Linux Bash脚本编程入门指南

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"Bash-Beginners-Guide.pdf" 本书《Bash Guide for Beginners》由Machtelt Garrels撰写，主要面向Linux用户，特别是对Bash脚本编程感兴趣的初学者。Bash是Linux系统中最常用的命令行解释器，也是许多系统管理任务的核心工具。通过学习这本书，读者可以掌握Bash脚本的基础知识，从而更高效地管理和自动化Linux环境。 1.1章节介绍了Bash及其相关概念。书中指出，Bash（Bourne-Again SHell）是一种广泛使用的shell程序，具有通用的shell功能。它包括一般shell函数，如变量、条件测试和循环结构。此外，还提到了不同类型的shell，如C shell、Korn shell和Bash，其中Bash因其作为GNU项目的官方shell而备受青睐。 1.2章节阐述了Bash的优势。Bash不仅继承了Bourne shell的功能，还引入了许多独特的特性，使其成为编写shell脚本的理想选择。例如，它支持数组、命令别名、历史扩展和命令行编辑等高级功能。在1.3章节中，作者详细讲解了如何执行命令。这包括了基本的命令执行方式、内置shell命令以及如何在脚本中执行外部程序。了解这些基础知识对于编写和运行Bash脚本至关重要。 1.4章节介绍了构建Bash脚本的基本元素，即"building blocks"。这些元素包括变量、控制结构（如if语句和for循环）、函数以及输入/输出重定向等，它们构成了脚本的核心部分。 1.5章节讨论了编写良好脚本的原则和实践。好的脚本应该具备可读性、可维护性和逻辑清晰等特点。作者强调了脚本的结构、术语定义和逻辑顺序的重要性，并给出了一个名为"mysystem.sh"的示例脚本来演示这些原则。此外，还提供了一个初始化脚本的例子，帮助读者更好地理解脚本的实际应用。 1.6章节对本章内容进行了总结，回顾了关键概念和技巧，为后续深入学习打下基础。通过阅读本书，读者将能够熟练掌握Bash脚本的基本语法和最佳实践，从而在Linux环境中编写出高效且易于维护的脚本。同时，书中鼓励读者积极反馈和参与，共同提升对Bash的理解和使用。

1.2.2.2.1. Invoked as an interactive login shell, or with `--login'

Interactive means you can enter commands. The shell is not running because a script has been activated. A

and password.

Files read:

/etc/profile•

~/.bash_profile, ~/.bash_login or ~/.profile: first existing readable file is read•

~/.bash_logout upon logout.•

Error messages are printed if configuration files exist but are not readable. If a file does not exist, bash

searches for the next.

1.2.2.2.2. Invoked as an interactive non-login shell

A non-login shell means that you did not have to authenticate to the system. For instance, when you open a

terminal using an icon, or a menu item, that is a non-login shell.

Files read:

~/.bashrc•

This file is usually referred to in ~/.bash_profile:

if [ -f ~/.bashrc ]; then . ~/.bashrc; fi

See Chapter 7 for more information on the if construct.

1.2.2.2.3. Invoked non-interactively

All scripts use non-interactive shells. They are programmed to do certain tasks and cannot be instructed to do

other jobs than those for which they are programmed.

Files read:

defined by BASH_ENV•

PATH is not used to search for this file, so if you want to use it, best refer to it by giving the full path and file

name.

1.2.2.2.4. Invoked with the sh command

Bash tries to behave as the historical Bourne sh program while conforming to the POSIX standard as well.

Files read:

/etc/profile•

~/.profile•

Bash Guide for Beginners

Chapter 1. Bash and Bash scripts 8

1.2.2.3.3. Interactive shell behavior

Differences in interactive mode:

Bash reads startup files.•

Job control enabled by default.•

Prompts are set, PS2 is enabled for multi-line commands, it is usually set to ">". This is also the

prompt you get when the shell thinks you entered an unfinished command, for instance when you

forget quotes, command structures that cannot be left out, etc.

•

Commands are by default read from the command line using readline.•

Bash interprets the shell option ignoreeof instead of exiting immediately upon receiving EOF

(End Of File).

•

Command history and history expansion are enabled by default. History is saved in the file pointed to

by HISTFILE when the shell exits. By default, HISTFILE points to ~/.bash_history.

•

Alias expansion is enabled.•

In the absence of traps, the SIGTERM signal is ignored.•

In the absence of traps, SIGINT is caught and handled. Thus, typing Ctrl+C, for example, will not

quit your interactive shell.

•

Sending SIGHUP signals to all jobs on exit is configured with the huponexit option.•

Commands are executed upon read.•

Bash checks for mail periodically.•

Bash can be configured to exit when it encounters unreferenced variables. In interactive mode this

behavior is disabled.

•

When shell built-in commands encounter redirection errors, this will not cause the shell to exit.•

Special built-ins returning errors when used in POSIX mode don't cause the shell to exit. The built-in

commands are listed in Section 1.3.2.

•

Failure of exec will not exit the shell.•

Parser syntax errors don't cause the shell to exit.•

Simple spell check for the arguments to the cd built-in is enabled by default.•

Automatic exit after the length of time specified in the TMOUT variable has passed, is enabled.•

More information:

Section 3.2•

Section 3.6•

See Chapter 12 for more about signals.•

Section 3.4 discusses the various expansions performed upon entering a command.•

1.2.2.4. Conditionals

Conditional expressions are used by the [[ compound command and by the test and [ built-in commands.

Expressions may be unary or binary. Unary expressions are often used to examine the status of a file. You

only need one object, for instance a file, to do the operation on.

There are string operators and numeric comparison operators as well; these are binary operators, requiring two

objects to do the operation on. If the FILE argument to one of the primaries is in the form /dev/fd/N, then

file descriptor N is checked. If the FILE argument to one of the primaries is one of /dev/stdin,

/dev/stdout or /dev/stderr, then file descriptor 0, 1 or 2 respectively is checked.

Conditionals are discussed in detail in Chapter 7.

Bash Guide for Beginners

Chapter 1. Bash and Bash scripts 10

More information about the file descriptors in Section 8.2.3.

1.2.2.5. Shell arithmetic

The shell allows arithmetic expressions to be evaluated, as one of the shell expansions or by the let built-in.

Evaluation is done in fixed-width integers with no check for overflow, though division by 0 is trapped and

flagged as an error. The operators and their precedence and associativity are the same as in the C language,

see Chapter 3.

1.2.2.6. Aliases

Aliases allow a string to be substituted for a word when it is used as the first word of a simple command. The

shell maintains a list of aliases that may be set and unset with the alias and unalias commands.

Bash always reads at least one complete line of input before executing any of the commands on that line.

Aliases are expanded when a command is read, not when it is executed. Therefore, an alias definition

appearing on the same line as another command does not take effect until the next line of input is read. The

commands following the alias definition on that line are not affected by the new alias.

Aliases are expanded when a function definition is read, not when the function is executed, because a function

definition is itself a compound command. As a consequence, aliases defined in a function are not available

until after that function is executed.

We will discuss aliases in detail in Section 3.5.

1.2.2.7. Arrays

Bash provides one-dimensional array variables. Any variable may be used as an array; the declare built-in

will explicitly declare an array. There is no maximum limit on the size of an array, nor any requirement that

members be indexed or assigned contiguously. Arrays are zero-based. See Chapter 10.

1.2.2.8. Directory stack

The directory stack is a list of recently-visited directories. The pushd built-in adds directories to the stack as it

changes the current directory, and the popd built-in removes specified directories from the stack and changes

the current directory to the directory removed.

Content can be displayed issuing the dirs command or by checking the content of the DIRSTACK variable.

More information about the workings of this mechanism can be found in the Bash info pages.

1.2.2.9. The prompt

Bash makes playing with the prompt even more fun. See the section Controlling the Prompt in the Bash info

pages.

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Chapter 1. Bash and Bash scripts 11

1.2.2.10. The restricted shell

When invoked as rbash or with the --restricted or -r option, the following happens:

The cd built-in is disabled.•

Setting or unsetting SHELL, PATH, ENV or BASH_ENV is not possible.•

Command names can no longer contain slashes.•

Filenames containing a slash are not allowed with the . (source) built-in command.•

The hash built-in does not accept slashes with the -p option.•

Import of functions at startup is disabled.•

SHELLOPTS is ignored at startup.•

Output redirection using >, >|, ><, >&, &> and >> is disabled.•

The exec built-in is disabled.•

The -f and -d options are disabled for the enable built-in.•

A default PATH cannot be specified with the command built-in.•

Turning off restricted mode is not possible.•

When a command that is found to be a shell script is executed, rbash turns off any restrictions in the shell

spawned to execute the script.

More information:

Section 3.2•

Section 3.6•

Info Bash->Basic Shell Features->Redirections•

Section 8.2.3: advanced redirection•

1.3. Executing commands

1.3.1. General

Bash determines the type of program that is to be executed. Normal programs are system commands that exist

in compiled form on your system. When such a program is executed, a new process is created because Bash

makes an exact copy of itself. This child process has the same environment as its parent, only the process ID

number is different. This procedure is called forking.

After the forking process, the address space of the child process is overwritten with the new process data. This

is done through an exec call to the system.

The fork-and-exec mechanism thus switches an old command with a new, while the environment in which the

new program is executed remains the same, including configuration of input and output devices, environment

variables and priority. This mechanism is used to create all UNIX processes, so it also applies to the Linux

operating system. Even the first process, init, with process ID 1, is forked during the boot procedure in the

so-called bootstrapping procedure.

1.3.2. Shell built-in commands

Built-in commands are contained within the shell itself. When the name of a built-in command is used as the

first word of a simple command, the shell executes the command directly, without creating a new process.

Bash Guide for Beginners

Chapter 1. Bash and Bash scripts 12

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Linux Bash脚本编程入门指南

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