Shell variables, system predefined variables $home, $pwd, $shell, $user, custom variables, special variables $n, $, $*, [email protected],

1. Variable

1.1 System predefined variables

1.1.1 Common system variables

$HOME、$PWD、$SHELL、$USER  etc. 

1.1.2 Look at all the global variable values

[[email protected] scripts]# env

If you want to see it conveniently, you can add | less

[[email protected] scripts]# printenv

[[email protected] scripts]# printenv USER

There is no need to add $ Symbol , He can print all or part shell Environmental Science

[[email protected] scripts]# set

Look at all variables, including global variables and custom variables, local variables

1.2 Custom variable

1.2.1 Basic grammar

1） Defining variables ： Variable name = A variable's value , Be careful ,= There must be no spaces before or after the sign
2） Revoke variables ：unset Variable name
3） Declare static variables ：readonly Variable , Be careful ： You can't unset

1.2.2 Variable definition rules

1） Variable names can be alphabetized 、 Numbers and underscores , But it can't start with a number , Environment variable name is recommended to be capitalized .
2） No spaces on both sides of equal sign
3） stay bash in , Variable default type is string type , No direct numerical operation .
4） Value of variable if there is a space , Double or single quotes are required .

1.2.3 Case study

1.2.3.1 Defining variables a

[[email protected] scripts]# a=2

1.2.3.2 View undefined sssshell

[[email protected] scripts]# echo $sssshell

1.2.3.3“=” There are spaces before and after the sign

1.2.3.4 Change the value of the variable

1.2.3.5 When there are spaces in the content to be assigned, you need to add single quotation marks or double quotation marks

1.2.3.6 Local and global variables

env You can only check the global variables set by the system , and set You can view all defined variables , At this stage, I can't completely know ssshell Is it a local variable or a global variable? Go on

Open a sub shell

explain ssshell It's a local variable

1.2.3.7 take ssshell Define as global variable

When we need to be in the son shell Use in ssshell variable , We need to ssshell Define as global variable , Definition ssshell When it becomes a global variable, you need to create ssshell The father of shell change , Not in other places shell change

1.2.3.8 In the child shell Change parent in shell Global variables are only in children shell It works , Back to the father shell It won't change

1.2.3.9 use vim Output the difference between global variables and local variables

stay hello.sh Add local variables to sssshell And then execute hello.sh As the result of the

If the absolute path is used, there is no hello,linux This one is , because sssshell It's a local variable

hold sssshell When the variable becomes a global variable, the result is

Both show

1.2.3.10 stay set Look at custom variables

Defined a Variable , stay set You can find

1.2.3.11 Declare static variables

[[email protected] ~]# readonly b=2

Both static and local variables are set Inside

1.2.3.12 Revoke variables a

[[email protected] ~]# unset a

set There's no a The variable

1.2.3.13 Static variables cannot be undone

1.2.3.14 stay bash in , Variable default type is string type , No direct numerical operation

1.2.3.15 If you need to use , Need to use shell Expression of operators in

[[email protected] ~]# a=$((1+4))

[[email protected] ~]# a=$[5+2]

1.3 Special variables

1.3.1$n

1.3.1.1 Basic grammar

$n （ Function description ：n  Is the number ,$0  Represents the script name ,$1-$9  Represents the first to ninth parameters ,
 More than ten parameters , More than ten parameters need to be enclosed in braces , Such as ${10}）

1.3.1.2 Case practice

1.3.1.2.1 stay vim Inside "" and ’' The difference between

 Enclose with double quotation marks $n And enclose with single quotation marks $n
 Double quotation mark expanding $n Code variables 
 No matter what single quotation marks surround, the output is intact 

1.3.1.2.2$n Use

If you use the absolute path, you need to give parameter.sh Plus Execution Authority

1.3.2$#

1.3.2.1 Basic grammar

$# （ Function description ： Get the number of all input parameters ,
 Commonly used in cycles , Judge whether the number of parameters is correct and   Enhance the robustness of scripts ）.

1.3.2.2$#

1.3.3 $*、[email protected]

1.3.3.1 Basic grammar

$* （ Function description ： This variable represents all the parameters on the command line ,$* Treat all parameters as a whole ） 
[email protected] （ Function description ： This variable also represents all the parameters on the command line , however [email protected] Treat each parameter differently ）

1.3.3.2$*[email protected] Use

1.3.3$？

1.3.3.1 Basic grammar

$？ （ Function description ： Return status of last executed command . If the value of this variable is  0, Prove the previous   A command is executed correctly ;
 If the value of this variable is not  0（ Which number is it , It's up to the order itself ）, Then prove   The last command was executed incorrectly .）

1.3.3.2 Judge parameter.sh Whether the script is executed correctly