Linux self-Study _shell variable Chapter

Introduction

3000 block of wages and 30000 wages, the gap lies in the latter has the accumulation of life experience, the summary of technical knowledge is also a life experience.

1) What is a shell variable?

A variable is a simple character to replace a complex character.

2) How many shell variables are there?

Environment variables and local variables

3) How are variables defined and referenced?

Variable name = variable Content # content can be path, command, string, number, etc.

Variable reference $ variable name # such as Cheng=30

# echo $cheng #结果就是30

4) Special variables (the system has pre-defined characters that represent fixed content)

The name of the shell script

$n the nth parameter of a shell script

$* [email protected] All the parameters of the shell script (slightly different from the data analysis can be checked)

$# the number of arguments to the shell script

Special variables that are not commonly used

$! The process number of the last script running in the background

$$ the process number of the script

$? The return value of the previous command or script in the shell script (what is the return value? can be used as a non-existent, only when the wrong time)

$_ the last parameter of the script

5) The substring of the variable (sometimes we need to get a part of the variable, at which time the variable quantum string is the fastest way to calculate)

Such as

Cheng=ruofei

echo ${#cheng} #计算变量的字符个数

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Echo ${cheng:2} #显示第2个字符以后的字符

Eng

6) Calculation of variables

When the content of a variable is a number, it involves the numerical calculation of the variable, which we use (()) to automatically calculate

Such as

Cheng=30

Zhang=cheng+1

Echo $zhang

Cheng+1

Zhang= ((cheng+1))

Echo $zhang

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Linux self-Study _shell variable Chapter