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~]#unseta[root@localhost~]#echo${a:-hello};echo$ahello#$a未赋值输出了空值#可以再脚本用于设置默认值
3、${VAR:=DEFAULT},VAR没有定义或空则为$DEFAULT的值
[root@localhost~]#echo${a:=hello};echoahellohello#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$ayemohello[root@localhost~]#unseta[root@localhost~]#echo${a:+yemo};echo$a
5、${VAR?MSG},VAR未定义则打印$MSG
[root@localhost~]#unseta[root@localhost~]#echo${a?thisisnull}-bash:a:thisisnull[root@localhost~]#typeset-ia[root@localhost~]#echo${a?thisisnull}-bash:a:thisisnull
6、${VAR:?MSG},VAR未定义或者为空则打印$MSG
[root@localhost~]#unseta[root@localhost~]#echo${a?thisisnull}-bash:a:thisisnull[root@localhost~]#typeset-ia[root@localhost~]#echo${a?thisisnull}-bash:a:thisisnull
7、${!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${!a}12345[root@localhost~]#evalecho\$$a12345
9、${#STR},返回STR长度
[root@localhost~]#b=12345[root@localhost~]#echo${#b}5
10、${STR^^},把STR中的所有小写字母转换为大写输出(不改变值)
[root@localhost~]#unsetab[root@localhost~]#a="helloRieKugimiya"#钉宫理惠老师,声音很萌很可爱[root@localhost~]#echo${a^^}HELLORIEKUGIMIYA[root@localhost~]#echo$ahelloRieKugimiya
11、${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${a,,}helloriekugimiya[root@localhost~]#echo$ahelloRieKugimiya
12、${STR:POSITON},从$POSITON位置提取子串
[root@localhost~]#echo$ahelloRieKugimiya[root@localhost~]#echo${a:5}RieKugimiya#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
${STR[@]:POSITON:LENGTH}数组的切片
CentOS_7[root@localhost~]#echo$ahelloRieKugimiya[root@localhost~]#echo${a:5:6}#从第5个开始向后取6个字符RieK[root@localhost~]#echo${a:5:-3}#从第五个开始去去掉倒数3个字符RieKugim[root@localhost~]#echo${a:-5:-3}#取倒数5个字符去掉倒数3个字符im#由于减号是前面的关键字所以POSITON前面得加空格CentOS_6[root@localhost~]#a="daisukiRieKugimiya"#依旧是可爱的钉宫理惠老师[root@localhost~]#echo${a:5:6}kiRie[root@localhost~]#echo${a:5:-4}-bash:-4:substringexpression<0[root@localhost~]#echo${a:-5:-2}-bash:-2:substringexpression<0[root@localhost~]#echo${a:-5:2}#从-5位置开始去2个字符长度的子串im
数组的切片:
[root@localhost~]#a=(123456789)[root@localhost~]#echo${a[@]:2:3}345[root@localhost~]#echo${a[@]:2:3}345[root@localhost~]#echo${a[@]:-2}89[root@localhost~]#echo${a[@]:2}3456789
14、${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}daisukiRieKugimiya[root@localhost~]#echo${a#*R}ieKugimiya#从左向右匹配*R匹配到daisukiR,删除并返回剩下的#数组[root@localhost~]#b=(daisukiRieKugimiya)[root@localhost~]#echo${b[@]}daisukiRieKugimiya[root@localhost~]#echo${b[@]#*i}#删除了b[0]的dai,b[1]的Ri,b[2]的Kugisukiemiya
15、${STR##SUBSTR},从$STR头部查找匹配,删除最长匹配$SUBSTR的子串
[root@localhost~]#echo$a#前面很详细了,后面就不解释了daisukiRieKugimiya[root@localhost~]#echo${a##*i}ya[root@localhost~]#echo${b[@]##*i}eya
16、${STR%SUBSTR},从$STR尾部查找匹配,删除最短匹配$SUBSTR的子串
[root@localhost~]#echo${a%i*}daisukiRieKugim[root@localhost~]#echo${b[@]%i*}daisukRKugim
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${b[@]%%i*}daRKug
18、${STR/SUBSTR/REPLACE},使用$REPALCE替换$STR第一个匹配的$SUBSTR
[root@localhost~]#echo${a/i/%%/}da%%/sukiRieKugimiya[root@localhost~]#echo${b[@]/i/%%/}da%%/sukiR%%/eKug%%/miya
19、${STR//SUBSTR/REPLACE},使用$REPALCE替换$STR中所有匹配的$SUBSTR
[root@localhost~]#echo${a//i/%%/}da%%/suk%%/R%%/eKug%%/m%%/ya[root@localhost~]#echo${b[@]//i/%%/}da%%/suk%%/R%%/eKug%%/m%%/ya
20、${STR/#SUBSTR/REPLACE},$STR以$STR开头的,则用$REPLACE来替换匹配到的$SUBSTR
[root@localhost~]#echo${a/#su/%%}daisukiRieKugimiya[root@localhost~]#echo${a/#dai/%%}%%sukiRieKugimiya[root@localhost~]#echo${b[@]/#K/@}daisukiRie@ugimiya[root@localhost~]#echo${b[@]/#K/$$}daisukiRie1723ugimiya[root@localhost~]#echo${b[@]/#K/$#}daisukiRie0ugimiya[root@localhost~]#echo${b[@]/#K/666}daisukiRie666ugimiya
21、${STR/%SUBSTR/REPLACE},$STR以$STR结尾的,则用$REPLACE来替换匹配到的$SUBSTR
[root@localhost~]#echo${a/%dai/##}daisukiRieKugimiya[root@localhost~]#echo${a/%miya/##}daisukiRieKugi##[root@localhost~]#echo${b[@]/%ie/666}daisukiR666Kugimiya[root@localhost~]#echo${b[@]/%ie/$$}daisukiR1723Kugimiya
Bash函数或者脚本接受连续参数到数组
#1、使用shifttypeset-aarryforiin`seq0$(($#-1))`;doarry[i]=$1shiftdone#2、使用变量的间接引用typeset-aarryforiin`seq1$$#`;do#arry[$((i-1))]=${!i}arry[$((i-1))]=`evalecho\$$i`done
参考文献:
Linux系统命令和Shell脚本实践指南
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