Loop statement

One ,for Loop statement structure

• In practice , There are often situations where a task needs to be performed many times , And each time it is executed, it is only processed Different objects , The other commands are the same

• for example , Create a system account according to the name list in the address book , Clear according to the server Check the survival status of each host , according to IP Address blacklist settings, access denied firewall policies, etc

• When repeating tasks in the face of various lists , Easy to use if Statement has been difficult to meet the requirements , Writing all the code in sequence is extremely cumbersome 、 The difficulties . And in different scenarios, use different loop statements , It can solve similar problems well

for Structure of statement

• Read different values of variables , Used to execute the same set of commands one by one ,
• for There were , List loop , Loop without list , class C Style for loop

1.1 Format ：

for  Variable name  in  Value list 
do
	 Command sequence 
done

1.2 example ：

List loop :

 List loop :
1. example ：
#!/bin/bash
a=3                    	// Defining variables 
for i in `seq $a`      //seq You can refer to variables 
do
    echo $i
done
[[email protected] opt]# sh 1.sh
1
2
3

2. example ：
#!/bin/bash
for i in a b c       //i There is actually no call here , So it's equivalent to in  The following parameters will be cycled several times 
do
  echo 123
done
[[email protected] opt]# sh 1.sh
123
123
123

3. example ：
#!/bin/bash
for i in a b c     // Used here i Variable , So it displays normally i Value  （a b c）  
do
  echo  $i
done
[[email protected] opt]# sh 1.sh
a
b
c

4. example ： Print 3 Time hello world, Although variables are defined i, But I didn't use it , It just controls the number of cycles 
#!/bin/bash
for i in {
    1..3}    or  `seq 5`  or   Corresponding numbers or letters （a b c,1 2 3 ）
do
  echo "hello world"
done

Summary ：

for In circulation for i in {list} Usage mode

i Name the custom variable

in First of all, will in What follows , Define as a list Then loop through the values of the elements in the list , And assign it to the variable after each value i ( Custom variable name )

{list} list ：① Define the number of cycles ② Every element , It will be taken out in turn at each cycle , Then copy it to the variable name

With do For division

command ： Define each cycle , Instructions to output

done ： end

Loop without list :

#!/bin/bash
for i
do
  echo "hello"
done 
[[email protected] opt]# sh 1.sh // No parameters were passed to the script, so there was no result 
[[email protected] opt]# sh 1.sh a // hold a Assigned to i,i With the value, it starts to execute do..done 了 
hello

 The second kind :
#!/bin/bash
for i
do
  echo "$i"       // Change the constant display to , Variable display 
done 
[[email protected] opt]# sh 1.sh 
[[email protected] opt]# sh 1.sh world
world

class C Style for loop :

 Format :
for ((expr1;expr2;expr3))
do
     command
done
expr1： Define variables and assign initial values                                      i=1
expr2： Decide whether to cycle or not （ Define the range of values , The purpose is to define the number of cycles ）      i<=3
expr3： Decide how the loop variable changes , Decide when the loop will exit （ Custom step size ）  i++ or i=+2

1. example ： Print 1-5
 iteration 
#!/bin/bash
for ((i=1;i<=3;i++))
do
  echo "$i"
done
[[email protected] opt] sh 1.sh 
1
2
3

2. example ： Print 1-10 The odd number 
#!/bin/bash
for ((i=1;i<=10;i+=2))
do
  echo "$i"
done
2. example ： Calculation 1-100 Odd and 
#!/bin/bash
sum=0
for ((i=1;i<=100;i+=2))
do
  let sum=$i+$sum
done
  echo "$sum"

Two ,while Statement loop structure

• while Loops are generally used for conditional judgment loops , If the judgment condition is true , Then enter the cycle , When the condition is false, jump out of the loop
• In short ： Test a condition repeatedly , Repeat as long as the conditions are met

while Writing ：
1. Grammatical structure

2. Dead cycle

2.1 Format ：

while  Conditional test operation 
do
	 Command sequence 
done

2.2 example ：

1. example ： Print 1-5
#!/bin/bash
i=1                          // Defining variables i=1
while [ $i -le 5 ]           // When i be equal to 5 Stop the cycle when 
do    
  echo "$i"                  // Output per cycle i Value 
  let i++                    // Each cycle +1
done                        // Cycle to greater than or equal 5 End cycle at 
 echo " Last i The value of is ： $i"     // Output last $i Value 
[[email protected] while]# sh 1.sh
1
2
3
4
5
 Last i The value of is ： 6

2. example ： Check the cycle several times based on the previous condition 
#!/bin/bash
i=1
sum=0
while [ $i -le 5 ]
do
  echo "$i"
  let i+=2
  let sum++
 echo " Last i The value of is $i, Circulated $sum  Time  "
done
[[email protected] while]# sh 1.sh
1
 Last i The value of is  3, Circulated  1  Time  
3
 Last i The value of is  5, Circulated  2  Time  
5
 Last i The value of is  7, Circulated  3  Time  

2.2 Loop control statement

Loops are generally executed together with conditional judgment statements and process control statements , Then it will be necessary to skip the loop and stop the loop , There are three kinds of ：

• exit： Support execution exited , The process executed by the current script , So you can end the script , You can define the return value to determine where the script went wrong （ Jump out of the program , It can be followed by the status return code , Jump out of the program directly, so the final... Will not be executed echo hi , And the return code is 100 adopt $? see ）

• break： End the current script loop , It will not affect other loops written under the script （ interrupt , Stop this cycle immediately , Execute the extracorporeal code ）

Print 1-10 The number of ,7 The back ones don't print

• continue： sign out （ At present ） This small cycle （ Nested inside ）, Until you jump to the next cycle , If the conditions are true, the cycle will continue （ continue , But I won't execute the following code in the loop body , Start and restart the next cycle ）

Print 1-5 The number of ,3 No printing , The results will be 1245,3 No output , Because jump out of the back echo Statement executes the next loop

while Dead cycle

i=1                       ## Defining variables  
while true [ $1 -eq 2 ]   ## Used true  Whether the judgment condition is true or not , Will execute in a loop 
do
  echo " Dead cycle "
 # exit ## If you add cycle to exit  Exit the current loop directly 
done

2. Example guessing numbers , Guess wrong and keep guessing 
#!/bin/bash
num=10
while true
do
read -p " Please enter a number ：" zi
 if [ $zi -eq $num ];then
     echo " You guessed it "
     break 1                 ## It means you guessed right and quit the current cycle , Does not affect other nested loops 
elif [ $zi -gt $num ];then
     echo " You guessed big "
elif [ $zi -lt $num ];then
     echo " You guess it's small "
fi
done

[[email protected] home]# vim while2.sh 

#!/bin/bash
i=1						// Defining variables i=1
while [ $i -le 100 ]	// Repeat the test $1 Less than or equal to 100, until $i be equal to 100
        do				// The list of commands 
        if [[ $i%3 -ne 0 ]]	// Conditions for testing  $i Remainder 3, Is not equal to 0
        then				// Conditions established 
                echo "$i"	// Output  $i Value 
        fi					// End judgment 
        let i++				// Each cycle i+1
done						// End of cycle 

3、 ... and ,until Loop statement

• Follow while contrary , If the condition is false, enter the loop , If the condition is true, exit the loop
• Test a condition repeatedly , As long as the condition is not established, it will be executed repeatedly

3.1 Format ：

until  Conditional test operation 
do
	 Command sequence 
done

3.2 example ：

[[email protected] home]# vim 50.sh 

#!/bin/bash
i=1				        // Defining variables i=1
sum=0			
until [ $i -eq 51 ]		//$i be equal to 51 Stop execution 
do	
        sum=$[$i+$sum]	// Definition sum be equal to $i + $sum
        let i++			// Each cycle i+1	

done
echo "$sum				// Print the results  $sum
[[email protected] home]# bash 50.sh 
1275

until Dead cycle

Four ,shell function

• Write command sequences together in format
• It is convenient to reuse the command sequence

4.1 Format ：

# Format 1 ：
[ function ]  Function name (){
    
	 Command sequence 
	[return x]         // Use return or exit You can end the function explicitly 	
}
## Standard writing 
#!/bin/bash
function dxj{
                      // Define the calling function name 
 cat /opt/name.txt             // The command to call （ Parameters ）
}
dxj                           // Call function name , Use the commands inside （ Parameters ）

[[email protected] ~]# sh 2.sh
1 zhangsan
2 lisi
3 wangwu


# Format two ：
 Function name  (){
    	           
 Command sequence 
}

#!/bin/bash
dxj(){
    
 cat /opt/name.txt
}
dxj
## The most commonly used , Because the most concise 

• After the function is defined, it will not execute automatically , You need to call , The advantage is that you can write a piece of function code as a function , If you need to call the definition directly, it doesn't matter even if there are syntax errors , If you don't call, you won't report an error
  Of course, the ultimate purpose of writing functions is to call , In order to implement a function block

The return value of the function ：

• return Represents the exit function and returns an exit value , You can use $? The variable displays the value

Usage principle ：

• 1. Take the return value as soon as the function ends , because $? Variable returns only the exit status code of the last command executed
• 2. Exit status code must be 0~255, If the value exceeds the value, it will be the remainder 256

4.2 Function call

Directly define the code block of the function in the script and write the function name to complete the call

#!/bin/bash
dxj(){
               // Defines a function called dxj
 echo "this is a dxj"    // The function is to print this is a dxj
}
dxj                 // Writing the function name directly will run the code in the function body 
[[email protected] ~]# sh 2.sh
this is a dxj

Be careful 1： Function name must be unique , If you define a , Use the same name to define , The second will override the function of the first , There are results you don't want , So don't use the same name （ The function name can be consistent, but it can only be executed in different environments ）

1 example ：

[[email protected] ~]# vim 2.sh
#!/bin/bash
xtz (){
    
 echo hello 
}
xtz (){
    
 echo world
}
xtz
[[email protected] ~]# sh 2.sh
world

Be careful 2： The function must be defined before calling

2 example ：

#!/bin/bash
dxj (){
    
 echo hello
}

xtz (){
    
 echo world
}

he (){
    
 echo "$(dxj) $(xtz)"
}
he
[[email protected] ~]# sh 2.sh
hello world

Be careful 3： You don't have to define the function at the beginning of the script , Just call the previously defined

• In case of nested calls in other places , You cannot write function values directly , Avoid being unable to recognize , You can use a reverse apostrophe , It is equivalent to the result of calling the function

3 example ：

#!/bin/bash
dxj (){
    
 echo hello
}

xtz (){
    
 echo world
}

he (){
    
 echo "`dxj` `xtz`"    // If it's directly dxj and xtz Words , It won't print out hello world And it will be dxj xtz
}
he
[[email protected] ~]# sh 2.sh
hello world

4 example ： Deploy local yum Source

[[email protected] ~]# vim yum.sh 

#!/bin/bash
backuprepo (){
    				// Function name 
cd /etc/yum.repos.d			
mkdir repo,bak
mv *.repo repo.bak
mount /dev/sr0 /mnt &>/dev/null
}

makelocalrepo (){
    
echo
[local]
name=local
baseurl=file:///mnt
enable=1
gpgcheck=0 > local.repo
}

userlocalrepo (){
    
yum clean all > /dev/null
yum makecache > /dev/null
yum install -y httpd > /dev/null
}

backuprepo			// call 
makelocalrepo		// call 
userlocalrepo		// call 

4 example ： Deploy local yum Source

[[email protected] ~]# vim yum.sh 

#!/bin/bash
backuprepo (){
    				// Function name 
cd /etc/yum.repos.d			
mkdir repo,bak
mv *.repo repo.bak
mount /dev/sr0 /mnt &>/dev/null
}

makelocalrepo (){
    
echo
[local]
name=local
baseurl=file:///mnt
enable=1
gpgcheck=0 > local.repo
}

userlocalrepo (){
    
yum clean all > /dev/null
yum makecache > /dev/null
yum install -y httpd > /dev/null
}

backuprepo			// call 
makelocalrepo		// call 
userlocalrepo		// call