Bash variable -- user defined variable

1、 What is a variable

Variables are units of computer memory , The stored values can be changed . When shell When a script needs to save some information , Such as a file name or a number , Just store it in a variable . Each variable has a name , So it's easy to quote it . Using variables can save useful information , Let the system know the user's relevant settings , Variables can also be used to hold temporary information .

2、 Variable setting rules

Variable names can be alphabetized 、 Numbers and underscores , But it can't start with a number . If the variable name is “2name“ It's wrong .

stay bash in , The default type of variables is string , If you want to do a numerical operation , You must revise the specified variable type to numeric .

Connecting values with equal signs for variables , There can be no spaces on the left and right sides of the equal sign .

Value of variable if there is a space , Single or double quotes are required to include .

In the value of the variable , You can use the escape character .

If you need to increase the value of a variable , Then you can stack the variable values . But variables need to be enclosed in double quotes “$ Variable name “ Or use ${ Variable name } contain .

If the result of the command is given to the variable as the value of the variable , You need to use backquotes or $() Include command .

Environment variable name is recommended to be capitalized , Easy to distinguish .

3、 Variable classification

User defined variables

environment variable ： This kind of variable mainly stores the data related to the operating environment of the system . The variable name part is fixed , But users can customize environment variables

Position parameter variable ： This variable is mainly used to pass parameters or data to the script , Variable name cannot be customized , Variable action is fixed .

Predefined variables ： yes bash Variables already defined in , Variable name cannot be customized , Variable action is also fixed .

4、 The local variable （ User defined variables ）

Variable definitions ----- #name="shen chao"

Variable superposition ----- #aa=123;bb=456

#aa="$aa"$bb --------aa After superposition of variable values echo by 123456

Variable call ---- #echo $name

Variable view ---- #set

Variable deletion ---- #unset name