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Advanced Bash-Scripting Guide 读书笔记

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看Advanced Bash-Scripting Guide这本书做的一些笔记，将书中一些比较主要的东西提取出来了，看完这本书之后用来复习还是很有用的。

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08/14/12 Edited by: xuzhezhao Last edited: 09/05/12

Advanced Bash-Scripting Guide

学习笔记

Specia Characters

1. 分号（colon）代表空命令（null command）

• 类似于 shell 内置的 true 功能，执行后的 exit 值是 0，即$?=0。

This is the shell equivalent of a "NOP" (no op, a do-nothing operation). It may

be considered a synonym for the shell builtin true. The ":" command is itself a

Bash builtin, and its exit status is true (0).

• Endless loop

while :

operation-1

operation-2

...

operation-n

done

# Same as:

while true

...

done

• 在 if/then 结构中作为占位符（Placeholder in if/then test）

if condition

then : # 注意“：”前面有空格，不加分号的话会产生语法错误

# Do nothing and branch ahead

else

# Or else ...

take-some-action

• 作为命令占位符（Provide a placeholder where a binary operation is

expected）

: ${username=`whoami`}

#可以用分号预留一个命令的位置

# ${username=`whoami`}

# Gives an error without the leading :

#+ unless "username" is a command or builtin...

• Evaluate string of variables using parameter substitution

: ${HOSTNAME?} ${USER?} ${MAIL?}

# Prints error message

#+ if one or more of essential environmental variables not set.

• 配合重定向，将文件变成空文件（truncates a file to zero length）

In combination with the > redirection operator, truncates a file to zero length,

without changing its permissions. If the file did not previously exist, creates it.

: > data.xxx

# File "data.xxx" now empty.

# Same effect as

cat /dev/null >data.xxx

# However, this does not fork a new process, since ":" is a builtin.

In combination with the >> redirection operator, has no effect on a

pre-existing target file (: >> target_file). If the file did not previously exist,

creates it.

• 还有一种不建议的用法，用作注释（May be used to begin a comment line,

although this is not recommended）

: This is a comment that generates an error, ( if [ $x -eq 3] ).

2. ！（bang）：取反 reverse (or negate) the sense of a test or exit

status

The ! operator inverts the exit status of the command to which it is applied (see

Example 6-2). It also inverts the meaning of a test operator. This can, for example,

change the sense of equal ( = ) to not-equal ( != ). The ! operator is a Bash keyword.

3. *（asterisk）：通配符（wild card）、乘号（multiplication）

在正则表达式（regular expression）中代表 0 个或任意多个前一个字符。

**：取幂运算符，或者是 extended file-match globbing.

4. ？：test operator

In a double-parentheses construct（双括号结构中）, the ? can serve as an element

of a C-style trinary operator（三目运算符）.

condition?result-if-true:result-if-false

(( var0 = var1<98?9:21 ))

if [ "$var1" -lt 98 ]

then

var0=9

else

var0=21

In a parameter substitution expression, the ? tests whether a variable has been set.

5. $：变量代换 Variable substitution (contents of a variable).

• $var：显示 var 内容

• $：正则表达式的结尾标志。end-of-line. In a regular expression, a "$" addresses the

end of a line of text.

• ${ }：参数代换（Parameter substitution）.

• $' … '：将 ascii 码转成字符形式，如 echo $'\111'。Quoted string expansion. This

construct expands single or multiple escaped octal or hex values into

ASCII [16] or Unicode characters.

• $*,$@：positional parameters.

• $?：上一命令的返回值。exit status variable. The $? variable holds the exit status of

a command, a function, or of the script

itself.

• $$：script 所在执行环境中的进程 ID，process ID variable.

The $$ variable holds the process ID of the script in which it appears.

6. （）：命令组（command group）

(a=hello; echo $a)

括号中的命令是在子 shell 中执行（A listing of commands within parentheses starts a

subshell），子 shell 要的变量在父 shell 中不可见。

数组初始化（array initialization）

Array=(element1 element2 element3)

echo ${Array[0]}

; echo ${Array[1]}; echo ${Array[2]}

7. { }：Brace expansion

echo \"{These,words,are,quoted}\"

# "These" "words" "are" "quoted"

# " prefix and suffix

cat {file1,file2,file3} > combined_file

# Concatenates the files file1, file2, and file3 into combined_file.

cp file22.{txt,backup}

# Copies "file22.txt" to "file22.backup"

注意：No spaces allowed within the braces unless the spaces are quoted or escaped.

echo {file1,file2}\ :{\ A," B",' C'}

• {a..z}：Extended Brace expansion

echo {a..z} # a b c d e f g h i j k l m n o p q r s t u v w x y z

# Echoes characters between a and z.

echo {0..3} # 0 1 2 3

# Echoes characters between 0 and 3.

base64_charset=( {A..Z} {a..z} {0..9} + / = )

# Initializing an array, using extended brace expansion.

# From vladz's "base64.sh" example script.

• { }：Block of code [curly brackets]. Also referred to as an inline group, this

construct, in effect, creates

an anonymous function (a function without a name). However, unlike in a "standard"

function, the

variables inside a code block remain visible to the remainder of the script.

a=123

{ a=321; } #大括号内的命令必须要以分号或者换行来结束

echo "a = $a"

# a = 321

(value inside code block)

# Thanks, S.C.

• 重定向 Code blocks and I/O redirection

#!/bin/bash

# Reading lines in /etc/fstab.

File=/etc/fstab

{

read line1

read line2

} < $File

echo "First line in $File is:"

echo

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滨野明日香

2014-06-25

有很好的辅助作用

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