掌握专业Bash编程：从入门到实践

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《Pro Bash Programming 2015》是一本专注于Bash shell编程的专业指南，它将bash shell视为一门完整的编程语言，而非仅仅是连接外部Linux命令的工具。通过深入探讨shell的内部机制，作者教你如何利用标准编程技术编写与C等编译语言编写的应用同样高效的bash 4.0程序。本书适合初学者和Linux/Unix系统管理员，帮助他们全面掌握系统控制。在本书中，读者可以从基础开始学习，第一章"Hello, World：你的第一个shell程序"介绍shell脚本的概念，让你理解shell脚本是由一系列用户在命令行中输入的命令组成，旨在实现自动化和简化任务处理。章节内容涵盖了以下几个关键主题： 1. **入门篇**：从基础入手，引导读者编写第一个shell脚本，理解基本语法结构和执行流程。 2. **输入、输出与性能**：章节探讨如何处理脚本中的数据输入和输出，优化脚本执行效率，提高程序的响应速度。 3. **循环与分支**：通过讲解while, for和if-else语句，教授如何根据条件执行不同的代码块，增强脚本的灵活性。 4. **命令行解析与扩展**：学会解析和处理命令行参数，以及利用shell变量进行参数扩展，减少对外部命令的依赖。 5. **参数与变量**：深入理解shell中的变量类型、作用域和操作，以及如何有效地存储和传递数据。 6. **shell函数**：掌握如何创建和调用shell函数，模块化脚本，提高代码重用性。 7. **字符串处理**：学习对字符串的操作，如查找、替换、分割等，这对于处理文本数据至关重要。 8. **文件操作与命令**：涵盖文件和目录操作，如读取、写入、复制、删除等，以及如何使用内建命令。 9. **保留词与内建命令**：了解bash特有的保留词和内建命令，这些都是脚本编程的基础。 10. **编写无误脚本与调试**：提供实用技巧来编写出少错误或不出错的脚本，并学习如何调试和解决潜在问题。 11. **面向命令行的编程**：针对命令行环境的特点，讨论如何设计用户友好的交互式脚本。 12. **运行时配置**：探索如何在脚本运行过程中动态调整配置，适应不同环境需求。 13. **数据处理**：应用shell脚本来处理大量数据，包括文件操作、文本处理等复杂任务。 14. **屏幕脚本**：如何利用脚本控制终端输出，增强用户体验。 15. **入门级编程**：为对shell编程感兴趣的读者提供一个起点，逐步提升编程技能。通过阅读本书，你将能够利用Bash shell的强大功能，编写出高效且专业的脚本，无论是作为系统管理工具还是自定义应用程序。书中详尽的内容确保了无论你是Linux/Unix新手还是进阶者，都能从中获益良多。

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.

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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掌握专业Bash编程：从入门到实践

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