Pro Bash编程第二版：入门与实战指南

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《Pro Bash Programming 2nd Edition》是一本专业的Bash Shell编程指南，全面覆盖了从基础入门到高级技巧的内容。该书适合希望提升Shell脚本技能的读者，无论是初学者还是经验丰富的开发者，都能从中找到宝贵的知识。本书结构清晰，分为多个章节： 1. **第1章：Hello, World：你的第一个Shell程序** - 开篇以经典的“Hello, World”示例引导读者进入Shell编程的世界，介绍脚本的基本概念和文件结构。 2. **第2章：输入、输出与吞吐量** - 探讨如何处理用户输入，控制程序输出，并优化命令执行效率，确保有效通信。 3. **第3章：循环和分支** - 学习如何通过while, for, if等结构实现循环和条件判断，增强脚本的逻辑控制能力。 4. **第4章：命令行解析与扩展** - 深入理解Shell的语法解析规则，以及变量替换、参数扩展等关键概念。 5. **第5章：参数和变量** - 介绍变量的作用，如何声明、赋值、引用以及管理环境变量，为脚本编写提供基础支持。 6. **第6章：Shell函数** - 学习如何创建和调用函数，提高代码重用性和模块化编程。 7. **第7章：字符串操作** - 掌握字符串处理技巧，包括搜索、替换、分割等操作，丰富脚本的文本处理能力。 8. **第8章：文件操作与命令** - 详细讲解文件I/O操作，如读写文件、目录操作，以及如何组合系统命令构建复杂任务。 9. **第9章：保留词和内建命令** - 分析Shell语言的关键保留词及其作用，了解内置命令的使用，提升对Shell特性的理解。 10. **第10章：编写无bug脚本与调试** - 学习如何设计健壮的脚本，以及如何使用调试工具找出并修复错误。 11. **第11章：面向命令行的编程** - 讨论如何利用Shell的特性进行高级的命令行工具开发。 12. **第12章：运行时配置** - 探索如何在脚本运行过程中动态调整配置，增强脚本的适应性。 13. **第13章：数据处理** - 学习如何使用Shell进行数据转换、过滤和格式化，处理大量数据的任务。 14. **第14章：屏幕编程** - 关注交互式脚本，包括图形用户界面（GUI）的创建和操作。 15. **第15章：入门级编程** - 针对初学者，提供基础知识的巩固和进阶技巧，帮助快速上手Shell编程。附录A提供了关于Shell变量的深入讲解，帮助读者巩固所学知识。书后的索引则方便查阅特定主题。这本书不仅是学习Bash编程的实用手册，也是提升Shell脚本编写能力的实用工具。

CHAPTER 2 ■ INPUT, OUTPUT, AND THROUGHPUT

## Width of divider

totalwidth=44

## Print categories

printf "$header" ITEM "PER UNIT" NUM TOTAL

## Print divider to match width of report

printf "%$totalwidth.${totalwidth}s\n" "$divider"

## Print lines of report

printf "$format" \

Chair 79.95 4 319.8 \

Table 209.99 1 209.99 \

Armchair 315.49 2 630.98

The resulting report looks like this:

ITEM PER UNIT NUM TOTAL

============================================

Chair 79.95 4 319.80

Table 209.99 1 209.99

Armchair 315.49 2 630.98

Note the use of braces around the second totalwidth variable name: ${totalwidth}. In the first

instance, the name is followed by a period, which cannot be part of a variable name. In the second, it is

followed by the letter s, which could be, so the totalwidth name must be separated from it by using braces.

Printing to a Variable

With version 3.1, bash added a -v option to store the output in a variable instead of printing it to the standard

output:

$ printf -v num4 "%04d" 4

$ printf "%s\n" "$num4"

0004

Line Continuation

At the end of the report script, the last four lines are read as a single line, using line continuation.

A backslash at the end of a line tells the shell to ignore the newline character, effectively joining the next line

to the current one.

Standard Input /Output Streams and Redirection

In Unix (of which Linux is a variety), everything is a stream of bytes. The streams are accessible as files, but

there are three streams that are rarely accessed by a filename. These are the input/output (I/O) streams

attached to every command: standard input, standard output, and standard error. By default, these streams

are connected to your terminal.

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CHAPTER 2 ■ INPUT, OUTPUT, AND THROUGHPUT

When a command reads a character or a line, it reads from the standard input stream, which is the

keyboard. When it prints information, it is sent to the standard output, your monitor. The third stream,

standard error, is also connected to your monitor; as the name implies, it is used for error messages. These

streams are referred to by numbers, called file descriptors (FDs). These are 0, 1, and 2, respectively.

The stream names are also often contracted to stdin, stdout, and stderr.

I/O streams can be redirected to (or from) a file or into a pipeline.

Redirection: >, >>, and <

In chapter 1, you redirected standard output to a file using the > redirection operator.

When redirecting using >, the file is created if it doesn’t exist. If it does exist, the file is truncated to

zero length before anything is sent to it. You can create an empty file by redirecting an empty string

(that is, nothing) to the file:

printf "" > FILENAME

or by simply using this:

> FILENAME

Redirection is performed before any command on the line is executed. If you redirect to the same file

you are reading from, that file will be truncated, and the command will have nothing to read.

The >> operator doesn’t truncate the destination file; it appends to it. You could add a line to the

hw command from the first chapter by doing the following:

echo exit 0 >> bin/hw

Redirecting standard output does not redirect standard error. Error messages will still be displayed on

your monitor. To send the error messages to a file – in other words, to redirect FD2 – the redirection operator

is preceded by the FD.

Both standard output and standard error can be redirected on the same line. The next command sends

standard output to FILE and standard error to ERRORFILE:

$ printf '%s\n%v\n' OK? Oops! > FILE 2> ERRORFILE

$ cat ERRORFILE

bash4: printf: `v': invalid format character

In this case, the error message is going to a special file, /dev/null. Sometimes called the bit bucket,

anything written to it is discarded.

printf '%s\n%v\n' OK? Oops! 2>/dev/null

Instead of sending output to a file, it can be redirected to another I/O stream by using >&N where N is the

number of the file descriptor. This command sends both standard output and standard error to FILE:

printf '%s\n%v\n' OK? Oops! > FILE 2>&1

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