wenchy/shell_cheatsheet.md

## shell_cheatsheet.md

      
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    shell变量的子串的删除/替换

${#string}
返回$string的长度
${string:position}
在$string中，从$position位置之后开始提取子串
${string:position:length}
在$string中，从$position位置之后开始提取$length长度的子串
[root@localhost SHELL]# NAME="ni hao, ming tian"
[root@localhost SHELL]# echo $NAME
ni hao, ming tian
[root@localhost SHELL]# echo ${NAME}
ni hao, ming tian
[root@localhost SHELL]# echo ${#NAME}
17
[root@localhost SHELL]# echo ${NAME:3}
hao, ming tian
[root@localhost SHELL]# echo ${NAME:3:6}
hao, m
[root@localhost SHELL]# 
${string#substring}
从变量$string开头开始删除最短匹配$substring子串
${string##substring}
从变量$string开头开始删除最长匹配$sunstring子串
[root@localhost SHELL]# echo $NAME
abcABC123ABCabcde
[root@localhost SHELL]# echo ${NAME#a*C}
123ABCabcde
[root@localhost SHELL]# echo ${NAME##a*C}
abcde
[root@localhost SHELL]# 
${string%substring}
从变量$string结尾开始删除最短匹配$substring子串
${string%%substring}
从变量$string结尾开始删除最长匹配$substring子串
[root@localhost SHELL]# NAME=abcABC123ABCabc
[root@localhost SHELL]# echo $NAME
abcABC123ABCabc
[root@localhost SHELL]# echo ${NAME%%a*c}

[root@localhost SHELL]# echo ${NAME%a*c}
abcABC123ABC
[root@localhost SHELL]# echo ${NAME#b*c}
abcABC123ABCabc
[root@localhost SHELL]# echo ${NAME#bc}
abcABC123ABCabc
[root@localhost SHELL]# echo ${NAME%bc}
abcABC123ABCa
[root@localhost SHELL]# echo ${NAME%%bc}
abcABC123ABCa
[root@localhost SHELL]# echo ${NAME%C*c}
abcABC123AB
[root@localhost SHELL]# echo ${NAME%%C*c}
abcAB
[root@localhost SHELL]# echo ${NAME%%C*b}
abcABC123ABCabc
[root@localhost SHELL]# echo ${NAME}
abcABC123ABCabc
[root@localhost SHELL]# 

注意：在进行#或##匹配时，$string的首字符必须是被删除子串$substring的第一个字符，不然无法匹配删除；
在进行%或者%%匹配时，$string的最后一个字符必须是被删除子串$substring的最后一个字符，不然无法进行删除操作；

${parameter/parttern/string}
用string来替换第一个匹配的pattern
${parameter/#parttern/string}
从开头匹配parameter变量中的pattern，匹配上后就用string来替换匹配的pattern
${parameter/%pattern/string}
从结尾开始匹配parameter变量中的pattern，匹配上后就用string替换匹配的pattern
${parameter//pattern/string}
用string来替换parameter变量中所有匹配的pattern
[root@localhost SHELL]# NAME="I am a student student"
[root@localhost SHELL]# echo ${NAME//student/teacher}
I am a teacher teacher
[root@localhost SHELL]# echo ${NAME/#student/teacher}
I am a student student
[root@localhost SHELL]# echo ${NAME/%student/teacher}
I am a student teacher
[root@localhost SHELL]# echo ${NAME/%I am /teacher}
I am a student student
[root@localhost SHELL]# echo ${NAME/#I am /teacher}
teachera student student
[root@localhost SHELL]# echo ${NAME/#I am a/teacher}
teacher student student
[root@localhost SHELL]# echo $NAME
I am a student student
[root@localhost SHELL]# 
shell 数组遍历的3种方法

首先创建一个数组

array=(A B C D 1 2 3 4)
IFS=' ' read -a array <<< "A B C D 1 2 3 4"


for

for(( i=0;i<${#array[@]};i++)) do
# ${#array[@]}获取数组长度用于循环
echo ${array[i]};
done;


for in

for i in "${!arr[@]}";   
do   
    printf "%s\t%s\n" "$i" "${arr[$i]}"  
done  


while

i=0  
while [ $i -lt ${#array[@]} ]  
# 当变量（下标）小于数组长度时进入循环体
do  
    echo ${ array[$i] }  
    # 按下标打印数组元素
    let i++  
done