 新手来说， 他们可能会感到非常困惑。

Another goal is to acquaint you with the Unix way of thinking, which is different from the Windows way of thinking. Along the way, we’ll go on a few side trips to help you understand why certain things work the way they do and how they got that way. Linux is not just a piece of software, it’s also a small part of the larger Unix culture, which has its own language and history. I might throw in a rant or two, as well.

​ 另一个目的，是想让读者熟悉 Unix 的思维方式，这种思维方式与 Windows 不同。在学习过程中， 我们会帮助你理解为什么某些命令那样工作，以及它们是如何工作的。 Linux 不仅是一款软件，也是 Unix 文化的一小部分，它有自己的语言和历史。关于这些，书中我会提到一些。

This book is divided into five parts, each covering some aspect of the command line experience. Besides the first part, which you are reading now, this book contains:

​ 这本书共分为五部分，每一部分讲述了不同方面的命令行知识。除了第一部分， 也就是你正在阅读的这一部分，这本书还包括：

• Part 2 – Learning The Shell starts our exploration of the basic language of the command line including such things as the structure of commands, file system navigation, command line editing, and finding help and documentation for commands.
• Part 3 – Configuration And The Environment covers editing configuration files that control the computer’s operation from the command line.
• Part 4 – Common Tasks And Essential Tools explores many of the ordinary tasks that are commonly performed from the command line. Unix-like operating systems, such as Linux, contain many “classic” command line programs that are used to perform powerful operations on data.
• Part 5 – Writing Shell Scripts introduces shell programming, an admittedly rudimentary, but easy to learn, technique for automating many common computing tasks. By learning shell programming, you will become familiar with concepts that can be applied to many other programming languages.
• 第二部分 — 学习 shell 开始探究命令行基本语言，包括命令组成结构， 文件系统浏览，编写命令行，查找命令帮助文档。
• 第三部分 — 配置文件及环境 讲述了如何编写配置文件，通过配置文件，用命令行来 操控计算机。
• 第四部分 — 常见任务及主要工具 探究了许多命令行经常执行的普通任务。类似于 Unix 的操作系统，例如 Linux, 包括许多经典的命令行程序，这些程序可以用来对数据进行 强大的操作。
• 第五部分 — 编写 Shell 脚本 介绍了 shell 编程，一个无可否认的基本技能，能够自动化许多 常见的计算任务，很容易学。通过学习 shell 编程，你会逐渐熟悉一些关于编程语言方面的概念， 这些概念也适用于其他的编程语言。

怎样阅读这本书

Start at the beginning of the book and follow it to the end. It isn’t written as a reference work, it’s really more like a story with a beginning, middle, and an end.

​ 从头到尾的阅读。它并不是一本技术参考手册，实际上它更像一本故事书，有开头，过程，结尾。

前提条件

To use this book, all you will need is a working Linux installation. You can get this in one of two ways:

​ 为了使用这本书，你需要安装 Linux 操作系统。你可以通过两种方式，来完成安装。

1. Install Linux on a (not so new) computer. It doesn’t matter which distribution you choose, though most people today start out with either Ubuntu, Fedora, or OpenSUSE. If in doubt, try Ubuntu first. Installing a modern Linux distribution can be ridiculously easy or ridiculously difficult depending on your hardware. I suggest a desktop computer that is a couple of years old and has at least 256 megabytes of RAM and 6 gigabytes of free hard disk space. Avoid laptops and wireless networks if at all possible, as these are often more difficult to get working.
2. Use a “Live CD.” One of the cool things you can do with many Linux distributions is run them directly from a CDROM without installing them at all. Just go into your BIOS setup and set your computer to “Boot from CDROM,” insert the live CD, and reboot. Using a live CD is a great way to test a computer for Linux compatibility prior to installation. The disadvantage of using a live CD is that it may be very slow compared to having Linux installed on your hard drive. Both Ubuntu and Fedora (among others) have live CD versions.
3. 在一台（不用很新）的电脑上安装 Linux。你选择哪个 Linux 发行版安装，是无关紧要的事。 虽然大多数人一开始选择安装 Ubuntu, Fedora, 或者 OpenSUSE。如果你拿不定主意，那就先试试 Ubuntu。 由于主机硬件配置不同，安装 Linux 时，你可能不费吹灰之力就装上了，也可能费了九牛二虎之力还装不上。 所以我建议，一台使用了几年的台式机，至少要有256M 的内存，6G 的硬盘可用空间。尽可能避免安装时使用 笔记本电脑和无线网络，它们经常不能工作。
4. 使用“Live CD.” 许多 Linux 发行版都自带一个比较酷的功能，你可以直接从系统安装盘 CDROM 中运行 Linux， 而不必安装 Linux。开机进入 BIOS 设置界面，更改引导项，设置为“从 CDROM 启动”。插入 live CD，然后重启。 采用 live CD 而非直接安装可以很好的测试你的电脑对 linux 的兼容性。缺点就是相较于在硬盘上安装 linux，这种方式 过程较慢。Ubuntu 和 Fedora 等都有 live CD 的版本。

Regardless of how you install Linux, you will need to have occasional superuser (i.e., administrative) privileges to carry out the lessons in this book.

​ 不管你怎样安装 Linux，为了练习书中介绍的知识，你需要有超级用户（管理员）权限。

After you have a working installation, start reading and follow along with your own computer. Most of the material in this book is “hands on,” so sit down and get typing!

​ 当你在自己的电脑上安装了 Linux 系统之后，就开始一边阅读本书，一边练习吧。本书大部分内容 都可以自己动手练习，坐下来，敲入命令，体验一下吧。

Why I Don’t Call It “GNU/Linux”

In some quarters, it’s politically correct to call the Linux operating system the “GNU/Linux operating system.” The problem with “Linux” is that there is no completely correct way to name it because it was written by many different people in a vast, distributed development effort. Technically speaking, Linux is the name of the operating system’s kernel, nothing more. The kernel is very important of course, since it makes the operating system go, but it’s not enough to form a complete operating system.

Enter Richard Stallman, the genius-philosopher who founded the Free Software movement, started the Free Software Foundation, formed the GNU Project, wrote the first version of the GNU C Compiler (gcc), created the GNU General Public License (the GPL), etc., etc., etc. He insists that you call it “GNU/Linux” to properly reflect the contributions of the GNU Project. While the GNU Project predates the Linux kernel, and the project’s contributions are extremely deserving of recognition, placing them in the name is unfair to everyone else who made significant contributions. Besides, I think “Linux/GNU” would be more technically accurate since the kernel boots first and everything else runs on top of it.

In popular usage, “Linux” refers to the kernel and all the other free and open source software found in the typical Linux distribution; that is, the entire Linux ecosystem, not just the GNU components. The operating system marketplace seems to prefer one-word names such as DOS, Windows, MacOS, Solaris, Irix, AIX. I have chosen to use the popular format. If, however, you prefer to use “GNU/Linux” instead, please perform a mental search and replace while reading this book. I won’t mind.

为什么我不叫它“GNU/Linux”

​ 在某些人眼里，把 Linux 操作系统称为“GNU/Linux 操作系统”，才是政治正确。但“Linux”的问题是， 没有一个完全正确的方式能命名它，因为它是由许许多多，分布在世界各地的贡献者们，合作开发而成的。 从技术层面讲，Linux 只是操作系统的内核名字。当然内核非常重要，因为有它， 操作系统才能运行起来，但它并不能构成一个完备的操作系统。

​ Richard Stallman 是一个天才的哲学家，自由软件运动创始人，自由软件基金会创办者，他创建了 GNU 项目， 编写了第一版 GNU C 编译器（gcc），创立了 GNU 通用公共协议（the GPL)等等。 他坚持把 Linux 称为“GNU/Linux”，为的是准确地反映 GNU 项目对 Linux 操作系统的贡献。 然而，尽管 GNU 项目早于 Linux 内核，项目的贡献应该得到极高的赞誉，但是把 GNU 用在 Linux 名字里， 这对其他为 Linux 的发展做出重大贡献的程序员来说，就不公平了。而且，我觉得要是叫也要叫 “Linux/GNU” 比较准确一些， 因为内核会先启动，其他一切都运行在内核之上。

​ 在目前流行的用法中，“Linux”指的是内核以及在一个典型的 Linux 发行版中所包含的所有免费及开源软件； 也就是说，整个 Linux 生态系统，不只有 GNU 项目软件。在操作系统商界，好像喜欢使用单个词的名字， 比如说 DOS, Windows, MacOS, Solaris, Irix, AIX. 所以我选择用流行的命名规则。然而， 如果你喜欢用“GNU/Linux”，当你读这本书时，可以在脑子里搜索并替换“Linux”。我不介意。

拓展阅读

Here are some Wikipedia articles on the famous people mentioned in this chapter:

​ Wikipedia 网站上有些介绍本章提到的名人的文章，以下是链接地址：

• http://en.wikipedia.org/wiki/Linux_Torvalds
• http://en.wikipedia.org/wiki/Richard_Stallman

The Free Software Foundation and the GNU Project:

​ 介绍自由软件基金会及 GNU 项目的网站和文章：

• http://en.wikipedia.org/wiki/Free_Software_Foundation
• http://www.fsf.org
• http://www.gnu.org

Richard Stallman has written extensively on the “GNU/Linux” naming issue:

​ Richard Stallman 用了大量的文字来叙述“GNU/Linux”的命名问题，可以浏览以下网页：

• http://www.gnu.org/gnu/why-gnu-linux.html
• http://www.gnu.org/gnu/gnu-linux-faq.html#tools

2 - 2 什么是 shell

什么是 shell

http://billie66.github.io/TLCL/book/chap02.html

When we speak of the command line, we are really referring to the shell. The shell is a program that takes keyboard commands and passes them to the operating system to carry out. Almost all Linux distributions supply a shell program from the GNU Project called bash. The name “bash” is an acronym for “Bourne Again SHell”, a reference to the fact bash is an enhanced replacement for sh, the original Unix shell program written by Steve Bourne.

​ 一说到命令行，我们真正指的是 shell。shell 就是一个程序，它接受从键盘输入的命令， 然后把命令传递给操作系统去执行。几乎所有的 Linux 发行版都提供一个名为 bash 的 来自 GNU 项目的 shell 程序。“bash” 是 “Bourne Again SHell” 的首字母缩写， 是最初在 Unix 上由 Steve Bourne 写成 shell 程序 sh 的增强版。

终端仿真器

When using a graphical user interface, we need another program called a terminal emulator to interact with the shell. If we look through our desktop menus, we will probably find one. KDE uses konsole and GNOME uses gnome-terminal, though it’s likely called simply “terminal” on our menu. There are a number of other terminal emulators available for Linux, but they all basically do the same thing; give us access to the shell. You will probably develop a preference for one or another based on the number of bells and whistles it has.

​ 当使用图形用户界面时，我们需要另一个和 shell 交互的叫做终端仿真器的程序。 如果我们浏览一下桌面菜单，可能会找到一个。虽然在菜单里它可能都 被简单地称为 “terminal”。 KDE 用的是 konsole , GNOME 则使用 gnome-terminal。 还有其他一些终端仿真器可供 Linux 使用，但基本上，它们都完成同样的事情， 让我们能访问 shell。也许，你可能会因为它附加的一系列花俏功能而喜欢上某个终端仿真器。

第一次按键

So let’s get started. Launch the terminal emulator! Once it comes up, we should see somehing like this:

​ 好，开始吧。启动终端仿真器！一旦它运行起来，我们应该看到一行像这样的文字：

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[me@linuxbox ~]$

This is called a shell prompt and it will appear whenever the shell is ready to accept input. While it may vary in appearance somewhat depending on the distribution, it will usually include your username@machinename, followed by the current working directory (more about that in a little bit) and a dollar sign.

​ 这叫做 shell 提示符，当 shell 准备好了去接受输入时，它就会出现。然而， 它可能会以各种各样的面孔显示，这则取决于不同的 Linux 发行版， 它通常包括你的用户名@主机名，紧接着当前工作目录（稍后会有更多介绍）和一个美元符号。

If the last character of the prompt is a pound sign (“#”) rather than a dollar sign, the terminal session has superuser privileges. This means either we are logged in as the root user or we selected a terminal emulator that provides superuser (administrative) privileges.

​ 如果提示符的最后一个字符是“#”, 而不是“$”, 那么这个终端会话就有超级用户权限。 这意味着，我们要么是以 root 用户的身份登录，要么是我们选择的终端仿真器提供超级用户（管理员）权限。

Assuming that things are good so far, let’s try some typing. Type some gibberish at the prompt like so:

假定到目前为止，所有事情都进展顺利，那我们试着键入字符吧。在提示符下敲入 一些像下面一样的乱七八糟的字符：

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[me@linuxbox ~]$ kaekfjaeifj

Since this command makes no sense, the shell will tell us so and give us another chance:

​ 因为这个命令没有任何意义，所以 shell 会提示错误信息，并让我们再试一下：

bash: kaekfjaeifj: command not found
[me@linuxbox ~]$

命令历史

If we press the up-arrow key, we will see that the previous command “kaekfjaeifj” reappears after the prompt. This is called command history. Most Linux distributions remember the last five hundred commands by default. Press the down-arrow key and the previous command disappears.

​ 如果按下上箭头按键，我们会看到刚才输入的命令“kaekfjaeifj”重新出现在提示符之后。 这就叫做命令历史。许多 Linux 发行版默认保存最后输入的500个命令。 按下下箭头按键，先前输入的命令就消失了。

移动光标

Recall the previous command with the up-arrow key again. Now try the left and right-arrow keys. See how we can position the cursor anywhere on the command line? This makes editing commands easy.

​ 可借助上箭头按键，来获得上次输入的命令。现在试着使用左右箭头按键。 看一下怎样把光标定位到命令行的任意位置？使用箭头按键可以使编辑命令变得轻松些。

关于鼠标和光标

While the shell is all about the keyboard, you can also use a mouse with your terminal emulator. There is a mechanism built into the X Window System (the underlying engine that makes the GUI go) that supports a quick copy and paste technique. If you highlight some text by holding down the left mouse button and dragging the mouse over it (or double clicking on a word), it is copied into a buffer maintained by X. Pressing the middle mouse button will cause the text to be pasted at the cursor location. Try it.

​ 虽然，shell 是和键盘打交道的，但你也可以在终端仿真器里使用鼠标。X 窗口系统 （使 GUI 工作的底层引擎）内建了一种机制，支持快速拷贝和粘贴技巧。 如果你按下鼠标左键，沿着文本拖动鼠标（或者双击一个单词）高亮了一些文本， 那么这些高亮的文本就被拷贝到了一个由 X 管理的缓冲区里面。然后按下鼠标中键， 这些文本就被粘贴到光标所在的位置。试试看。

Note: Don’t be tempted to use Ctrl-c and Ctrl-v to perform copy and paste inside a terminal window. They don’t work. These control codes have different meanings to the shell and were assigned many years before Microsoft Windows.

​ 注意： 不要在一个终端窗口里使用 Ctrl-c 和 Ctrl-v 快捷键来执行拷贝和粘贴操作。 它们不起作用。对于 shell 来说，这两个控制代码有着不同的含义，它们在早于 Microsoft Windows （定义复制粘贴的含义）许多年之前就赋予了不同的意义。

Your graphical desktop environment (most likely KDE or GNOME), in an effort to behave like Windows, probably has its focus policy set to “click to focus.” This means for a window to get focus (become active) you need to click on it. This is contrary to the traditional X behavior of “focus follows mouse” which means that a window gets focus by just passing the mouse over it. The window will not come to the foreground until you click on it but it will be able to receive input. Setting the focus policy to “focus follows mouse” will make the copy and paste technique even more useful. Give it a try. I think if you give it a chance you will prefer it. You will find this setting in the configuration program for your window manager.

​ 你的图形桌面环境（像 KDE 或 GNOME），努力想和 Windows 一样，可能会把它的聚焦策略 设置成“单击聚焦”。这意味着，为了让窗口聚焦（变成活动窗口）你需要单击它。 这与“焦点跟随着鼠标”的传统 X 行为不同，传统 X 行为是指只要把鼠标移动到一个窗口的上方。 它能接受输入， 但是直到你单击窗口之前它都不会成为前端窗口。 设置聚焦策略为“聚焦跟随着鼠标”，可以使拷贝和粘贴更方便易用。尝试一下。 我想如果你试了一下你会喜欢上它的。你能在窗口管理器的配置中找到这个设置。

试试运行一些简单命令

Now that we have learned to type, let’s try a few simple commands. The first one is date. This command displays the current time and date.

​ 现在，我们学习了怎样输入命令，那我们执行一些简单的命令吧。第一个命令是 date。 这个命令显示系统当前时间和日期。

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[me@linuxbox ~]$ date
Thu Oct 25 13:51:54 EDT 2007

A related command is cal which, by default, displays a calendar of the current month.

​ 一个相关联的命令，cal，它默认显示当前月份的日历。

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[me@linuxbox ~]$ cal
October 2007
Su Mo Tu We Th Fr Sa
1 2 3 4 5 6
7 8 9 10 11 12 13
14 15 16 17 18 19 20
21 22 23 24 25 26 27
28 29 30 31

To see the current amount of free space on your disk drives, type df:

​ 查看磁盘剩余空间的数量，输入 df:

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[me@linuxbox ~]$ df
Filesystem           1K-blocks      Used Available Use% Mounted on
/dev/sda2             15115452   5012392   9949716  34% /
/dev/sda5             59631908  26545424  30008432  47% /home
/dev/sda1               147764     17370   122765   13% /boot
tmpfs                   256856         0   256856    0% /dev/shm

Likewise, to display the amount of free memory, type the free command.

​ 同样地，显示空闲内存的数量，输入命令 free 。

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[me@linuxbox ~]$ free
total       used       free     shared    buffers     cached
Mem:       2059676     846456    1213220          0
44028      360568
-/+ buffers/cache:     441860    1617816
Swap:      1042428          0    1042428

结束终端会话

We can end a terminal session by either closing the terminal emulator window, or by entering the exit command at the shell prompt:

​ 我们可以通过关闭终端仿真器窗口，或者是在 shell 提示符下输入 exit 命令来终止一个终端会话：

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[me@linuxbox ~]$ exit

幕后控制台

Even if we have no terminal emulator running, several terminal sessions continue to run behind the graphical desktop. Called virtual terminals or virtual consoles, these sessions can be accessed on most Linux distributions by pressing Ctrl- Alt-F1 through Ctrl-Alt-F6 on most systems. When a session is accessed, it presents a login prompt into which we can enter our user name and password. To switch from one virtual console to another, press Alt and F1-F6. To return to the graphical desktop, press Alt-F7.

​ 即使终端仿真器没有运行，在后台仍然有几个终端会话运行着。它们叫做虚拟终端 或者是虚拟控制台。在大多数 Linux 发行版中，这些终端会话都可以通过按下 Ctrl-Alt-F1 到 Ctrl-Alt-F6 访问。当一个会话被访问的时候， 它会显示登录提示框，我们需要输入用户名和密码。要从一个虚拟控制台转换到另一个， 按下 Alt 和 F1-F6(中的一个)。返回图形桌面，按下 Alt-F7。

拓展阅读

To learn more about Steve Bourne, father of the Bourne Shell, see this Wikipedia article:

​ 想了解更多关于 Steve Bourne 的故事，Bourne Shell 之父，读一下这篇文章：

http://en.wikipedia.org/wiki/Steve_Bourne

Here is an article about the concept of shells in computing:

​ 这是一篇关于在计算机领域里，shell 概念的文章：

http://en.wikipedia.org/wiki/Shell_(computing)

3 - 3 文件系统中跳转

文件系统中跳转

http://billie66.github.io/TLCL/book/chap03.html

The first thing we need to learn to do (besides just typing) is how to navigate the file system on our Linux system. In this chapter we will introduce the following commands:

​ 我们需要学习的第一件事（除了打字之外）是如何在 Linux 文件系统中跳转。 在这部分，我们将介绍以下命令：

• pwd - Print name of current working directory
• cd - Change directory
• ls - List directory contents
• pwd — 打印出当前工作目录名
• cd — 更改目录
• ls — 列出目录内容

理解文件系统树

Like Windows, a Unix-like operating system such as Linux organizes its files in what is called a hierarchical directory structure. This means that they are organized in a tree-like pattern of directories (sometimes called folders in other systems), which may contain files and other directories. The first directory in the file system is called the root directory. The root directory contains files and subdirectories, which contain more files and subdirectories and so on and so on.

​ 类似于 Windows，一个“类 Unix” 的操作系统，比如说 Linux，以分层目录结构来组织所有文件。 这就意味着所有文件组成了一棵树型目录（有时候在其它系统中叫做文件夹）， 这个目录树可能包含文件和其它的目录。文件系统中的第一级目录称为根目录。 根目录包含文件和子目录，子目录包含更多的文件和子目录，依此类推。

Note that unlike Windows, which has a separate file system tree for each storage device, Unix-like systems such as Linux always have a single file system tree, regardless of how many drives or storage devices are attached to the computer. Storage devices are attached (or more correctly, mounted) at various points on the tree according to the whims of the system administrator, the person (or persons) responsible for the maintenance of the system.

​ 注意(类 Unix 系统)不像 Windows ，每个存储设备都有一个独自的文件系统树。类 Unix 操作系统， 比如 Linux，总是只有一个单一的文件系统树，不管有多少个磁盘或者存储设备连接到计算机上。 根据负责维护系统安全的系统管理员的兴致，存储设备连接到（或着更精确些，是挂载到）目录树的各个节点上。

当前工作目录

Most of us are probably familiar with a graphical file manager which represents the file system tree as in Figure 1. Notice that the tree is usually shown upended, that is, with the root at the top and the various branches descending below.

图1: 由图形化文件管理器显示的文件系统树

​ 大多数人都可能熟悉如图1所示描述文件系统树的图形文件管理器。注意， 通常这是一棵 倒置的树，也就是说，树根在最上面，而各个枝干在下面展开。

However, the command line has no pictures, so to navigate the file system tree we need to think of it in a different way.

​ 然而，命令行不能显示图像，所以我们需要把文件系统树想象成别的样子（而不是图片中的这个形象）。

Imagine that the file system is a maze shaped like an upside-down tree and we are able to stand in the middle of it. At any given time, we are inside a single directory and we can see the files contained in the directory and the pathway to the directory above us (called the parent directory) and any subdirectories below us. The directory we are standing in is called the current working directory. To display the current working directory, we use the pwd (print working directory) command.

​ 把文件系统想象成一个迷宫形状，就像一棵倒立的大树，我们站在迷宫的中间位置。 在任意时刻，我们处于一个目录里面，我们能看到这个目录包含的所有文件， 以及通往上面目录（父目录）的路径，和下面的各个子目录。我们所在的目录则称为 当前工作目录。我们使用 pwd（print working directory(的缩写)）命令，来显示当前工作目录。

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[me@linuxbox ~]$ pwd
/home/me

When we first log in to our system (or start a terminal emulator session) our current working directory is set to our home directory. Each user account is given its own home directory and when operating as a regular user, the home directory is the only place the user is allowed to write files.

​ 当我们首次登录系统（或者启动终端仿真器会话）后，当前工作目录是我们的家目录。 每个用户都有他自己的家目录，当用户以普通用户的身份操控系统时，家目录是唯一 允许用户写入文件的地方。

列出目录内容

To list the files and directories in the current working directory, we use the ls command.

​ 列出一个目录包含的文件及子目录，使用 ls 命令。

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

Actually, we can use the ls command to list the contents of any directory, not just the current working directory, and there are many other fun things it can do as well. We’ll spend more time with ls in the next chapter.

​ 实际上，用 ls 命令可以列出任一个目录的内容，而不只是当前工作目录的内容。 ls 命令还能完成许多有趣的事情。在下一章节，我们将介绍更多关于 ls 的知识。

更改当前工作目录

To change your working directory (where we are standing in our tree-shaped maze) we use the cd command. To do this, type cd followed by the pathname of the desired working directory. A pathname is the route we take along the branches of the tree to get to the directory we want. Pathnames can be specified in one of two different ways; as absolute pathnames or as relative pathnames. Let’s deal with absolute pathnames first.

​ 要更改工作目录（此刻，我们站在树形迷宫里面），我们用 cd 命令。输入 cd, 然后输入你想要去的工作目录的路径名。路径名就是沿着目录树的分支 到达想要的目录期间所经过的路线。路径名可通过两种方式来指定，一种是绝对路径， 另一种是相对路径。我们先来介绍绝对路径。

绝对路径

An absolute pathname begins with the root directory and follows the tree branch by branch until the path to the desired directory or file is completed. For example, there is a directory on your system in which most of your system’s programs are installed. The pathname of the directory is /usr/bin. This means from the root directory (represented by the leading slash in the pathname) there is a directory called “usr” which contains a directory called “bin”.

​ 绝对路径开始于根目录，紧跟着目录树的一个个分支，一直到达所期望的目录或文件。 例如，你的系统中有一个目录，大多数系统程序都安装在这个目录下。这个目录的 路径名是 /usr/bin。它意味着从根目录（用开头的”/”表示）开始，有一个叫 “usr” 的 目录包含了目录 “bin”。

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[me@linuxbox ~]$ cd /usr/bin
[me@linuxbox bin]$ pwd
/usr/bin
[me@linuxbox bin]$ ls
...Listing of many, many files ...

Now we can see that we have changed the current working directory to /usr/bin and that it is full of files. Notice how the shell prompt has changed? As a convenience, it is usually set up to automatically display the name of the working directory.

​ 我们把工作目录转到 /usr/bin 目录下，里面装满了文件。注意 shell 提示符是怎样改变的吗？ 为了方便，通常终端提示符自动显示工作目录。

相对路径

Where an absolute pathname starts from the root directory and leads to its destination, a relative pathname starts from the working directory. To do this, it uses a couple of special symbols to represent relative positions in the file system tree. These special symbols are “.” (dot) and “..” (dot dot).

​ 绝对路径从根目录开始，直到它的目的地，而相对路径开始于工作目录。 为了做到这个（用相对路径表示）， 我们在文件系统树中用一对特殊符号来表示相对位置。 这对特殊符号是 “.” (点) 和 “..” (点点)。

The “.” symbol refers to the working directory and the “..” symbol refers to the working directory’s parent directory. Here is how it works. Let’s change the working directory to /usr/bin again:

​ 符号 “.” 指的是工作目录，”..” 指的是工作目录的父目录。举个例子， 让我们再次把工作目录切换到 /usr/bin：

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[me@linuxbox ~]$ cd /usr/bin
[me@linuxbox bin]$ pwd
/usr/bin

Okay, now let’s say that we wanted to change the working directory to the parent of /usr/bin which is /usr. We could do that two different ways. Either with an absolute pathname:

​ 好了，比方说我们想更改工作目录到 /usr/bin 的父目录 /usr。可以通过两种方法来实现。可以使用以下绝对路径名：

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[me@linuxbox bin]$ cd /usr
[me@linuxbox usr]$ pwd
/usr

Or, with a relative pathname:

​ 或者， 也可以使用相对路径：

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[me@linuxbox bin]$ cd ..
[me@linuxbox usr]$ pwd
/usr

Two different methods with identical results. Which one should we use? The one that requires the least typing!

​ 两种不同的方法，一样的结果。我们应该选哪一个呢？ 选输入量最少的那个！

Likewise, we can change the working directory from /usr to /usr/bin in two different ways. Either using an absolute pathname:

​ 同样地，从目录 /usr/ 到 /usr/bin 也有两种途径。可以使用绝对路径：

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[me@linuxbox usr]$ cd /usr/bin
[me@linuxbox bin]$ pwd
/usr/bin

Or, with a relative pathname:

​ 或者，也可以用相对路径：

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[me@linuxbox usr]$ cd ./bin
[me@linuxbox bin]$ pwd
/usr/bin

Now, there is something important that I must point out here. In almost all cases, you can omit the “./”. It is implied. Typing:

​ 有一件很重要的事，我必须指出来。在几乎所有的情况下，你可以省略”./”。它是隐含的。输入：

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[me@linuxbox usr]$ cd bin

does the same thing. In general, if you do not specify a pathname to something, the working directory will be assumed.

​ 可以实现相同的效果。总的来说，如果不指定一个文件的路径，那它被默认为在当前工作目录下。

有用的快捷键

In table 3-1 we see some useful ways the current working directory can be quickly changed.

​ 在表3-1中，列举出了一些快速改变当前工作目录的有效方法。

Important Facts About Filenames

1. Filenames that begin with a period character are hidden. This only means that ls will not list them unless you say ls -a. When your account was created, several hidden files were placed in your home directory to configure things for your account. Later on we will take a closer look at some of these files to see how you can customize your environment. In addition, some applications place their configuration and settings files in your home directory as hidden files.
2. Filenames and commands in Linux, like Unix, are case sensitive. The filenames “File1” and “file1” refer to different files.
3. Linux has no concept of a “file extension” like some other operating systems. You may name files any way you like. The contents and/or purpose of a file is determined by other means. Although Unix-like operating system don’t use file extensions to determine the contents/purpose of files, some application programs do.
4. Though Linux supports long filenames which may contain embedded spaces and punctuation characters, limit the punctuation characters in the names of files you create to period, dash, and underscore. Most importantly, do not embed spaces in filenames. If you want to represent spaces between words in a filename, use underscore characters. You will thank yourself later.

关于文件名的重要规则

1. 以 “.” 字符开头的文件名是隐藏文件。这仅表示，ls 命令不能列出它们， 用 ls -a 命令就可以了。当你创建帐号后，几个配置帐号的隐藏文件被放置在 你的家目录下。稍后，我们会仔细研究一些隐藏文件，来定制你的系统环境。 另外，一些应用程序也会把它们的配置文件以隐藏文件的形式放在你的家目录下面。
2. 文件名和命令名是大小写敏感的。文件名 “File1” 和 “file1” 是指两个不同的文件名。
3. Linux 没有“文件扩展名”的概念，不像其它一些系统。可以用你喜欢的任何名字 来给文件起名。文件内容或用途由其它方法来决定。虽然类 Unix 的操作系统， 不用文件扩展名来决定文件的内容或用途，但是有些应用程序会。
4. 虽然 Linux 支持长文件名，文件名可能包含空格，标点符号，但标点符号仅限 使用 “.”，“－”，下划线。最重要的是，不要在文件名中使用空格。如果你想表示词与 词间的空格，用下划线字符来代替。将来你就会明白这样做的好处。

4 - 4 探究操作系统

探究操作系统

http://billie66.github.io/TLCL/book/chap04.html

Now that we know how to move around the file system, it’s time for a guided tour of our Linux system. Before we start however, we’re going to learn some more commands that will 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, we can see directory contents and determine a variety of important file and directory attributes. As we have seen, we can simply type ls to see a list of files and subdirectories 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。

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[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 home directory (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：

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[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：

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[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 often followed 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，看看大多数命令都是如何工作的。命令名经常会带有一个或多个用来改变命令行为的选项， 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” option to 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。

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[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：

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

The ls command has a large number of possible options. The most common are listed in the 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。

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. This format contains a great deal of useful information. Here is the Examples directory from 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：

确定文件类型

As we explore the system it will be useful to know what files contain. To do this 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 not required 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’s contents. 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：

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[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 operating systems such as Linux is that “everything is a file.” As we proceed with our lessons, we will 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, there are 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, there are many files that contain human-readable text. The less program provides a convenient 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 methods involve defining a relationship between the information and some numbers that will be used to represent it. Computers, after all, only understand numbers and all data 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 compressed video files), while others are rather simple. One of the earliest and simplest is called ASCII text. ASCII (pronounced “As-Key”) is short for American Standard Code for Information Interchange. This is a simple encoding scheme that was first used on Teletype machines to map keyboard characters to numbers.

​ 有些表达法非常复杂（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 very compact. Fifty characters of text translates to fifty bytes of data. It is important to understand that text only contains a simple mapping of characters to numbers. It is not the same as a word processor document such as one created by Microsoft Word or OpenOffice.org Writer. Those files, in contrast to simple ASCII text, contain many non-text elements that are used to describe its structure and formatting. Plain ASCII text files contain only the characters themselves and a few rudimentary control codes like tabs, carriage returns and line feeds. Throughout a Linux system, many files are stored in text format and there are many Linux tools that work with text files. Even Windows recognizes the importance of this format. The well-known NOTEPAD.EXE program 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。它和一些常见办公文档编辑软件，比如说由微软 Office 或 OpenOffice.org，创建的文字内容不同。和简单的 ASCII 文件形成鲜明对比，这些文档内容中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 system settings (called configuration files) are stored in this format, and being able to read them gives us insight about how the system works. In addition, many of the actual programs that the system uses (called scripts) are stored in this format. In later chapters, we will learn 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 a text 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：

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

Once the less program starts, we may view the contents of the file. If the file is longer than 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。

少就是多

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

​ less 程序是早期 Unix 程序 more 的改进版。“less” 这个名字，套用习语 “less is more” ， 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 the easy viewing of long text documents in a page by page manner. Whereas the more program could only page forward, the less program allows paging both forward 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-like systems. The design is actually specified in a published standard called the Linux Filesystem Hierarchy Standard. Not all Linux distributions conform to the standard exactly 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 文件系统层级标准，详细说明了这种设计模式。不是所有 Linux 发行版都遵守这个标准，但 大多数都是。

Next, we are going to wander around the file system ourselves to see what makes our Linux system tick. This will give you a chance to practice your navigation skills. One of the 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游览，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

5. cd 到给定目录

6. 列出目录内容 ls -l

7. 如果看到一个有趣的文件，用 file 命令确定文件内容

8. 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 double click 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 largely prohibited from messing things up. That’s the system administrators job! If a command complains about something, just move on to something else. Spend some time looking around. 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!

在系统中游览时，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！

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 or symlink.) In most Unix-like systems it is possible to have a file referenced by multiple names. 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。这个特性实际中真的很有用。

Picture this scenario: a program requires the use of a shared resource of some kind contained in a file named “foo,” but “foo” has frequent version changes. It would be good to include the version number in the filename so the administrator or other interested party could see what version of “foo” is installed. This presents a problem. If we change the name of the shared resource, we have to track down every program that might use it and change it to look for a new resource name every time a new version of the 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” that points to “foo-2.6.” This means that when a program opens the file “foo”, it is actually opening 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 to upgrade 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 problem of 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 the old version. Again, we just delete the symbolic link pointing to the new version and create a new symbolic link pointing to the old version.

​ 符号链接避免了这种情况。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 a symbolic 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 of link called a hard link. Hard links also allow files to have multiple names, but they do it in a different way. We’ll talk more about the differences between symbolic and hard links 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 文件系统层级标准可通过以下链接找到：

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

5 - 5 操作文件和目录

操作文件和目录

http://billie66.github.io/TLCL/book/chap05.html

At this point, we are ready for some real work! This chapter will introduce the following commands:

​ 此时此刻，我们已经准备好了做些真正的工作！这一章节将会介绍以下命令：

• cp – Copy files and directories
• mv – Move/rename files and directories
• mkdir – Create directories
• rm – Remove files and directories
• ln – Create hard and symbolic links
• cp — 复制文件和目录
• mv — 移动/重命名文件和目录
• mkdir — 创建目录
• rm — 删除文件和目录
• ln — 创建硬链接和符号链接

These five commands are among the most frequently used Linux commands. They are used for manipulating both files and directories.

​ 这五个命令属于最常使用的 Linux 命令之列。它们用来操作文件和目录。

Now, to be frank, some of the tasks performed by these commands are more easily done with a graphical file manager. With a file manager, we can drag and drop a file from one directory to another, cut and paste files, delete files, etc. So why use these old command line programs?

​ 现在，坦诚地说，用图形文件管理器来完成一些由这些命令执行的任务会更容易些。使用文件管理器， 我们可以把文件从一个目录拖放到另一个目录、剪贴和粘贴文件、删除文件等等。那么， 为什么还使用早期的命令行程序呢？

The answer is power and flexibility. While it is easy to perform simple file manipulations with a graphical file manager, complicated tasks can be easier with the command line programs. For example, how could we copy all the HTML files from one directory to another, but only copy files that do not exist in the destination directory or are newer than the versions in the destination directory? Pretty hard with a file manager. Pretty easy with the command line:

​ 答案是命令行程序，功能强大灵活。虽然图形文件管理器能轻松地实现简单的文件操作，但是对于 复杂的文件操作任务，则使用命令行程序比较容易完成。例如，怎样拷贝一个目录下的 HTML 文件到目标目录，同时保证只拷贝目标目录不存在或者版本比目标目录的文件更新的文件？ 要完成这个任务，使用文件管理器相当难，使用命令行相当容易：

cp -u *.html destination

通配符

Before we begin using our commands, we need to talk about a shell feature that makes these commands so powerful. Since the shell uses filenames so much, it provides special characters to help you rapidly specify groups of filenames. These special characters are called wildcards. Using wildcards (which is also known as globbing) allow you to select filenames based on patterns of characters. The table below lists the wildcards and what they select:

​ 在开始使用命令之前，我们需要介绍一个使命令行变得非常强大的 shell 特性。因为 shell 频繁地使用 文件名，shell 提供了特殊字符来帮助你快速指定一组文件名。这些特殊字符叫做通配符。 通配符允许你依据字符的组合模式来选择文件名。下表列出这些通配符 以及它们所选择的对象：

Table 5-2 lists the most commonly used character classes:

​ 表5-2列出了最常使用的字符类：

Using wildcards makes it possible to construct very sophisticated selection criteria for filenames. Here are some examples of patterns and what they match:

借助通配符，为文件名构建非常复杂的选择标准成为可能。下面是一些类型匹配的范例:

Wildcards can be used with any command that accepts filenames as arguments, but we’ll talk more about that in Chapter 8.

​ 接受文件名作为参数的任何命令，都可以使用通配符，我们会在第八章更深入地谈到这个知识点。

Character Ranges

字符范围

If you are coming from another Unix-like environment or have been reading some other books on this subject, you may have encountered the [A-Z] or the [a-z] character range notations. These are traditional Unix notations and worked in older versions of Linux as well. They can still work, but you have to be very careful with them because they will not produce the expected results unless properly configured. For now, you should avoid using them and use character classes instead.

如果你用过别的类 Unix 系统的操作环境，或者是读过这方面的书籍，你可能遇到过[A-Z]或 [a-z]形式的字符范围表示法。这些都是传统的 Unix 表示法，并且在早期的 Linux 版本中仍有效。 虽然它们仍然起作用，但是你必须小心地使用它们，因为它们不会产生你期望的输出结果，除非 你合理地配置它们。从现在开始，你应该避免使用它们，并且用字符类来代替它们。

Wildcards Work In The GUI Too

通配符在 GUI 中也有效

Wildcards are especially valuable not only because they are used so frequently on the command line, but are also supported by some graphical file managers.

通配符非常重要，不仅因为它们经常用在命令行中，而且一些图形文件管理器也支持它们。

• In Nautilus (the file manager for GNOME), you can select files using the Edit/Select Pattern menu item. Just enter a file selection pattern with wildcards and the files in the currently viewed directory will be highlighted for selection.
• In Dolphin and Konqueror (the file managers for KDE), you can enter wildcards directly on the location bar. For example, if you want to see all the files starting with a lowercase “u” in the /usr/bin directory, type “/usr/bin/u*” into the location bar and it will display the result.
• 在 Nautilus (GNOME 文件管理器）中，可以通过 Edit/Select 模式菜单项来选择文件。 输入一个用通配符表示的文件选择模式后，那么当前所浏览的目录中，所匹配的文件名就会高亮显示。
• 在 Dolphin 和 Konqueror（KDE 文件管理器）中，可以在地址栏中直接输入通配符。例如， 如果你想查看目录 /usr/bin 中，所有以小写字母 ‘u’ 开头的文件， 在地址栏中敲入 ‘/usr/bin/u*‘，则 文件管理器会显示匹配的结果。

Many ideas originally found in the command line interface make their way into the graphical interface, too. It is one of the many things that make the Linux desktop so powerful.

最初源于命令行界面中的想法，在图形界面中也适用。这就是 Linux 桌面系统 如此强大的众多原因之一。

mkdir - 创建目录

The mkdir command is used to create directories. It works like this:

​ mkdir 命令是用来创建目录的。它这样工作：

mkdir directory...

A note on notation: When three periods follow an argument in the description of a command (as above), it means that the argument can be repeated, thus:

​ 注意: 在描述一个命令时（如上所示），当有三个圆点跟在一个命令的参数后面， 这意味着那个参数可以跟多个，就像这样：

mkdir dir1

would create a single directory named “dir1”, while

​ 会创建一个名为”dir1”的目录，而

mkdir dir1 dir2 dir3

would create three directokries named “dir1”, “dir2”, “dir3”.

会创建三个目录，名为 dir1, dir2, dir3。

cp - 复制文件和目录

The cp command copies files or directories. It can be used two different ways:

​ cp 命令，复制文件或者目录。它有两种使用方法：

cp item1 item2

to copy the single file or directory “item1” to file or directory “item2” and:

​ 复制单个文件或目录”item1”到文件或目录”item2”，和：

cp item... directory

to copy multiple items (either files or directories) into a directory.

​ 复制多个项目（文件或目录）到一个目录下。

有用的选项和实例

Here are some of the commonly used options (the short option and the equivalent long option) for cp:

​ 这里列举了 cp 命令一些有用的选项（短选项和等效的长选项）：

mv - 移动和重命名文件

The mv command performs both file moving and file renaming, depending on how it is used. In either case, the original filename no longer exists after the operation. mv is used in much the same way as cp:

​ mv 命令可以执行文件移动和文件命名任务，这取决于你怎样使用它。任何一种 情况下，完成操作之后，原来的文件名不再存在。mv 使用方法与 cp 很相像：

mv item1 item2

to move or rename file or directory “item1” to “item2” or:

​ 把文件或目录 “item1” 移动或重命名为 “item2”, 或者：

mv item... directory

to move one or more items from one directory to another.

​ 把一个或多个条目从一个目录移动到另一个目录中。

有用的选项和实例

mv shares many of the same options as cp:

​ mv 与 cp 共享了很多一样的选项：

rm - 删除文件和目录

The rm command is used to remove(delete)files and directories:

​ rm 命令用来删除文件和目录：

rm item...

where “item” is one or more files or directories.

​ “item”代表一个或多个文件或目录。

有用的选项和实例

Here are some of the common options for rm:

​ 下表是一些普遍使用的 rm 选项：

Be Careful With rm!

小心 rm!

Unix-like operating systems such as Linux do not have an undelete command. Once you delete something with rm, it’s gone. Linux assumes you’re smart and you know what you’re doing.

​ 类 Unix 的操作系统，比如说 Linux，没有复原命令。一旦你用 rm 删除了一些东西， 它就消失了。Linux 假定你很聪明，你知道你在做什么。

Be particularly careful with wildcards. Consider this classic example. Let’s say you want to delete just the HTML files in a directory. To do this, you type:

​ 尤其要小心通配符。思考一下这个经典的例子。假如说，你只想删除一个目录中的 HTML 文件。输入：

rm *.html

which is correct, but if you accidentally place a space between the “*” and the “.html” like so:

​ 这是正确的，如果你不小心在 “*” 和 “.html” 之间多输入了一个空格，就像这样：

rm * .html

the rm command will delete all the files in the directory and then complain that there is no file called “.html”.

​ 这个 rm 命令会删除目录中的所有文件，还会抱怨没有文件叫做 “.html”。

Here is a useful tip. Whenever you use wildcards with rm (besides carefully checking your typing!), test the wildcard first with ls. This will let you see the files that will be deleted. Then press the up arrow key to recall the command and replace the ls with rm.

小贴士。 当你使用带有通配符的 rm 命令时（除了仔细检查输入的内容外）， 先用 ls 命令来测试通配符。这会让你看到将要被删除的文件是什么。然后按下上箭头按键，重新调用 刚刚执行的命令，用 rm 替换 ls。

ln — 创建链接

The ln command is used to create either hard or symbolic links. It is used in one of two ways:

​ ln 命令既可创建硬链接，也可以创建符号链接。可以用两者中的任意一种形式来使用它：

ln file link

to create a hard link, and:

​ 创建硬链接，和：

ln -s item link

to create a symbolic link where “item” is either a file or a directory.

​ 创建符号链接，”item” 可以是一个文件或是一个目录。

硬链接

Hard links are the original Unix way of creating links; symbolic links are more modern. By default, every file has a single hard link that gives the file its name. When we create a hard link, we create an additional directory entry for a file. Hard links have two important limitations:

​ 与更加现代的符号链接相比，硬链接是最初 Unix 创建链接的方式。每个文件默认会有一个硬链接， 这个硬链接给予文件名字。我们每创建一个硬链接，就为一个文件创建了一个额外的目录项。 硬链接有两个重要局限性：

1. A hard link cannot reference a file outside its own file system. This means a link may not reference a file that is not on the same disk partition as the link itself.

2. A hard link may not reference a directory.

3. 一个硬链接不能关联它所在文件系统之外的文件。这是说一个链接不能关联 与链接本身不在同一个磁盘分区上的文件。

4. 一个硬链接不能关联一个目录。

A hard link is indistinguishable from the file itself. Unlike a symbolic link, when you list a directory containing a hard link you will see no special indication of the link. When a hard link is deleted, the link is removed but the contents of the file itself continue to exist (that is, its space is not deallocated) until all links to the file are deleted. It is important to be aware of hard links because you might encounter them from time to time, but modern practice prefers symbolic links, which we will cover next.

​ 一个硬链接和文件本身表面上看不出什么区别。它跟符号链接很不一样，当你列出一个包含硬链接的目录 内容时，你会看不到有什么特殊说明来表示这是一个链接。当一个硬链接被删除时，这个链接 被删除，但是文件本身的内容仍然存在（这是说，它所占的磁盘空间不会被释放）， 直到所有关联这个文件的链接都删除掉。知道硬链接很重要，因为你可能有时 会遇到它们，但现在实际中更喜欢使用符号链接，下一步我们会讨论符号链接。

符号链接

Symbolic links were created to overcome the limitations of hard links. Symbolic links work by creating a special type of file that contains a text pointer to the referenced file or directory. In this regard, they operate in much the same way as a Windows shortcut though of course, they predate the Windows feature by many years ;-)

​ 创建符号链接是为了克服硬链接的局限性。符号链接生效，是通过创建一个 特殊类型的文件，这个文件包含一个关联文件或目录的文本指针。在这一方面， 它们和 Windows 的快捷方式差不多，当然，符号链接早于 Windows 的快捷方式 很多年;-)

A file pointed to by a symbolic link, and the symbolic link itself are largely indistinguishable from one another. For example, if you write some something to the symbolic link, the referenced file is also written to. However when you delete a symbolic link, only the link is deleted, not the file itself. If the file is deleted before the symbolic link, the link will continue to exist, but will point to nothing. In this case, the link is said to be broken. In many implementations, the ls command will display broken links in a distinguishing color, such as red, to reveal their presence.

​ 一个符号链接指向一个文件，而且这个符号链接本身与其它的符号链接几乎没有区别。 例如，如果你往一个符号链接里面写入东西，那么相关联的文件也被写入。然而， 当你删除一个符号链接时，只有这个链接被删除，而不是文件自身。如果先于符号链接 删除文件，这个链接仍然存在，但是不指向任何东西。在这种情况下，这个链接被称为 坏链接。在许多实现中，ls 命令会以不同的颜色展示坏链接，比如说红色，来显示它们 的存在。

The concept of links can seem very confusing, but hang in there. We’re going to try all this stuff and it will, hopefully, become clear.

​ 关于链接的概念，看起来很迷惑，但不要胆怯。我们将要努力地练习 这些命令而且尽可能得使它变得清晰。

创建游戏场（实战演习）

Since we are going to do some real file manipulation, let’s build a safe place to “play” with our file manipulation commands. First we need a directory to work in. We’ll create one in our home directory and call it “playground.”

​ 下面我们将要做些真正的文件操作，让我们先建立一个安全地带， 来玩一下文件操作命令。首先，我们需要一个工作目录。在我们的 家目录下创建一个叫做“playground”的目录。

创建目录

The mkdir command is used to create a directory. To create our playground directory we will first make sure we are in our home directory and will then create the new directory:

​ mkdir 命令被用来创建目录。首先确定我们在我们的家目录下，然后创建 playground 目录：

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[me@linuxbox ~]$ cd
[me@linuxbox ~]$ mkdir playground

To make our playground a little more interesting, let’s create a couple of directories inside it called “dir1” and “dir2”. To do this, we will change our current working directory to playground and execute another mkdir:

​ 为了让我们的游戏场更加有趣，在 playground 目录下创建一对目录 ，分别叫做 “dir1” 和 “dir2”。更改我们的当前工作目录到 playground，然后 执行 mkdir 命令：

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[me@linuxbox ~]$ cd playground
[me@linuxbox playground]$ mkdir dir1 dir2

Notice that the mkdir command will accept multiple arguments allowing us to create both directories with a single command.

​ 注意到 mkdir 命令可以接受多个参数，它允许我们用一个命令来创建这两个目录。

复制文件

Next, let’s get some data into our playground. We’ll do this by copying a file. Using the cp command, we’ll copy the passwd file from the /etc directory to the current working directory:

​ 下一步，让我们输入一些数据到我们的游戏场中。我们可以通过复制一个文件来实现目的。 我们使用 cp 命令从 /etc 目录复制 passwd 文件到当前工作目录下：

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[me@linuxbox playground]$ cp /etc/passwd .

Notice how we used the shorthand for the current working directory, the single trailing period. So now if we perform an ls, we will see our file:

​ 请注意，我们使用命令末尾的一个圆点来简化当前工作目录的写法。如果我们执行 ls 命令， 可以看到我们的文件：

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[me@linuxbox playground]$ ls -l
total 12
drwxrwxr-x 2  me  me   4096 2008-01-10 16:40 dir1
drwxrwxr-x 2  me  me   4096 2008-01-10 16:40 dir2
-rw-r--r-- 1  me  me   1650 2008-01-10 16:07 passwd

Now, just for fun, let’s repeat the copy using the “-v” option (verbose) to see what it does:

​ 现在，仅仅是为了好玩，重复操作复制命令，使用”-v”选项（详细），看看它做了些什么：

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[me@linuxbox playground]$ cp -v /etc/passwd .
`/etc/passwd` -> `./passwd`

The cp command performed the copy again, but this time displayed a concise message indicating what operation it was performing. Notice that cp overwrote the first copy without any warning. Again this is a case of cp assuming that you know what you’re are doing. To get a warning, we’ll include the “-i” (interactive) option:

​ cp 命令再一次执行了复制操作，但是这次显示了一条简洁的信息，指明它 进行了什么操作。注意，cp 没有警告，就覆盖了第一次复制的文件。这是一个案例， cp 会假设你知道自己在做什么。如果希望得到警告的话，需要加入“-i”（互动）选项：

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[me@linuxbox playground]$ cp -i /etc/passwd .
cp: overwrite `./passwd`?

Responding to the prompt by entering a “y” will cause the file to be overwritten, any other character (for example, “n”) will cause cp to leave the file alone.

​ 在提示信息后输入”y”，文件就会被覆盖，输入其它的字符（例如，”n”） cp 命令会保留原文件。

移动和重命名文件

Now, the name “passwd” doesn’t seem very playful and this is a playground, so let’s change it to something else:

​ 现在，”passwd” 这个名字，看起来不怎么有趣，这是个游戏场，所以我们给它改个名字：

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[me@linuxbox playground]$ mv passwd fun

Let’s pass the fun around a little by moving our renamed file to each of the directories and back again:

​ 让我们来传送 fun 文件，通过移动重命名的文件到各个子目录， 然后再把它移回到当前目录：

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[me@linuxbox playground]$ mv fun dir1

to move it first to directory dir1, then:

​ 首先，把 fun 文件移动目录 dir1 中，然后：

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[me@linuxbox playground]$ mv dir1/fun dir2

to move it from dir1 to dir2, then:

​ 再把 fun 文件从 dir1 移到目录 dir2, 然后：

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[me@linuxbox playground]$ mv dir2/fun .

to finally bringing it back to the current working directory. Next, let’s see the effect of mv on directories. First we will move our data file into dir1 again:

​ 最后，再把 fun 文件带回到当前工作目录。接下来，来看看移动目录的效果。 首先，我们先移动我们的数据文件到 dir1 目录：

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[me@linuxbox playground]$ mv fun dir1

then move dir1 into dir2 and confirm it with ls:

​ 然后移动 dir1 到 dir2 目录，用 ls 来确认执行结果:

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[me@linuxbox playground]$ mv dir1 dir2
[me@linuxbox playground]$ ls -l dir2
total 4
drwxrwxr-x 2 me me 4096 2008-01-11 06:06 dir1
[me@linuxbox playground]$ ls -l dir2/dir1
total 4
-rw-r--r-- 1 me me 1650 2008-01-10 16:33 fun

Note that since dir2 already existed, mv moved dir1 into dir2. If dir2 had not existed, mv would have renamed dir1 to dir2. Lastly, let’s put everything back:

​ 注意：因为目录 dir2 已经存在，mv 命令会把 dir1 移动到 dir2 目录中。如果 dir2 不存在， mv 会把 dir1 重命名为 dir2。最后，让我们把所有的东西放回原处：

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[me@linuxbox playground]$ mv dir2/dir1 .
[me@linuxbox playground]$ mv dir1/fun .

创建硬链接

Now we’ll try some links. First the hard links. We’ll create some links to our data file like so:

​ 现在，我们试着创建链接。首先是硬链接。我们创建一些关联我们 数据文件的链接：

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[me@linuxbox playground]$ ln fun fun-hard
[me@linuxbox playground]$ ln fun dir1/fun-hard
[me@linuxbox playground]$ ln fun dir2/fun-hard

So now we have four instances of the file “fun”. Let’s take a look our playground directory:

​ 所以现在，我们有四个文件”fun”的实例。看一下目录 playground 中的内容：

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[me@linuxbox playground]$ ls -l
total 16
drwxrwxr-x 2 me  me 4096 2008-01-14 16:17 dir1
drwxrwxr-x 2 me  me 4096 2008-01-14 16:17 dir2
-rw-r--r-- 4 me  me 1650 2008-01-10 16:33 fun
-rw-r--r-- 4 me  me 1650 2008-01-10 16:33 fun-hard

One thing you notice is that the second field in the listing for fun and fun-hard both contain a “4” which is the number of hard links that now exist for the file. You’ll remember that a file will always have at least one because the file’s name is created by a link. So, how do we know that fun and fun-hard are, in fact, the same file? In this case, ls is not very helpful. While we can see that fun and fun-hard are both the same size (field 5), our listing provides no way to be sure. To solve this problem, we’re going to have to dig a little deeper.

​ 注意到一件事，列表中，文件 fun 和 fun-hard 的第二个字段是”4”，这个数字 是文件”fun”的硬链接数目。你要记得一个文件至少有一个硬链接，因为文件 名就是由链接创建的。那么，我们怎样知道实际上 fun 和 fun-hard 是同一个文件呢？ 在这个例子里，ls 不是很有用。虽然我们能够看到 fun 和 fun-hard 文件大小一样 （第五字段），但我们的列表没有提供可靠的信息来确定（这两个文件一样）。 为了解决这个问题，我们更深入的研究一下。

When thinking about hard links, it is helpful to imagine that files are made up of two parts: the data part containing the file’s contents and the name part which holds the file’s name. When we create hard links, we are actually creating additional name parts that all refer to the same data part. The system assigns a chain of disk blocks to what is called an inode, which is then associated with the name part. Each hard link therefore refers to a specific inode containing the file’s contents.

​ 当考虑到硬链接的时候，我们可以假设文件由两部分组成：包含文件内容的数据部分和持有文件名的名字部分 ，这将有助于我们理解这个概念。当我们创建文件硬链接的时候，实际上是为文件创建了额外的名字部分， 并且这些名字都关联到相同的数据部分。这时系统会分配一连串的磁盘块给所谓的索引节点，然后索引节点与文 件名字部分相关联。因此每一个硬链接都指向一个包含文件内容的索引节点。

The ls command has a way to reveal this information. It is invoked with the “-i” option:

​ ls 命令有一种方法，来展示（文件索引节点）的信息。在命令中加上”-i”选项：

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[me@linuxbox playground]$ ls -li
total 16
12353539 drwxrwxr-x 2 me  me 4096  2008-01-14  16:17  dir1
12353540 drwxrwxr-x 2 me  me 4096  2008-01-14  16:17  dir2
12353538 -rw-r--r-- 4 me  me 1650  2008-01-10  16:33  fun
12353538 -rw-r--r-- 4 me  me 1650  2008-01-10  16:33  fun-hard

In this version of the listing, the first field is the inode number and, as we can see, both fun and fun-hard share the same inode number, which confirms they are the same file.

​ 在这个版本的列表中，第一字段表示文件索引节点号，正如我们所见到的， fun 和 fun-hard 共享一样的索引节点号，这就证实这两个文件是同一个文件。

创建符号链接

Symbolic links were created to overcome the two disadvantages of hard links: hard links cannot span physical devices and hard links cannot reference directories, only files. Symbolic links are a special type of file that contains a text pointer to the target file or directory.

​ 建立符号链接的目的是为了克服硬链接的两个缺点：硬链接不能跨越物理设备， 硬链接不能关联目录，只能是文件。符号链接是文件的特殊类型，它包含一个指向 目标文件或目录的文本指针。

Creating symbolic links is similar to creating hard links:

​ 符号链接的建立过程相似于创建硬链接：

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[me@linuxbox playground]$ ln -s fun fun-sym
[me@linuxbox playground]$ ln -s ../fun dir1/fun-sym
[me@linuxbox playground]$ ln -s ../fun dir2/fun-sym

The first example is pretty straightforward, we simply add the “-s” option to create a symbolic link rather than a hard link. But what about the next two? Remember, when we create a symbolic link, we are creating a text description of where the target file is relative to the symbolic link. It’s easier to see if we look at the ls output:

​ 第一个例子相当直接，在 ln 命令中，简单地加上”-s”选项就可以创建一个符号链接， 而不是一个硬链接。下面两个例子又是怎样呢？ 记住，当我们创建一个符号链接 的时候，会建立一个文本，其中描述了目标文件的具体位置。如果我们看看 ls 命令的输出结果，比较容易理解。

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[me@linuxbox playground]$ ls -l dir1
total 4
-rw-r--r-- 4 me  me 1650 2008-01-10 16:33 fun-hard
lrwxrwxrwx 1 me  me    6 2008-01-15 15:17 fun-sym -> ../fun

The listing for fun-sym in dir1 shows that is it a symbolic link by the leading “l” in the first field and that it points to “../fun”, which is correct. Relative to the location of fun-sym, fun is in the directory above it. Notice too, that the length of the symbolic link file is 6, the number of characters in the string “../fun” rather than the length of the file to which it is pointing.

​ 目录 dir1 中，fun-sym 的列表说明了它是一个符号链接，通过在第一字段中的首字符”l” 可知，并且它还指向”../fun”，也是正确的。相对于 fun-sym 的存储位置，fun 在它的 上一个目录。同时注意，符号链接文件的长度是6，这是字符串”../fun”所包含的字符数， 而不是符号链接所指向的文件长度。

When creating symbolic links, you can either use absolute pathnames:

​ 当建立符号链接时，你既可以使用绝对路径名：

ln -s /home/me/playground/fun dir1/fun-sym

or relative pathnames, as we did in our earlier example. Using relative pathnames is more desirable because it allows a directory containing symbolic links to be renamed and/or moved without breaking the links.

​ 也可用相对路径名，正如前面例题所展示的。使用相对路径名更令人满意， 因为它允许一个包含符号链接的目录重命名或移动，而不会破坏链接。

In addition to regular files, symbolic links can also reference directories:

​ 除了普通文件，符号链接也能关联目录：

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[me@linuxbox playground]$ ln -s dir1 dir1-sym
[me@linuxbox playground]$ ls -l
total 16
...省略

移动文件和目录

As we covered earlier, the rm command is used to delete files and directories. We are going to use it to clean up our playground a little bit. First, let’s delete one of our hard links:

​ 正如我们之前讨论的，rm 命令被用来删除文件和目录。我们将要使用它 来清理一下我们的游戏场。首先，删除一个硬链接：

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[me@linuxbox playground]$ rm fun-hard
[me@linuxbox playground]$ ls -l
total 12
...省略

That worked as expected. The file fun-hard is gone and the link count shown for fun is reduced from four to three, as indicated in the second field of the directory listing. Next, we’ll delete the file fun, and just for enjoyment, we’ll include the “-i” option to show what that does:

​ 结果不出所料。文件 fun-hard 消失了，文件 fun 的链接数从4减到3，正如 目录列表第二字段所示。下一步，我们会删除文件 fun，仅为了娱乐，我们会加入”-i” 选项，看一看它的作用：

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[me@linuxbox playground]$ rm -i fun
rm: remove regular file `fun`?

Enter “y” at the prompt and the file is deleted. But let’s look at the output of ls now. Noticed what happened to fun-sym? Since it’s a symbolic link pointing to a now- nonexistent file, the link is broken:

​ 在提示符下输入”y”，删除文件。让我们看一下 ls 的输出结果。注意，fun-sym 发生了 什么事? 因为它是一个符号链接，指向已经不存在的文件，链接已经坏了：

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[me@linuxbox playground]$ ls -l
total 8
drwxrwxr-x 2 me  me     4096 2008-01-15 15:17 dir1
lrwxrwxrwx 1 me  me        4 2008-01-16 14:45 dir1-sym -> dir1
drwxrwxr-x 2 me  me     4096 2008-01-15 15:17 dir2
lrwxrwxrwx 1 me  me        3 2008-01-15 15:15 fun-sym -> fun

Most Linux distributions configure ls to display broken links. On a Fedora box, broken links are displayed in blinking red text! The presence of a broken link is not, in and of itself dangerous but it is rather messy. If we try to use a broken link we will see this:

​ 大多数 Linux 的发行版本配置 ls 显示损坏的链接。在 Fedora 系统中，坏的链接以闪烁的 红色文本显示！损坏链接的出现，并不危险，但是相当混乱。如果我们试着使用 损坏的链接，会看到以下情况：

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[me@linuxbox playground]$ less fun-sym
fun-sym: No such file or directory

Let’s clean up a little. We’ll delete the symbolic links:

​ 稍微清理一下现场。删除符号链接：

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[me@linuxbox playground]$ rm fun-sym dir1-sym
[me@linuxbox playground]$ ls -l
total 8
drwxrwxr-x 2 me  me    4096 2008-01-15 15:17 dir1
drwxrwxr-x 2 me  me    4096 2008-01-15 15:17 dir2

One thing to remember about symbolic links is that most file operations are carried out on the link’s target, not the link itself. rm is an exception. When you delete a link, it is the link that is deleted, not the target.

​ 对于符号链接，有一点值得记住，执行的大多数文件操作是针对链接的对象，而不是链接本身。 而 rm 命令是个特例。当你删除链接的时候，删除链接本身，而不是链接的对象。

Finally, we will remove our playground. To do this, we will return to our home directory and use rm with the recursive option (-r) to delete playground and all of its contents, including its subdirectories:

​ 最后，我们将删除我们的游戏场。为了完成这个工作，我们将返回到 我们的家目录，然后用 rm 命令加上选项(-r)，来删除目录 playground， 和目录下的所有内容，包括子目录：

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[me@linuxbox playground]$ cd
[me@linuxbox ~]$ rm -r playground

Creating Symlinks With The GUI

用 GUI 来创建符号链接

The file managers in both GNOME and KDE provide an easy and automatic method of creating symbolic links. With GNOME, holding the Ctrl+Shift keys while dragging a file will create a link rather than copying (or moving) the file. In KDE, a small menu appears whenever a file is dropped, offering a choice of copying, moving, or linking the file.

​ 文件管理器 GNOME 和 KDE 都提供了一个简单而且自动化的方法来创建符号链接。 在 GNOME 里面，当拖动文件时，同时按下 Ctrl+Shift 按键会创建一个链接，而不是 复制（或移动）文件。在 KDE 中，无论什么时候放下一个文件，会弹出一个小菜单， 这个菜单会提供复制，移动，或创建链接文件选项。

总结

We’ve covered a lot of ground here and it will take a while to fully sink in. Perform the playground exercise over and over until it makes sense. It is important to get a good understanding of basic file manipulation commands and wildcards. Feel free to expand on the playground exercise by adding more files and directories, using wildcards to specify files for various operations. The concept of links is a little confusing at first, but take the time to learn how they work. They can be a real lifesaver.

​ 在这一章中，我们已经研究了许多基础知识。我们得花费一些时间来全面地理解。 反复练习 playground 例题，直到你觉得它有意义。能够良好地理解基本文件操作 命令和通配符，非常重要。随意通过添加文件和目录来拓展 playground 练习， 使用通配符来为各种各样的操作命令指定文件。关于链接的概念，在刚开始接触 时会觉得有点迷惑，值得花些时间来学习它们是怎样工作的，因为它们有时候真的特别有用。

6 - 6 使用命令

使用命令

http://billie66.github.io/TLCL/book/chap06.html

Up to this point, we have seen a series of mysterious commands, each with its own mysterious options and arguments. In this chapter, we will attempt to remove some of that mystery and even create some of our own commands. The commands introduced in this chapter are:

​ 在这之前，我们已经知道了一系列神秘的命令，每个命令都有自己奇妙的 选项和参数。在这一章中，我们将试图去掉一些神秘性，甚至创建我们自己 的命令。这一章将介绍以下命令：

• type – Indicate how a command name is interpreted
• type – 说明一个命令名是如何被解释的（这里的“解释”是一个计算机术语，例如，解释型语言）
• which – Display which executable program will be executed
• which – 显示会执行哪个可执行程序
• man – Display a command’s manual page
• man – 显示命令手册页
• apropos – Display a list of appropriate commands
• apropos – 显示一系列适合的命令
• info – Display a command’s info entry
• info – 显示命令 info
• whatis – Display a very brief description of a command
• whatis – 显示一个命令的简洁描述
• alias – Create an alias for a command
• alias – 创建命令别名

到底什么是命令？

A command can be one of four different things:

​ 命令可以是下面四种形式之一：

1. An executable program like all those files we saw in /usr/bin. Within this category, programs can be compiled binaries such as programs written in C and C++, or programs written in scripting languages such as the shell, perl, python, ruby, etc.

2. A command built into the shell itself. bash supports a number of commands internally called shell builtins. The cd command, for example, is a shell builtin.

3. A shell function. These are miniature shell scripts incorporated into the environment. We will cover configuring the environment and writing shell functions in later chapters, but for now, just be aware that they exist.

4. An alias. Commands that we can define ourselves, built from other commands.

5. 一个可执行程序，就像我们所看到的位于目录/usr/bin 中的文件一样。 这一类程序可以是用诸如 C 和 C++ 语言写成的程序然后编译得到的二进制文件, 也可以是由诸如 shell，perl，python，ruby 等等脚本语言写成的程序。

6. 一个内建于 shell 自身的命令。bash 支持若干命令，内部叫做 shell 内部命令 (builtins)。例如，cd 命令，就是一个 shell 内部命令。

7. 一个 shell 函数。这些是小规模的 shell 脚本，它们混合到环境变量中。 在后续的章节里，我们将讨论配置环境变量以及书写 shell 函数。但是现在， 仅仅意识到它们的存在就可以了。

8. 一个命令别名。我们可以定义自己的命令，建立在其它命令之上。

识别命令

It is often useful to know exactly which of the four kinds of commands is being used and Linux provides a couple of ways to find out.

​ 准确地知道正在使用的四种命令中的哪一种通常很有用 Linux 提供了几种查找方法。

type － 显示命令的类型

The type command is a shell builtin that displays the kind of command the shell will execute, given a particular command name. It works like this:

​ type 命令是 shell 内部命令，它会显示命令的类型，给出一个特定的命令名（做为参数）。 它像这样工作：

type command

Where “command” is the name of the command you want to examine. Here are some examples:

​ command 是你要检测的命令名。这里有些例子：

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[me@linuxbox ~]$ type type
type is a shell builtins
[me@linuxbox ~]$ type ls
ls is aliased to `ls --color=tty`
[me@linuxbox ~]$ type cp
cp is /bin/cp

Here we see the results for three different commands. Notice that the one for ls (taken from a Fedora system) and how the ls command is actually an alias for the ls command with the “--color=tty” option added. Now we know why the output from ls is displayed in color!

​ 我们看到这三个不同命令的检测结果。注意，ls 命令（在 Fedora 系统中）的检查结果，ls 命令实际上 是 ls 命令加上选项”--color=tty”的别名。现在我们知道为什么 ls 的输出结果是有颜色的！

which － 显示一个可执行程序的位置

Sometimes there is more than one version of an executable program installed on a system. While this is not very common on desktop systems, it’s not unusual on large servers. To determine the exact location of a given executable, the which command is used:

​ 有时候在一个操作系统中，不只安装了可执行程序的一个版本。虽然在桌面系统中这并不普遍， 但在大型服务器中却很平常。为了确定所给定的执行程序的准确位置，使用 which 命令：

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[me@linuxbox ~]$ which ls
/bin/ls

which only works for executable programs, not builtins nor aliases that are substitutes for actual executable programs. When we try to use which on a shell builtin, for example, cd, we either get no response or an error message:

​ which 命令只对可执行程序有效，不包括内建命令和命令别名。 当我们试着使用 shell 内建命令时，例如，cd 命令，我们或者得不到回应，或者是个错误信息：

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[me@linuxbox ~]$ which cd
/usr/bin/which: no cd in
(/opt/jre1.6.0_03/bin:/usr/lib/qt-3.3/bin:/usr/kerberos/bin:/opt/jre1
.6.0_03/bin:/usr/lib/ccache:/usr/local/bin:/usr/bin:/bin:/home/me/bin)

which is a fancy way of saying “command not found.”

​ 这些信息真正的意思就是“命令没有找到”。

得到命令文档

With this knowledge of what a command is, we can now search for the documentation available for each kind of command.

​ 知道了什么是命令，现在我们来查找每一类命令的文档。

help － 得到 shell 内建命令的帮助文档

bash has a built-in help facility available for each of the shell builtins. To use it, type “help” followed by the name of the shell builtin. For example:

​ bash 有一个内建的 help 命令，可查找每一个 shell 内建命令的文档。输入“help”，接着是 shell 内部命令名。例如：

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[me@linuxbox ~]$ help cd
cd: cd [-L|-P] [dir]
Change ...

A note on notation: When square brackets appear in the description of a command’s syntax, they indicate optional items. A vertical bar character indicates mutually exclusive items. In the case of the cd command above:

​ 注意：出现在命令语法说明中的方括号证的内容是可选的项目。一个竖杠字符 表示互斥选项。在上面 cd 命令的例子中：

cd [-L|-P] [dir]

This notation says that the command cd may be followed optionally by either a “-L” or a “-P” and further, optionally followed by the argument “dir”.

​ 这种表示法说明，cd 命令可以跟一个“-L”选项“-P”选项其中之一或者什么都不跟，“dir”也是可选参数。

While the output of help for the cd commands is concise and accurate, it is by no means tutorial and as we can see, it also seems to mention a lot of things we haven’t talked about yet! Don’t worry. We’ll get there.

​ 虽然 cd 命令的帮助文档很简洁准确，但它决不是教程。正如我们所看到的，它似乎提到了许多 我们还没有谈论到的东西！不要担心，我们会学到的。

--help - 显示用法信息

Many executable programs support a “--help” option that displays a description of the command’s supported syntax and options. For example:

​ 许多可执行程序支持一个 --help 选项，这个选项是显示命令所支持的语法和选项说明。例如：

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[me@linuxbox ~]$ mkdir --help
Usage: mkdir [OPTION] DIRECTORY...
Create ...

Some programs don’t support the “--help” option, but try it anyway. Often it results in an error message that will reveal the same usage information.

​ 一些程序不支持 --help 选项，但不管怎样试一下。通常输出的错误提示也同样能 揭示命令的用法信息。

man － 显示用户手册

Most executable programs intended for command line use provide a formal piece of documentation called a manual or man page. A special paging program called man is used to view them. It is used like this:

​ 许多希望被命令行使用的可执行程序，提供了一个正式的文档，叫做手册或手册页(man page)。一个特殊的叫做 man 的分页程序，可用来浏览他们。它是这样使用的：

man program

where “program” is the name of the command to view.

“program”是要浏览的命令名。

Man pages vary somewhat in format but generally contain a title, a synopsis of the command’s syntax, a description of the command’s purpose, and a listing and description of each of the command’s options. Man pages, however, do not usually include examples, and are intended as a reference, not a tutorial. As an example, let’s try viewing the man page for the ls command:

​ 手册文档的格式有点不同，一般地包含一个标题、命令语法的纲要、命令用途的说明、 以及每个命令选项的列表和说明。然而，手册文档通常并不包含实例，它打算 作为一本参考手册，而不是教程。作为一个例子，浏览一下 ls 命令的手册文档：

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[me@linuxbox ~]$ man ls

On most Linux systems, man uses less to display the manual page, so all of the familiar less commands work while displaying the page.

​ 在大多数 Linux 系统中，man 使用 less 工具来显示参考手册，所以当浏览文档时，你所熟悉的 less 命令都能有效。

The “manual” that man displays is broken into sections and not only covers user commands but also system administration commands, programming interfaces, file formats and more. The table below describes the layout of the manual:

​ man 所显示的参考手册，被分成几个章节，它们不仅仅包括用户命令，也包括系统管理员 命令、程序接口、文件格式等等。下表描绘了手册的布局：

Sometimes we need to look in a specific section of the manual to find what we are looking for. This is particularly true if we are looking for a file format that is also the name of a command. Without specifying a section number, we will always get the first instance of a match, probably in section 1. To specify a section number, we use man like this:

​ 有时候，我们需要查看参考手册的特定章节，从而找到我们需要的信息。 如果我们要查找一种文件格式，而同时它也是一个命令名时,这种情况尤其正确。 没有指定章节号，我们总是得到第一个匹配项，可能在第一章节。我们这样使用 man 命令， 来指定章节号：

man section search_term

For example:

​ 例如：

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[me@linuxbox ~]$ man 5 passwd

This will display the man page describing the file format of the /etc/passwd file.

​ 命令运行结果会显示文件 /etc/passwd 的文件格式说明手册。

apropos － 显示适合的命令

It is also possible to search the list of man pages for possible matches based on a search term. It’s very crude but sometimes helpful. Here is an example of a search for man pages using the search term “floppy”:

​ 我们也可以搜索全部参考手册来找到自己需要的命令，这个方法虽然很粗糙但有时很有用。 下面是一个以”floppy”为关键词来搜索参考手册的例子：

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[me@linuxbox ~]$ apropos floppy
create_floppy_devices (8)   - udev callout to create all possible
...

The first field in each line of output is the name of the man page, the second field shows the section. Note that the man command with the “-k” option performs the exact same function as apropos.

​ 输出结果每行的第一个字段是手册页的名字，第二个字段展示章节。注意，man 命令加上”-k”选项， 和 apropos 完成一样的功能。

whatis － 显示非常简洁的命令说明

The whatis program displays the name and a one line description of a man page matching a specified keyword:

​ whatis 程序显示匹配特定关键字的手册页的名字和一行命令说明：

The Most Brutal Man Page Of Them All

最晦涩难懂的手册页

As we have seen, the manual pages supplied with Linux and other Unix-like systems are intended as reference documentation and not as tutorials. Many man pages are hard to read, but I think that the grand prize for difficulty has got to go to the man page for bash. As I was doing my research for this book, I gave it careful review to ensure that I was covering most of its topics. When printed, it’s over eighty pages long and extremely dense, and its structure makes absolutely no sense to a new user.

正如我们所看到的，Linux 和类 Unix 的系统提供的手册页，只是打算作为参考手册使用， 而不是教程。许多手册页都很难阅读，但是我认为由于阅读难度而能拿到特等奖的手册页应该是 bash 手册页。因为我正在为这本书做我的研究，所以我很仔细地浏览了整个 bash 手册，为的是确保我讲述了 大部分的 bash 主题。当把 bash 参考手册整个打印出来，其篇幅有八十多页且内容极其紧密， 但对于初学者来说，其结构安排毫无意义。

On the other hand, it is very accurate and concise, as well as being extremely complete. So check it out if you dare and look forward to the day when you can read it and it all makes sense.

另一方面，bash 参考手册的内容非常简明精确，同时也非常完善。所以，如果你有胆量就查看一下， 并且期望有一天你能读懂它。

info － 显示程序 Info 条目

The GNU Project provides an alternative to man pages for their programs, called “info.” Info pages are displayed with a reader program named, appropriately enough, info. Info pages are hyperlinked much like web pages. Here is a sample:

​ GNU 项目提供了一个命令程序手册页的替代物，称为”info”。info 内容可通过 info 阅读器 程序读取。info 页是超级链接形式的，和网页很相似。这有个例子：

File: coreutils.info,    Node: ls invocation,    Next: dir invocation,
 Up: Directory listing

10.1 `ls`: List directory contents
==================================
...

The info program reads info files, which are tree structured into individual nodes, each containing a single topic. Info files contain hyperlinks that can move you from node to node. A hyperlink can be identified by its leading asterisk, and is activated by placing the cursor upon it and pressing the enter key.

​ info 程序读取 info 文件，info 文件是树型结构，分化为各个结点，每一个包含一个题目。 info 文件包含超级链接，它可以让你从一个结点跳到另一个结点。一个超级链接可通过 它开头的星号来辨别出来，把光标放在它上面并按下 enter 键，就可以激活它。

To invoke info, type “info” followed optionally by the name of a program. Below is a table of commands used to control the reader while displaying an info page:

​ 输入”info”，接着输入程序名称，启动 info。当显示一个 info 页面时，下表中的命令 用来控制阅读器。

Most of the command line programs we have discussed so far are part of the GNU Project’s “coreutils” package, so typing:

​ 到目前为止，我们所讨论的大多数命令行程序，属于 GNU 项目”coreutils”包，所以输入：

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[me@linuxbox ~]$ info coreutils

will display a menu page with hyperlinks to each program contained in the coreutils package.

​ 将会显示一个包含超级链接的手册页，这些超级链接指向包含在 coreutils 包中的各个程序。

README 和其它程序文档

Many software packages installed on your system have documentation files residing in the /usr/share/doc directory. Most of these are stored in plain text format and can be viewed with less. Some of the files are in HTML format and can be viewed with a web browser. We may encounter some files ending with a “.gz” extension. This indicates that they have been compressed with the gzip compression program. The gzip package includes a special version of less called zless that will display the contents of gzip-compressed text files.

​ 许多安装在你系统中的软件，都有自己的文档文件，这些文件位于/usr/share/doc 目录下。 这些文件大多数是以文本文件的形式存储的，可用 less 阅读器来浏览。一些文件是 HTML 格式， 可用网页浏览器来阅读。我们可能遇到许多以”.gz”结尾的文件。这表示 gzip 压缩程序 已经压缩了这些文件。gzip 软件包包括一个特殊版本的 less ，叫做 zless，zless 可以显示由 gzip 压缩的文本文件的内容。

用别名（alias）创建你自己的命令

Now for our very first experience with programming! We will create a command of our own using the alias command. But before we start, we need to reveal a small command line trick. It’s possible to put more than one command on a line by separating each command with a semicolon character. It works like this:

​ 现在是时候，感受第一次编程经历了！我们将用 alias 命令创建我们自己的命令。但在 开始之前，我们需要展示一个命令行小技巧。可以把多个命令放在同一行上，命令之间 用”;”分开。它像这样工作：

command1; command2; command3...

Here’s the example we will use:

​ 我们会用到下面的例子：

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[me@linuxbox ~]$ cd /usr; ls; cd -
bin  games    kerberos  lib64    local  share  tmp
...
[me@linuxbox ~]$

As we can see, we have combined three commands on one line. First we change directory to /usr then list the directory and finally return to the original directory (by using ‘cd -‘) so we end up where we started. Now let’s turn this sequence into a new command using alias. The first thing we have to do is dream up a name for our new command. Let’s try “test”. Before we do that, it would be a good idea to find out if the name “test” is already being used. To find out, we can use the type command again:

​ 正如我们看到的，我们在一行上联合了三个命令。首先更改目录到/usr，然后列出目录 内容，最后回到之前的目录（用命令”cd -“）,结束在开始的地方。现在，通过 alias 命令 把这一串命令转变为一个命令。我们要做的第一件事就是为我们的新命令构想一个名字。 比方说”test”。在使用”test”之前，最好先查明”test”命令名是否已经存在于系统中。 为此，可以使用 type 命令：

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[me@linuxbox ~]$ type test
test is a shell builtin

Oops! The name “test” is already taken. Let’s try “foo”:

​ 哦！”test”名字已经被使用了。试一下”foo”:

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[me@linuxbox ~]$ type foo
bash: type: foo: not found

Great! “foo” is not taken. So let’s create our alias:

​ 太棒了！”foo”还没被占用。创建命令别名：

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[me@linuxbox ~]$ alias foo='cd /usr; ls; cd -'

Notice the structure of this command:

​ 注意命令结构：

alias name='string'

After the command “alias” we give alias a name followed immediately (no whitespace allowed) by an equals sign, followed immediately by a quoted string containing the meaning to be assigned to the name. After we define our alias, it can be used anywhere the shell would expect a command. Let’s try it:

​ 在命令”alias”之后，输入“name”，紧接着（没有空格）是一个等号，等号之后是 一串用引号引起的字符串，字符串的内容要赋值给 name。我们定义了别名之后， 这个命令别名可以使用在任何地方。试一下：

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[me@linuxbox ~]$ foo
bin   games   kerberos  lib64    local   share  tmp
...
[me@linuxbox ~]$

We can also use the type command again to see our alias:

​ 我们也可以使用 type 命令来查看我们的别名：

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[me@linuxbox ~]$ type foo
foo is aliased to `cd /usr; ls ; cd -'

To remove an alias, the unalias command is used, like so:

​ 删除别名，使用 unalias 命令，像这样：

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[me@linuxbox ~]$ unalias foo
[me@linuxbox ~]$ type foo
bash: type: foo: not found

While we purposefully avoided naming our alias with an existing command name, it is not uncommon to do so. This is often done to apply a commonly desired option to each invocation of a common command. For instance, we saw earlier how the ls command is often aliased to add color support:

​ 虽然我们有意避免使用已经存在的命令名来命名我们的别名，但有时候也会故意这么做。通常， 会把一个普遍用到的选项加到一个经常使用的命令后面。例如，之前见到的 ls 命令，会 带有色彩支持：

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[me@linuxbox ~]$ type ls
ls is aliased to 'ls --color=tty'

To see all the aliases defined in the environment, use the alias command without arguments. Here are some of the aliases defined by default on a Fedora system. Try and figure out what they all do:

​ 要查看所有定义在系统环境中的别名，可使用不带参数的 alias 命令。下面是 Fedora 系统中 默认定义的别名。试着弄明白它们是做什么的：

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[me@linuxbox ~]$ alias
alias l.='ls -d .* --color=tty'
...

There is one tiny problem with defining aliases on the command line. They vanish when your shell session ends. In a later chapter, we will see how to add our own aliases to the files that establish the environment each time we log on, but for now, enjoy the fact that we have taken our first, albeit tiny, step into the world of shell programming!

​ 在命令行中定义别名有点个小问题。当你的 shell 会话结束时，它们会消失。随后的章节里， 我们会了解怎样把自己的别名添加到文件中去，每次我们登录系统，这些文件会建立系统环境。 现在，好好享受我们刚经历过的，步入 shell 编程世界的第一步吧，虽然是小小的一步。

拜访老朋友

Now that we have learned how to find the documentation for commands, go and look up the documentation for all the commands we have encountered so far. Study what additional options are available and try them out!

​ 既然我们已经学习了怎样找到命令的帮助文档，那就试着查阅，到目前为止，我们学到的所有 命令的文档。学习命令其它可用的选项，练习一下！

拓展阅读

• There are many online sources of documentation for Linux and the command line. Here are some of the best:

• 在网上，有许多关于 Linux 和命令行的文档。以下是其中最好的一些：

• The Bash Reference Manual is a reference guide to the bash shell. It’s still a reference work but contains examples and is easier to read than the bash man page.

• Bash 参考手册是一本 bash shell 的参考指南。它仍然是一本参考书，但是包含了很多 实例，而且它比 bash 手册页容易阅读。

  http://www.gnu.org/software/bash/manual/bashref.html

• The Bash FAQ contains answers to frequently asked questions regarding bash. This list is aimed at intermediate to advanced users, but contains a lot of good information.

• Bash FAQ 包含关于 bash，而经常提到的问题的答案。这个列表面向 bash 的中高级用户， 但它包含了许多有帮助的信息。

  http://mywiki.wooledge.org/BashFAQ

• The GNU Project provides extensive documentation for its programs, which form the core of the Linux command line experience. You can see a complete list here:

• GNU 项目为它的程序提供了大量的文档，这些文档组成了 Linux 命令行实验的核心。 这里你可以看到一个完整的列表：

  http://www.gnu.org/manual/manual.html

• Wikipedia has an interesting article on man pages:

• Wikipedia 有一篇关于手册页的有趣文章：

  http://en.wikipedia.org/wiki/Man_page

7 - 7 重定向

重定向

http://billie66.github.io/TLCL/book/chap07.html

In this lesson we are going to unleash what may be the coolest feature of the command line. It’s called I/O redirection. The “I/O” stands for input/output and with this facility you can redirect the input and output of commands to and from files, as well as connect multiple commands together into powerful command pipelines. To show off this facility, we will introduce the following commands:

​ 这堂课，我们来介绍可能是命令行最酷的特性。它叫做 I/O 重定向。”I/O”代表输入/输出， 通过这个机制，你可以将命令的输入来源以及输出地点重定向为文件。也可以把多个命令连接起来组成一个强大的命令管道。为了展示这个工具，我们将用到 以下命令：

• cat - Concatenate files
• sort - Sort lines of text
• uniq - Report or omit repeated lines
• grep - Print lines matching a pattern
• wc - Print newline, word, and byte counts for each file
• head - Output the first part of a file
• tail - Output the last part of a file
• tee - Read from standard input and write to standard output and files
• cat － 连接文件
• sort － 排序文本行
• uniq － 报道或省略重复行
• grep － 打印匹配行
• wc － 打印文件中换行符，字，和字节个数
• head － 输出文件第一部分
• tail - 输出文件最后一部分
• tee - 从标准输入读取数据，并同时写到标准输出和文件

标准输入、标准输出和标准错误输出

Many of the programs that we have used so far produce output of some kind. This output often consists of two types. First, we have the program’s results; that is, the data the program is designed to produce, and second, we have status and error messages that tell us how the program is getting along. If we look at a command like ls, we can see that it displays its results and its error messages on the screen.

​ 到目前为止，我们用到的许多程序都会产生某种输出。这种输出，经常由两种类型组成。 第一，程序运行结果；这是说，程序要完成的功能。第二，我们得到状态和错误信息， 这些告诉我们程序进展。如果我们观察一个命令，例如 ls，会看到它的运行结果和错误信息 显示在屏幕上。

Keeping with the Unix theme of “everything is a file,” programs such as ls actually send their results to a special file called standard output (often expressed as stdout) and their status messages to another file called standard error (stderr). By default, both standard output and standard error are linked to the screen and not saved into a disk file. In addition, many programs take input from a facility called standard input (stdin) which is, by default, attached to the keyboard.

​ 与 Unix 主题“任何东西都是一个文件”保持一致，像 ls这样的程序实际上把他们的运行结果 输送到一个叫做标准输出的特殊文件（经常用 stdout 表示），而它们的状态信息则送到另一个 叫做标准错误输出的文件（stderr）。默认情况下，标准输出和标准错误输出都连接到屏幕，而不是 保存到磁盘文件。除此之外，许多程序从一个叫做标准输入（stdin）的设备得到输入，默认情况下， 标准输入连接到键盘。

I/O redirection allows us to change where output goes and where input comes from. Normally, output goes to the screen and input comes from the keyboard, but with I/O redirection, we can change that.

​ I/O 重定向允许我们更改输出地点和输入来源。一般来说，输入来自键盘，输出送到屏幕， 但是通过 I/O 重定向，我们可以做出改变。

标准输出重定向

I/O redirection allows us to redefine where standard output goes. To redirect standard output to another file besides the screen, we use the “>” redirection operator followed by the name of the file. Why would we want to do this? It’s often useful to store the output of a command in a file. For example, we could tell the shell to send the output of the ls command to the file ls-output.txt instead of the screen:

​ I/O 重定向允许我们来重定义标准输出的地点。我们使用 “>” 重定向符后接文件名将标准输出重定向到除屏幕 以外的另一个文件。为什么我们要这样做呢？因为有时候把一个命令的运行结果存储到 一个文件很有用处。例如，我们可以告诉 shell 把 ls 命令的运行结果输送到文件 ls-output.txt 中去， 由文件代替屏幕。

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[me@linuxbox ~]$ ls -l /usr/bin > ls-output.txt

Here, we created a long listing of the /usr/bin directory and sent the results to the file ls-output.txt. Let’s examine the redirected output of the command:

​ 这里，我们创建了一个长长的目录 /usr/bin 列表，并且输送程序运行结果到文件 ls-output.txt 中。 我们检查一下重定向的命令输出结果：

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[me@linuxbox ~]$ ls -l ls-output.txt
-rw-rw-r-- 1   me   me    167878 2008-02-01 15:07 ls-output.txt

Good; a nice, large, text file. If we look at the file with less, we will see that the file ls-output.txt does indeed contain the results from our ls command:

​ 好；一个不错的大型文本文件。如果我们用 less 阅读器来查看这个文件，我们会看到文件 ls-output.txt 的确包含 ls 命令的执行结果。

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[me@linuxbox ~]$ less ls-output.txt

Now, let’s repeat our redirection test, but this time with a twist. We’ll change the name of the directory to one that does not exist:

​ 现在，重复我们的重定向测试，但这次有改动。我们把目录换成一个不存在的目录。

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[me@linuxbox ~]$ ls -l /bin/usr > ls-output.txt
ls: cannot access /bin/usr: No such file or directory

We received an error message. This makes sense since we specified the non-existent directory /bin/usr, but why was the error message displayed on the screen rather than being redirected to the file ls-output.txt? The answer is that the ls program does not send its error messages to standard output. Instead, like most well-written Unix programs, it sends its error messages to standard error. Since we only redirected standard output and not standard error, the error message was still sent to the screen. We’ll see how to redirect standard error in just a minute, but first, let’s look at what happened to our output file:

​ 我们收到一个错误信息。这讲得通，因为我们指定了一个不存在的目录 /bin/usr ， 但是为什么这条错误信息显示在屏幕上而不是被重定向到文件 ls-output.txt？答案是， ls 程序不把它的错误信息输送到标准输出。像许多写得正规的 Unix 程序，ls 会把 错误信息送到标准错误输出。因为我们只是重定向了标准输出，而没有重定向标准错误输出， 所以错误信息被送到屏幕。马上，我们将知道怎样重定向标准错误输出，但是首先看一下 我们的输出文件发生了什么事情。

me@linuxbox ~]$ ls -l ls-output.txt
-rw-rw-r-- 1 me   me    0 2008-02-01 15:08 ls-output.txt

The file now has zero length! This is because, when we redirect output with the “>” redirection operator, the destination file is always rewritten from the beginning. Since our ls command generated no results and only an error message, the redirection operation started to rewrite the file and then stopped because of the error, resulting in its truncation. In fact, if we ever need to actually truncate a file (or create a new, empty file) we can use a trick like this:

​ 文件长度为零！这是因为，当我们使用 “>” 重定向符来重定向输出结果时，目标文件总是从开头被重写。 因为我们 ls 命令没有产生运行结果，只有错误信息，重定向操作开始重写文件，然后 由于错误而停止，导致文件内容清空。事实上，如果我们需要清空一个文件内容（或者创建一个 新的空文件），可以使用这样的技巧：

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[me@linuxbox ~]$ > ls-output.txt

Simply using the redirection operator with no command preceding it will truncate an existing file or create a new, empty file.

​ 简单地使用重定向符，没有命令在它之前，这会清空一个已存在文件的内容或是 创建一个新的空文件。

So, how can we append redirected output to a file instead of overwriting the file from the beginning? For that, we use the “»” redirection operator, like so:

​ 所以，怎样才能把重定向结果追加到文件内容后面，而不是从开头重写文件？为了这个目的， 我们使用”»“重定向符，像这样：

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[me@linuxbox ~]$ ls -l /usr/bin >> ls-output.txt

Using the “»” operator will result in the output being appended to the file. If the file does not already exist, it is created just as though the “>” operator had been used. Let’s put it to the test:

​ 使用”»“操作符，将导致输出结果添加到文件内容之后。如果文件不存在，文件会 被创建，就如使用了”>”操作符。来试一下：

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[me@linuxbox ~]$ ls -l /usr/bin >> ls-output.txt
[me@linuxbox ~]$ ls -l /usr/bin >> ls-output.txt
[me@linuxbox ~]$ ls -l /usr/bin >> ls-output.txt
[me@linuxbox ~]$ ls -l ls-output.txt
-rw-rw-r-- 1 me   me    503634 2008-02-01 15:45 ls-output.txt

We repeated the command three times resulting in an output file three times as large.

​ 我们重复执行命令三次，导致输出文件大小是原来的三倍。

标准错误输出重定向

Redirecting standard error lacks the ease of a dedicated redirection operator. To redirect standard error we must refer to its file descriptor. A program can produce output on any of several numbered file streams. While we have referred to the first three of these file streams as standard input, output and error, the shell references them internally as file descriptors zero, one and two, respectively. The shell provides a notation for redirecting files using the file descriptor number. Since standard error is the same as file descriptor number two, we can redirect standard error with this notation:

​ 标准错误输出重定向没有专用的重定向操作符。为了重定向标准错误输出，我们必须用到其文件描述符。 一个程序的输出会流入到几个带编号的文件中。这些文件的前 三个称作标准输入、标准输出和标准错误输出，shell 内部分别将其称为文件描述符0、1和2。shell 使用文件描述符提供 了一种表示法来重定向文件。因为标准错误输出和文件描述符2一样，我们用这种 表示法来重定向标准错误输出：

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[me@linuxbox ~]$ ls -l /bin/usr 2> ls-error.txt

The file descriptor “2” is placed immediately before the redirection operator to perform the redirection of standard error to the file ls-error.txt.

​ 文件描述符”2”，紧挨着放在重定向操作符之前，来执行重定向标准错误输出到文件 ls-error.txt 任务。

重定向标准输出和错误到同一个文件

There are cases in which we may wish to capture all of the output of a command to a single file. To do this, we must redirect both standard output and standard error at the same time. There are two ways to do this. First, the traditional way, which works with old versions of the shell:

​ 有时我们希望将一个命令的所有输出保存到一个文件。为此，我们 必须同时重定向标准输出和标准错误输出。有两种方法来完成任务。第一个是传统的方法， 在旧版本 shell 中也有效：

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[me@linuxbox ~]$ ls -l /bin/usr > ls-output.txt 2>&1

Using this method, we perform two redirections. First we redirect standard output to the file ls-output.txt and then we redirect file descriptor two (standard error) to file descriptor one (standard output) using the notation 2>&1.

​ 使用这种方法，我们完成两个重定向。首先重定向标准输出到文件 ls-output.txt，然后 重定向文件描述符2（标准错误输出）到文件描述符1（标准输出）使用表示法2>&1。

Notice that the order of the redirections is significant. The redirection of standard error must always occur after redirecting standard output or it doesn’t work. In the example above,

​ 注意重定向的顺序安排非常重要。标准错误输出的重定向必须总是出现在标准输出 重定向之后，要不然它不起作用。上面的例子，

>ls-output.txt 2>&1

redirects standard error to the file ls-output.txt, but if the order is changed to

​ 重定向标准错误输出到文件 ls-output.txt，但是如果命令顺序改为：

2>&1 >ls-output.txt

standard error is directed to the screen.

​ 则标准错误输出会定向到屏幕。

Recent versions of bash provide a second, more streamlined method for performing this combined redirection:

​ 现在的 bash 版本提供了第二种方法，更精简合理的方法来执行这种联合的重定向。

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[me@linuxbox ~]$ ls -l /bin/usr &> ls-output.txt

In this example, we use the single notation &> to redirect both standard output and standard error to the file ls-output.txt.

​ 在这个例子里面，我们使用单单一个表示法 &> 来重定向标准输出和错误到文件 ls-output.txt。

处理不需要的输出

Sometimes “silence is golden,” and we don’t want output from a command, we just want to throw it away. This applies particularly to error and status messages. The system provides a way to do this by redirecting output to a special file called “/dev/null”. This file is a system device called a bit bucket which accepts input and does nothing with it. To suppress error messages from a command, we do this:

​ 有时候“沉默是金”，我们不想要一个命令的输出结果，只想把它们扔掉。这种情况 尤其适用于错误和状态信息。具体做法是重定向输出结果到一个叫做”/dev/null”的特殊文件。这个文件是系统设备，叫做数字存储桶，它可以 接受输入，并且对输入不做任何处理。为了丢掉命令错误信息，我们这样做：

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[me@linuxbox ~]$ ls -l /bin/usr 2> /dev/null

/dev/null in Unix Culture

Unix 文化中的 /dev/null

The bit bucket is an ancient Unix concept and due to its universality, has appeared in many parts of Unix culture. When someone says he/she is sending your comments to /dev/null, now you know what it means. For more examples, see the Wikipedia article on “/dev/null”.

​ 数字存储桶是个古老的 Unix 概念，由于它的普遍性，它的身影出现在 Unix 文化的很多角落。当有人说我把你的评论送到/dev/null 了，现在你应该知道那是 什么意思了。更多的例子，可以阅读 Wikipedia 关于”/dev/null”的文章。

标准输入重定向

Up to now, we haven’t encountered any commands that make use of standard input (actually we have, but we’ll reveal that surprise a little bit later), so we need to introduce one.

​ 到目前为止，我们还没有遇到一个命令是利用标准输入的（实际上我们遇到过了，但是 一会儿再揭晓谜底），所以我们需要介绍一个。

cat － 连接文件

The cat command reads one or more files and copies them to standard output like so:

​ cat 命令读取一个或多个文件，然后复制它们到标准输出，就像这样:

cat [file]

In most cases, you can think of cat as being analogous to the TYPE command in DOS. You can use it to display files without paging, for example:

​ 在大多数情况下，你可以认为 cat 命令相似于 DOS 中的 TYPE 命令。你可以使用 cat 来显示 文件而没有分页，例如：

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[me@linuxbox ~]$ cat ls-output.txt

will display the contents of the file ls-output.txt. cat is often used to display short text files. Since cat can accept more than one file as an argument, it can also be used to join files together. Say we have downloaded a large file that has been split into multiple parts (multimedia files are often split this way on USENET), and we want to join them back together. If the files were named:

​ 将会显示文件 ls-output.txt 的内容。cat 经常被用来显示简短的文本文件。因为 cat 可以 接受不只一个文件作为参数，所以它也可以用来把文件连接在一起。比方说我们下载了一个 大型文件，这个文件被分离成多个部分（USENET 中的多媒体文件经常以这种方式分离）， 我们想把它们连起来。如果文件命名为：

movie.mpeg.001 movie.mpeg.002 … movie.mpeg.099

we could join them back together with this command:

​ 我们能用这个命令把它们连接起来：

cat movie.mpeg.0* > movie.mpeg

Since wildcards always expand in sorted order, the arguments will be arranged in the correct order.

​ 因为通配符总是以有序的方式展开，所以这些参数会以正确顺序安排。

This is all well and good, but what does this have to do with standard input? Nothing yet, but let’s try something else. What happens if we type “cat” with no arguments:

​ 这很好，但是这和标准输入有什么关系呢？没有任何关系，让我们试着做些其他的工作。 如果我们输入不带参数的”cat”命令，会发生什么呢：

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[me@linuxbox ~]$ cat

Nothing happens, it just sits there like it’s hung. It may seem that way, but it’s really doing exactly what it’s supposed to.

​ 似乎没有发生任何事情，但是它正在做它该做的事情：

If cat is not given any arguments, it reads from standard input and since standard input is, by default, attached to the keyboard, it’s waiting for us to type something! Try this:

​ 如果 cat 没有给出任何参数，它会从标准输入读入数据，又因为标准输入默认情况下连接到键盘， 它正在等待我们输入数据！试试这个：

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[me@linuxbox ~]$ cat
The quick brown fox jumped over the lazy dog.

Next, type a Ctrl-d (i.e., hold down the Ctrl key and press “d”) to tell cat that it has reached end of file (EOF) on standard input:

​ 下一步，输入 Ctrl-d（按住 Ctrl 键同时按下”d”），来告诉 cat，在标准输入中， 它已经到达文件末尾（EOF）：

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[me@linuxbox ~]$ cat
The quick brown fox jumped over the lazy dog.
The quick brown fox jumped over the lazy dog.

In the absence of filename arguments, cat copies standard input to standard output, so we see our line of text repeated. We can use this behavior to create short text files. Let’s say that we wanted to create a file called “lazy_dog.txt” containing the text in our example. We would do this:

​ 由于没有文件名参数，cat 复制标准输入到标准输出，所以我们看到文本行重复出现。 我们可以使用这种行为来创建简短的文本文件。比方说，我们想创建一个叫做”lazy_dog.txt” 的文件，这个文件包含例子中的文本。我们这样做：

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[me@linuxbox ~]$ cat > lazy_dog.txt
The quick brown fox jumped over the lazy dog.

Type the command followed by the text we want in to place in the file. Remember to type Ctrl-d at the end. Using the command line, we have implemented the world’s dumbest word processor! To see our results, we can use cat to copy the file to stdout again:

​ 输入命令，其后输入要放入文件中的文本。记住，最后输入 Ctrl-d。通过使用这个命令，我们 实现了世界上最低能的文字处理器！看一下运行结果，我们使用 cat 来复制文件内容到 标准输出：

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[me@linuxbox ~]$ cat lazy_dog.txt
The quick brown fox jumped over the lazy dog.

Now that we know how cat accepts standard input, in addition to filename arguments, let’s try redirecting standard input:

​ 现在我们知道 cat 怎样接受标准输入，除了文件名参数，让我们试着重定向标准输入：

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[me@linuxbox ~]$ cat < lazy_dog.txt
The quick brown fox jumped over the lazy dog.

Using the “<” redirection operator, we change the source of standard input from the keyboard to the file lazy_dog.txt. We see that the result is the same as passing a single filename argument. This is not particularly useful compared to passing a filename argument, but it serves to demonstrate using a file as a source of standard input. Other commands make better use of standard input, as we shall soon see.

​ 使用“<”重定向操作符，我们把标准输入源从键盘改到文件 lazy_dog.txt。我们看到结果 和传递单个文件名作为参数的执行结果一样。把这和传递一个文件名参数作比较，不是特别有意义， 但它是用来说明把一个文件作为标准输入源。有其他的命令更好地利用了标准输入，我们不久将会看到。

Before we move on, check out the man page for cat, as it has several interesting options.

​ 在我们继续之前，请查看 cat 的手册页，因为它有几个有趣的选项。

管道线

The ability of commands to read data from standard input and send to standard output is utilized by a shell feature called pipelines. Using the pipe operator “|” (vertical bar), the standard output of one command can be piped into the standard input of another:

​ 命令从标准输入读取数据并输送到标准输出的能力被一个称为管道线的 shell 功能所利用。 使用管道操作符”|”（竖杠），一个命令的标准输出可以通过管道送至另一个命令的标准输入：

command1 | command2

To fully demonstrate this, we are going to need some commands. Remember how we said there was one we already knew that accepts standard input? It’s less. We can use less to display, page-by-page, the output of any command that sends its results to standard output:

​ 为了全面地说明这个命令，我们需要一些命令。是否记得我们说过，我们已经知道有一个 命令接受标准输入？它是 less 命令。我们用 less 来一页一页地显示任何命令的输出，命令把 它的运行结果输送到标准输出：

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[me@linuxbox ~]$ ls -l /usr/bin | less

This is extremely handy! Using this technique, we can conveniently examine the output of any command that produces standard output.

​ 这极其方便！使用这项技术，我们可以方便地检测会产生标准输出的任一命令的运行结果。

过滤器

Pipelines are often used to perform complex operations on data. It is possible to put several commands together into a pipeline. Frequently, the commands used this way are referred to as filters. Filters take input, change it somehow and then output it. The first one we will try is sort. Imagine we wanted to make a combined list of all of the executable programs in /bin and /usr/bin, put them in sorted order and view it:

​ 管道线经常用来对数据完成复杂的操作。有可能会把几个命令放在一起组成一个管道线。 通常，以这种方式使用的命令被称为过滤器。过滤器接受输入，以某种方式改变它，然后 输出它。第一个我们想试验的过滤器是 sort。想象一下，我们想把目录/bin 和/usr/bin 中 的可执行程序都联合在一起，再把它们排序，然后浏览执行结果：

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[me@linuxbox ~]$ ls /bin /usr/bin | sort | less

Since we specified two directories (/bin and /usr/bin), the output of ls would have consisted of two sorted lists, one for each directory. By including sort in our pipeline, we changed the data to produce a single, sorted list.

​ 因为我们指定了两个目录（/bin 和/usr/bin），ls 命令的输出结果由有序列表组成， 各自针对一个目录。通过在管道线中包含 sort，我们改变输出数据，从而产生一个 有序列表。

uniq - 报道或忽略重复行

The uniq command is often used in conjunction with sort. uniq accepts a sorted list of data from either standard input or a single filename argument (see the uniq man page for details) and, by default, removes any duplicates from the list. So, to make sure our list has no duplicates (that is, any programs of the same name that appear in both the /bin and /usr/bin directories) we will add uniq to our pipeline:

​ uniq 命令经常和 sort 命令结合在一起使用。uniq 从标准输入或单个文件名参数接受数据有序 列表（详情查看 uniq 手册页），默认情况下，从数据列表中删除任何重复行。所以，为了确信 我们的列表中不包含重复句子（这是说，出现在目录/bin 和/usr/bin 中重名的程序），我们添加 uniq 到我们的管道线中：

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[me@linuxbox ~]$ ls /bin /usr/bin | sort | uniq | less

In this example, we use uniq to remove any duplicates from the output of the sort command. If we want to see the list of duplicates instead, we add the “-d” option to uniq like so:

​ 在这个例子中，我们使用 uniq 从 sort 命令的输出结果中，来删除任何重复行。如果我们想看到 重复内容，让 uniq 命令带上”-d”选项，就像这样：

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[me@linuxbox ~]$ ls /bin /usr/bin | sort | uniq -d | less

wc － 打印行数、字数和字节数

The wc (word count) command is used to display the number of lines, words, and bytes contained in files. For example:

​ wc（字数统计）命令是用来显示文件所包含的行数、字数和字节数。例如：

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[me@linuxbox ~]$ wc ls-output.txt
7902 64566 503634 ls-output.txt

In this case it prints out three numbers: lines, words, and bytes contained in ls- output.txt. Like our previous commands, if executed without command line arguments, wc accepts standard input. The “-l” option limits its output to only report lines. Adding it to a pipeline is a handy way to count things. To see the number of programs we have in our sorted list, we can do this:

​ 在这个例子中，wc 打印出来三个数字：包含在文件 ls-output.txt 中的行数，单词数和字节数， 正如我们先前的命令，如果 wc 不带命令行参数，它接受标准输入。”-l”选项限制命令输出只能 报道行数。添加 wc 到管道线来统计数据，是个很便利的方法。查看我们的有序列表中程序个数， 我们可以这样做：

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[me@linuxbox ~]$ ls /bin /usr/bin | sort | uniq | wc -l
2728

grep － 打印匹配行

grep is a powerful program used to find text patterns within files. It’s used like this:

​ grep 是个很强大的程序，用来找到文件中的匹配文本。这样使用 grep 命令：

grep pattern [file...]

When grep encounters a “pattern” in the file, it prints out the lines containing it. The patterns that grep can match can be very complex, but for now we will concentrate on simple text matches. We’ll cover the advanced patterns, called regular expressions in a later chapter.

​ 当 grep 遇到一个文件中的匹配”模式”，它会打印出包含这个类型的行。grep 能够匹配的模式可以 很复杂，但是现在我们把注意力集中在简单文本匹配上面。在后面的章节中，我们将会研究 高级模式，叫做正则表达式。

Let’s say we want to find all the files in our list of programs that had the word “zip” embedded in the name. Such a search might give us an idea of some of the programs on our system that had something to do with file compression. We would do this:

比如说，我们想在我们的程序列表中，找到文件名中包含单词”zip”的所有文件。这样一个搜索， 可能让我们了解系统中的一些程序与文件压缩有关系。这样做：

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[me@linuxbox ~]$ ls /bin /usr/bin | sort | uniq | grep zip
bunzip2
bzip2
gunzip
...

There are a couple of handy options for grep: “-i” which causes grep to ignore case when performing the search (normally searches are case sensitive) and “-v” which tells grep to only print lines that do not match the pattern.

​ grep 有一些方便的选项：”-i”使得 grep 在执行搜索时忽略大小写（通常，搜索是大小写 敏感的），”-v”选项会告诉 grep 只打印不匹配的行。

head / tail － 打印文件开头部分/结尾部分

Sometimes you don’t want all of the output from a command. You may only want the first few lines or the last few lines. The head command prints the first ten lines of a file and the tail command prints the last ten lines. By default, both commands print ten lines of text, but this can be adjusted with the “-n” option:

​ 有时候你不需要一个命令的所有输出。可能你只想要前几行或者后几行的输出内容。 head 命令打印文件的前十行，而 tail 命令打印文件的后十行。默认情况下，两个命令 都打印十行文本，但是可以通过”-n”选项来调整命令打印的行数。

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[me@linuxbox ~]$ head -n 5 ls-output.txt
total 343496
...
[me@linuxbox ~]$ tail -n 5 ls-output.txt
...

These can be used in pipelines as well:

​ 它们也能用在管道线中：

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[me@linuxbox ~]$ ls /usr/bin | tail -n 5
znew
...

tail has an option which allows you to view files in real-time. This is useful for watching the progress of log files as they are being written. In the following example, we will look at the messages file in /var/log. Superuser privileges are required to do this on some Linux distributions, since the /var/log/messages file may contain security information:

​ tail 有一个选项允许你实时地浏览文件。当观察日志文件的进展时，这很有用，因为 它们同时在被写入。在以下的例子里，我们要查看目录/var/log 里面的信息文件。在 一些 Linux 发行版中，要求有超级用户权限才能阅读这些文件，因为文件 /var/log/messages 可能包含安全信息。

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[me@linuxbox ~]$ tail -f /var/log/messages
Feb 8 13:40:05 twin4 dhclient: DHCPACK from 192.168.1.1
....

Using the “-f” option, tail continues to monitor the file and when new lines are appended, they immediately appear on the display. This continues until you type Ctrl-c.

​ 使用”-f”选项，tail 命令继续监测这个文件，当新的内容添加到文件后，它们会立即 出现在屏幕上。这会一直继续下去直到你输入 Ctrl-c。

tee － 从 Stdin 读取数据，并同时输出到 Stdout 和文件

In keeping with our plumbing metaphor, Linux provides a command called tee which creates a “tee” fitting on our pipe. The tee program reads standard input and copies it to both standard output (allowing the data to continue down the pipeline) and to one or more files. This is useful for capturing a pipeline’s contents at an intermediate stage of processing. Here we repeat one of our earlier examples, this time including tee to capture the entire directory listing to the file ls.txt before grep filters the pipeline’s contents:

​ 为了和我们的管道隐喻保持一致，Linux 提供了一个叫做 tee 的命令，这个命令制造了 一个”tee”（三通管件，做水管工人会对这个非常熟悉），安装到我们的管道上。tee 程序从标准输入读入数据，并且同时复制数据 到标准输出（允许数据继续随着管道线流动）和一个或多个文件。当在某个中间处理 阶段来捕捉一个管道线的内容时，这很有帮助。这里，我们重复执行一个先前的例子， 这次包含 tee 命令，在 grep 过滤管道线的内容之前，来捕捉整个目录列表到文件 ls.txt：

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[me@linuxbox ~]$ ls /usr/bin | tee ls.txt | grep zip
bunzip2
bzip2
....

总结归纳

As always, check out the documentation of each of the commands we have covered in this chapter. We have only seen their most basic usage. They all have a number of interesting options. As we gain Linux experience, we will see that the redirection feature of the command line is extremely useful for solving specialized problems. There are many commands that make use of standard input and output, and almost all command line programs use standard error to display their informative messages.

​ 一如既往，查看这章学到的每一个命令的文档。我们已经知道了他们最基本的用法。 它们还有很多有趣的选项。随着我们 Linux 经验的积累，我们会了解命令行重定向特性 在解决特殊问题时非常有用处。有许多命令利用标准输入和标准输出，而几乎所有的命令行 程序都使用标准错误输出来显示特别重要的信息。

Linux Is About Imagination

Linux 可以激发我们的想象

When I am asked to explain the difference between Windows and Linux, I often use a toy analogy.

​ 当我被要求解释 Windows 与 Linux 之间的差异时，我经常拿玩具来作比喻。

Windows is like a Game Boy. You go to the store and buy one all shiny new in the box. You take it home, turn it on and play with it. Pretty graphics, cute sounds. After a while though, you get tired of the game that came with it so you go back to the store and buy another one. This cycle repeats over and over. Finally, you go back to the store and say to the person behind the counter, “I want a game that does this!” only to be told that no such game exists because there is no “market demand” for it. Then you say, “But I only need to change this one thing!” The person behind the counter says you can’t change it. The games are all sealed up in their cartridges. You discover that your toy is limited to the games that others have decided that you need and no more.

​ Windows 就像一个游戏机。你去商店，买了一个包装在盒子里面的全新的游戏机。 你把它带回家，打开盒子，开始玩游戏。精美的画面，动人的声音。玩了一段时间之后， 你厌倦了它自带的游戏，所以你返回商店，又买了另一个游戏机。这个过程反复重复。 最后，你玩腻了游戏机自带的游戏，你回到商店，告诉售货员，“我想要一个这样的游戏！” 但售货员告诉你没有这样的游戏存在，因为它没有“市场需求”。然后你说，“但是我只 需要修改一下这个游戏！“，售货员又告诉你不能修改它。所有游戏都被封装在它们的 存储器中。到头来，你发现你的玩具只局限于别人为你规定好的游戏。

Linux, on the other hand, is like the world’s largest Erector Set. You open it up and it’s just a huge collection of parts. A lot of steel struts, screws, nuts, gears, pulleys, motors, and a few suggestions on what to build. So you start to play with it. You build one of the suggestions and then another. After a while you discover that you have your own ideas of what to make. You don’t ever have to go back to the store, as you already have everything you need. The Erector Set takes on the shape of your imagination. It does what you want.

​ 另一方面，Linux 就像一个全世界上最大的零件盒子。你打开它，发现它只是一个巨大的 部件集合。有许多钢支柱、螺钉、螺母、齿轮、滑轮、发动机和一些怎样来建造它的说明书。 然后你开始摆弄它。你建造了一个又一个样板模型。过了一会儿，你发现你要建造自己的模型。 你不必返回商店，因为你已经拥有了你需要的一切。建造模型以你构想的形状为模板，搭建 你想要的模型。

Your choice of toys is, of course, a personal thing, so which toy would you find more satisfying?

​ 当然，选择哪一个玩具，是你的事情，那么你觉得哪个玩具更令人满意呢？

8 - 8 从 shell 眼中看世界

从 shell 眼中看世界

http://billie66.github.io/TLCL/book/chap08.html

In this chapter we are going to look at some of the “magic” that occurs on the command line when you press the enter key. While we will examine several interesting and complex features of the shell, we will do it with just one new command:

​ 在这一章我们将看到，当你按下 enter 键后，发生在命令行中的一些“魔法”。我们会 深入研究几个复杂而有趣的 shell 特性，需要使用一个新命令：

• echo - Display a line of text
• echo － 显示一行文本

(字符)展开

Each time you type a command line and press the enter key, bash performs several processes upon the text before it carries out your command. We have seen a couple of cases of how a simple character sequence, for example “*”, can have a lot of meaning to the shell. The process that makes this happen is called expansion. With expansion, you type something and it is expanded into something else before the shell acts upon it. To demonstrate what we mean by this, let’s take a look at the echo command. echo is a shell builtin that performs a very simple task. It prints out its text arguments on standard output:

​ 每当你输入一个命令并按下 enter 键，bash 会在执行你的命令之前对输入 的字符完成几个步骤的处理。我们已经见过几个例子：例如一个简单的字符序列”*”, 对 shell 来说有着多么丰富的涵义。这背后的的过程叫做（字符）展开。通过展开， 你输入的字符，在 shell 对它起作用之前，会展开成为别的字符。为了说明这一点 ，让我们看一看 echo 命令。echo 是一个 shell 内建命令，可以完成非常简单的任务。 它将它的文本参数打印到标准输出中。

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[me@linuxbox ~]$ echo this is a test
this is a test

That’s pretty straightforward. Any argument passed to echo gets displayed. Let’s try another example:

​ 这个命令的作用相当简单明了。传递到 echo 命令的任一个参数都会在（屏幕上）显示出来。 让我们试试另一个例子：

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[me@linuxbox ~]$ echo *
Desktop Documents ls-output.txt Music Pictures Public Templates Videos

So what just happened? Why didn’t echo print “”? As you recall from our work with wildcards, the “” character means match any characters in a filename, but what we didn’t see in our original discussion was how the shell does that. The simple answer is that the shell expands the “” into something else (in this instance, the names of the files in the current working directory) before the echo command is executed. When the enter key is pressed, the shell automatically expands any qualifying characters on the command line before the command is carried out, so the echo command never saw the “”, only its expanded result. Knowing this, we can see that echo behaved as expected.

​ 那么刚才发生了什么事情呢？ 为什么 echo 不打印”“呢？如果你回忆起我们所学过的 关于通配符的内容，这个”“字符意味着匹配文件名中的任意字符，但在原先的讨论 中我们并不知道 shell 是怎样实现这个功能的。简单的答案就是 shell 在 echo 命 令被执行前把”“展开成了另外的东西（在这里，就是在当前工作目录下的文件名字）。 当回车键被按下时，shell 在命令被执行前在命令行上自动展开任何符合条件的字符， 所以 echo 命令的实际参数并不是”“，而是它展开后的结果。知道了这个以后， 我们就能明白 echo 的行为符合预期。

路径名展开

The mechanism by which wildcards work is called pathname expansion. If we try some of the techniques that we employed in our earlier chapters, we will see that they are really expansions. Given a home directory that looks like this:

​ 通配符所依赖的工作机制叫做路径名展开。如果我们试一下在之前的章节中使用的技巧， 我们会看到它们实际上是展开。给定一个家目录，它看起来像这样：

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[me@linuxbox ~]$ ls
Desktop   ls-output.txt   Pictures   Templates
....

we could carry out the following expansions:

​ 我们能够执行以下的展开：

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[me@linuxbox ~]$ echo D*
Desktop  Documents

and:

​ 和：

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[me@linuxbox ~]$ echo *s
Documents Pictures Templates Videos

or even:

​ 甚至是：

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[me@linuxbox ~]$ echo [[:upper:]]*
Desktop Documents Music Pictures Public Templates Videos

and looking beyond our home directory:

​ 查看家目录之外的目录：

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[me@linuxbox ~]$ echo /usr/*/share
/usr/kerberos/share  /usr/local/share

Pathname Expansion Of Hidden Files

隐藏文件路径名展开

As we know, filenames that begin with a period character are hidden. Pathname expansion also respects this behavior. An expansion such as:

​ 正如我们知道的，以圆点字符开头的文件名是隐藏文件。路径名展开也尊重这种 行为。像这样的展开：

*echo **

does not reveal hidden files.

​ 不会显示隐藏文件

It might appear at first glance that we could include hidden files in an expansion by starting the pattern with a leading period, like this:

​ 直觉告诉我们，如果展开模式以一个圆点开头，我们就能够在展开中包含隐藏文件， 就像这样：

echo .*

It almost works. However, if we examine the results closely, we will see that the names “.” and “..” will also appear in the results. Since these names refer to the current working directory and its parent directory, using this pattern will likely produce an incorrect result. We can see this if we try the command:

​ 它几乎要起作用了。然而，如果我们仔细检查一下输出结果，我们会看到名字”.” 和”..”也出现在结果中。由于它们是指当前工作目录和父目录，使用这种 模式可能会产生不正确的结果。我们可以通过这个命令来验证：

ls -d .* | less

To correctly perform pathname expansion in this situation, we have to employ a more specific pattern. This will work correctly:

​ 为了在这种情况下正确地完成路径名展开，我们应该使用一个更精确的模式。 这个模式会正确地工作：

ls -d .[!.]?*

This pattern expands into every filename that begins with a period, does not include a second period, contains at least one additional character and can be followed by any other characters. This will work correctly with most hidden files (though it still won’t include filenames with multiple leading periods). The ls command with the -A option (“almost all”) will provide a correct listing of hidden files:

​ 这种模式展开成所有以圆点开头，第二个字符不包含圆点，再包含至少一个字符， 并且这个字符之后紧接着任意多个字符的文件名。这个命令将正确列出大多数的隐藏文件 （但仍不能包含以多个圆点开头的文件名）。带有 -A 选项（“几乎所有”）的 ls 命令能够提供一份正确的隐藏文件清单：

ls -A

波浪线展开

As you may recall from our introduction to the cd command, the tilde character (“~”) has a special meaning. When used at the beginning of a word, it expands into the name of the home directory of the named user, or if no user is named, the home directory of the current user:

​ 可能你从我们对 cd 命令的介绍中回想起来，波浪线字符(“~”)有特殊的含义。当它用在 一个单词的开头时，它会展开成指定用户的家目录名，如果没有指定用户名，则展开成当前用户的家目录：

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[me@linuxbox ~]$ echo ~
/home/me

If user “foo” has an account, then:

​ 如果有用户”foo”这个帐号，那么：

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[me@linuxbox ~]$ echo ~foo
/home/foo

算术表达式展开

The shell allows arithmetic to be performed by expansion. This allow us to use the shell prompt as a calculator:

​ shell 在展开中执行算数表达式。这允许我们把 shell 当作计算器来使用：

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[me@linuxbox ~]$ echo $((2 + 2))
4

Arithmetic expansion uses the form:

​ 算术表达式展开使用这种格式：

$((expression))

where expression is an arithmetic expression consisting of values and arithmetic operators.

​ （以上括号中的）表达式是指算术表达式，它由数值和算术操作符组成。

Arithmetic expansion only supports integers (whole numbers, no decimals), but can perform quite a number of different operations. Here are a few of the supported operators:

​ 算术表达式只支持整数（全部是数字，不带小数点），但是能执行很多不同的操作。这里是 一些它支持的操作符：

Spaces are not significant in arithmetic expressions and expressions may be nested. For example, to multiply five squared by three:

​ 在算术表达式中空格并不重要，并且表达式可以嵌套。例如，5的平方乘以3：

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[me@linuxbox ~]$ echo $(($((5**2)) * 3))
75

Single parentheses may be used to group multiple subexpressions. With this technique, we can rewrite the example above and get the same result using a single expansion instead of two:

​ 一对括号可以用来把多个子表达式括起来。通过这个技术，我们可以重写上面的例子， 同时用一个展开代替两个，来得到一样的结果：

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[me@linuxbox ~]$ echo $(((5**2) * 3))
75

Here is an example using the division and remainder operators. Notice the effect of integer division:

​ 这是一个使用除法和取余操作符的例子。注意整数除法的结果：

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[me@linuxbox ~]$ echo Five divided by two equals $((5/2))
Five divided by two equals 2
[me@linuxbox ~]$ echo with $((5%2)) left over.
with 1 left over.

Arithmetic expansion is covered in greater detail in Chapter 35.

​ 在35章会更深入地讨论算术表达式的内容。

花括号展开

Perhaps the strangest expansion is called brace expansion. With it, you can create multiple text strings from a pattern containing braces. Here’s an example:

​ 可能最奇怪的展开是花括号展开。通过它，你可以从一个包含花括号的模式中 创建多个文本字符串。这是一个例子：

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[me@linuxbox ~]$ echo Front-{A,B,C}-Back
Front-A-Back Front-B-Back Front-C-Back

Patterns to be brace expanded may contain a leading portion called a preamble and a trailing portion called a postscript. The brace expression itself may contain either a comma-separated list of strings, or a range of integers or single characters. The pattern may not contain embedded whitespace. Here is an example using a range of integers:

​ 花括号展开模式可能包含一个开头部分叫做前言，一个结尾部分叫做附言。花括号表达式本身可 能包含一个由逗号分开的字符串列表，或者一个整数区间，或者单个的字符的区间。这种模式不能 嵌入空白字符。这个例子中使用了一个整数区间：

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[me@linuxbox ~]$ echo Number_{1..5}
Number_1  Number_2  Number_3  Number_4  Number_5

A range of letters in reverse order:

​ 倒序排列的字母区间：

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[me@linuxbox ~]$ echo {Z..A}
Z Y X W V U T S R Q P O N M L K J I H G F E D C B A

Brace expansions may be nested:

​ 花括号展开可以嵌套：

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[me@linuxbox ~]$ echo a{A{1,2},B{3,4}}b
aA1b aA2b aB3b aB4b

So what is this good for? The most common application is to make lists of files or directories to be created. For example, if we were photographers and had a large collection of images that we wanted to organize into years and months, the first thing we might do is create a series of directories named in numeric “Year-Month” format. This way, the directory names will sort in chronological order. We could type out a complete list of directories, but that’s a lot of work and it’s error-prone too. Instead, we could do this:

​ 那这个有啥用呢？最常见的应用是，创建一系列的文件或目录列表。例如， 如果我们是摄影师，有大量的相片。我们想把这些相片按年月先后组织起来。首先， 我们要创建一系列以数值”年－月”形式命名的目录。通过这种方式，可以使目录名按照 年代顺序排列。我们可以手动键入整个目录列表，但是工作量太大了，并且易于出错。 反之，我们可以这样做：

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[me@linuxbox ~]$ mkdir Pics
[me@linuxbox ~]$ cd Pics
[me@linuxbox Pics]$ mkdir {2007..2009}-0{1..9} {2007..2009}-{10..12}
[me@linuxbox Pics]$ ls
2007-01 2007-07 2008-01 2008-07 2009-01 2009-07
2007-02 2007-08 2008-02 2008-08 2009-02 2009-08
2007-03 2007-09 2008-03 2008-09 2009-03 2009-09
2007-04 2007-10 2008-04 2008-10 2009-04 2009-10
2007-05 2007-11 2008-05 2008-11 2009-05 2009-11
2007-06 2007-12 2008-06 2008-12 2009-06 2009-12

Pretty slick!

棒极了！

参数展开

We’re only going to touch briefly on parameter expansion in this chapter, but we’ll be covering it extensively later. It’s a feature that is more useful in shell scripts than directly on the command line. Many of its capabilities have to do with the system’s ability to store small chunks of data and to give each chunk a name. Many such chunks, more properly called variables, are available for your examination. For example, the variable named “USER” contains your user name. To invoke parameter expansion and reveal the contents of USER you would do this:

​ 在这一章我们将会简单介绍参数展开，会在后续章节中进行详细讨论。这个特性在 shell 脚本中比直接在命令行中更有用。 它的许多功能和系统存储小块数据，并给每块数据命名的能力有关系。许多像这样的小块数据， 更恰当的称呼应该是变量，可供你方便地检查它们。例如，叫做”USER”的变量包含你的 用户名。可以这样做来调用参数，并查看 USER 中的内容，：

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[me@linuxbox ~]$ echo $USER
me

To see a list of available variables, try this:

​ 要查看有效的变量列表，可以试试这个：

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[me@linuxbox ~]$ printenv | less

You may have noticed that with other types of expansion, if you mistype a pattern, the expansion will not take place and the echo command will simply display the mistyped pattern. With parameter expansion, if you misspell the name of a variable, the expansion will still take place, but will result in an empty string:

​ 你可能注意到在其它展开类型中，如果你误输入一个字符串，展开就不会发生。这时 echo 命令只简单地显示误键入的字符串。但在参数展开中，如果你拼写错了一个变量名， 展开仍然会进行，只是展开的结果是一个空字符串：

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[me@linuxbox ~]$ echo $SUER

[me@linuxbox ~]$

命令替换

Command substitution allows us to use the output of a command as an expansion:

​ 命令替换允许我们把一个命令的输出作为另一个命令的一部分来使用：

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[me@linuxbox ~]$ echo $(ls)
Desktop Documents ls-output.txt Music Pictures Public Templates
Videos

One of my favorites goes something like this:

​ 我最喜欢用的一行命令是像这样的：

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[me@linuxbox ~]$ ls -l $(which cp)
-rwxr-xr-x 1 root root 71516 2007-12-05 08:58 /bin/cp

Here we passed the results of which cp as an argument to the ls command, thereby getting the listing of of the cp program without having to know its full pathname. We are not limited to just simple commands. Entire pipelines can be used (only partial output shown):

​ 这里我们把 which cp 的执行结果作为一个参数传递给 ls 命令，因此可以在不知道 cp 命令 完整路径名的情况下得到它的文件属性列表。我们不只限于简单命令。也可以使用整个管道线 （只展示部分输出）：

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[me@linuxbox ~]$ file $(ls /usr/bin/* | grep zip)
/usr/bin/bunzip2:     symbolic link to `bzip2`
....

In this example, the results of the pipeline became the argument list of the file command.

​ 在这个例子中，管道线的输出结果成为 file 命令的参数列表。

There is an alternate syntax for command substitution in older shell programs which is also supported in bash. It uses back-quotes instead of the dollar sign and parentheses:

​ 在旧版 shell 程序中，有另一种语法也支持命令替换，可与刚提到的语法轮换使用。 bash 也支持这种语法。它使用倒引号来代替美元符号和括号：

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[me@linuxbox ~]$ ls -l `which cp`
-rwxr-xr-x 1 root root 71516 2007-12-05 08:58 /bin/cp

引用

Now that we’ve seen how many ways the shell can perform expansions, it’s time to learn how we can control it. Take for example:

​ 我们已经知道 shell 有许多方式可以完成展开，现在是时候学习怎样来控制展开了。 以下面例子来说明：

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[me@linuxbox ~]$ echo this is a    test
this is a test

or:

​ 或者：

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[me@linuxbox ~]$ echo The total is $100.00
The total is 00.00

In the first example, word-splitting by the shell removed extra whitespace from the echo command’s list of arguments. In the second example, parameter expansion substituted an empty string for the value of “$1” because it was an undefined variable. The shell provides a mechanism called quoting to selectively suppress unwanted expansions.

​ 在第一个例子中，shell 利用单词分割删除掉 echo 命令的参数列表中多余的空格。在第二个例子中， 参数展开把 $1 的值替换为一个空字符串，因为 1 是没有定义的变量。shell 提供了一种 叫做引用的机制，来有选择地禁止不需要的展开。

双引号

The first type of quoting we will look at is double quotes. If you place text inside double quotes, all the special characters used by the shell lose their special meaning and are treated as ordinary characters. The exceptions are $, \ (backslash), and ` (back-quote). This means that word-splitting, pathname expansion, tilde expansion, and brace expansion are suppressed, but parameter expansion, arithmetic expansion, and command substitution are still carried out. Using double quotes, we can cope with filenames containing embedded spaces. Say we were the unfortunate victim of a file called two words.txt.If we tried to use this on the command line, word-splitting would cause this to be treated as two separate arguments rather than the desired single argument:

​ 我们将要看一下引用的第一种类型，双引号。如果你把文本放在双引号中， shell 使用的特殊字符，都失去它们的特殊含义，被当作普通字符来看待。 有几个例外： $，\ (反斜杠），和 `（倒引号）。这意味着单词分割、路径名展开、 波浪线展开和花括号展开都将失效，然而参数展开、算术展开和命令替换 仍然执行。使用双引号，我们可以处理包含空格的文件名。比方说 two words.txt 文件，如果我们试图在命令行中使用这个 文件，单词分割机制会导致这个文件名被看作两个独自的参数，而不是所期望 的单个参数：

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[me@linuxbox ~]$ ls -l two words.txt
ls: cannot access two: No such file or directory
ls: cannot access words.txt: No such file or directory

By using double quotes, we stop the word-splitting and get the desired result; further, we can even repair the damage:

​ 使用双引号，我们可以阻止单词分割，得到期望的结果；进一步，我们甚至可以修复 破损的文件名。

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[me@linuxbox ~]$ ls -l "two words.txt"
-rw-rw-r-- 1 me   me   18 2008-02-20 13:03 two words.txt
[me@linuxbox ~]$ mv "two words.txt" two_words.txt

There! Now we don’t have to keep typing those pesky double quotes.

​ 用来下划线，现在我们不必一直输入那些讨厌的双引号了。

Remember, parameter expansion, arithmetic expansion, and command substitution still take place within double quotes:

​ 记住，在双引号中，参数展开、算术表达式展开和命令替换仍然有效：

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[me@linuxbox ~]$ echo "$USER $((2+2)) $(cal)"
me 4    February 2008
Su Mo Tu We Th Fr Sa
....

We should take a moment to look at the effect of double quotes on command substitution. First let’s look a little deeper at how word splitting works. In our earlier example, we saw how word-splitting appears to remove extra spaces in our text:

​ 我们应该花费一点时间来看一下双引号在命令替换中的效果。首先仔细研究一下单词分割 是怎样工作的。在之前的范例中，我们已经看到单词分割机制是怎样来删除文本中额外空格的：

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[me@linuxbox ~]$ echo this is a   test
this is a test

By default, word-splitting looks for the presence of spaces, tabs, and newlines (linefeed characters) and treats them as delimiters between words. This means that unquoted spaces, tabs, and newlines are not considered to be part of the text. They only serve as separators. Since they separate the words into different arguments, our example command line contains a command followed by four distinct arguments. If we add double quotes:

​ 在默认情况下，单词分割机制会在单词中寻找空格，制表符，和换行符，并把它们看作 单词之间的界定符。这意味着无引用的空格，制表符和换行符都不是文本的一部分， 它们只作为分隔符使用。由于它们把单词分为不同的参数，所以在上面的例子中， 命令行包含一个带有四个不同参数的命令。如果我们加上双引号：

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[me@linuxbox ~]$ echo "this is a    test"
this is a    test

word-splitting is suppressed and the embedded spaces are not treated as delimiters, rather they become part of the argument. Once the double quotes are added, our command line contains a command followed by a single argument.

单词分割被禁止，内嵌的空格也不会被当作界定符，它们成为参数的一部分。 一旦加上双引号，我们的命令行就包含一个带有一个参数的命令。

The fact that newlines are considered delimiters by the word-splitting mechanism causes an interesting, albeit subtle, effect on command substitution. Consider the following:

​ 事实上，单词分割机制把换行符看作界定符，对命令替换产生了一个虽然微妙但有趣的影响。 考虑下面的例子：

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[me@linuxbox ~]$ echo $(cal)
February 2008 Su Mo Tu We Th Fr Sa 1 2 3 4 5 6 7 8 9 10 11 12 13 14
15 16 17 18 19 20 21 22 23 24 25 26 27 28 29
[me@linuxbox ~]$ echo "$(cal)"
February 2008
....

In the first instance, the unquoted command substitution resulted in a command line containing thirty-eight arguments. In the second, a command line with one argument that includes the embedded spaces and newlines.

​ 在第一个例子，没有引用的命令替换导致命令行包含38个参数。在第二个例子中， 命令行只有一个参数，参数中包括嵌入的空格和换行符。

单引号

If we need to suppress all expansions, we use single quotes. Here is a comparison of unquoted, double quotes, and single quotes:

​ 如果需要禁止所有的展开，我们要使用单引号。以下例子是无引用，双引号，和单引号的比较结果：

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[me@linuxbox ~]$ echo text ~/*.txt {a,b} $(echo foo) $((2+2)) $USER
text /home/me/ls-output.txt a b foo 4 me
[me@linuxbox ~]$ echo "text ~/*.txt {a,b} $(echo foo) $((2+2)) $USER"
text ~/*.txt   {a,b} foo 4 me
[me@linuxbox ~]$ echo 'text ~/*.txt {a,b} $(echo foo) $((2+2)) $USER'
text ~/*.txt  {a,b} $(echo foo) $((2+2)) $USER

As we can see, with each succeeding level of quoting, more and more of the expansions are suppressed.

​ 正如我们所看到的，随着引用程度加强，越来越多的展开被禁止。

转义字符

Sometimes we only want to quote a single character. To do this, we can precede a character with a backslash, which in this context is called the escape character. Often this is done inside double quotes to selectively prevent an expansion:

​ 有时候我们只想引用单个字符。我们可以在字符之前加上一个反斜杠，在这里叫做转义字符。 经常在双引号中使用转义字符，来有选择地阻止展开。

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[me@linuxbox ~]$ echo "The balance for user $USER is: \$5.00"
The balance for user me is: $5.00

It is also common to use escaping to eliminate the special meaning of a character in a filename. For example, it is possible to use characters in filenames that normally have special meaning to the shell. These would include “$”, “!”, “&”, “ “, and others. To include a special character in a filename you can to this:

​ 使用转义字符来消除文件名中一个字符的特殊含义，是很普遍的。例如，在文件名中可能使用 一些对于 shell 来说有特殊含义的字符。这些字符包括”$”, “!”, “ “等字符。在文件名 中包含特殊字符，你可以这样做：

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[me@linuxbox ~]$ mv bad\&filename good_filename

To allow a backslash character to appear, escape it by typing “\”. Note that within single quotes, the backslash loses its special meaning and is treated as an ordinary character.

​ 为了允许反斜杠字符出现，输入”\“来转义。注意在单引号中，反斜杠失去它的特殊含义，它 被看作普通字符。

Backslash Escape Sequences

反斜杠转义字符序列

In addition to its role as the escape character, the backslash is also used as part of a notation to represent certain special characters called control codes. The first thirty-two characters in the ASCII coding scheme are used to transmit commands to teletype-like devices. Some of these codes are familiar (tab, backspace, linefeed, and carriage return), while others are not (null, end-of-transmission, and acknowledge).

​ 反斜杠除了作为转义字符外，也可以构成一种表示法，来代表某种 特殊字符，这些特殊字符叫做控制码。ASCII 编码表中前32个字符被用来把命令转输到电报机 之类的设备。一些编码是众所周知的（制表符，退格符，换行符，和回车符），而其它 一些编码就不熟悉了（空值，传输结束码，和确认）。

Escape Sequence	Meaning
\a	Bell(“Alert”-causes the computer to beep)
\b	Backspace
\n	Newline. On Unix-like systems, this produces a linefeed.
\r	Carriage return
\t	Tab

转义序列	含义
\a	响铃（”警告”－导致计算机嘟嘟响）
\b	退格符
\n	新的一行。在类 Unix 系统中，产生换行。
\r	回车符
\t	制表符

The table above lists some of the common backslash escape sequences. The idea behind this representation using the backslash originated in the C programming language and has been adopted by many others, including the shell.

​ 上表列出了一些常见的反斜杠转义字符序列。这种利用反斜杠的表示法背后的思想来源于 C 编程语言， 许多其它语言也采用了这种表示方法，包括 shell。

Adding the ‘-e’ option to echo will enable interpretation of escape sequences. You may also place them inside $’ ‘. Here, using the sleep command, a simple program that just waits for the specified number of seconds and then exits, we can create a primitive countdown timer:

​ echo 命令带上 ‘-e’ 选项，能够解释转义序列。你可以把转义序列放在 $’ ’ 里面。 以下例子中，我们可以使用 sleep 命令创建一个简单的倒数计数器（ sleep 是一个简单的程序， 它会等待指定的秒数，然后退出）：

sleep 10; echo -e “Time’s up\a”

We could also do this: 我们也可以这样做：

sleep 10; echo “Time’s up” $’\a’

总结归纳

As we move forward with using the shell, we will find that expansions and quoting will be used with increasing frequency, so it makes sense to get a good understanding of the way they works. In fact, it could be argued that they are the most important subjects to learn about the shell. Without a proper understanding of expansion, the shell will always be a source of mystery and confusion, and much of it potential power wasted.

​ 随着我们继续学习 shell，你会发现使用展开和引用的频率逐渐多起来，所以能够很好的 理解它们的工作方式很有意义。事实上，可以这样说，它们是学习 shell 的最重要的主题。 如果没有准确地理解展开模式，shell 总是神秘和混乱的源泉，并且 shell 潜在的能力也 浪费掉了。

拓展阅读

• The bash man page has major sections on both expansion and quoting which cover these topics in a more formal manner.

• Bash 手册页有主要段落是关于展开和引用的，内容更全面。

• The Bash Reference Manual also contains chapters on expansion and quoting:

• Bash 参考手册也包含绍展开和引用的相关内容：

  http://www.gnu.org/software/bash/manual/bashref.html

9 - 9 键盘高级操作技巧

键盘高级操作技巧

http://billie66.github.io/TLCL/book/chap09.html

I often kiddingly describe Unix as “the operating system for people who like to type.” Of course, the fact that it even has a command line is a testament to that. But command line users don’t like to type that much. Why else would so many commands have such short names like cp, ls, mv, and rm? In fact, one of the most cherished goals of the command line is laziness; doing the most work with the fewest number of keystrokes. Another goal is never having to lift your fingers from the keyboard, never reaching for the mouse. In this chapter, we will look at bash features that make keyboard use faster and more efficient.

​ 开玩笑地说，我经常把 Unix 描述为“这个操作系统是为喜欢敲键盘的人们而生的。” 当然，Unix 有命令行这件事证明了我所说的话。但是命令行用户不喜欢敲入 那么多字。要不为什么会有如此多的命令有这样简短的命令名，像cp、ls、mv和 rm？事实上 ，命令行最为珍视的目标之一就是懒惰；用最少的击键次数来完成最多的工作。另一个 目标是你的手指永远不必离开键盘，永不触摸鼠标。在这一章节，我们将看一下 bash 特性 ，这些特性使键盘使用起来更加迅速，更加高效。

The following commands will make an appearance:

​ 以下命令将会露面：

• clear - Clear the screen
• history - Display the contents of the history list
• clear － 清空屏幕
• history － 显示历史列表内容

命令行编辑

bash uses a library (a shared collection of routines that different programs can use) called Readline to implement command line editing. We have already seen some of this. We know, for example, that the arrow keys move the cursor but there are many more features. Think of these as additional tools that we can employ in our work. It’s not important to learn all of them, but many of them are very useful. Pick and choose as desired.

​ Bash 使用了一个名为 Readline 的库（一系列功能的集合，可以被不同的程序使用）， 来实现命令行编辑。我们已经用过一些这个库所带的功能了。例如，用箭头按键可以移动光标等等。这些功能可以在工作中帮我们提高命令行编辑的效率。不一定要把所有的功能都学会，但会一些是非常有帮助的。选择自己常用的学吧。

Note: Some of the key sequences below (particularly those which use the Alt key) may be intercepted by the GUI for other functions. All of the key sequences should work properly when using a virtual console.

​ 注意：当我们处于图形化环境时，下面一些按键组合（尤其使用 Alt 键的组合），可能会被图形界面拦截来触发其它的功能。 但是当切换到虚拟控制台时，所有的按键组合都会正常工作。

移动光标

The following table lists the keys used to move the cursor:

​ 下表列出了移动光标所使用的按键：

修改文本

Table 9-2 lists keyboard commands that are used to edit characters on the command line.

​ 表9－2列出了键盘命令，这些命令用来在命令行中编辑字符。

剪切和粘贴文本

The Readline documentation uses the terms killing and yanking to refer to what we would commonly call cutting and pasting. Items that are cut are stored in a buffer called the kill-ring.

​ Readline 的文档使用术语 killing 和 yanking 来指我们平常所说的剪切和粘贴。 剪切下来的本文被存储在一个叫做剪切环(kill-ring)的缓冲区中。

The Meta Key

元键

If you venture into the Readline documentation, which can be found in the READLINE section of the bash man page, you will encounter the term “meta key.” On modern keyboards this maps to the Alt key but it wasn’t always so.

​ 如果你冒险进入到 Readline 的文档中，你会在 bash 手册页的 READLINE 段落， 遇到一个术语”元键”（meta key）。在当今的键盘上，这个元键是指 Alt 键，但 并不总是这样。

Back in the dim times (before PCs but after Unix) not everybody had their own computer. What they might have had was a device called a terminal. A terminal was a communication device that featured a text display screen and a keyboard and just enough electronics inside to display text characters and move the cursor around. It was attached (usually by serial cable) to a larger computer or the communication network of a larger computer. There were many different brands of terminals and they all had different keyboards and display feature sets. Since they all tended to at least understand ASCII, software developers wanting portable applications wrote to the lowest common denominator. Unix systems have a very elaborate way of dealing with terminals and their different display features. Since the developers of Readline could not be sure of the presence of a dedicated extra control key, they invented one and called it “meta.” While the Alt key serves as the meta key on modern keyboards, you can also press and release the Esc key to get the same effect as holding down the Alt key if you’re still using a terminal (which you can still do in Linux!).

​ 回到昏暗的年代（在 PC 之前 Unix 之后），并不是每个人都有他们自己的计算机。 他们可能有一个叫做终端的设备。一个终端是一种通信设备，它以一个文本显示 屏幕和一个键盘作为其特色，它里面有足够的电子器件来显示文本字符和移动光标。 它连接到（通常通过串行电缆）一个更大的计算机或者是一个大型计算机的通信 网络。有许多不同的终端产品商标，它们有着不同的键盘和特征显示集。因为它们 都倾向于至少能理解 ASCII，所以软件开发者想要符合最低标准的可移植的应用程序。 Unix 系统有一个非常精巧的方法来处理各种终端产品和它们不同的显示特征。因为 Readline 程序的开发者们，不能确定一个专用多余的控制键的存在，他们发明了一个 控制键，并把它叫做”元”（”meta”）。然而在现代的键盘上，Alt 键作为元键来服务。 如果你仍然在使用终端（在 Linux 中，你仍然可以得到一个终端），你也可以按下和 释放 Esc 键来得到如控制 Alt 键一样的效果。

自动补全

Another way that the shell can help you is through a mechanism called completion. Completion occurs when you press the tab key while typing a command. Let’s see how this works. Given a home directory that looks like this:

​ shell 能帮助你的另一种方式是通过一种叫做自动补全的机制。当你敲入一个命令时， 按下 tab 键，自动补全就会发生。让我们看一下这是怎样工作的。给出一个看起来 像这样的家目录：

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[me@linuxbox ~]$ ls
Desktop   ls-output.txt   Pictures   Templates   Videos
....

Try typing the following but don’t press the Enter key:

​ 试着输入下面的命令，但不要按下 Enter 键：

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[me@linuxbox ~]$ ls l

Now press the tab key:

​ 现在按下 tab 键：

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[me@linuxbox ~]$ ls ls-output.txt

See how the shell completed the line for you? Let’s try another one. Again, don’t press Enter:

​ 看一下 shell 是怎样补全这一行的？让我们再试试另一个例子。这回，也 不要按下 Enter:

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[me@linuxbox ~]$ ls D

Press tab:

​ 按下 tab:

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[me@linuxbox ~]$ ls D

No completion, just a beep. This happened because “D” matches more than one entry in the directory. For completion to be successful, the “clue” you give it has to be unambiguous. If we go further:

​ 没有补全，只是嘟嘟响。因为”D”不止匹配目录中的一个条目。为了自动补全执行成功， 你给它的”线索”不能模棱两可。如果我们继续输入：

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[me@linuxbox ~]$ ls Do

Then press tab:

​ 然后按下 tab：

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[me@linuxbox ~]$ ls Documents

The completion is successful.

​ 自动补全成功了。

While this example shows completion of pathnames, which is its most common use, completion will also work on variables (if the beginning of the word is a “$”), user names (if the word begins with “~”), commands (if the word is the first word on the line.) and host names (if the beginning of the word is “@”). Host name completion only works for host names listed in /etc/hosts.

​ 这个实例展示了路径名自动补全，这是最常用的形式。自动补全也能对变量（如果 字的开头是一个”$”）、用户名字（单词以”~”开始）、命令（如果单词是一行的第一个单词） 和主机名（如果单词的开头是”@”）起作用。主机名自动补全只对包含在文件/etc/hosts 中的主机名有效。

There are a number of control and meta key sequences that are associated with completion:

​ 有一系列的控制和元键序列与自动补全相关联：

Programmable Completion

可编程自动补全

Recent versions of bash have a facility called programmable completion. Programmable completion allows you (or more likely, your distribution provider) to add additional completion rules. Usually this is done to add support for specific applications. For example it is possible to add completions for the option list of a command or match particular file types that an application supports. Ubuntu has a fairly large set defined by default. Programmable completion is implemented by shell functions, a kind of mini shell script that we will cover in later chapters. If you are curious, try:

​ 目前的 bash 版本有一个叫做可编程自动补全工具。可编程自动补全允许你（更可能是，你的 发行版提供商）来加入额外的自动补全规则。通常需要加入对特定应用程序的支持，来完成这个 任务。例如，有可能为一个命令的选项列表，或者一个应用程序支持的特殊文件类型加入自动补全。 默认情况下，Ubuntu 已经定义了一个相当大的规则集合。可编程自动补全是由 shell 函数实现的，shell 函数是一种小巧的 shell 脚本，我们会在后面的章节中讨论到。如果你感到好奇，试一下：

set | less

and see if you can find them. Not all distributions include them by default.

​ 查看一下如果你能找到它们的话。默认情况下，并不是所有的发行版都包括它们。

利用历史命令

As we discovered in Chapter 2, bash maintains a history of commands that have been entered. This list of commands is kept in your home directory in a file called .bash_history. The history facility is a useful resource for reducing the amount of typing you have to do, especially when combined with command line editing.

​ 正如我们在第二章中讨论到的，bash 维护着一个已经执行过的命令的历史列表。这个命令列表 被保存在你家目录下，一个叫做 .bash_history 的文件里。这个 history 工具是个有用资源， 因为它可以减少你敲键盘的次数，尤其当和命令行编辑联系起来时。

搜索历史命令

At any time, we can view the contents of the history list by:

​ 在任何时候，我们都可以浏览历史列表的内容，通过：

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[me@linuxbox ~]$ history | less

By default, bash stores the last five hundred commands you have entered. We will see how to adjust this value in a later chapter. Let’s say we want to find the commands we used to list /usr/bin. One way we could do this:

​ 在默认情况下，bash 会存储你所输入的最后 500 个命令。在随后的章节里，我们会知道 怎样调整这个数值。比方说我们想在自己曾经用过的命令中，找出和/usr/bin这一目录相关的。那么我们就可以这样做：

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[me@linuxbox ~]$ history | grep /usr/bin

And let’s say that among our results we got a line containing an interesting command like this:

​ 比方说在我们的搜索结果之中，我们得到一行，包含了有趣的命令，像这样；

88  ls -l /usr/bin > ls-output.txt

The number “88” is the line number of the command in the history list. We could use this immediately using another type of expansion called history expansion. To use our discovered line we could do this:

​ 数字 “88” 是这个命令在历史列表中的行号。我们可以使用另一种叫做 历史命令展开的方式，来调用“88”所代表的这一行命令：

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[me@linuxbox ~]$ !88

bash will expand “!88” into the contents of the eighty-eighth line in the history list. There are other forms of history expansion that we will cover a little later. bash also provides the ability to search the history list incrementally. This means that we can tell bash to search the history list as we enter characters, with each additional character further refining our search. To start incremental search type Ctrl-r followed by the text you are looking for. When you find it, you can either type Enter to execute the command or type Ctrl-j to copy the line from the history list to the current command line. To find the next occurrence of the text (moving “up” the history list), type Ctrl-r again. To quit searching, type either Ctrl-g or Ctrl-c. Here we see it in action:

​ bash 会把 “!88” 展开成为历史列表中88行的内容。还有其它的历史命令展开形式，我们一会儿 讨论它们。bash 也具有增量搜索历史列表的能力。意思是在字符输入的同时，bash 会去搜索历史列表（直接出结果，并高亮匹配的第一个字），每多输入一个字符都会使搜索结果更接近目标。输入 Ctrl-r来启动增量搜索， 接着输入你要寻找的字。当你找到它以后，你可以敲入 Enter 来执行命令， 或者输入 Ctrl-j，从历史列表中复制这一行到当前命令行。再次输入 Ctrl-r，来找到下一个 匹配项（历史列表中向上移动）。输入 Ctrl-g 或者 Ctrl-c，退出搜索。现在看看它的实际效果：

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[me@linuxbox ~]$

First type Ctrl-r:

​ 首先输入 Ctrl-r:

(reverse-i-search)`':

The prompt changes to indicate that we are performing a reverse incremental search. It is “reverse” because we are searching from “now” to some time in the past. Next, we start typing our search text. In this example “/usr/bin”:

​ 提示符改变，显示我们正在执行反向增量搜索。搜索过程是”反向的”，因为我们按照从”现在”到过去 某个时间段的顺序来搜寻。下一步，我们开始输入要查找的文本。在这个例子里是 “/usr/bin”：

(reverse-i-search)`/usr/bin`: ls -l /usr/bin > ls-output.txt

上面这一行冒号后面的第一个”/”会高亮显示。

Immediately, the search returns our result. With our result, we can execute the command by pressing Enter, or we can copy the command to our current command line for further editing by typing Ctrl-j. Let’s copy it. Type Ctrl-j:

​ 即刻，搜索返回我们需要的结果。我们可以按下 Enter 键来执行这个命令，或者我们可以按下Ctrl-j复制 这个命令到我们当前的命令行，来进一步编辑它。好了现在我们复制它，输入 Ctrl-j：

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[me@linuxbox ~]$ ls -l /usr/bin > ls-output.txt

Our shell prompt returns and our command line is loaded and ready for action! The table below lists some of the keystrokes used to manipulate the history list:

​ 我们的 shell 提示符重新出现，命令行加载完毕，准备接受下一命令！ 下表列出了一些按键组合， 这些按键可以用来操作历史列表：

历史命令展开

The shell offers a specialized type of expansion for items in the history list by using the “!” character. We have already seen how the exclamation point can be followed by a number to insert an entry from the history list. There are a number of other expansion features:

​ 通过使用 “!” 字符，shell 为历史列表中的命令，提供了一个特殊的展开类型。我们已经知道一个感叹号 ，其后再加上一个数字，可以把来自历史列表中的命令插入到命令行中。这里还有一些其它的展开特性：

I would caution against using the “!string” and “!?string” forms unless you are absolutely sure of the contents of the history list items.

​ 应该小心谨慎地使用 “!string” 和 “!?string” 格式，除非你完全确信历史列表条目的内容。

There are many more elements available in the history expansion mechanism, but this subject is already too arcane and our heads may explode if we continue. The HISTORY EXPANSION section of the bash man page goes into all the gory details. Feel free to explore!

​ 在历史展开机制中，还有许多可利用的特点，但是这个题目已经太晦涩难懂了， 如果我们再继续讨论的话，我们的头可能要爆炸了。bash 手册页的 HISTORY EXPANSION 部分详尽地讲述了所有要素。

script

脚本

In addition to the command history feature in bash, most Linux distributions include a program called script that can be used to record an entire shell session and store it in a file. The basic syntax of the command is:

​ 除了 bash 中的命令历史特性，许多 Linux 发行版包括一个叫做 script 的程序， 这个程序可以记录整个 shell 会话，并把 shell 会话存在一个文件里面。这个命令的基本语法是：

script [file]

where file is the name of the file used for storing the recording. If no file is specified, the file typescript is used. See the script man page for a complete list of the program’s options and features.

​ 命令中的 file 是指用来存储 shell 会话记录的文件名。如果没有指定文件名，则使用文件 typescript。查看脚本的手册页，可以得到一个关于 script 程序选项和特点的完整列表。

总结归纳

In this chapter we have covered some of the keyboard tricks that the shell provides to help hardcore typists reduce their workloads. I suspect that as time goes by and you become more involved with the command line, you will refer back to this chapter to pick up more of these tricks. For now, consider them optional and potentially helpful.

​ 在这一章中，我们已经讨论了一些由 shell 提供的键盘操作技巧，这些技巧是来帮助打字员减少工作量的。 随着时光流逝，你和命令行打交道越来越多，我猜想你会重新翻阅这一章的内容，学会更多的技巧。现在不用一下子全记住。

拓展阅读

• The Wikipedia has a good article on computer terminals:

• Wikipedia 上有一篇关于计算机终端的好文章：

  http://en.wikipedia.org/wiki/Computer_terminal

10 - 10 权限

权限

http://billie66.github.io/TLCL/book/chap10.html

Operating systems in the Unix tradition differ from those in the MS-DOS tradition in that they are not only multitasking systems, but also multi-user systems, as well. What exactly does this mean? It means that more than one person can be using the computer at the same time. While a typical computer will likely have only one keyboard and monitor, it can still be used by more than one user. For example, if a computer is attached to a network or the Internet, remote users can log in via ssh (secure shell) and operate the computer. In fact, remote users can execute graphical applications and have the graphical output appear on a remote display. The X Window System supports this as part of its basic design.

​ Unix 传统中的操作系统不同于那些 MS-DOS 传统中的系统，区别在于它们不仅是多任务系统，而且也是 多用户系统。这到底意味着什么？它意味着多个用户可以在同一时间使用同一台计算机。然而一个 典型的计算机可能只有一个键盘和一个监视器，但是它仍然可以被多个用户使用。例如，如果一台 计算机连接到一个网络或者因特网，那么远程用户通过 ssh（安全 shell）可以登录并操纵这台电脑。 事实上，远程用户也能运行图形界面应用程序，并且图形化的输出结果会出现在远端的显示器上。 X 窗口系统把这个作为基本设计理念的一部分，并支持这种功能。

The multi-user capability of Linux is not a recent “innovation,” but rather a feature that is deeply embedded into the design of the operating system. Considering the environment in which Unix was created, this makes perfect sense. Years ago, before computers were “personal,” they were large, expensive, and centralized. A typical university computer system, for example, consisted of a large central computer located in one building and terminals which were located throughout the campus, each connected to the large central computer. The computer would support many users at the same time.

​ Linux 系统的多用户性能，不是最近的“创新”，而是一种深深地嵌入到了 Linux 操作系统的 设计中的特性。想想 Unix 系统的诞生环境，这一点就很好理解了。多年前，在个人电脑出现之前，计算机 都是大型、昂贵的、集中化的。例如一个典型的大学计算机系统，是由坐落在一座建筑中的一台 大型中央计算机和许多散布在校园各处的终端机组成，每个终端都连接到这台大型中央计算机。 这台计算机可以同时支持很多用户。

In order to make this practical, a method had to be devised to protect the users from each other. After all, the actions of one user could not be allowed to crash the computer, nor could one user interfere with the files belonging to another user.

​ 为了使多用户特性付诸实践，那么必须发明一种方法来阻止用户彼此之间受到影响。毕竟，一个 用户的行为不能导致计算机崩溃，也不能乱动属于另一个用户的文件。

In this chapter we are going to look at this essential part of system security and introduce the following commands:

​ 在这一章中，我们将看看这一系统安全的本质部分，会介绍以下命令：

• id – Display user identity
• id – 显示用户身份号
• chmod – Change a file’s mode
• chmod – 更改文件模式
• umask – Set the default file permissions
• umask – 设置默认的文件权限
• su – Run a shell as another user
• su – 以另一个用户的身份来运行 shell
• sudo – Execute a command as another user
• sudo – 以另一个用户的身份来执行命令
• chown – Change a file’s owner
• chown – 更改文件所有者
• chgrp – Change a file’s group ownership
• chgrp – 更改文件组所有权
• passwd – Change a user’s password
• passwd – 更改用户密码

拥有者、组成员和其他人

When we were exploring the system back in Chapter 4, we may have encountered a problem when trying to examine a file such as /etc/shadow:

​ 在第四章探究文件系统时，当我们试图查看一个像/etc/shadow 那样的文件的时候，我们会遇到一个问题。

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[me@linuxbox ~]$ file /etc/shadow
/etc/shadow:  regular file, no read permission
[me@linuxbox ~]$ less /etc/shadow
/etc/shadow:  Permission denied

The reason for this error message is that, as regular users, we do not have permission to read this file.

​ 产生这种错误信息的原因是，作为一个普通用户，我们没有权限来读取这个文件。

In the Unix security model, a user may own files and directories. When a user owns a file or directory, the user has control over its access. Users can, in turn, belong to a group consisting of one or more users who are given access to files and directories by their owners. In addition to granting access to a group, an owner may also grant some set of access rights to everybody, which in Unix terms is referred to as the world. To find out information about your identity, use the id command:

​ 在 Unix 安全模型中，一个用户可能拥有文件和目录。当一个用户拥有一个文件或目录时， 用户对这个文件或目录的访问权限拥有控制权。用户反过来又属于一个由一个或多个 用户组成的用户组，用户组成员由文件和目录的所有者授予对文件和目录的访问权限。除了 对一个用户组授予权限之外，文件所有者可能会给所有的人授权，在 Unix 术语中，”所有的人“ 也被称作“整个世界”（ world ）。可以用 id 命令，来找到关于你自己身份的信息：

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[me@linuxbox ~]$ id
uid=500(me) gid=500(me) groups=500(me)

Let’s look at the output. When user accounts are created, users are assigned a number called a user ID or uid which is then, for the sake of the humans, mapped to a user name. The user is assigned a primary group ID or gid and may belong to additional groups. The above example is from a Fedora system. On other systems, such as Ubuntu, the output may look a little different:

​ 让我们看一下输出结果。当用户创建帐户之后，系统会给用户分配一个号码，叫做用户 ID 或者 uid，然后，为了符合人类的习惯，这个 ID 映射到一个用户名。系统又会给这个用户 分配一个原始的组 ID(即 gid)。一个用户可以属于多个组。上面的例子来自于 Fedora 系统， 下面 Ubuntu 的输出结果看起来有点儿不同：

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[me@linuxbox ~]$ id
uid=1000(me) gid=1000(me)
groups=4(adm),20(dialout),24(cdrom),25(floppy),29(audio),30(dip),44(v
ideo),46(plugdev),108(lpadmin),114(admin),1000(me)

As we can see, the uid and gid numbers are different. This is simply because Fedora starts its numbering of regular user accounts at 500, while Ubuntu starts at 1000. We can also see that the Ubuntu user belongs to a lot more groups. This has to do with the way Ubuntu manages privileges for system devices and services.

​ 正如我们能看到的，两个系统中用户的 uid 和 gid 号码是不同的。原因很简单，因为 Fedora 系统 从500开始进行普通用户帐户的编号，而 Ubuntu 从1000开始。我们也能看到 Ubuntu 的用户属于 更多的用户组。这和 Ubuntu 管理系统设备和服务权限的方式有关系。

So where does this information come from? Like so many things in Linux, from a couple of text files. User accounts are defined in the /etc/passwd file and groups are defined in the /etc/group file. When user accounts and groups are created, these files are modified along with /etc/shadow which holds information about the user’s password. For each user account, the /etc/passwd file defines the user (login) name, uid, gid, the account’s real name, home directory, and login shell. If you examine the contents of /etc/passwd and /etc/group, you will notice that besides the regular user accounts, there are accounts for the superuser (uid 0) and various other system users.

​ 那么这些信息存在哪里呢？像 Linux 系统中的许多东西一样，存到了一系列的文本文件。用户帐户 定义在 /etc/passwd 文件里面，用户组定义在 /etc/group 文件里面。当用户帐户和用户组创建以后， 这些文件随着文件 /etc/shadow 的变动而修改，文件 /etc/shadow 包含了关于用户密码的信息。 对于每个用户帐号，文件 /etc/passwd 定义了用户（登录）名、uid、gid、帐号的真实姓名、家目录 和登录 shell。如果你查看一下文件 /etc/passwd 和文件 /etc/group 的内容，你会注意到除了普通 用户帐号之外，还有超级用户（uid 0）帐号，和各种各样的系统用户。

In the next chapter, when we cover processes, you will see that some of these other “users” are, in fact, quite busy.

​ 在下一章中，当我们讨论进程时，你会知道这些其他的“用户”是谁，实际上，他们相当忙碌。

While many Unix-like systems assign regular users to a common group such as “users”, modern Linux practice is to create a unique, single-member group with the same name as the user. This makes certain types of permission assignment easier.

然而许多像 Unix 的系统会把普通用户分配到一个公共的用户组中，例如 users ，现在的 Linux 会创建一个独一无二的，只有一个成员的用户组，这个用户组与用户同名。这样使某种类型的 权限分配更容易些。

读取、写入和执行

Access rights to files and directories are defined in terms of read access, write access, and execution access. If we look at the output of the ls command, we can get some clue as to how this is implemented:

​ 对于文件和目录的访问权力是根据”读权限“、”写权限“和“执行权限“来定义的。如果我们看一下 ls 命令的输出结果，我们能得到一些线索，这是怎样实现的：

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[me@linuxbox ~]$ > foo.txt
[me@linuxbox ~]$ ls -l foo.txt
-rw-rw-r-- 1 me   me   0 2008-03-06 14:52 foo.txt

The first ten characters of the listing are the file attributes. The first of these characters is the file type. Here are the file types you are most likely to see (there are other, less common types too):

​ 列表的前十个字符是文件的属性。这十个字符的第一个字符表明文件类型。下表是你可能经常看到 的文件类型（还有其它的，不常见类型）：

The remaining nine characters of the file attributes, called the file mode, represent the read, write, and execute permissions for the file’s owner, the file’s group owner, and everybody else:

​ 剩下的九个字符叫做文件模式，代表着文件所有者、文件组所有者和其他人的读、写和执行权限。

图 1: 权限属性

When set, the r, w, and x mode attributes have the following effect on files and directories:

​ 当设置文件模式后，r、w和x 模式属性对文件和目录会产生以下影响：

Here are some examples of file attribute settings:

​ 下面是权限属性的一些例子：

chmod － 更改文件模式

To change the mode (permissions) of a file or directory, the chmod command is used. Be aware that only the file’s owner or the superuser can change the mode of a file or directory. chmod supports two distinct ways of specifying mode changes: octal number representation, or symbolic representation. We will cover octal number representation first.

​ 更改文件或目录的模式（权限），可以利用 chmod 命令。注意只有文件的所有者或者超级用户才 能更改文件或目录的模式。chmod 命令支持两种不同的方法来改变文件模式：八进制数字表示法或 符号表示法。首先我们讨论一下八进制数字表示法。

What The Heck Is Octal?

究竟什么是八进制？

Octal (base 8), and its cousin, hexadecimal (base 16) are number systems often used to express numbers on computers. We humans, owing to the fact that we (or at least most of us) were born with ten fingers, count using a base 10 number system. Computers, on the other hand, were born with only one finger and thus do all their counting in binary (base 2). Their number system only has two numerals, zero and one. So in binary, counting looks like this:

​ 八进制（以8为基数）及其亲戚十六进制（以16为基数）都是数字系统，通常 被用来表示计算机中的数字。我们人类，因为（或者至少大多数人）天生有 十个手指的事实，利用以10为基数的数字系统来计数。计算机，从另一方面讲，生来只有一个 手指，因此它以二进制（以2为基数）来计数。它们的数字系统只有两个数值，0和1。 因此在二进制中，计数看起来像这样：

0, 1, 10, 11, 100, 101, 110, 111, 1000, 1001, 1010, 1011…

In octal, counting is done with the numerals zero through seven, like so:

​ 在八进制中，逢八进一，用数字0到7来计数，像这样：

0, 1, 2, 3, 4, 5, 6, 7, 10, 11, 12, 13, 14, 15, 16, 17, 20, 21…

Hexadecimal counting uses the numerals zero through nine plus the letters “A” through “F”:

​ 十六进制中，使用数字0到9，加上大写字母”A”到”F”来计数，逢16进一：

0, 1, 2, 3, 4, 5, 6, 7, 8, 9, A, B, C, D, E, F, 10, 11, 12, 13…

While we can see the sense in binary (since computers only have one finger), what are octal and hexadecimal good for? The answer has to do with human convenience. Many times, small portions of data are represented on computers as bit patterns. Take for example an RGB color. On most computer displays, each pixel is composed of three color components: eight bits of red, eight bits of green, and eight bits of blue. A lovely medium blue would be a twenty-four digit number:

​ 虽然我们能知道二进制的意义（因为计算机只有一个手指），但是八进制和十六进制对什么 有好处呢？ 答案是为了人类的便利。许多时候，在计算机中，一小部分数据以二进制的形式表示。 以 RGB 颜色为例来说明。大多数的计算机显示器，每个像素由三种颜色组成：8位红色，8位绿色， 8位蓝色。这样，一种可爱的中蓝色就由24位数字来表示：

010000110110111111001101

How would you like to read and write those kinds of numbers all day? I didn’t think so. Here’s where another number system would help. Each digit in a hexadecimal number represents four digits in binary. In octal, each digit represents three binary digits. So our twenty-four digit medium blue could be condensed down to a six digit hexadecimal number:

​ 我不认为你每天都喜欢读写这类数字。另一种数字系统对我们更有帮助。每个十六进制 数字代表四个二进制。在八进制中，每个数字代表三个二进制数字。那么代表中蓝色的24位 二进制能够压缩成6位十六进制数：

436FCD

Since the digits in the hexadecimal number “line up” with the bits in the binary number we can see that the red component of our color is “43”, the green “6F”, and the blue “CD”.

​ 因为十六进制中的两个数字对应二进制的8位数字，我们可以看到”43“代表红色，“6F” 代表绿色，“CD”代表蓝色。 † These days, hexadecimal notation (often spoken as “hex”) is more common than octal, but as we shall soon see, octal’s ability to express three bits of binary will be very useful…

​ 现在，十六进制表示法（经常叫做“hex”）比八进制更普遍，但是我们很快会看到，用八进制 来表示3个二进制数非常有用处…

With octal notation we use octal numbers to set the pattern of desired permissions. Since each digit in an octal number represents three binary digits, this maps nicely to the scheme used to store the file mode. This table shows what we mean:

​ 通过八进制表示法，我们使用八进制数字来设置所期望的权限模式。因为每个八进制数字代表了 3个二进制数字，这种对应关系，正好映射到用来存储文件模式所使用的方案上。下表展示了 我们所要表达的意思：

By using three octal digits, we can set the file mode for the owner, group owner, and world:

​ 通过使用3个八进制数字，我们能够设置文件所有者、用户组和其他人的权限：

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[me@linuxbox ~]$ > foo.txt
[me@linuxbox ~]$ ls -l foo.txt
-rw-rw-r-- 1 me    me    0  2008-03-06 14:52 foo.txt
[me@linuxbox ~]$ chmod 600 foo.txt
[me@linuxbox ~]$ ls -l foo.txt
-rw------- 1 me    me    0  2008-03-06 14:52 foo.txt

By passing the argument “600”, we were able to set the permissions of the owner to read and write while removing all permissions from the group owner and world. Though remembering the octal to binary mapping may seem inconvenient, you will usually only have to use a few common ones: 7 (rwx), 6 (rw-), 5 (r-x), 4 (r–), and 0 (—).

​ 通过传递参数 “600”，我们能够设置文件所有者的权限为读写权限，而删除用户组和其他人的所有 权限。虽然八进制到二进制的映射看起来不方便，但通常只会用到一些常见的映射关系： 7 (rwx)，6 (rw-)，5 (r-x)，4 (r–)，和 0 (—)。

chmod also supports a symbolic notation for specifying file modes. Symbolic notation is divided into three parts: who the change will affect, which operation will be performed, and what permission will be set. To specify who is affected, a combination of the characters “u”, “g”, “o”, and “a” is used as follows:

​ chmod 命令支持一种符号表示法，来指定文件模式。符号表示法分为三部分：更改会影响谁， 要执行哪个操作，要设置哪种权限。通过字符 “u”、“g”、“o”和 “a” 的组合来指定 要影响的对象，如下所示：

If no character is specified, “all” will be assumed. The operation may be a “+” indicating that a permission is to be added, a “-” indicating that a permission is to be taken away, or a “=” indicating that only the specified permissions are to be applied and that all others are to be removed.

​ 如果没有指定字符，则假定使用”all”。执行的操作可能是一个“＋”字符，表示加上一个权限， 一个“－”，表示删掉一个权限，或者是一个“＝”，表示只有指定的权限可用，其它所有的权限被删除。

Permissions are specified with the “r”, “w”, and “x” characters. Here are some examples of symbolic notation:

​ 权限由 “r”、“w”和 “x” 来指定。这里是一些符号表示法的实例：

Some people prefer to use octal notation, some folks really like the symbolic. Symbolic notation does offer the advantage of allowing you to set a single attribute without disturbing any of the others.

​ 一些人喜欢使用八进制表示法，而另一些人则非常喜欢符号表示法。符号表示法的优点是， 允许你设置文件模式的某个属性，而不影响其他的属性。

Take a look at the chmod man page for more details and a list of options. A word of caution regarding the “–recursive” option: it acts on both files and directories, so it’s not as useful as one would hope since, we rarely want files and directories to have the same permissions.

​ 看一下 chmod 命令的手册页，可以得到更详尽的信息和 chmod 命令的各个选项。要注意”--recursive”选项： 它可以同时作用于文件和目录，所以它并不是如我们期望的那么有用处，因为我们很少希望文件和 目录拥有同样的权限。

借助 GUI 来设置文件模式

Now that we have seen how the permissions on files and directories are set, we can better understand the permission dialogs in the GUI. In both Nautilus (GNOME) and Konqueror (KDE), right-clicking a file or directory icon will expose a properties dialog. Here is an example from KDE 3.5:

​ 现在我们已经知道了怎样设置文件和目录的权限，这样我们就可以更好的理解 GUI 中的设置 权限对话框。在 Nautilus (GNOME)和 Konqueror (KDE)中，右击一个文件或目录图标将会弹出一个属性对话框。下面这个例子来自 KDE 3.5：

图 2: KDE 3.5 文件属性对话框

Here we can see the settings for the owner, group, and world. In KDE, clicking on the “Advanced Permissions” button brings up another dialog that allows you to set each of the mode attributes individually. Another victory for understanding brought to us by the command line!

​ 从这个对话框中，我们看到可以设置文件所有者、用户组和其他人的访问权限。 在 KDE 中，右击”Advanced Permissions”按钮，会打开另一个对话框，这个对话框允许 你单独设置各个模式属性。这也可以通过命令行来理解！

umask － 设置默认权限

The umask command controls the default permissions given to a file when it is created. It uses octal notation to express a mask of bits to be removed from a file’s mode attributes. Let’s take a look:

​ 当创建一个文件时，umask 命令控制着文件的默认权限。umask 命令使用八进制表示法来表达 从文件模式属性中删除一个位掩码。大家看下面的例子：

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[me@linuxbox ~]$ rm -f foo.txt
[me@linuxbox ~]$ umask
0002
[me@linuxbox ~]$ > foo.txt
[me@linuxbox ~]$ ls -l foo.txt
-rw-rw-r-- 1 me   me   0 2008-03-06 14:53 foo.txt

We first removed any old copy of foo.txt to make sure we were starting fresh. Next, we ran the umask command without an argument to see the current value. It responded with the value 0002 (the value 0022 is another common default value), which is the octal representation of our mask. We next create a new instance of the file foo.txt and observe its permissions.

​ 首先，删除文件 foo.txt，以此确定我们从新开始。下一步，运行不带参数的 umask 命令， 看一下当前的掩码值，数值是0002（0022是另一个常用的默认值），这个数值是掩码的八进制 表示形式。下一步，我们创建文件 foo.txt，看看它的权限。

We can see that both the owner and group both get read and write permission, while everyone else only gets read permission. The reason that world does not have write permission is because of the value of the mask. Let’s repeat our example, this time setting the mask ourselves:

​ 我们可以看到文件所有者和用户组都得到读权限和写权限，而其他人只是得到读权限。 其他人没有得到写权限的原因是由掩码值决定的。重复我们的实验，这次自己设置掩码值：

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[me@linuxbox ~]$ rm foo.txt
[me@linuxbox ~]$ umask 0000
[me@linuxbox ~]$ > foo.txt
[me@linuxbox ~]$ ls -l foo.txt
-rw-rw-rw- 1 me   me    0 2008-03-06 14:58 foo.txt

When we set the mask to 0000 (effectively turning it off), we see that the file is now world writable. To understand how this works, we have to look at octal numbers again. If we take the mask and expand it into binary, then compare it to the attributes we can see what happens:

​ 当掩码设置为0000（实质上是关掉它）之后，我们看到其他人能够读写文件。为了弄明白这是 怎么回事，我们需要看一下掩码的八进制形式。把掩码展开成二进制形式，然后与文件属性 相比较，看看有什么区别：

Ignore for the moment the leading zeros (we’ll get to those in a minute) and observe that where the 1 appears in our mask, an attribute was removed—in this case, the world write permission. That’s what the mask does. Everywhere a 1 appears in the binary value of the mask, an attribute is unset. If we look at a mask value of 0022, we can see what it does:

​ 此刻先忽略掉开头的三个零（我们一会儿再讨论），注意掩码中若出现一个数字1，则 删除文件模式中和这个1在相同位置的权限，在这是其他人的写权限。这就是掩码要完成的 任务。掩码的二进制形式中，出现数字1的位置，相应地关掉一个文件模式属性。看一下 掩码0022的作用：

Again, where a 1 appears in the binary value, the corresponding attribute is unset. Play with some values (try some sevens) to get used to how this works. When you’re done, remember to clean up:

​ 又一次，二进制中数字1出现的位置，相对应的属性被删除。再试一下其它的掩码值（一些带数字7的） ，习惯于掩码的工作原理。当你实验完成之后，要记得清理现场：

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[me@linuxbox ~]$ rm foo.txt; umask 0002

Most of the time you won’t have to change the mask; the default provided by your distribution will be fine. In some high-security situations, however, you will want to control it.

​ 大多数情况下，你不必修改掩码值，系统提供的默认掩码值就很好了。然而，在一些高 安全级别下，你要能控制掩码值。

Some Special Permissions

一些特殊权限

Though we usually see an octal permission mask expressed as a three digit number, it is more technically correct to express it in four digits. Why? Because, in addition to read, write, and execute permission, there are some other, less used, permission settings.

​ 虽然我们通常看到一个八进制的权限掩码用三位数字来表示，但是从技术层面上来讲， 用四位数字来表示它更确切些。为什么呢？因为除了读取、写入和执行权限之外，还有 其它较少用到的权限设置。

The first of these is the setuid bit (octal 4000). When applied to an executable file, it sets the effective user ID from that of real user (the user actually running the program) to that of the program’s owner. Most often this is given to a few programs owned by the superuser. When an ordinary user runs a program that is “setuid root” , the program runs with the effective privileges of the superuser. This allows the program to access files and directories that an ordinary user would normally be prohibited from accessing. Clearly, because this raises security concerns, number of setuid programs must be held to an absolute minimum.

​ 其中之一是 setuid 位（八进制4000）。当应用到一个可执行文件时，它把有效用户 ID 从真正的用户（实际运行程序的用户）设置成程序所有者的 ID。这种操作通常会应用到 一些由超级用户所拥有的程序。当一个普通用户运行一个程序，这个程序由根用户(root) 所有，并且设置了 setuid 位，这个程序运行时具有超级用户的特权，这样程序就可以 访问普通用户禁止访问的文件和目录。很明显，因为这会引起安全方面的问题，所有可以 设置 setuid 位的程序个数，必须控制在绝对小的范围内。

The second is the setgid bit (octal 2000) which, like the setuid bit, changes the effective group ID from the real group ID of the user to that of the file owner. If the setgid bit is set on a directory, newly created files in the directory will be given the group ownership of the directory rather the group ownership of the file’s creator. This is useful in a shared directory when members of a common group need access to all the files in the directory, regardless of the file owner’s primary group.

​ 第二个是 setgid 位（八进制2000），这个相似于 setuid 位，把有效用户组 ID 从真正的 用户组 ID 更改为文件所有者的组 ID。如果设置了一个目录的 setgid 位，则目录中新创建的文件 具有这个目录用户组的所有权，而不是文件创建者所属用户组的所有权。对于共享目录来说， 当一个普通用户组中的成员，需要访问共享目录中的所有文件，而不管文件所有者的主用户组时， 那么设置 setgid 位很有用处。

The third is called the sticky bit (octal 1000). This is a holdover from ancient Unix, where it was possible to mark an executable file as “not swappable.” On files, Linux ignores the sticky bit, but if applied to a directory, it prevents users from deleting or renaming files unless the user is either the owner of the directory, the owner of the file, or the superuser. This is often used to control access to a shared directory, such as /tmp.

​ 第三个是 sticky 位（八进制1000）。这个继承于 Unix，在 Unix 中，它可能把一个可执行文件 标志为“不可交换的”。在 Linux 中，会忽略文件的 sticky 位，但是如果一个目录设置了 sticky 位， 那么它能阻止用户删除或重命名文件，除非用户是这个目录的所有者，或者是文件所有者，或是 超级用户。这个经常用来控制访问共享目录，比方说/tmp。

Here are some examples of using chmod with symbolic notation to set these special permissions. First assigning setuid to a program:

​ 这里有一些例子，使用 chmod 命令和符号表示法，来设置这些特殊的权限。首先， 授予一个程序 setuid 权限。

chmod u+s program

Next, assigning setgid to a directory:

​ 下一步，授予一个目录 setgid 权限：

chmod g+s dir

Finally, assigning the sticky bit to a directory:

​ 最后，授予一个目录 sticky 权限：

chmod +t dir

When viewing the output from ls, you can determine the special permissions. Here are some examples. First, a program that is setuid:

​ 当浏览 ls 命令的输出结果时，你可以确认这些特殊权限。这里有一些例子。首先，一个程序被设置为setuid属性：

-rwsr-xr-x

A directory that has the setgid attribute:

​ 具有 setgid 属性的目录：

drwxrwsr-x

A directory with the sticky bit set:

​ 设置了 sticky 位的目录：

drwxrwxrwt

更改身份

At various times, we may find it necessary to take on the identity of another user. Often we want to gain superuser privileges to carry out some administrative task, but it is also possible to “become” another regular user for such things as testing an account. There are three ways to take on an alternate identity:

​ 很多时候，我们会发现很有必要使用另一个用户的身份来执行一些操作。经常地，我们想要得到超级 用户特权，来执行一些管理任务，但是也有可能”变为”另一个普通用户，比如说测试一个帐号。 有三种方式，可以拥有多重身份：

1. Log out and log back in as the alternate user.

2. Use the su command.

3. Use the sudo command.

4. 注销系统并以其他用户身份重新登录系统。

5. 使用 su 命令。

6. 使用 sudo 命令。

We will skip the first technique since we know how to do it and it lacks the convenience of the other two. From within our own shell session, the su command allows you to assume the identity of another user, and either start a new shell session with that user’s IDs, or to issue a single command as that user. The sudo command allows an administrator to set up a configuration file called /etc/sudoers, and define specific commands that particular users are permitted to execute under an assumed identity. The choice of which command to use is largely determined by which Linux distribution you use. Your distribution probably includes both commands, but its configuration will favor either one or the other. We’ll start with su.

​ 我们将跳过第一种方法，因为我们知道怎样使用它，并且它缺乏其它两种方法的方便性。 在我们自己的 shell 会话中，su 命令允许你假定为另一个用户的身份，以这个用户的 ID 启动一个新的 shell 会话，或者是以这个用户的身份来发布一个命令。sudo 命令允许一个管理员 设置一个叫做 /etc/sudoers 的配置文件，并且定义了一些具体命令，允许变身用户 执行这些命令。选择使用哪个命令，很大程度上是由你使用的 Linux 发行版来决定的。 你的发行版可能这两个命令都包含，但系统配置可能会禁用其中一个。我们先介绍 su 命令。

su － 以其他用户身份和组 ID 运行一个 shell

The su command is used to start a shell as another user. The command syntax looks like this:

​ su 命令用来以另一个用户的身份来启动 shell。这个命令语法看起来像这样：

su [-[l]] [user]

If the “-l” option is included, the resulting shell session is a login shell for the specified user. This means that the user’s environment is loaded and the working directory is changed to the user’s home directory. This is usually what we want. If the user is not specified, the superuser is assumed. Notice that (strangely) the “-l” may be abbreviated “-”, which is how it is most often used. To start a shell for the superuser, we would do this:

​ 如果包含”-l”选项，那么会为指定用户启动一个需要登录的 shell。这意味着会加载此用户的 shell 环境， 并且工作目录会更改到这个用户的家目录。这通常是我们所需要的。如果不指定用户，那么就假定是 超级用户。注意（不可思议地），选项”-l”可以缩写为”-“，这是经常用到的形式。启动超级用户的 shell， 我们可以这样做：

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[me@linuxbox ~]$ su -
Password:
[root@linuxbox ~]#

After entering the command, we are prompted for the superuser’s password. If it is successfully entered, a new shell prompt appears indicating that this shell has superuser privileges (the trailing “#” rather than a “$”) and the current working directory is now the home directory for the superuser (normally /root.) Once in the new shell, we can carry out commands as the superuser. When finished, type “exit” to return to the previous shell:

​ 按下回车符之后，shell 提示我们输入超级用户的密码。如果密码输入正确，出现一个新的 shell 提示符， 这表明这个 shell 具有超级用户特权（提示符的末尾字符是”#”而不是”$”），并且当前工作目录是超级用户的家目录 （通常是/root）。一旦进入一个新的 shell，我们能执行超级用户所使用的命令。当工作完成后， 输入”exit”，则返回到原来的 shell:

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[root@linuxbox ~]# exit
[me@linuxbox ~]$

It is also possible to execute a single command rather than starting a new interactive command by using su this way:

​ 以这样的方式使用 su 命令，也可以只执行单个命令，而不是启动一个新的可交互的 shell：

su -c 'command'

Using this form, a single command line is passed to the new shell for execution. It is important to enclose the command in quotes, as we do not want expansion to occur in our shell, but rather in the new shell:

​ 使用这种模式，命令传递到一个新 shell 中执行。把命令用单引号引起来很重要，因为我们不想 命令在我们的 shell 中展开，但需要在新 shell 中展开。

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[me@linuxbox ~]$ su -c 'ls -l /root/*'
Password:
-rw------- 1 root root    754 2007-08-11 03:19 /root/anaconda-ks.cfg

/root/Mail:
total 0
[me@linuxbox ~]$

sudo － 以另一个用户身份执行命令

The sudo command is like su in many ways, but has some important additional capabilities. The administrator can configure sudo to allow an ordinary user to execute commands as a different user (usually the superuser) in a very controlled way. In particular, a user may be restricted to one or more specific commands and no others. Another important difference is that the use of sudo does not require access to the superuser’s password. To authenticate using sudo, the user uses his/her own password. Let’s say, for example, that sudo has been configured to allow us to run a fictitious backup program called “backup_script”, which requires superuser privileges. With sudo it would be done like this:

​ sudo 命令在很多方面都相似于 su 命令，但是 sudo 还有一些非常重要的功能。管理员能够配置 sudo 命令，从而允许一个普通用户以不同的身份（通常是超级用户），通过一种非常可控的方式 来执行命令。尤其是，只有一个用户可以执行一个或多个特殊命令时，（更体现了 sudo 命令的方便性）。 另一个重要差异是 sudo 命令不要求超级用户的密码。使用 sudo 命令时，用户使用他/她自己的密码 来认证。比如说，例如，sudo 命令经过配置，允许我们运行一个虚构的备份程序，叫做”backup_script”， 这个程序要求超级用户权限。通过 sudo 命令，这个程序会像这样运行：

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[me@linuxbox ~]$ sudo backup_script
Password:
System Backup Starting...

After entering the command, we are prompted for our password (not the superuser’s) and once the authentication is complete, the specified command is carried out. One important difference between su and sudo is that sudo does not start a new shell, nor does it load another user’s environment. This means that commands do not need to be quoted any differently than they would be without using sudo. Note that this behavior can be overridden by specifying various options. See the sudo man page for details.

​ 按下回车键之后，shell 提示我们输入我们的密码（不是超级用户的）。一旦认证完成，则执行 具体的命令。su 和 sudo 之间的一个重要区别是 sudo 不会重新启动一个 shell，也不会加载另一个 用户的 shell 运行环境。这意味者命令不必用单引号引起来。注意通过指定各种各样的选项，这 种行为可以被推翻。详细信息，阅读 sudo 手册页。

To see what privileges are granted by sudo, use the “-l” option to list them:

​ 想知道 sudo 命令可以授予哪些权限，使用”-l”选项，列出所有权限：

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[me@linuxbox ~]$ sudo -l
User me may run the following commands on this host:
(ALL) ALL

Ubuntu And sudo

Ubuntu 与 sudo

One of the recurrent problems for regular users is how to perform certain tasks that require superuser privileges. These tasks include installing and updating software, editing system configuration files, and accessing devices. In the Windows world, this is often done by giving users administrative privileges. This allows users to perform these tasks. However, it also enables programs executed by the user to have the same abilities. This is desirable in most cases, but it also permits malware (malicious software) such as viruses to have free reign of the computer.

​ 普通用户经常会遇到这样的问题，怎样完成某些需要超级用户权限的任务。这些任务 包括安装和更新软件，编辑系统配置文件，和访问设备。在 Windows 世界里，这些任务是 通过授予用户管理员权限来完成的。这允许用户执行这些任务。然而，这也会导致用户所 执行的程序拥有同样的能力。在大多数情况下，这是我们所期望的，但是它也允许 malware （恶意软件），比方说电脑病毒，自由地支配计算机。

In the Unix world, there has always been a larger division between regular users and administrators, owing to the multi-user heritage of Unix. The approach taken in Unix is to grant superuser privileges only when needed. To do this, the su and sudo commands are commonly used.

​ 在 Unix 世界中，由于 Unix 是多用户系统，所以在普通用户和管理员之间总是存在很大的 差别。Unix 采取的方法是只有在需要的时候，才授予普通用户超级用户权限。这样，普遍会 用到 su 和 sudo 命令。

Up until a couple of years ago, most Linux distributions relied on su for this purpose. su didn’t require the configuration that sudo required, and having a root account is traditional in Unix. This introduced a problem. Users were tempted to operate as root unnecessarily. In fact, some users operated their systems as the root user exclusively, since it does away with all those annoying “permission denied” messages. This is how you reduce the security of a Linux system to that of a Windows system. Not a good idea.

​ 几年前，大多数的 Linux 发行版都依赖于 su 命令，来达到目的。su 命令不需要 sudo 命令 所要求的配置，su 命令拥有一个 root 帐号，是 Unix 中的传统。但这会引起问题。所有用户 会企图以 root 用户帐号来操纵系统。事实上，一些用户专门以 root 用户帐号来操作系统， 因为这样做，的确消除了所有那些讨厌的“权限被拒绝”的消息。你这样做就会使得 Linux 系统的 安全性能被降低到和 Windows 系统相同的级别。不是一个好主意。

When Ubuntu was introduced, its creators took a different tack. By default, Ubuntu disables logins to the root account (by failing to set a password for the account), and instead uses sudo to grant superuser privileges. The initial user account is granted full access to superuser privileges via sudo and may grant similar powers to subsequent user accounts.

​ 当引进 Ubuntu 的时候，它的创作者们采取了不同的策略。默认情况下，Ubuntu 不允许用户登录 到 root 帐号（因为不能为 root 帐号设置密码），而是使用 sudo 命令授予普通用户超级用户权限。 通过 sudo 命令，最初的用户可以拥有超级用户权限，也可以授予随后的用户帐号相似的权力。

chown － 更改文件所有者和用户组

The chown command is used to change the owner and group owner of a file or directory. Superuser privileges are required to use this command. The syntax of chown looks like this:

​ chown 命令被用来更改文件或目录的所有者和用户组。使用这个命令需要超级用户权限。chown 命令 的语法看起来像这样：

chown [owner][:[group]] file...

chown can change the file owner and/or the file group owner depending on the first argument of the command. Here are some examples:

​ chown 可以根据这个命令的第一个参数更改文件所有者和/或文件用户组。这里有 一些例子：

Let’s say that we have two users; janet, who has access to superuser privileges and tony, who does not. User janet wants to copy a file from her home directory to the home directory of user tony. Since user janet wants tony to be able to edit the file, janet changes the ownership of the copied file from janet to tony:

​ 比方说，我们有两个用户，janet 拥有超级用户访问权限，而 tony 没有。用户 janet 想要从 她的家目录复制一个文件到用户 tony 的家目录。因为用户 janet 想要 tony 能够编辑这个文件， janet 把这个文件的所有者更改为 tony:

[janet@linuxbox ~]$ sudo cp myfile.txt ~tony
Password:
[janet@linuxbox ~]$ sudo ls -l ~tony/myfile.txt
-rw-r--r-- 1 root  root 8031 2008-03-20 14:30 /home/tony/myfile.txt
[janet@linuxbox ~]$ sudo chown tony: ~tony/myfile.txt
[janet@linuxbox ~]$ sudo ls -l ~tony/myfile.txt
-rw-r--r-- 1 tony  tony 8031 2008-03-20 14:30 /home/tony/myfile.txt

Here we see user janet copy the file from her directory to the home directory of user tony. Next, janet changes the ownership of the file from root (a result of using sudo) to tony. Using the trailing colon in the first argument, janet also changed the group ownership of the file to the login group of tony, which happens to be group tony.

​ 这里，我们看到用户 janet 把文件从她的目录复制到 tony 的家目录。下一步，janet 把文件所有者 从 root（使用 sudo 命令的原因）改到 tony。通过在第一个参数中使用末尾的”:”字符，janet 同时把 文件用户组改为 tony 登录系统时，所属的用户组，碰巧是用户组 tony。

Notice that after the first use of sudo, janet was not prompted for her password? This is because sudo, in most configurations, “trusts” you for several minutes until its timer runs out.

​ 注意，第一次使用 sudo 命令之后，为什么（shell）没有提示 janet 输入她的密码？这是因为，在 大多数的配置中，sudo 命令会相信你几分钟，直到计时结束。

chgrp － 更改用户组所有权

In older versions of Unix, the chown command only changed file ownership, not group ownership. For that purpose, a separate command, chgrp was used. It works much the same way as chown, except for being more limited.

​ 在旧版 Unix 系统中，chown 命令只能更改文件所有权，而不是用户组所有权。为了达到目的， 使用一个独立的命令，chgrp 来完成。除了限制多一点之外，chgrp 命令与 chown 命令使用起来很相似。

练习使用权限

Now that we have learned how this permissions thing works, it’s time to show it off. We are going to demonstrate the solution to a common problem — setting up a shared directory. Let’s imagine that we have two users named “bill” and “karen.” They both have music CD collections and wish to set up a shared directory, where they will each store their music files as Ogg Vorbis or MP3. User bill has access to superuser privileges via sudo.

​ 到目前为止，我们已经知道了权限这类东西是怎样工作的，现在是时候炫耀一下了。我们 将展示一个常见问题的解决方案，这个问题是如何设置一个共享目录。假想我们有两个用户， 他们分别是 “bill” 和 “karen”。他们都有音乐 CD 收藏品，也愿意设置一个共享目录，在这个 共享目录中，他们分别以 Ogg Vorbis 或 MP3 的格式来存储他们的音乐文件。通过 sudo 命令， 用户 bill 具有超级用户访问权限。

The first thing that needs to happen is creating a group that will have both bill and karen as members. Using the graphical user management tool, bill creates a group called music and adds users bill and karen to it:

​ 我们需要做的第一件事，是创建一个以 bill 和 karen 为成员的用户组。使用图形化的用户管理工具， bill 创建了一个叫做 music 的用户组，并且把用户 bill 和 karen 添加到用户组 music 中：

图 3: 用 GNOME 创建一个新的用户组

Next, bill creates the directory for the music files:

​ 下一步，bill 创建了存储音乐文件的目录：

[bill@linuxbox ~]$ sudo mkdir /usr/local/share/Music
password:

Since bill is manipulating files outside his home directory, superuser privileges are required. After the directory is created, it has the following ownerships and permissions:

​ 因为 bill 正在他的家目录之外操作文件，所以需要超级用户权限。这个目录创建之后，它具有 以下所有权和权限：

[bill@linuxbox ~]$ ls -ld /usr/local/share/Music
drwxr-xr-x 2 root root 4096 2008-03-21 18:05 /usr/local/share/Music

As we can see, the directory is owned by root and has 755 permissions. To make this directory sharable, bill needs to change the group ownership and the group permissions to allow writing:

​ 正如我们所见到的，这个目录由 root 用户拥有，并且具有权限755。为了使这个目录共享，允许（用户 karen）写入，bill 需要更改目录用户组所有权和权限：

[bill@linuxbox ~]$ sudo chown :music /usr/local/share/Music
[bill@linuxbox ~]$ sudo chmod 775 /usr/local/share/Music
[bill@linuxbox ~]$ ls -ld /usr/local/share/Music
drwxrwxr-x 2 root music 4096 2008-03-21 18:05 /usr/local/share/Music

So what does this all mean? It means that we now have a directory, /usr/local/share/Music that is owned by root and allows read and write access to group music. Group music has members bill and karen, thus bill and karen can create files in directory /usr/local/share/Music. Other users can list the contents of the directory but cannot create files there.

​ 那么这是什么意思呢？ 它的意思是，现在我们拥有一个目录，/usr/local/share/Music，这个目录由 root 用户拥有，并且 允许用户组 music 读取和写入。用户组 music 有两个成员 bill 和 karen，这样 bill 和 karen 能够在目录 /usr/local/share/Music 中创建文件。其他用户能够列出目录中的内容，但是不能在其中创建文件。

But we still have a problem. With the current permissions, files and directories created within the Music directory will have the normal permissions of the users bill and karen:

​ 但是我们仍然会遇到问题。通过我们目前所拥有的权限，在 Music 目录中创建的文件，只具有用户 bill 和 karen 的普通权限：

[bill@linuxbox ~]$ > /usr/local/share/Music/test_file
[bill@linuxbox ~]$ ls -l /usr/local/share/Music
-rw-r--r-- 1 bill    bill    0 2008-03-24 20:03 test_file

Actually there are two problems. First, the default umask on this system is 0022 which prevents group members from writing files belonging to other members of the group. This would not be a problem if the shared directory only contained files, but since this directory will store music, and music is usually organized in a hierarchy of artists and albums, members of the group will need the ability to create files and directories inside directories created by other members. We need to change the umask used by bill and karen to 0002 instead.

​ 实际上，存在两个问题。第一个，系统中默认的掩码值是0022，这会禁止用户组成员编辑属于同 组成员的文件。如果共享目录中只包含文件，这就不是个问题，但是因为这个目录将会存储音乐， 通常音乐会按照艺术家和唱片的层次结构来组织分类。所以用户组成员需要在同组其他成员创建的 目录中创建文件和目录。我们将把用户 bill 和 karen 使用的掩码值改为0002。

Second, each file and directory created by one member will be set to the primary group of the user rather than the group music. This can be fixed by setting the setgid bit on the directory:

​ 第二个问题是，用户组成员创建的文件和目录的用户组，将会设置为用户的主要组，而不是用户组 music。 通过设置此目录的 setgid 位来解决这个问题：

[bill@linuxbox ~]$ sudo chmod g+s /usr/local/share/Music
[bill@linuxbox ~]$ ls -ld /usr/local/share/Music
drwxrwsr-x 2 root music 4096 2008-03-24 20:03 /usr/local/share/Music

Now we test to see if the new permissions fix the problem. bill sets his umask to 0002, removes the previous test file, creates a new test file and directory:

​ 现在测试一下，看看是否新的权限解决了这个问题。bill 把他的掩码值设为0002，删除 先前的测试文件，并创建了一个新的测试文件和目录：

[bill@linuxbox ~]$ umask 0002

[bill@linuxbox ~]$ rm /usr/local/share/Music/test_file

[bill@linuxbox ~]$ > /usr/local/share/Music/test_file
[bill@linuxbox ~]$ mkdir /usr/local/share/Music/test_dir
[bill@linuxbox ~]$ ls -l /usr/local/share/Music
drwxrwsr-x 2 bill   music 4096 2008-03-24 20:24 test_dir
-rw-rw-r-- 1 bill   music 0 2008-03-24 20:22 test_file
[bill@linuxbox ~]$

Both files and directories are now created with the correct permissions to allow all members of the group music to create files and directories inside the Music directory.

​ 现在，创建的文件和目录都具有正确的权限，允许用户组 music 的所有成员在目录 Music 中创建 文件和目录。

The one remaining issue is umask. The necessary setting only lasts until the end of session and must be reset. In the next part of the book, we’ll look at making the change to umask permanent.

​ 剩下一个问题是关于 umask 命令的。umask 命令设置的掩码值只能在当前 shell 会话中生效，若当前 shell 会话结束后，则必须重新设置。在这本书的第三部分，我们将看一下，怎样使掩码值永久生效。

更改用户密码

The last topic we’ll cover in this chapter is setting passwords for yourself (and for other users if you have access to superuser privileges.) To set or change a password, the passwd command is used. The command syntax looks like this:

​ 这一章最后一个话题，我们将讨论自己帐号的密码（和其他人的密码，如果你具有超级用户权限）。 使用 passwd 命令，来设置或更改用户密码。命令语法如下所示：

passwd [user]

To change your password, just enter the passwd command. You will be prompted for your old password and your new password:

​ 只要输入 passwd 命令，就能更改你的密码。shell 会提示你输入你的旧密码和你的新密码：

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[me@linuxbox ~]$ passwd
(current) UNIX password:
New UNIX password:

The passwd command will try to enforce use of “strong” passwords. This means the it will refuse to accept passwords that are too short, too similar to previous passwords, are dictionary words, or too easily guessed:

​ passwd 命令将会试着强迫你使用“强”密码。这意味着它会拒绝接受太短的密码、与先前相似的密码、 字典中的单词作为密码或者是太容易猜到的密码：

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[me@linuxbox ~]$ passwd
(current) UNIX password:
New UNIX password:
BAD PASSWORD: is too similar to the old one
New UNIX password:
BAD PASSWORD: it is WAY too short
New UNIX password:
BAD PASSWORD: it is based on a dictionary word

If you have superuser privileges, you can specify a user name as an argument to the passwd command to set the password for another user. There are other options available to the superuser to allow account locking, password expiration, etc. See the passwd man page for details.

​ 如果你具有超级用户权限，你可以指定一个用户名作为 passwd 命令的参数，这样可以设置另一个 用户的密码。还有其它的 passwd 命令选项对超级用户有效，允许帐号锁定，密码失效，等等。 详细内容，参考 passwd 命令的手册页。

拓展阅读

• Wikipedia has a good article on malware:

• Wikipedia 上面有一篇关于 malware（恶意软件）好文章：

  http://en.wikipedia.org/wiki/Malware

There are number of command line programs used to create and maintain users and groups. For more information, see the man pages for the following commands:

​ 还有一系列的命令行程序，可以用来创建和维护用户和用户组。更多信息，查看以下命令的手册页：

• adduser
• useradd
• groupadd

11 - 11 进程

进程

http://billie66.github.io/TLCL/book/chap11.html

Modern operating systems are usually multitasking, meaning that they create the illusion of doing more than one thing at once by rapidly switching from one executing program to another. The Linux kernel manages this through the use of processes. Processes are how Linux organizes the different programs waiting for their turn at the CPU.

​ 通常，现在的操作系统都支持多任务，意味着操作系统通过在一个执行中的程序和另一个 程序之间快速地切换造成了一种它同时能够做多件事情的假象。Linux 内核通过使用进程来 管理多任务。进程，就是 Linux 组织安排正在等待使用 CPU 的各种程序的方式。

Sometimes a computer will become sluggish or an application will stop responding. In this chapter, we will look at some of the tools available at the command line that let us examine what programs are doing, and how to terminate processes that are misbehaving.

​ 有时候，计算机变得呆滞，运行缓慢，或者一个应用程序停止响应。在这一章中，我们将看一些 可用的命令行工具，这些工具帮助我们查看程序的执行状态，以及怎样终止行为不当的进程。

This chapter will introduce the following commands:

​ 这一章将介绍以下命令：

• ps– Report a snapshot of current processes
• top – Display tasks
• jobs – List active jobs
• bg – Place a job in the background
• fg – Place a job in the foreground
• kill – Send a signal to a process
• killall – Kill processes by name
• shutdown – Shutdown or reboot the system
• ps – 报告当前进程快照
• top – 显示任务
• jobs – 列出活跃的任务
• bg – 把一个任务放到后台执行
• fg – 把一个任务放到前台执行
• kill – 给一个进程发送信号
• killall – 杀死指定名字的进程
• shutdown – 关机或重启系统

进程是怎样工作的

When a system starts up, the kernel initiates a few of its own activities as processes and launches a program called init. init, in turn, runs a series of shell scripts (located in /etc) called init scripts, which start all the system services. Many of these services are implemented as daemon programs, programs that just sit in the background and do their thing without having any user interface. So even if we are not logged in, the system is at least a little busy performing routine stuff.

​ 当系统启动的时候，内核先把一些它自己的活动初始化为进程，然后运行一个叫做 init 的程序。init， 依次地，再运行一系列的称为 init 脚本的 shell 脚本（位于 /etc ），它们可以启动所有的系统服务。 其中许多系统服务以守护进程（daemon）的形式实现，守护进程仅在后台运行，没有任何用户接口(User Interface)。 即使我们没有登录系统，系统也在执行一些例行事务。

The fact that a program can launch other programs is expressed in the process scheme as a parent process producing a child process.

​ 从进程的角度而言，一个程序启动另一个程序可以被表述为一个父进程可以产生一个子进程。

The kernel maintains information about each process to help keep things organized. For example, each process is assigned a number called a process ID or PID. PIDs are assigned in ascending order, with init always getting PID 1. The kernel also keeps track of the memory assigned to each process, as well as the processes’ readiness to resume execution. Like files, processes also have owners and user IDs, effective user IDs, etc.

​ 内核维护每个进程的信息，以此来保持事情有序。例如，系统分配给每个进程一个数字，这个数字叫做 进程(process) ID 或 PID。PID 号按升序分配，init 进程的 PID 总是1。内核也对分配给每个进程的内存和就绪状态进行跟踪以便继续执行这个进程。 像文件一样，进程也有所有者和用户 ID，有效用户 ID，等等。

查看进程

The most commonly used command to view processes (there are several) is ps. The ps program has a lot of options, but in it simplest form it is used like this:

​ 查看进程，最常使用地命令（有几个命令）是 ps(process status)。ps 程序有许多选项，它最简单地使用形式是这样的：

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[me@linuxbox ~]$ ps
PID TTY           TIME CMD
5198 pts/1    00:00:00 bash
10129 pts/1   00:00:00 ps

The result in this example lists two processes, process 5198 and process 10129, which are bash and ps respectively. As we can see, by default, ps doesn’t show us very much, just the processes associated with the current terminal session. To see more, we need to add some options, but before we do that, let’s look at the other fields produced by ps. TTY is short for “Teletype,” and refers to the controlling terminal for the process. Unix is showing its age here. The TIME field is the amount of CPU time consumed by the process. As we can see, neither process makes the computer work very hard.

​ 上例中，列出了两个进程，进程 5198 和进程 10129，各自代表命令 bash 和 ps。正如我们所看到的， 默认情况下，ps 不会显示很多进程信息，只是列出与当前终端会话相关的进程。为了得到更多信息， 我们需要加上一些选项，但是在这样做之前，我们先看一下 ps 命令运行结果的其它字段。 TTY 是 “Teletype”(直译电传打字机) 的简写，是指进程的控制终端。TTY 体现了 Unix 的年代久远。TIME 字段表示 进程所消耗的 CPU 时间数量。正如我们所看到的，这两个进程使计算机工作起来很轻松。

If we add an option, we can get a bigger picture of what the system is doing:

​ 如果给 ps 命令加上选项，我们可以得到更多关于系统运行状态的信息：

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[me@linuxbox ~]$ ps x
PID TTY   STAT   TIME COMMAND
2799 ?    Ssl    0:00 /usr/libexec/bonobo-activation-server –ac
2820 ?    Sl     0:01 /usr/libexec/evolution-data-server-1.10 --

and many more...

Adding the “x” option (note that there is no leading dash) tells ps to show all of our processes regardless of what terminal (if any) they are controlled by. The presence of a “?” in the TTY column indicates no controlling terminal. Using this option, we see a list of every process that we own.

​ 加上 “x” 选项（注意没有开头的 “-“ 字符），告诉 ps 命令，展示所有进程，不管它们由什么 终端（如果有的话）控制。在 TTY 一栏中出现的 “?” ，表示没有控制终端。使用这个 “x” 选项，可以 看到我们所拥有的每个进程的信息。

Since the system is running a lot of processes, ps produces a long list. It is often helpful to pipe the output from ps into less for easier viewing. Some option combinations also produce long lines of output, so maximizing the terminal emulator window may be a good idea, too.

​ 因为系统中正运行着许多进程，所以 ps 命令的输出结果很长。为了方便查看，将 ps 的输出管道 到 less 中通常很有帮助。一些选项组合也会产生很长的输出结果，所以最大化 终端仿真器窗口可能也是一个好主意。

A new column titled STAT has been added to the output. STAT is short for “state” and reveals the current status of the process:

​ 输出结果中，新添加了一栏，标题为 STAT 。STAT 是 “state” 的简写，它揭示了进程当前状态：

The process state may be followed by other characters. These indicate various exotic process characteristics. See the ps man page for more detail.

​ 进程状态信息之后，可能还跟随其他的字符，来表示各种外来进程的特性。详细信息请看 ps 手册页。

Another popular set of options is “aux” (without a leading dash). This gives us even more information:

​ 另一个流行的选项组合是 “aux”（不带开头的”-“字符）。这会给我们更多信息：

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[me@linuxbox ~]$ ps aux
USER   PID  %CPU  %MEM     VSZ    RSS  TTY   STAT   START   TIME  COMMAND
root     1   0.0   0.0    2136    644  ?     Ss     Mar05   0:31  init
root     2   0.0   0.0       0      0  ?     S<     Mar05   0:00  [kt]

and many more...

This set of options displays the processes belonging to every user. Using the options without the leading dash invokes the command with “BSD style” behavior. The Linux version of ps can emulate the behavior of the ps program found in several different Unix implementations. With these options, we get these additional columns:

​ 这个选项组合，能够显示属于每个用户的进程信息。使用这个选项，可以唤醒 “BSD 风格” 的输出结果。 Linux 版本的 ps 命令，可以模拟几个不同 Unix 版本中的 ps 程序的行为。通过这些选项，我们得到 这些额外的列。

用 top 命令动态查看进程

While the ps command can reveal a lot about what the machine is doing, it provides only a snapshot of the machine’s state at the moment the ps command is executed. To see a more dynamic view of the machine’s activity, we use the top command:

​ 虽然 ps 命令能够展示许多计算机运行状态的信息，但是它只是提供 ps 命令执行时刻的机器状态快照。 为了看到更多动态的信息，我们使用 top 命令：

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[me@linuxbox ~]$ top

The top program displays a continuously updating (by default, every 3 seconds) display of the system processes listed in order of process activity. The name “top” comes from the fact that the top program is used to see the “top” processes on the system. The top display consists of two parts: a system summary at the top of the display, followed by a table of processes sorted by CPU activity:

​ top 程序以进程活动顺序显示连续更新的系统进程列表。（默认情况下，每三秒钟更新一次），”top”这个名字 来源于 top 程序是用来查看系统中“顶端”进程的。top 显示结果由两部分组成： 最上面是系统概要，下面是进程列表，以 CPU 的使用率排序。

top - 14:59:20 up 6:30, 2 users, load average: 0.07, 0.02, 0.00
Tasks: 109 total,   1 running,  106 sleeping,    0 stopped,    2 zombie
Cpu(s): 0.7%us, 1.0%sy, 0.0%ni, 98.3%id, 0.0%wa, 0.0%hi, 0.0%si
Mem:   319496k total,   314860k used,   4636k free,   19392k buff
Swap:  875500k total,   149128k used,   726372k free,  114676k cach

 PID  USER       PR   NI   VIRT   RES   SHR  S %CPU  %MEM   TIME+    COMMAND
6244  me         39   19  31752  3124  2188  S  6.3   1.0   16:24.42 trackerd
....

The system summary contains a lot of good stuff. Here’s a rundown:

其中系统概要包含许多有用信息。下表是对系统概要的说明：

The top program accepts a number of keyboard commands. The two most interesting are h, which displays the program’s help screen, and q, which quits top.

​ top 程序接受一系列从键盘输入的命令。两个最有趣的命令是 h 和 q。h，显示程序的帮助屏幕，q， 退出 top 程序。

Both major desktop environments provide graphical applications that display information similar to top (in much the same way that Task Manager in Windows works), but I find that top is better than the graphical versions because it is faster and it consumes far fewer system resources. After all, our system monitor program shouldn’t be the source of the system slowdown that we are trying to track.

​ 两个主要的桌面环境都提供了图形化应用程序，来显示与 top 程序相似的信息 （和 Windows 中的任务管理器类似），但是我觉得 top 程序要好于图形化的版本， 因为它运行速度快，并且消费很少的系统资源。总不至于因为启动了监控界面，让那被监控的系统都变慢。

控制进程

Now that we can see and monitor processes, let’s gain some control over them. For our experiments, we’re going to use a little program called xlogo as our guinea pig. The xlogo program is a sample program supplied with the X Window System (the underlying engine that makes the graphics on our display go) which simply displays a re- sizable window containing the X logo. First, we’ll get to know our test subject:

​ 现在我们可以看到和监测进程，让我们得到一些对它们的控制权。为了我们的实验，我们将使用 一个叫做 xlogo 的小程序，作为我们的实验品。这个 xlogo 程序是 X 窗口系统 （使图形界面显示在屏幕上的底层引擎）提供的示例程序，这个程序仅显示一个大小可调的 包含 X 标志的窗口。首先，我们需要知道测试的实验对象：

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[me@linuxbox ~]$ xlogo

After entering the command, a small window containing the logo should appear somewhere on the screen. On some systems, xlogo may print a warning message, but it may be safely ignored.

​ 命令执行之后，一个包含 X 标志的小窗口应该出现在屏幕的某个位置上。在一些系统中，xlogo 命令 会打印一条警告信息，但是不用理会它。

Tip: If your system does not include the xlogo program, try using gedit or kwrite instead.

​ 小贴士：如果你的系统不包含 xlogo 程序，试着用 gedit 或者 kwrite 来代替。

We can verify that xlogo is running by resizing its window. If the logo is redrawn in the new size, the program is running.

​ 通过调整它的窗口大小，我们能够证明 xlogo 程序正在运行。如果这个标志以新的尺寸被重画， 则这个程序正在运行。

Notice how our shell prompt has not returned? This is because the shell is waiting for the program to finish, just like all the other programs we have used so far. If we close the xlogo window, the prompt returns.

​ 注意，为什么我们的 shell 提示符还没有返回？这是因为 shell 正在等待这个程序结束， 就像到目前为止我们用过的其它所有程序一样。如果我们关闭 xlogo 窗口，shell 提示符就返回了。

中断一个进程

Let’s observe what happens when we run xlogo again. First, enter the xlogo command and verify that the program is running. Next, return to the terminal window and type Ctrl-c.

​ 我们再运行 xlogo 程序一次，观察一下发生了什么事。首先，执行 xlogo 命令，并且 证实这个程序正在运行。下一步，回到终端窗口，按下 Ctrl-c。

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[me@linuxbox ~]$ xlogo
[me@linuxbox ~]$

In a terminal, typing Ctrl-c, interrupts a program. This means that we politely asked the program to terminate. After typing Ctrl-c, the xlogo window closed and the shell prompt returned.

​ 在一个终端中，输入 Ctrl-c，中断一个程序。这意味着，我们礼貌地要求终止这个程序。 输入 Ctrl-c 之后，xlogo 窗口关闭，shell 提示符返回。

Many (but not all) command line programs can be interrupted by using this technique.

​ 通过这个技巧，许多（但不是全部）命令行程序可以被中断。

把一个进程放置到后台(执行)

Let’s say we wanted to get the shell prompt back without terminating the xlogo program. We’ll do this by placing the program in the background. Think of the terminal as having a foreground (with stuff visible on the surface like the shell prompt) and a background (with hidden stuff behind the surface.) To launch a program so that it is immediately placed in the background, we follow the command with an- “&” character:

​ 假如说我们想让 shell 提示符返回，却不终止 xlogo 程序。我们可以把 这个程序放到后台(background)执行。把终端想象是一个有前台（包含在表层可见的事物，像 shell 提示符） 和后台（包含表层之下的隐藏的事物）的设备。为了启动一个程序并让它立即在后台 运行，我们在程序命令之后，加上”&”字符：

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[me@linuxbox ~]$ xlogo &
[1] 28236
[me@linuxbox ~]$

After entering the command, the xlogo window appeared and the shell prompt returned, but some funny numbers were printed too. This message is part of a shell feature called job control. With this message, the shell is telling us that we have started job number 1 (“[1]”) and that it has PID 28236. If we run ps, we can see our process:

​ 执行命令之后，这个 xlogo 窗口出现，并且 shell 提示符返回，同时打印一些有趣的数字。 这条信息是 shell 特性的一部分，叫做任务控制 (job control)。通过这条信息，shell 告诉我们，已经启动了 任务号(job number)为1（“［1］”），PID 为28236的程序。如果我们运行 ps 命令，可以看到我们的进程：

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[me@linuxbox ~]$ ps
  PID TTY         TIME   CMD
10603 pts/1   00:00:00   bash
28236 pts/1   00:00:00   xlogo
28239 pts/1   00:00:00   ps

The shell’s job control facility also gives us a way to list the jobs that are have been launched from our terminal. Using the jobs command, we can see this list:

​ shell 的任务控制功能给出了一种列出从我们终端中启动了的任务的方法。执行 jobs 命令，我们可以看到这个输出列表：

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[me@linuxbox ~]$ jobs
[1]+ Running            xlogo &

The results show that we have one job, numbered “1”, that it is running, and that the command was xlogo &.

​ 结果显示我们有一个任务，编号为“1”，它正在运行，并且这个任务的命令是 xlogo ＆。

进程返回到前台

A process in the background is immune from keyboard input, including any attempt interrupt it with a Ctrl-c. To return a process to the foreground, use the fg command, this way:

​ 一个在后台运行的进程对一切来自键盘的输入都免疫，也不能用 Ctrl-c 来中断它。 为了让一个进程返回前台 (foreground)，这样使用 fg 命令：

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[me@linuxbox ~]$ jobs
[1]+ Running        xlogo &
[me@linuxbox ~]$ fg %1
xlogo

The command fg followed by a percent sign and the job number (called a jobspec) does the trick. If we only have one background job, the jobspec is optional. To terminate xlogo, type Ctrl-c.

​ fg 命令之后，跟随着一个百分号和任务序号（叫做 jobspec ，如此处的 %1）就可以了。如果我们只有一个后台任务，那么 jobspec(job specification) 是可有可无的。输入 Ctrl-c 来终止 xlogo 程序。

停止一个进程

Sometimes we’ll want to stop a process without terminating it. This is often done to allow a foreground process to be moved to the background. To stop a foreground process, type Ctrl-z. Let’s try it. At the command prompt, type xlogo, the Enter key, then Ctrl-z:

​ 有时候，我们想要停下一个进程，而不是终止它。我们这么做通常是为了允许前台进程被移动到后台。 输入 Ctrl-z，可以停下一个前台进程。让我们试一下。在命令提示符下，执行 xlogo 命令， 然后输入 Ctrl-z:

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[me@linuxbox ~]$ xlogo
[1]+ Stopped                 xlogo
[me@linuxbox ~]$

After stopping xlogo, we can verify that the program has stopped by attempting to resize the xlogo window. We will see that it appears quite dead. We can either restore the program to the foreground, using the fg command, or move the program to the background with the bg command:

​ 停止 xlogo 程序之后，通过调整 xlogo 的窗口大小，我们可以证实这个程序已经停止了。 它看起来像死掉了一样。使用 fg 命令，可以恢复程序到前台运行，或者用 bg 命令把程序移到后台。

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[me@linuxbox ~]$ bg %1
[1]+ xlogo &
[me@linuxbox ~]$

As with the fg command, the jobspec is optional if there is only one job.

​ 和 fg 命令一样，如果只有一个任务的话，jobspec 参数是可选的。

Moving a process from the foreground to the background is handy if we launch a graphical program from the command, but forget to place it in the background by appending the trailing “&”.

​ 如果我们从命令行启动一个图形程序，但是忘了在命令后加字符 “&”， 将一个进程从前台移动到后台也是很方便的。

Why would you want to launch a graphical program from the command line? There are two reasons. First, the program you wish to run might not be listed on the window manager’s menus (such as xlogo). Secondly, by launching a program from the command line, you might be able to see error messages that would otherwise be invisible if the program were launched graphically. Sometimes, a program will fail to start up when launched from the graphical menu. By launching it from the command line instead, we may see an error message that will reveal the problem. Also, some graphical programs have many interesting and useful command line options.

​ 为什么要从命令行启动一个图形界面程序呢？有两个原因。第一个，你想要启动的程序，可能 没有在窗口管理器的菜单中列出来（比方说 xlogo）。第二个，从命令行启动一个程序， 你能够看到一些错误信息，如果从图形界面中运行程序的话，这些信息是不可见的。有时候， 一个程序不能从图形界面菜单中启动。通过从命令行中启动它，我们可能会看到 能揭示问题的错误信息。一些图形界面程序还有许多有意思并且有用的命令行选项。

Signals

The kill command is used to “kill” programs. This allows us to terminate programs that need killing. Here’s an example:

​ kill 命令被用来“杀死”程序。这样我们就可以终止需要杀死的程序。这里有一个例子：

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[me@linuxbox ~]$ xlogo &
[1] 28401
[me@linuxbox ~]$ kill 28401
[1]+ Terminated               xlogo

We first launch xlogo in the background. The shell prints the jobspec and the PID of the background process. Next, we use the kill command and specify the PID of the process we want to terminate. We could have also specified the process using a jobspec (for example, “%1”) instead of a PID.

​ 首先，我们在后台启动 xlogo 程序。shell 打印出这个后台进程的 jobspec 和 PID。下一步，我们使用 kill 命令，并且指定我们想要终止的进程 PID。也可以用 jobspec（例如，“％1”）来代替 PID。

While this is all very straightforward, there is more to it than that. The kill command doesn’t exactly “kill” programs, rather it sends them signals. Signals are one of several ways that the operating system communicates with programs. We have already seen signals in action with the use of Ctrl-c and Ctrl-z. When the terminal receives one of these keystrokes, it sends a signal to the program in the foreground. In the case of Ctrl-c, a signal called INT (Interrupt) is sent; with Ctrl-z, a signal called TSTP (Terminal Stop.) Programs, in turn, “listen” for signals and may act upon them as they are received. The fact that a program can listen and act upon signals allows a program to do things like save work in progress when it is sent a termination signal.

​ 虽然这个命令看上去很直白， 但是它的含义不止于此。这个 kill 命令不是真的“杀死”程序，而是给程序 发送信号。信号是操作系统与程序之间进行通信时所采用的几种方式中的一种。 Ctrl-c 和 Ctrl-z 就是信号的实际例子。当终端接受了其中一个按键组合后，它会给在前端运行 的程序发送一个信号。在使用 Ctrl-c 的情况下，会发送一个叫做 INT（Interrupt ，中断）的信号；当使用 Ctrl-z 时，则发送一个叫做 TSTP（Terminal Stop ，终端停止）的信号。程序，相应地，监听信号的到来，当程序 接到信号之后，则做出响应。一个程序能够监听和响应信号这件事允许一个程序做些事情， 比如，当程序接到一个终止信号时，它可以保存所做的工作。

通过 kill 命令给进程发送信号

The kill command is used to send signals to programs. Its most common syntax looks like this:

​ kill 命令被用来给程序发送信号。它最常见的语法形式看起来像这样：

kill [-signal] PID...

If no signal is specified on the command line, then the TERM (Terminate) signal is sent by default. The kill command is most often used to send the following signals:

​ 如果在命令行中没有指定信号，那么默认情况下，发送 TERM（Terminate，终止）信号。kill 命令被经常 用来发送以下命令：

Let’s try out the kill command:

​ 让我们试一下 kill 命令：

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[me@linuxbox ~]$ xlogo &
[1] 13546
[me@linuxbox ~]$ kill -1 13546
[1]+ Hangup         xlogo

In this example, we start the xlogo program in the background and then send it a HUP signal with kill. The xlogo program terminates and the shell indicates that the background process has received a hangup signal. You may need to press the enter key a couple of times before you see the message. Note that signals may be specified either by number or by name, including the name prefixed with the letters “SIG”:

​ 在这个例子里，我们在后台启动 xlogo 程序，然后通过 kill 命令，发送给它一个 HUP 信号。 这个 xlogo 程序终止运行，并且 shell 指示这个后台进程已经接受了一个挂起信号。在看到这条 信息之前，你可能需要多按几次 enter 键。注意，信号既可以用号码，也可以用名字来指定， 包括在前面加上字母 “SIG” 的名字。

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[me@linuxbox ~]$ xlogo 1] 13601
[me@linuxbox ~]$ kill -INT 13601
[1]+ Interrupt                    xlogo
[me@linuxbox ~]$ xlogo &
[1] 13608
[me@linuxbox ~]$ kill -SIGINT 13608
[1]+ Interrupt                    xlogo

Repeat the example above and try out the other signals. Remember, you can also use jobspecs in place of PIDs.

​ 重复上面的例子，试着使用其它的信号。记住，你也可以用 jobspecs 来代替 PID。

Processes, like files, have owners, and you must be the owner of a process (or the superuser) in order to send it signals with kill.

​ 进程，和文件一样，拥有所有者，所以为了能够通过 kill 命令来给进程发送信号， 你必须是进程的所有者（或者是超级用户）。

In addition to the list of signals above, which are most often used with kill, there are other signals frequently used by the system. Here is a list of other common signals:

​ 除了上表列出的 kill 命令最常使用的信号之外，还有一些系统频繁使用的信号。以下是其它一些常用 信号列表：

For the curious, a complete list of signals can be seen with the following command:

​ 为了满足读者的好奇心，通过下面的命令可以得到一个完整的信号列表：

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[me@linuxbox ~]$ kill -l

通过 killall 命令给多个进程发送信号

It’s also possible to send signals to multiple processes matching a specified program or user name by using the killall command. Here is the syntax:

​ 也有可能通过 killall 命令，给匹配特定程序或用户名的多个进程发送信号。下面是 killall 命令的语法形式：

killall [-u user] [-signal] name...

To demonstrate, we will start a couple of instances of the xlogo program and then terminate them:

​ 为了说明情况，我们将启动一对 xlogo 程序的实例，然后再终止它们：

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[me@linuxbox ~]$ xlogo &
[1] 18801
[me@linuxbox ~]$ xlogo &
[2] 18802
[me@linuxbox ~]$ killall xlogo
[1]- Terminated                xlogo
[2]+ Terminated                xlogo

Remember, as with kill, you must have superuser privileges to send signals to processes that do not belong to you.

​ 记住，和 kill 命令一样，你必须拥有超级用户权限才能给不属于你的进程发送信号。

更多和进程相关的命令

Since monitoring processes is an important system administration task, there are a lot of commands for it. Here are some to play with:

​ 因为监测进程是一个很重要的系统管理任务，所以有许多命令与它相关。玩玩下面几个命令：

12 - 12 shell 环境

shell 环境

http://billie66.github.io/TLCL/book/chap12.html

As we discussed earlier, the shell maintains a body of information during our shell session called the environment. Data stored in the environment is used by programs to determine facts about our configuration. While most programs use configuration files to store program settings, some programs will also look for values stored in the environment to adjust their behavior. Knowing this, we can use the environment to customize our shell experience.

​ 恰如我们之前所讲的，shell 在 shell 会话中保存着大量信息。这些信息被称为 (shell 的) 环境。 程序获取环境中的数据（即环境变量）来了解本机的配置。虽然大多数程序用配置文件来存储程序设置， 一些程序会根据环境变量来调整他们的行为。知道了这些，我们就可以用环境变量来自定制 shell 体验。

In this chapter, we will work with the following commands:

• printenv – Print part or all of the environment
• set – Set shell options
• export – Export environment to subsequently executed programs
• alias – Create an alias for a command

​ 在这一章，我们将用到以下命令：

• printenv - 打印部分或所有的环境变量
• set - 设置 shell 选项
• export — 导出环境变量，让随后执行的程序知道。
• alias - 创建命令别名

什么存储在环境变量中？

The shell stores two basic types of data in the environment, though, with bash, the types are largely indistinguishable. They are environment variables and shell variables. Shell variables are bits of data placed there by bash, and environment variables are basically everything else. In addition to variables, the shell also stores some programmatic data, namely aliases and shell functions. We covered aliases in Chapter 6, and shell functions (which are related to shell scripting) will be covered in Part 5.

​ shell 在环境中存储了两种基本类型的数据，虽然在 bash 里，我们几乎无法区分它们。 它们是环境变量和 shell 变量。Shell 变量是 bash 存放的少量数据。剩下的都是 环境变量。除了变量，shell 也存储了一些可编程的数据，即别名和 shell 函数。我们 已经在第六章讨论了别名，而 shell 函数（涉及到 shell 脚本）将会在本章第五部分叙述。

检查环境变量

We can use either the set builtin in bash or the printenv program to see what is stored in the environment. The set command will show both the shell and environment variables, while printenv will only display the latter. Since the list of environment contents will be fairly long, it is best to pipe the output of either command into less:

​ 我们可以用 bash 的内建命令 set，或者是 printenv 程序来查看环境变量。set 命令可以 显示 shell 或环境变量，而 printenv 只是显示环境变量。因为环境变量列表比较长，最好 把每个命令的输出通过管道传递给 less 来阅读：

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[me@linuxbox ~]$ printenv | less

Doing so, we should get something that looks like this:

​ 执行以上命令之后，我们应该能得到类似以下内容：

KDE_MULTIHEAD=false
SSH_AGENT_PID=6666
HOSTNAME=linuxbox
GPG_AGENT_INFO=/tmp/gpg-PdOt7g/S.gpg-agent:6689:1
SHELL=/bin/bash
TERM=xterm
XDG_MENU_PREFIX=kde-
HISTSIZE=1000
XDG_SESSION_COOKIE=6d7b05c65846c3eaf3101b0046bd2b00-1208521990.996705
-1177056199
GTK2_RC_FILES=/etc/gtk-2.0/gtkrc:/home/me/.gtkrc-2.0:/home/me/.kde/sh
are/config/gtkrc-2.0
GTK_RC_FILES=/etc/gtk/gtkrc:/home/me/.gtkrc:/home/me/.kde/share/confi
g/gtkrc
GS_LIB=/home/me/.fonts
WINDOWID=29360136
QTDIR=/usr/lib/qt-3.3
QTINC=/usr/lib/qt-3.3/include
KDE_FULL_SESSION=true
USER=me
LS_COLORS=no=00:fi=00:di=00;34:ln=00;36:pi=40;33:so=00;35:bd=40;33;01
:cd=40;33;01:or=01;05;37;41:mi=01;05;37;41:ex=00;32:\*.cmd=00;32:\*.exe:

What we see is a list of environment variables and their values. For example, we see a variable called USER, which contains the value “me”. The printenv command can also list the value of a specific variable:

​ 我们所看到的是环境变量及其值的列表。例如，我们看到一个叫做 USER 的变量，这个变量值是 “me”。printenv 命令也能够列出特定变量的值：

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[me@linuxbox ~]$ printenv USER
me

The set command, when used without options or arguments, will display both the shell and environment variables, as well as any defined shell functions. Unlike printenv, its output is courteously sorted in alphabetical order:

​ 当使用没有带选项和参数的 set 命令时，shell 变量，环境变量，和定义的 shell 函数 都会被显示。不同于 printenv 命令，set 命令的输出很友好地按照首字母顺序排列：

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[me@linuxbox ~]$ set | less

It is also possible to view the contents of a variable using the echo command, like this:

​ 也可以通过 echo 命令来查看一个变量的内容，像这样：

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[me@linuxbox ~]$ echo $HOME
/home/me

One element of the environment that neither set nor printenv displays is aliases. To see them, enter the alias command without arguments:

​ 别名无法通过使用 set 或 printenv 来查看。 用不带参数的 alias 来查看别名:

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[me@linuxbox ~]$ alias
alias l.='ls -d .* --color=tty'
alias ll='ls -l --color=tty'
alias ls='ls --color=tty'
alias vi='vim'
alias which='alias | /usr/bin/which --tty-only --read-alias --show-dot --show-tilde'

一些有趣的环境变量

The environment contains quite a few variables, and though your environment may differ from the one presented here, you will likely see the following variables in your environment:

​ shell 环境中包含相当多的变量。虽然你的 shell 环境可能与我这里的不同，但也大概率会看到 以下的环境变量：

Don’t worry if some of these values are missing. They vary by distribution.

​ 如果缺失了一些变量，不要担心，这些变量会因发行版本的不同而不同。

如何建立 shell 环境？

When we log on to the system, the bash program starts, and reads a series of configuration scripts called startup files, which define the default environment shared by all users. This is followed by more startup files in our home directory that define our personal environment. The exact sequence depends on the type of shell session being started. There are two kinds: a login shell session and a non-login shell session.

​ 当我们登录系统后， bash 程序启动，并且会读取一系列称为启动文件的配置脚本， 这些文件定义了默认的可供所有用户共享的 shell 环境。然后是读取更于当前用户自己家目录中 的启动文件，这些启动文件定义了用户个人的 shell 环境。确切的启动顺序依赖于要运行的 shell 会话 类型。有两种 shell 会话类型：一个是登录 shell 会话，另一个是非登录 shell 会话。

A login shell session is one in which we are prompted for our user name and password; when we start a virtual console session, for example. A non-login shell session typically occurs when we launch a terminal session in the GUI.

​ 登录 shell 会话会在其中提示用户输入用户名和密码；例如，我们启动一个虚拟控制台会话。 非登录 shell 会话通常当我们在 GUI 下启动终端会话时出现。

Login shells read one or more startup files as shown in Table 12-2:

​ 登录 shell 会读取一个或多个启动文件，正如表12－2所示：

Non-login shell sessions read the following startup files:

​ 非登录 shell 会话会读取以下启动文件：

In addition to reading the startup files above, non-login shells also inherit the environment from their parent process, usually a login shell.

​ 除了读取以上启动文件之外，非登录 shell 会话也会继承它们父进程的环境设置，通常是一个登录 shell。

Take a look at your system and see which of these startup files you have. Remember— since most of the filenames listed above start with a period (meaning that they are hidden), you will need to use the “-a” option when using ls.

​ 浏览一下你的系统，看一看系统中有哪些启动文件。记住－因为上面列出的大多数文件名都以圆点开头 （意味着它们是隐藏文件），你需要使用带”-a”选项的 ls 命令。

The ~/.bashrc file is probably the most important startup file from the ordinary user’s point of view, since it is almost always read. Non-login shells read it by default and most startup files for login shells are written in such a way as to read the ~/.bashrc file as well.

​ 在普通用户看来，文件 ~/.bashrc 可能是最重要的启动文件，因为它几乎总是被读取。非登录 shell 默认 会读取它，并且大多数登录 shell 的启动文件会以能读取 ~/.bashrc 文件的方式来书写。

一个启动文件的内容

If we take a look inside a typical .bash_profile (taken from a CentOS 4 system), it looks something like this:

​ 如果我们看一下典型的 .bash_profile 文件（来自于 CentOS 4 系统），它看起来像这样：

# .bash_profile
# Get the aliases and functions
if [ -f ~/.bashrc ]; then
. ~/.bashrc
fi
# User specific environment and startup programs
PATH=$PATH:$HOME/bin
export PATH

Lines that begin with a “#” are comments and are not read by the shell. These are there for human readability. The first interesting thing occurs on the fourth line, with the following code:

​ 以”#”开头的行是注释，shell 不会读取它们。它们在那里是为了方便人们阅读。第一件有趣的事情 发生在第四行，伴随着以下代码：

if [ -f ~/.bashrc ]; then
. ~/.bashrc
fi

This is called an if compound command, which we will cover fully when we get to shell scripting in Part 5, but for now we will translate:

​ 这叫做一个 if 复合命令，我们将会在第五部分详细地介绍它，现在我们对它翻译一下：

If the file ~/.bashrc exists, then
read the ~/.bashrc file.

We can see that this bit of code is how a login shell gets the contents of .bashrc. The next thing in our startup file has to do with the PATH variable.

​ 我们可以看到这一小段代码就是一个登录 shell 得到 .bashrc 文件内容的方式。在我们启动文件中， 下一件有趣的事与 PATH 变量有关系。

Ever wonder how the shell knows where to find commands when we enter them on the command line? For example, when we enter ls, the shell does not search the entire computer to find /bin/ls (the full pathname of the ls command), rather, it searches a list of directories that are contained in the PATH variable.

​ 是否曾经对 shell 怎样知道在哪里找到我们在命令行中输入的命令感到迷惑？例如，当我们输入 ls 后， shell 不会查找整个计算机系统来找到 /bin/ls（ls 命令的全路径名），相反，它查找一个目录列表， 这个列表就包含在 PATH 变量中。

The PATH variable is often (but not always, depending on the distribution) set by the /etc/profile startup file and with this code:

​ PATH 变量经常（但不总是，依赖于发行版）在 /etc/profile 启动文件中设置，通过这些代码：

PATH=$PATH:$HOME/bin

PATH is modified to add the directory $HOME/bin to the end of the list. This is an example of parameter expansion, which we touched on in Chapter 8. To demonstrate how this works, try the following:

​ 修改 PATH 变量，添加目录 $HOME/bin 到目录列表的末尾。这是一个参数展开的实例， 参数展开我们在第八章中提到过。为了说明这是怎样工作的，试试下面的例子：

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[me@linuxbox ~]$ foo="This is some"
[me@linuxbox ~]$ echo $foo
This is some
[me@linuxbox ~]$ foo="$foo text."
[me@linuxbox ~]$ echo $foo
This is some text.