Linux variable operator "${}"

Table of contents

1. What is "${}":

2. Specific usage:

 1. Take the length:

2. Intercept the string

Example one:

Example two:

 Example three:

 3. Replace characters

Single replace:

 Example:

Full replacement:

 Example: 

Four: Extended utilization

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1. What is "${}":

"${}" is a function to process variables, it can take length, intercept, replace and other operations on variable values

2. Specific usage:

The variable we use for the experiment is the environment variable $PATH

The content of $PATH is as follows

 1. Take the length:

Order :

echo ${#PATH}  

 We can see that the length of the $PATH variable is 142

2. Intercept the string

The format is:

${PATH:start:length}  

• The first position of the string is 0
• The start parameter can not be added, the default is 0
• You can add the '~' symbol before the start parameter, which means starting from the end

Example one:

        Order :

echo ${PATH:0:1} 
从第0位开始，截取一个字符

         result: 

Example two:

        Order :

echo ${PATH::1} 
这次没加start参数，默认从0位开始

         result: 

 Example three:

        Order :

echo ${PATH: ~0:1}
从导数第0位返回1个字符

         result: 

 3. Replace characters

Single replace:

${file/a/b}
将字符串中第一个a替换为b

 Example:

        Order :

echo ${PATH/usr/root}

         result:

 We can see that the first "usr" has changed to "root"

Full replacement:

${file//a/b}
将字符串所有的a替换为b

 Example: 

        Order :

echo ${PATH//usr/root}

        result:

 We can see that all "usr" in the string becomes "root"

Four: Extended utilization

This function can be used for splicing instructions in RCE vulnerabilities . I will talk about the specific method in the next article.