Bash快速参考手册：Shell脚本必备知识点

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本篇文档是"Bash快速参考手册"，由Arnold Robbins撰写，版权归属O'Reilly Media，出版编号为0596527764。该手册旨在为用户提供深入理解Bash shell所需的所有知识，无论你是Linux、macOS X环境下的主要用户，还是在Windows上使用Bash的开发者，都能从中获益匪浅。首先，历史部分介绍了Bash shell的历史背景，以及它在GNU/Linux和MacOS X系统中的核心地位，特别关注的是版本3.1。Bash作为主要shell，对于Solaris平台也是可选的，并且可以通过简单的编译过程安装。手册涵盖了以下几个主要主题： 1. **历史**：这部分解释了Bash shell的历史演变，以及它在不同操作系统中的应用和地位。 2. **概述与功能**：详细阐述了Bash shell的核心特性，包括其在交互式和脚本环境中的表现，以及为什么它是开发者的首选工具。 3. **启动shell**：介绍了如何调用和运行Bash shell，可能涉及命令行选项和环境设置。 4. **语法**：深入解析Bash shell的语法结构，帮助用户了解如何编写正确的命令和语句。 5. **函数**：讨论了Bash中的函数定义和使用，这对于模块化编程和代码重用至关重要。 6. **变量**：讲解了变量的作用、类型（如环境变量、局部变量等）、赋值以及如何进行复杂的变量替换和操作。 7. **算术表达式**：展示了如何在Bash中执行算术运算，这对于处理数值数据很有帮助。 8. **命令历史**：介绍如何利用Bash的命令历史记录功能，提高工作效率。 9. **作业控制**：讲解了如何使用背景和前台任务管理，以及进程控制命令。 10. **shell选项**：列出并解释了Bash提供的一系列可选参数，这些参数可以定制shell的行为和工作模式。 11. **命令执行**：探讨了如何执行命令、管道、重定向和输入输出操作，这是编写高效脚本的基础。 12. **受限shell**：讨论了安全模式（restricted shell）的概念，如何在特定场景下限制用户的权限。 13. **内建命令**：列出了Bash内置的一系列命令，如`cd`, `echo`, `export`等，这些都是Bash环境中的基础工具。 14. **资源**：提供了进一步学习和参考资料的链接，帮助读者扩展知识面。这份快速参考手册是Bash shell的实用指南，无论是初学者还是高级用户，都能从中找到解决常见问题的答案，提升Shell编程技能。通过掌握这些内容，用户可以在日常工作中更加得心应手，尤其是在编写脚本和优化工作流程时。

Redirection using file descriptors

cmd

Send cmd output to ﬁle descriptor n.

cmd m

Same as previous, except that output that would normally go to ﬁle descriptor m

is sent to ﬁle descriptor n instead.

cmd

>&- Close standard output.

cmd

Take input for cmd from ﬁle descriptor n.

cmd m

Same as previous, except that input that would normally come from ﬁle descrip-

tor m comes from ﬁle descriptor n instead.

cmd

<&- Close standard input.

cmd

- Mo ve input ﬁle descriptor n instead of duplicating it.

cmd

- Mo ve output ﬁle descriptor n instead of duplicating it.

Multiple redirection

cmd

file

Send standard error to ﬁle; standard output remains the same

(e.g., the screen).

cmd

file

2>&1 Send both standard error and standard output to ﬁle.

cmd

file

Same as previous. Preferred form.

cmd

file

Same as previous.

cmd

Send standard output to ﬁle f1 and standard error to ﬁle f2.

cmd

|tee

files

Send output of cmd to standard output (usually the terminal) and

to ﬁles.

cmd

2>&1 | tee

files

Send standard output and error output of cmd to standard output

(usually the terminal) and to ﬁles.

No space should appear between ﬁle descriptors and a redirection symbol; spacing is optional

in the other cases.

Bash allows multidigit ﬁle descriptor numbers. Other shells do not.

Examples

$ cat part1 > book

$ cat part2 part3 >> book

$ mail tim < report

$ sed ’s/ˆ/XX /g’ << END_ARCHIVE

> This is often how a shell archive is "wrapped",

> bundling text for distribution. You would normally

> run sed from a shell program, not from the command line.

> END_ARCHIVE

XX This is often how a shell archive is "wrapped",

XX bundling text for distribution. You would normally

XX run sed from a shell program, not from the command line.

To redirect standard output to standard error:

$ echo "Usage error: see administrator" 1>&2

The following command sends output (ﬁles found) to filelist, and error messages (inac-

cessible ﬁles) to ﬁle no_access:

$ find / -print > filelist 2>no_access

Syntax 9

var

value

... Set each variable var to a value.

var

} Use value of var; braces are optional if var is separated from the

following text. They are required for array variables.

var

value

} Use var if set; otherwise, use value.

var

value

} Use var if set; otherwise, use value and assign value to var.

var

value

} Use var if set; otherwise, print value and exit (if not interactive). If

value isn’t supplied, print the phrase “parameter null or not set.”

var

value

} Use value if var is set; otherwise, use nothing.

${#

var

} Use the length of var.

${#*} Use the number of positional parameters.

${#@} Same as previous.

var

pattern

} Use value of var after removing patter n from the left. Remove the

shor test matching piece.

var

pattern

} Same as #patter n, but remove the longest matching piece.

var

pattern

} Use value of var after removing patter n from the right. Remove

the shortest matching piece.

var

pattern

} Same as %patter n, but remove the longest matching piece.

${!

prefix

*}, ${!

prefix

@} List of variables whose names begin with preﬁx.

var

pos

}, ${

var

pos

len

} Star ting at position pos (0-based) in variable var, extract len char-

acters, or extract rest of string if no len. pos and len may be arith-

metic expressions.

var

pat

repl

} Use value of var, with ﬁrst match of pat replaced with repl.

var

pat

} Use value of var, with ﬁrst match of pat deleted.

var

pat

repl

} Use value of var, with ever y match of pat replaced with repl.

var

pat

repl

} Use value of var, with match of pat replaced with repl. Match

must occur at beginning of the value.

var

pat

repl

} Use value of var, with match of pat replaced with repl. Match

must occur at end of the value.

Bash provides a special syntax that lets one variable indirectly reference another:

$ greet="hello, world" Create initial variable

$ friendly_message=greet Aliasing variable

$ echo ${!friendly_message} Use the alias

hello, world

Examples

$ u=up d=down blank= Assign values to three variables (last is null)

$ echo ${u}root Braces are needed here

uproot

$ echo ${u-$d} Display value of u or d; since u is set, it’s printed

$ echo ${tmp-‘date‘} If tmp is not set, the date command is executed

Sun Jun 11 13:14:54 EDT 2006

$ echo ${blank="no data"} blank is set, so it is printed (a blank line)

$ echo ${blank:="no data"} blank is set but null, so the string is printed

no data

$ echo $blank blank now has a new value

no data

$ tail=${PWD##*/} Take the current director y name and remove the

longest character string ending with /, which

removes the leading pathname and leaves the tail

12 Chapter 1 – The Bash Shell

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