Bash中${}的用法数组字符串的切片和变量的间接引用

在bash中${}用于设置变量默认值和字符串取值切片以及变量的间接引用，详细用法如下图，在Centos6下字符串取子${STR:POSITON:LENGTH},LENGTH为负数会报错。

1、${VAR}，取出变量VAR值

[root@localhost ~]# a=hjks
[root@localhost ~]# echo ${a}
hjks

2、${VAR:-DEFAULT}，VAR没有定义或者为空则输出$DEFAULT的值（VAR不变）

[root@localhost ~]# unset a
[root@localhost ~]# echo ${a:-hello};echo $a
hello
#$a未赋值输出了空值
#可以再脚本用于设置默认值

3、${VAR:=DEFAULT}，VAR没有定义或空则为$DEFAULT的值

[root@localhost ~]# echo ${a:=hello};echo a
hello
hello              #a被赋值

4、${VAR+VALUE}，${VAR+VALUE}，VAR定义则输出$VALUE,否则为空字符串（VAR不变）

[root@localhost ~]# a=hello
[root@localhost ~]# echo ${a:+yemo};echo $a
yemo
hello
[root@localhost ~]# unset a
[root@localhost ~]# echo ${a:+yemo};echo $a

5、${VAR?MSG}，VAR未定义则打印$MSG

[root@localhost ~]# unset a
[root@localhost ~]# echo ${a?this is null}
-bash: a: this is null
[root@localhost ~]# typeset -i a
[root@localhost ~]# echo ${a?this is null}
-bash: a: this is null

6、${VAR:?MSG}，VAR未定义或者为空则打印$MSG

[root@localhost ~]# unset a
[root@localhost ~]# echo ${a?this is null}
-bash: a: this is null
[root@localhost ~]# typeset -i a
[root@localhost ~]# echo ${a?this is null}
-bash: a: this is null

7、${!PERFIX*} ${!PERFIX@}匹配所有以PERFIX开头的变量

[root@localhost ~]# echo ${!RAND*}
RANDOM
[root@localhost ~]# echo ${!RAND@}
RANDOM

8、${！VAR}变量间接引用，也可以用eval echo \\$$VAR

[root@localhost ~]# a=b
[root@localhost ~]# b=12345
[root@localhost ~]# echo ${!a}
12345
[root@localhost ~]# eval echo \$$a
12345

9、${#STR}，返回STR长度

[root@localhost ~]# b=12345
[root@localhost ~]# echo ${#b}
5

10、${STR^^}，把STR中的所有小写字母转换为大写输出(不改变值)

[root@localhost ~]# unset a b
[root@localhost ~]# a="hello Rie Kugimiya"   #钉宫理惠老师，声音很萌很可爱
[root@localhost ~]# echo ${a^^}
HELLO RIE KUGIMIYA
[root@localhost ~]# echo $a
hello Rie Kugimiya

11、${STR,,}，把STR中的所有大写字母转换为小写输出（不改变值）

[root@localhost ~]# echo ${a,,}
hello rie kugimiya
[root@localhost ~]# echo $a
hello Rie Kugimiya

12、${STR:POSITON},从$POSITON位置提取子串

[root@localhost ~]# echo $a
hello Rie Kugimiya
[root@localhost ~]# echo ${a:5}
Rie Kugimiya                        #正向取子串
[root@localhost ~]# echo ${a: -5}   #反向从末尾开始计数$POSITON位置的子串
imiya

13、${STR:POSITON:LENGTH}，从$POSITON位置提取长度为$LENGTH子串
${STR[@]:POSITON:LENGTH}数组的切片

CentOS_7
[root@localhost ~]# echo $a
hello Rie Kugimiya
[root@localhost ~]# echo ${a:5:6}                #从第5个开始向后取6个字符
Rie K
[root@localhost ~]# echo ${a:5:-3}          #从第五个开始去去掉倒数3个字符
Rie Kugim
[root@localhost ~]# echo ${a: -5:-3}        #取倒数5个字符去掉倒数3个字符
im                                    #由于减号是前面的关键字所以POSITON前面得加空格
CentOS_6
[root@localhost ~]# a="daisuki Rie Kugimiya"    #依旧是可爱的钉宫理惠老师
[root@localhost ~]# echo ${a:5:6}
ki Rie
[root@localhost ~]# echo ${a:5:-4}
-bash: -4: substring expression < 0
[root@localhost ~]# echo ${a: -5:-2}
-bash: -2: substring expression < 0
[root@localhost ~]# echo ${a: -5:2}            #从-5位置开始去2个字符长度的子串
im

数组的切片：

[root@localhost ~]# a=(1 2 3 4 5 6 7 8 9)
[root@localhost ~]# echo ${a[@]:2:3}
3 4 5
[root@localhost ~]# echo ${a[@]:2:3}
3 4 5
[root@localhost ~]# echo ${a[@]: -2}
8 9
[root@localhost ~]# echo ${a[@]:2}
3 4 5 6 7 8 9

14、${STR#SUBSTR}，从$STR头查找匹配，删除最短匹配$SUBSTR的子串
${arry#SUBSTR}，对于数组，对每个数组元素单独匹配，执行删除子串操作

[root@localhost ~]# echo ${a}
daisuki Rie Kugimiya
[root@localhost ~]# echo ${a#*R}
ie Kugimiya                            #从左向右匹配*R匹配到daisuki R，删除并返回剩下的
#数组
[root@localhost ~]# b=(daisuki Rie Kugimiya)
[root@localhost ~]# echo ${b[@]}
daisuki Rie Kugimiya
[root@localhost ~]# echo ${b[@]#*i}   #删除了b[0]的dai,b[1]的Ri,b[2]的 Kugi
suki e miya

15、${STR##SUBSTR}，从$STR头部查找匹配，删除最长匹配$SUBSTR的子串

[root@localhost ~]# echo $a                    #前面很详细了，后面就不解释了
daisuki Rie Kugimiya
[root@localhost ~]# echo ${a##*i}
ya
[root@localhost ~]# echo ${b[@]##*i}
e ya

16、${STR%SUBSTR}，从$STR尾部查找匹配，删除最短匹配$SUBSTR的子串

[root@localhost ~]# echo ${a%i*}
daisuki Rie Kugim
[root@localhost ~]# echo ${b[@]%i*}
daisuk R Kugim

17、${STR%%SUBSTR}，从$STR尾部查找匹配，删除最长匹配$SUBSTR的子串

[root@localhost ~]# echo ${a%%i*}
da
[root@localhost ~]# echo ${b[@]%%i*}
da R Kug

18、${STR/SUBSTR/REPLACE}，使用$REPALCE替换$STR第一个匹配的$SUBSTR

[root@localhost ~]# echo ${a/i/%%/}
da%%/suki Rie Kugimiya
[root@localhost ~]# echo ${b[@]/i/%%/}
da%%/suki R%%/e Kug%%/miya

19、${STR//SUBSTR/REPLACE}，使用$REPALCE替换$STR中所有匹配的$SUBSTR

[root@localhost ~]# echo ${a//i/%%/}
da%%/suk%%/ R%%/e Kug%%/m%%/ya
[root@localhost ~]# echo ${b[@]//i/%%/}
da%%/suk%%/ R%%/e Kug%%/m%%/ya

20、${STR/#SUBSTR/REPLACE}，$STR以$STR开头的，则用$REPLACE来替换匹配到的$SUBSTR

[root@localhost ~]# echo ${a/#su/%%}
daisuki Rie Kugimiya
[root@localhost ~]# echo ${a/#dai/%%}
%%suki Rie Kugimiya
[root@localhost ~]# echo ${b[@]/#K/@}
daisuki Rie @ugimiya
[root@localhost ~]# echo ${b[@]/#K/$$}
daisuki Rie 1723ugimiya
[root@localhost ~]# echo ${b[@]/#K/$#}
daisuki Rie 0ugimiya
[root@localhost ~]# echo ${b[@]/#K/666}
daisuki Rie 666ugimiya

21、${STR/%SUBSTR/REPLACE}，$STR以$STR结尾的，则用$REPLACE来替换匹配到的$SUBSTR

[root@localhost ~]# echo ${a/%dai/##}
daisuki Rie Kugimiya
[root@localhost ~]# echo ${a/%miya/##}
daisuki Rie Kugi##
[root@localhost ~]# echo ${b[@]/%ie/666}
daisuki R666 Kugimiya
[root@localhost ~]# echo ${b[@]/%ie/$$}
daisuki R1723 Kugimiya

Bash函数或者脚本接受连续参数到数组

#1、使用shift
typeset -a arry
for i in `seq 0 $(($#-1))`;do
arry[i]=$1
shift
done
#2、使用变量的间接引用
typeset -a arry
for i in `seq 1 $$#`;do
#arry[$((i-1))]=${!i}
arry[$((i-1))]=`eval echo \$$i`
done

参考文献：

Linux系统命令和Shell脚本实践指南

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