It is said that the operation and maintenance of shell scripts are paid tens of thousands of yuan a month!!!

Preface

        Shell It's a use. C A program written in a language , It is used by users Linux The bridge .Shell It's a command language , Another programming language .
        Shell It's an application , This application provides an interface , Users access the services of the operating system kernel through this interface .
        Ken Thompson Of sh It's the first one Unix Shell,Windows Explorer It's a typical graphic interface Shell.

One 、Shell Variable

 The rules ：
·  Only English letters can be used for naming 、 Numbers and underscores , The first character cannot begin with a number .
·  No spaces in between , You can use underscores （_）
·  You can't use punctuation 
·  Out of commission bash Keywords in （ You can use help Command to view reserved keywords ）

1、 Custom variable

         Define the format of the variable ：
                Variable name = A variable's value

         Unformatted variable ：
                unset Variable name
matters needing attention ：
        √ = There must be no spaces on either side , Do not use keywords as variable names , Such as ：ls 、 cd etc. ;
        √ If the variable name already exists, overwrite the previous variable value
        √ Variable names are ： Letter / Numbers / Underline , Don't start with numbers

2、 System preset variables

① environment variable （ Variable names are usually capitalized , Operating system maintenance ）

      Stored in /etc/profile or ~/.bash_profile
      Common environment variables are ：PATH \ PWD \ USER \ UID \ HOME \ SHELL

② Positional variable （bash Built-in variables , Store parameters during script execution ）

      Store parameters during script execution
      Use $n Express ,n Is a numeric serial number
      Create a system account through the location variable , Configure a password

[[email protected] ~]# vim /root/user.sh
#!/bin/bash
useradd "$1"	// Create user 
echo "$2" | passwd --stdin "$1"		// Set the password for the user 
[[email protected] ~]# sh /root/user.sh abc 123

③ Predefined variables （bash Built-in variables , Can be called but cannot be assigned or modified ）

      It is used to save the execution information of the script program
      Use these variables directly , You can't assign values to these variables directly

Variable name	meaning
$0	Name of the current process or script
$$	Of the currently running process PID Number
$?	Return status after command execution ,0 Is normal ,1 Or other values indicate an exception
$#	Number of location variables loaded
$*	The value of all position variables

3、 The difference between quotation marks in variables

① The difference between multiple quotation marks

quotes	difference
Double quotes “”	Allow extension , With $ Reference other variables
Single quotation marks ‘’	Disable extensions , Even if $ It is also regarded as a common symbol
The quotation marks ``	Take the execution output of the command as the variable value ,$() Equivalent to backquotes

② Global variables and local variables

local variable ： The newly defined variable defaults only to the current variable shell Effective in the environment , Can't be in a child shell Use in the environment Global variables ： Any in the system shell Effective in all environments

Two 、Shell sentence

1、 Conditions for testing

① Introduction to comparison operators

The operator	meaning
-eq	be equal to (equal)
-ne	It's not equal to (not equal)
-ge	Greater than or equal to (greater or equal)
-le	Less than or equal to (less or equal)
-gt	Greater than (greater than)
-lt	Less than (less than)

② Introduction to file status operator

The operator	meaning
-e	Determines whether an object exists (exist), If it exists, the result is true
-d	Determine whether the object is a directory (directory), Yes, it is true
-f	Determine whether the object exists as a general file (file), Yes, it is true
-r	Determine whether the object has read permission (read), Yes, it is true
-w	Determine whether the object has writable permissions (write), Yes, it is true
-x	Determine whether the object has executable permissions (excute), Yes, it is true

③ Introduction to control operators

The operator	meaning
;	Separate multiple commands . Do not interfere with each other during execution
&&	Both the previous and subsequent commands were executed successfully , The whole command is right , Otherwise it's a mistake
II	Two commands before and after , Any one is executed successfully , The overall command is correct

2、if Judgment statement

①if Single branch statements

Grammar format ：
if Conditions ;then
       command
fi

example ：

[[email protected] ~]# vim /root/shell/user.sh
#!/bin/bash
read -p " Please enter a user name :" user
read -s -p " Please input a password ：" pass
if [ ! -z "$user"] && [ ! -z "$pass"];then
	useradd "$user"
	echo "$pass" | passwd --stdin	"$user"
fi

②if Two branch statements

if Conditions ;then
       command 1
else
       command 2
fi

③if Multi branch statement

Grammar format ：
if Conditions 1;then
       command 1
elif Conditions 2;then
       command 2
else
       command 3
fi
example ：

[[email protected] ~]# vim /root/shell/number.sh
#!/bin/bash
clear
num=$[RANDOM%10+1]
read -p " Please enter 10 A random integer within ："guess
if [ $guess -gt $num ];then
	echo "Oops, Guess the "
elif [ $guess -lt $num ];then
	echo "Oops, Guess a little "
else
	echo " Congratulations , Guessed it , Namely :$num"
fi

3、while loop

√ Test conditions repeatedly , As long as it is established, execute the order

Grammar format ：
while Conditions
do
       command
done

example ：

[[email protected] ~]# vim /root/while.sh
#!/bin/bash
i=1
while [ $i -le 5 ]
do
	echo $i
done

4、for loop

effect ： Reduce code redundancy and duplication

Grammar format 1：
for Variable in List of values
do
       command
done

Grammar format 2：
for （ initial value ; Conditions ; step ）
do
       Command sequence
done

example ：

[[email protected] ~]# vim /root/for.sh
for i in 1 2 3 4 5
do
	echo "this is a number $i"
done