shell中的特殊字符

* 星号

作为匹配文件名扩展的一个通配符，能自动匹配给定目录下的每一个文件； bash$ echo * abs-book.sgml add-drive.sh agram.sh alias.sh 正则表达式中可以作为字符限定符，表示其前面的匹配规则匹配任意次；

-算术运算中表示乘法。

** 双星号

算术运算中表示求幂运算。

？ 问号

表示条件测试；

在双括号内表示C风格的三元操作符

(( var0 = var1<98?9:21 )) # ^ ^ # if [ "$var1" -lt 98 ] # then # var0=9 # else # var0=21 # fi 参数替换表达式中用来测试一个变量是否设置了值； #!/bin/bash # Check some of the system's environmental variables. # This is good preventative maintenance. # If, for example, $USER, the name of the person at the console, is not set, #+ the machine will not recognize you. : ${HOSTNAME?} ${USER?} ${HOME?} ${MAIL?} echo echo "Name of the machine is $HOSTNAME." echo "You are $USER." echo "Your home directory is $HOME." echo "Your mail INBOX is located in $MAIL." echo echo "If you are reading this message," echo "critical environmental variables have been set." echo echo

作为通配符，用于匹配文件名扩展特性中，用于匹配单个字符；

正则表达式中，表示匹配其前面规则0次或者1次。

$ 美元符号

作为变量的前导符，用作变量替换，即引用一个变量的内容 var1=5 var2=23skidoo echo $var1 # 5 echo $var2 # 23skidoo 在正则表达式中被定义为行末（End of line）。

“${}”

参数替换 ，基本作用和$一致，在链接字符串方面稍有差别 your_id=${USER}-on-${HOSTNAME} echo "$your_id" # echo "Old \$PATH = $PATH" PATH=${PATH}:/opt/bin # Add /opt/bin to $PATH for duration of script. echo "New \$PATH = $PATH"

$’…’

引用展开 执行单引号内的转义内容（单引号原本是原样引用的），这种方式会将引号内的一个或者多个[]转义后的八进制，十六进制值展开到ASCII或Unicode字符。 quote=$'\042'.

$*, $@

位置参数，这个在使用脚本文件的时候，在传递参数的时候会用到,两者都能返回调用脚本文件的所有参数

$* 是将所有参数作为一个整体返回（字符串）

$@是将每个参数作为单元返回一个参数列表

index=1 # Initialize count. echo "Listing args with \"\$*\":" for arg in "$*" # Doesn't work properly if "$*" isn't quoted. do echo "Arg #$index = $arg" let "index+=1" done # $* sees all arguments as single word. echo "Entire arg list seen as single word." echo index=1 # Reset count. # What happens if you forget to do this? echo "Listing args with \"\$@\":" for arg in "$@" do echo "Arg #$index = $arg" let "index+=1" done # $@ sees arguments as separate words. echo "Arg list seen as separate words." echo index=1 # Reset count. echo "Listing args with \$* (unquoted):" for arg in $* do echo "Arg #$index = $arg" let "index+=1" done # Unquoted $* sees arguments as separate words. echo "Arg list seen as separate words." exit 0

代码输出：

$?

此变量值在使用的时候，返回的是最后一个命令、函数、或脚本的退出状态码值，如果没有错误则是0，如果为非0，则表示在此之前的最后一次执行有错误。 true 的返回码是0 false的返回码是非0

$$

进程ID变量，这个变量保存了运行当前脚本的进程ID值。

() 括号

命令组。 因为是在子shell内运行，因此在括号外面是没有办法获取括号内变量的值，但反过来，命令组内是可以获取到外面的值，这点有点像局部变量和全局变量的关系，在实作中，如果碰到要cd到子目录操作，并在操作完成后要返回到当前目录的时候，可以考虑使用subshell来处理； a=123 ( a=321; ) echo "a = $a" # a = 123 # "a" within parentheses acts like a local variable. 数组的初始化 Array=(element1 element2 element3)

{xxx,yyy,zzz,…}

在命令中可以用这种扩展来扩展参数列表 注意的一点是，这花括号扩展中不能有空格存在，如果确实有必要空格，则必须被转义或者使用引号来引用 echo \"{These,words,are,quoted}\" # " prefix and suffix # "These" "words" "are" "quoted" 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

{a..z}

列举字符 #!/bin/bash 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.

{} 花括号

代码块 这个是匿名函数，但是又与函数不同，在代码块里面的变量在代码块后面仍能访问。注意：花括号内侧需要有空格与语句分隔。 #!/bin/bash a=123 { a=321; } echo "a = $a" # a = 321 (value inside code block) # Thanks, S.C. {} 代码块里的内容可以通过IO重定向 #!/bin/bash # Reading lines in /etc/fstab. File=/etc/fstab { read line1 read line2 } < $File echo "First line in $File is:" echo "$line1" echo echo "Second line in $File is:" echo "$line2" exit 0 # Now, how do you parse the separate fields of each line? # Hint: use awk, or . . . # . . . Hans-Joerg Diers suggests using the "set" Bash builtin. 在xargs -i中的话，还可以作为文本的占位符，用以标记输出文本的位置。 #!/bin/bash # copydir.sh # Copy (verbose) all files in current directory ($PWD) #+ to directory specified on command-line. E_NOARGS=85 if [ -z "$1" ] # Exit if no argument given. then echo "Usage: `basename $0` directory-to-copy-to" exit $E_NOARGS fi ls . | xargs -i -t cp ./{} $1 # ^^ ^^ ^^ # -t is "verbose" (output command-line to stderr) option. # -i is "replace strings" option. # {} is a placeholder for output text. # This is similar to the use of a curly-bracket pair in "find." # # List the files in current directory (ls .), #+ pass the output of "ls" as arguments to "xargs" (-i -t options), #+ then copy (cp) these arguments ({}) to new directory ($1). # # The net result is the exact equivalent of #+ cp * $1 #+ unless any of the filenames has embedded "whitespace" characters. exit 0

{} \;

这个{}是表示路径名，现在接触到的情况看，好像只用在find命令里 find ~/ -name 'core*' -exec rm {} \; # Removes all core dump files from user's home directory.

[ ] 中括号

测试的表示 Shell会测试在[]内的表达式

在数组的上下文中，表示数组元素，方括号内填上数组元素的位置就能获得对应位置的内容

Array[1]=xxx echo ${Array[1]}; 表示字符集的范围，在正表达式中，方括号表示该位置可以匹配的字符集范围。 "[xyz]" matches any one of the characters x, y, or z. "[c-n]" matches any one of the characters in the range c to n. "[B-Pk-y]" matches any one of the characters in the ranges B to P and k to y. "[a-z0-9]" matches any single lowercase letter or any digit. Combined sequences of bracketed characters match common word patterns. "[Yy][Ee][Ss]" matches yes, Yes, YES, yEs, and so forth. "[0-9][0-9][0-9]-[0-9][0-9]-[0-9][0-9][0-9][0-9]" matches any Social Security number. "[^b-d]" matches any character except those in the range b to d. This is an instance of ^ negating(否定) or inverting the meaning of the following RE (taking on a role similar to ! in a different context).

[[ ]]

这个结构也是测试，测试[[]]之中的表达式(Shell的关键字)。这个比单中括号更能防止脚本里面的逻辑错误，比如：&&,||,<,>操作符能在一个[[]]里面测试通过，但是在[]却不能通过。[[]]里面没有文件名扩展(filename expansion）或是词分隔符(Word splitting)，但是可以用参数扩展(Parameter expansion)和命令替换(command substitution)。不用文件名通配符和像空白这样的分隔符。注意，这里面如果出现了八进制，十六进制等，shell会自动执行转换比较。 # [[ Octal and hexadecimal evaluation ]] # Thank you, Moritz Gronbach, for pointing this out. decimal=15 octal=017 # = 15 (decimal) hex=0x0f # = 15 (decimal) if [ "$decimal" -eq "$octal" ] then echo "$decimal equals $octal" else echo "$decimal is not equal to $octal" # 15 is not equal to 017 fi # Doesn't evaluate within [ single brackets ]! if [[ "$decimal" -eq "$octal" ]] then echo "$decimal equals $octal" # 15 equals 017 else echo "$decimal is not equal to $octal" fi # Evaluates within [[ double brackets ]]! if [[ "$decimal" -eq "$hex" ]] then echo "$decimal equals $hex" # 15 equals 0x0f else echo "$decimal is not equal to $hex" fi # [[ $hexadecimal ]] also evaluates!

$[ … ]

在方括号里面执行整数表达式 a=3 b=7 echo $[$a+$b] # 10 echo $[$a*$b] # 21

(( ))

功能和上面的 []差不多，但是需要注意的是， []是会返回里面表达式的值的，而(())只是执行，并不会返回值。两者执行后如果变量值发生变化，都会影响到后继代码的运行。可对变量赋值，可以对变量进行一目操作符操作，也可以是二目，三目操作符。 #!/bin/bash # c-vars.sh # Manipulating a variable, C-style, using the (( ... )) construct. echo (( a = 23 )) # Setting a value, C-style, #+ with spaces on both sides of the "=". echo "a (initial value) = $a" # 23 (( a++ )) # Post-increment 'a', C-style. echo "a (after a++) = $a" # 24 (( a-- )) # Post-decrement 'a', C-style. echo "a (after a--) = $a" # 23 (( ++a )) # Pre-increment 'a', C-style. echo "a (after ++a) = $a" # 24 (( --a )) # Pre-decrement 'a', C-style. echo "a (after --a) = $a" # 23 echo echo (( t = a<45?7:11 )) # C-style trinary operator. # ^ ^ ^ echo "If a < 45, then t = 7, else t = 11." # a = 23 echo "t = $t " # t = 7 echo