Shell (8) cycle

Shell（8） loop

Preface

Mentioned in the previous chapter if Judge sentences and case Sentences can only be judged once , When we want to automatically execute this script repeatedly many times, we need to use circular statements .

There are three main types of circular statements ：for loop ,while Circulation and until loop .

One 、for loop

At work , 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 , Check the survival status of each host according to the server list .

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 .

for loop , Some people call it conditional circulation , perhaps for i in , In fact, that is for The nature of the cycle , The number of times is proportional to the condition of giving .

Grammatical structure ：（1） List loop （2） Loop without list （3） class C Style for loop

1. List loop

for $ Variable  in {list}    # take list The value in is assigned to the variable in a traversal way 
do
    command           # Use each variable to execute commands , Until it's all done 
done

for i in {1..5}          #{} Instead of identifying variables, it will 1 To 5 Assign to respectively i
do
        echo $i          # The output is 1 2 3 4 5
done
 Or use seq command （seq Variables can be used after the command ）
for i in `seq 5`         #seq 5=1、2、3、4、5 Sequence 
do      
        echo $i          # The output is the same 1 2 3 4 5
done

for i in {1..5}
  do
        echo hello world
  done
# Although we define a variable i, But I didn't use it , It just controls the number of cycles , So it will output 5 All over hello world

# Output 0-50 Even number between 
for i in {0..50..2}      #..2 The representative step size is 2, Said every 2 individual 
do
        echo $i
done
 Or use seq command 
for i in `seq 0 2 50`    # From 0 Start , every other 2 individual , Until 50
do
        echo $i
done

Two 、 Loop without list

When the loop without list is executed, it is determined by the parameters specified by the user and the number of parameters .

for $ Variable  
do
    command
done
 Execute the script + Variable parameters 

example 1： take {list} The parameter in is written in the command line as the number of times

hold a Assign a value to a variable i,i When it has value, it starts to execute do…done 了 .

example 2： take {list} The parameters in are written on the command line as variables

3. class C Style for loop

class C Style representation is similar to C Language , The grammatical structure is ：

for ((i=1;i<=5;i++))     #i from 1 Start ,i Final less than or equal to 5,i Self add every time 1
do
     echo $i             # Output 1 1+1 1+1+1 1+1+1+1 1+1+1+1+1
done
 The execution result is 1 2 3 4 5
 notes ：i++ : i=1+1  Assign first and then calculate  i=1  after   Again  +1
   ++i : 1+1=i  Operation before assignment  1+1  after   Again  =i

class C Style operator usage ：

++	Self variable +1
–	Self variable -1
+=n	Self variable +n
-=n	Self variable -n
*=n	Self variable *n
/=n	Self variable /n
%=n	Self variable %n

example ： To calculate the 10 Odd sum within , And print .

Two 、while loop

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 .

Grammatical structure ：

while  expression 
do
    command
done

example ： Print 1-10 And

while Dead cycle ： When we write an expression that is always true ,while It will go through a cycle without jumping out , because while The criterion for ending the loop is when the result output is false . for example

while [ 1 -eq 1 ]   # When 1 be equal to 1 When 
do 
 echo "1"           # Output 1
done
 Once executed, this script will continue until the process is stopped .

example ： use while Write a number guessing game in a circle

3、 ... and 、until loop

Follow while contrary , If the condition is false, enter the loop , If the condition is true, exit the loop .

Grammatical structure ：

until  expression 
do
    command
done

example ： Show 1-10 The integer of

Four 、 Loop control statement

for Loops are generally executed together with conditional judgment statements and process control statements , Then you need to skip the loop and abort the loop , The commands to control the loop are as follows 3 individual .

1.continue： continue , But the code below this loop will not be executed , Start and restart the next cycle .

2.break： interrupt , Stop this cycle immediately , Execute code outside the loop .

3.exit： Jump out of the program , It can be followed by a status return code, such as exit 1 wait .

summary

for Generally used for fixed number of cycles , When you value the list to 4 When you get a value , Just cycle 4 Time ,5 A value is 5 Time .

while and until Generally used for non fixed number of cycles , As long as the condition test is true , It's been circulating do and done The command .

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