Shell tutorial circular statements for, while, until usage

Loop statement :

Bash Shell There are three main ways of circulation in ：for、while and until.

One 、for loop for The way circulation works , It's about serial elements , Put in the specified variable in order , Then repeat the enclosed command area （ stay do and done Between ）, Until all the elements are exhausted .

among , Serialization is a combination of strings , Use each other $IFS The separator defined （ Such as the space character ） separate , These strings are called fields .

for The grammatical structure of is as follows ：

for  Variable  in  Serial 
do
    Carry out orders 
done

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explain ：

That's ok 1, The fields in the serial are iterated into the variables

That's ok 2-4, This will then be repeated do and done The command area between , Until each field in the serial is processed .

flow chart

example 1：

use for The loop is created in the home directory aaa1-aaa10, And then in aaa1-aaa10 establish bbb1-bbb10 The catalog of

#!/bin/bash
for a in {
   1..10}
do
        mkdir /datas/aaa$a
        cd /datas/aaa$a
        for b in {
   1..10}
        do
                mkdir bbb$b
        done
done

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#!/bin/bash
for k in $( seq 1 10 )
do
   mkdir /home/kuangl/aaa${k}
   cd /home/kuangl/aaa${k}
   for l in $( seq 1 10 )
   do
   mkdir bbb${l}
   cd /home/kuangl/aaa${k}
   done
   cd ..
done

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explain ：

That's ok 2,seq Used to generate all integers from one number to another .

That's ok 4, Create a folder in your home directory .

That's ok 6, Using a for Loop to create a folder

example 2

List var The size of the disk space occupied by the subdirectories under the directory .

#!/bin/bash
DIR="/var"
cd $DIR
for k in $(ls $DIR)
do
  [ -d $k ] && du -sh $k
done

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explain ：

That's ok 4, Yes /var Every file in the directory , Conduct for Loop processing .

That's ok 6, If /var The file below is the directory , Then use du -sh Calculate the amount of disk space occupied by the directory .

Two 、while loop while Circular grammar ：

1 while  Conditions for testing 
2 do
3    Carry out orders 
4 done

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explain :

That's ok 1, First, do the conditional test , If the return value is 0（ The conditional test is true ）, Then enter the cycle , Execute command area , otherwise

Don't go into the cycle , Introduce while command

That's ok 3, Execute command area , In these orders , There should be a command to change the conditional test , such , To have a chance to be in

Finish execution after limited steps while loop （ Unless you want to execute an infinite loop ）.

That's ok 4, Back to the line 1, perform while command

flow chart :

example 1

while loop , The classic usage is with turn input , Read the contents of the file , Doing this ：

 #!/bin/bash
while read a
do
        echo $a
done < /datas/6files

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1 #!/bin/bash
2 while read kuangl
3 do
4   echo ${kuangl}
5 done < /home/kuangl/scripts/testfile

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explain ：

That's ok 2, Use read There are standard inputs to read data , Put variables kuangl in , If the data read is not empty , Go into the cycle .

That's ok 4, Show the data of changing profession

That's ok 5, take /home/kuangl/scripts/testfile The content steering input will give read Read .

example 2

#!/bin/bash
declare -i i=1
declare -i sum=0
while ((i<=10))
do
　　let sum+=i
　　let i++
done
echo $sum

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#!/bin/bash
declare -i i=1
declare -i sum=0
while ((i<=10))
do
  let sum+=i
  let ++i
done
echo $sum

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explain ：

That's ok 2-3, Statement i and sum It's an integer

That's ok 4, If the condition test ： as long as i The value is less than or equal to 10, Just execute the loop .

That's ok 6,sum+=i and sum=sum+i It's the same ,sum Add up i.

That's ok 7,i Increasing the value of the 1, This line is the command to change the conditional test , once i Greater than 10, Can terminate the cycle .

That's ok 8, encounter done, Back to the line 6 To perform conditional tests

That's ok 9, Show sum The value of is 55

example 3 while99 Multiplication table

#!/bin/bash
a=1
b=1
while ((a <=9))
do
        while ((b<=a))
        do
                let "c=a*b"   # Declare variables c
                echo -n  "$a*$b=$c "
                let b++
        done
        let a++
        let b=1  # Because every multiplication table is 1 Start to multiply , therefore b To reset 
        echo "" # Display to screen wrap 
done

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explain ：

That's ok 8, Declare variables c

That's ok 9,echo The format of the output display ,-n Don't wrap output

That's ok 13,let b=1 Because every multiplication table is 1 Start to multiply , therefore b To reset

3、 ... and 、until loop while The conditional test of the loop is the true value ,until A loop is a false value .

until Circular grammar ：

1 until  Conditions for testing 
2 do
3   Carry out orders 
4 done

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explain ： That's ok 1, If the conditional test result is false （ The return value is not 0）, Go into the cycle .

That's ok 3, Execute command area . In these orders , There should be a command to change the conditional test , It looks like , Only then has the opportunity to finish the execution after the limited steps until loop （ Unless you want to execute an infinite loop ）.

That's ok 4, Back to the line 1, perform until command .

flow chart ：

example 1

1 #!/bin/bash
2 declare -i i=10
3 declare -i sum=0
4 until ((i>10))
5 do
6   let sum+=i
7   let ++i
8 done
9 echo $sum

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explain ： That's ok 2-3, Statement i and sum It's an integer

That's ok 4, If the condition test ： as long as i The value does not exceed 10, Go into the cycle .

That's ok 6,sum+=i and sum=sum+i It's the same ,sum Add up i.

That's ok 7,i Increasing the value of the 1, This line is the command to change the conditional test , once i Greater than 10, Can terminate the cycle .

That's ok 8, encounter done, Back to the line 6 To perform conditional tests

That's ok 9, Show sum The value of is 10

example 2 until99 Multiplication table

#!/bin/bash
a=1
b=1
until ((a>9)) #until  and while contrary , Conditional execution , The conditions are really done end 
do
        until ((b>a))
        do
                let "c=a*b"
                echo -n "$a*$b=$c "
                let b++
        done
        let a++
        let b=1
        echo ""
done

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explain ：

That's ok 4, If the condition test ： as long as a The value does not exceed 9, Go into the cycle , Once you surpass 9 No execution ,until and while Opposite conditions , The conditions are really done end

That's ok 6,b>a, once b Greater than a I'm not going to implement it

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