Circular structure of shell system learning

for Loop statement

With a list for Loop statement

  Without a list for sentence

class C Style for Loop statement

Use for Loop through arrays  

until Loop statement

while Loop statement

break and continue

  Summary

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for Loop statement

With a list for Loop statement

The grammar is as follows ：

for variable in {list}
do
    statement1
    ...
done

In the grammar above ,variable It is called a cyclic variable ,list It's a list , It can be a series of numbers or strings , The elements are separated by spaces ,do and done The structure between them is called circular body .

Here are a few examples ：

#!/bin/bash

for var in 1 2 3 4 5 6
do
 printf "$var"; printf " "
done

echo 

for var in {1..6}
do
 printf "$var"; printf " "
done

echo 

for day in Mon Tue Wed Thu Fri Sat Sun
do
 printf "$day"; printf " "
done

echo 

for day in { Mon Tue Wed Thu Fri Sat Sun }
do
 printf "$day"; printf " "
done

give the result as follows :

[email protected]:~/Documents/Shell/5$ ./5-1.sh 
1 2 3 4 5 6 
1 2 3 4 5 6 
Mon Tue Wed Thu Fri Sat Sun 
{ Mon Tue Wed Thu Fri Sat Sun }

  Note that strings cannot be used {} Curly brackets indicate a list .

In the above example for The steps of the cycle are 1, actually ,Shell Allow the user to set the step size , The grammar is as follows ：

for variable in {start..end..step}
do
    statement1
    ...
done

start Indicates the starting value ,end Indicates the end value ,step Indicating step size

Let's look at an example ：

#!/bin/bash

sum=0
for i in {1..100..2}
do
 ((sum+=i))
done
echo "$sum"

give the result as follows ：

[email protected]:~/Documents/Shell/5$ ./5-2.sh 
2500

for The sentence is list It can also be Shell The result of the command output , such as ls The command will output a list

  At present ls The command output of is as follows ：

[email protected]:~/Documents/Shell/5$ ls
5-1.sh  5-2.sh  5-3.sh

Shell The code is as follows ： 

#!/bin/bash
for file in `ls`
do 
 echo "$file"
done

give the result as follows ：

[email protected]:~/Documents/Shell/5$ ./5-3.sh 
5-1.sh
5-2.sh
5-3.sh

Also use wildcards * As a condition list , This * The function of number is to list all files in the current directory , The code is as follows ：

#!/bin/bash
for file in *
do 
 echo "$file"
done

  give the result as follows ： 

[email protected]:~/Documents/Shell/5$ ./5-4.sh 
5-1.sh
5-2.sh
5-3.sh
5-4.sh

  Without a list for sentence

In some special cases ,for The condition list of the loop can be completely omitted , It is called without list for Loop statement . here ,Shell A list of conditions will be obtained from the command line , Take a look at the sample code

 

#!/bin/bash
for arg
do
 echo "$arg"
done

We output parameters a b c, give the result as follows ：

[email protected]:~/Documents/Shell/5$ ./5-5.sh a b c
a
b
c

  And use $* The result of doing the condition parameter is consistent .

class C Style for Loop statement

stay Linux On ,C/C++ Is the most mainstream development language , In order to adapt to the habits of these users ,bash shell Classes are also provided C Style for loop , The grammar is as follows ：

for ((expression1;expression2;expression3))
do
    statement1
    ...
done

Illustrate with examples

#!/bin/bash
for ((i=1;i<5;++i))
do
 echo "$i"
done

give the result as follows ：

[email protected]:~/Documents/Shell/5$ ./5-6.sh 
1
2
3
4

Use for Loop through arrays  

The grammar is as follows ：

for var in ${array[*]}
do
 statement1
 ...
done

  among array Represents the name of the array , See the example ：

#!/bin/bash

alist=(Mon Tue Med)
for day in ${alist[*]}
do
 echo "$day"
done

give the result as follows ：

[email protected]:~/Documents/Shell/5$ ./5-7.sh 
Mon
Tue
Med

until Loop statement

until The syntax of the sentence is ：

until expression
do
 statement1
 ...
done

Among them expression It's true return 0 when , Jump out of the loop .

while Loop statement

while The syntax of a sentence is related to until almost ：

while expression
do
 statement1
 ...
done

The difference lies in , while The statement is when expression Conditions For false Return non 0 when , Jump out of the loop . And until Just the opposite

break and continue

Shell The statement provides break and continue Used to control the cycle .

break Just jump out of the loop ,continue Is to skip the current loop body continue Subsequent statements , Execute again from the beginning of the loop statement . Here you are C If you know the language experience, you won't talk much . add ,Shell Of break and continue Numerical parameters can be added later , The grammar is as follows ：

break n
continue n

break n To jump out of n Layer cycle ,continue n To skip n Layer cycle , Feel for example ： 

#!/bin/bash

for i in {1..6}
do
    for j in {1..6}
    do
        if [ &j -eq 3 ]
        then
            break 2
        fi
        
        echo "$i $j"
    done
done

give the result as follows :

[email protected]:~/Documents/Shell/5$ ./5-8.sh 
1 1
1 2

  Summary

This article mainly talks about Shell The cycle structure of , Include for,until,while, And control loop statements break and continue Usage of