Bash新手指南：入门与实践

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本资源是一份名为"Bash-Beginners-Guide"的指南，由Machtelt Garrels编写，专注于向初学者介绍Bash脚本编程。该指南的目的是为了帮助那些想要学习或入门Bash语言的人，特别是对于Linux脚本爱好者来说，它提供了深入浅出的教学内容。首先，作者在"Introduction"部分解释了编写此指南的原因，可能是出于对Bash作为GNU shell的强大功能和易用性的赞赏，以及认识到初学者在理解和使用这个Shell时可能遇到的困难。指南适合那些对命令行操作感兴趣，希望提升技能，或者想要开发自动化脚本的新手。章节1详细介绍了Bash及其脚本。这部分包括常见的shell程序类型，如通用shell函数，以及Bash作为Bourne Again Shell（bash）的优势。Bash的特点在于其丰富的功能，如内置命令、执行外部程序的能力，以及作为GNU工具链的一部分所具有的广泛兼容性。在"Executing commands"章节，作者阐述了如何执行一般性命令，区分了Shell内置命令与外部程序调用。此外，还强调了编写脚本时的基本构建块，如使用变量、条件语句、循环结构等。 "Developing good scripts"是核心内容，强调了编写高效、可读性强的脚本的重要性。作者提出好脚本应具备的特性，如清晰的结构、术语规范，以及逻辑组织。通过提供一个实例"mysystem.sh"，读者可以直观地了解脚本设计和实现的实践过程。总结来说，这份Bash-Beginners-Guide是一份详尽的入门指南，覆盖了Bash的基本概念、命令执行、脚本开发的关键要素，对于刚接触Bash的新手来说，是系统学习和实践Bash脚本编程的理想资源。无论是为了提高Linux系统管理效率，还是探索Shell脚本的潜力，这份指南都是一个不错的起点。

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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