bash自带的切片操作的实现

echo ${#a} 	# $a中字符串的个数
echo ${a:3} 	# $a中从第三个开始取直到结束
echo ${a:3:2}	# $a中从第三个开始去两个字符
echo ${a: -1}	# 取最后一个字符
echo ${a#*.} 	# 从左边开始删除直到遇到.为止
echo ${a##*.} 	# 从左边开始删除直到遇到最后一个.为止
echo ${a%.*}	# 从右面开始删除直到遇到.为止
echo ${a%%.*}	# 从右面开始删除直到遇到最后一个.为止
echo ${a//abc/xyz}	# 将变量a中的所有的abc替换成xyz
echo ${1#--check=}  #删除第1个位置变量里的--check=字符串
echo ${1##[-+]}   #删除第1个位置变量的内容开头的-或者+

[root@rocky ~]# str="pythoniloveyou"
[root@rocky ~]# echo $str
pythoniloveyou
[root@rocky ~]# echo $str|cut -c 1-6
python
[root@rocky ~]# echo $str|cut -c 1,6
pn
[root@rocky ~]# echo $str|cut -c 1,6-9
pnilo

[root@rocky ~]# echo ${#str}
14
[root@rocky ~]# echo $str|wc -L
14

[root@rocky ~]# echo ${str:0:6}
python
[root@rocky ~]# echo ${str:7:4}
love
[root@rocky ~]# echo ${str: -3}
you

[root@rocky ~]# bigcity="beijing.shanghai.guangzhou.shenzhen.changsha"
[root@rocky ~]# echo ${bigcity#*.}
shanghai.guangzhou.shenzhen.changsha
[root@rocky ~]# echo ${bigcity##*.}
changsha

[root@rocky ~]# echo ${bigcity%.*}
beijing.shanghai.guangzhou.shenzhen
[root@rocky ~]# echo ${bigcity%%.*}
beijing

[root@rocky shell]# vim slice.sh
echo ${1##[-+]}

[root@rocky shell]# bash slice.sh abc+def-ghi+jkl-mno
abc+def-ghi+jkl-mno
[root@rocky shell]# bash slice.sh +abc+def-ghi+jkl-mno
abc+def-ghi+jkl-mno
[root@rocky shell]# bash slice.sh -abc+def-ghi+jkl-mno
abc+def-ghi+jkl-mno

[root@rocky shell]# cat slice.sh 
echo ${1##[+-]}
echo ${1#--check=}
[root@rocky shell]# bash slice.sh --check=root
-check=root
root

[root@rocky shell]# nicelevel=100
[root@rocky shell]# echo ${nicelevel:-}
100

[root@rocky shell]# echo ${nicelevel1:-}

[root@rocky shell]# echo ${nicelevel1:-0}
0

[root@rocky shell]# num="123123456123"
[root@rocky shell]# echo ${num/123/abc}
abc123456123
[root@rocky shell]# echo ${num//123/abc}
abcabc456abc