Shell process control - 39. Special process control statements

Catalog

1、 Introduction to special process control statements
2、exit sentence
3、break sentence
4、continue sentence

1、 Introduction to special process control statements

Shell Programs or other programs , It's all sequential , That is, execute the second line after the first line is executed , And so on , Execute sequentially .

Process control statement , Such as ：

if Conditional statements , You only have the right conditions , To be able to execute , Otherwise, the program will not be executed , This will skip some execution commands .
for A loop is also a process control statement , Is to repeat the same code a specified number of times .

In addition to the above conditional judgment statements and loop statements , There are also special process control statements .

such as ：exit sentence 、break sentence 、continue sentence .

2、exit sentence

The system has a exit command , Used to exit the login status of the current user .

But in the Shell Script ,exit Statement is used to exit the current script . in other words , stay Shell Script , As long as I meet exit sentence , Subsequent procedures are no longer executed , And just exit the script .

exit The grammar is as follows ：

exit [ Return value ]

If exit The return value is defined after the statement , The return value after the execution of this script is the return value defined by us . By querying $? This variable , To see the return value .
If exit No return value is defined after the statement , The return value after the script execution is , perform exit The statement before , The return value of the last command executed .

Write a exit Example ：

demand ： Determine whether the input is a pure number .

#!/bin/bash

#  demonstration exit The role of 

#  Receive user input , And assign the input to the variable num

read -p "Please input a number:" -t 30 num

#  If the variable num The value of is a number , Then put num Replace the value of with empty , Otherwise don't replace 

#  Assign the replaced value to the variable y

y=$(echo $num | sed 's/[0-9]//g' )

    #  explain ：

    #  It is through sed command , Put variables num Every character in the value , As long as it is 0-9 The content of , Replace with empty .

    #  Finally, if y The last value of is null , prove num The contents of variables are all numbers , Because they were all replaced .

    #  If y The last value of is not empty , prove num The contents of the variable are non numeric , namely num Impure number .

#  Judgment variable y If the value of is not empty , Output error message ,

#  Exit script , The return value of exit is 18

if [ -n "$y" ]

    then

        echo "Error！Please input a number！"

        exit 18

        #  explain ：

        #  If the input is not a number , The above two script commands will execute ,

        # exit Once the statement is executed, the script will terminate .

else

    #  If you do not exit the script , Then print the variable num Number in 

    echo "The number is：$num"

Execute the script

#  to Shell Scripts give execution permission 

[[email protected] sh]# chmod 755 exit.sh

#  Execute the script 

[[email protected] sh]# ./exit.sh

#  Input abc

please input num: abc

#  Please enter a number when the script returns 

please input number, error!!!!

#  see $? Variable , return 18, As set in our script .

[[email protected] sh]# echo $?

18

#  Execute the script again , Input 123

[[email protected] sh]# ./exit.sh

please input num:123

#  The script returns the input number 

#  Prove that the script meets the requirements .

123

3、break sentence

Special process control statements break The function of sentences , When the program is executed to break When the sentence is , Will end the whole cycle （ That is to jump out of this cycle , Continue with the following commands ）.

and continue Statement is also a statement that ends a loop , however continue Statement skips the current loop , Continue with next cycle .

Look at the diagram to explain break sentence ：

for instance ：

First, write a program without break Script for statement break1.sh.

[[email protected] sh]# vim sh/break1.sh

#!/bin/bash

#  Output 10 Secondary variable i Value 

# Cycle ten times 

for((i=1;i<=10;i=i+1))

    do

        #  Output variables i Value 

        echo $i

done

Execute the script to view the results ：

[[email protected] sh]# chmod 755 break1.sh

[[email protected] sh]#./break1.sh

1

2

3

4

5

6

7

8

9

10

Write another plus break Script for statement break2.sh.

[[email protected] ~]# vim sh/break2.sh

#!/bin/bash

#  Output 10 Secondary variable i Value 

#  Cycle ten times 

for((i=1;i<=10;i=i+1))

    do

        #  If the variable i The value is equal to the 4

        if[ "$i" -eq 4 ]

            then

                #  Exit the whole cycle 

                break

        fi

        #  Output variables i Value 

        echo $i

done

Execute the script to view the results ：

[[email protected] sh]# chmod 755 break2.sh

[[email protected] sh]#./break2.sh

1

2

3

Comparison of the above two scripts , It can be seen that when the execution is finished break After the statement , Directly jump out of the whole for loop .

4、continue sentence

continue Statements are also statements that end process control . If in a loop ,continue Statement will only end the current single loop ,

Draw a diagram to illustrate continue sentence ：

for instance ：

Write a plus directly continue Script for statement continue1.sh.

Come with the one above break Statement .

[[email protected] ~]# vim sh/continue1.sh

#!/bin/bash

#  Output 10 Secondary variable i Value 

#  Cycle ten times 

for((i=1;i<=10;i=i+1))

    do

        #  If the variable i The value is equal to the 4

        if[ "$i" -eq 4 ]

            then

                #  Exit the whole cycle 

                continue

        fi

        #  Output variables i Value 

        echo $i

done

Execute the script to view the results ：

[[email protected] sh]# chmod 755 continue1.sh

[[email protected] sh]#./continue1.sh

1

2

3

5

6

7

8

9

10

As can be seen from the above results ,continue Statement is to skip the fourth loop , Then proceed to step 5 Secondary cycle .
This is it. continue Statement and break The difference between sentences .

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