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I. for Loop statement

Second, use while loop statement

Third, use case branch statements

I. for Loop statement

When faced with various list repetitive tasks, it is difficult to meet the requirements with simple if statements, and it is even more tedious and difficult to write all the code sequentially. This will make it possible to use for loop statements to solve similar problems.

1. The structure of for statement

When using the for loop statement, you need to specify a variable and a list of possible values, and repeat the same command sequence for each different value until the variable value runs out of the loop. Here, the "value list" is called the execution condition of the for statement, which includes multiple objects with the same properties that need to be specified in advance (such as an address book).

The syntax structure of the for loop statement is as follows:

For variable name in value list do command sequence done

In the above statement structure, the operation object of the for statement is a variable with a user-specified name, and a list of values is pre-set for the variable through the in keyword, and multiple values are separated by spaces. The sequence of commands between do...done is called loop body, in which the execution statement needs to refer to variables to complete the corresponding task.

2. The execution flow of for statement

First, the first value in the list is assigned to the variable, and the command sequence in the do...done loop body is executed; then the second value in the list is assigned to the variable, and the command sequence in the loop body is executed. And so on until all the values in the list are used up, and finally set the

Skip to the done statement to end the loop, as shown in the following figure:

3. For statement application example 1) add users in batches according to the name list

According to the pinyin list of employee names given by the company's personnel department, add the corresponding user account to the Linux server, and the initial password is set to "pwd@123". Among them, the number of accounts in the employee name list is not fixed, and there is no other special rule except that the account name is pinyin.

In response to the above requirements, you can first specify the employee list file user.txt, and then write a shell script named useradd.sh, read the user names from the user.txt file, repeat the operations related to adding users and setting the initial password.

[root@centos01 ~] # vim user.txt zhangsanlisiwangwuzhaoliu [root@centos01 ~] # vim useradd.sh #! / bin/bashuser=$ (cat / root/user.txt) for username in $userdouseradd $usernameecho "pwd@123" | passwd-- stdin $username & > / dev/nulldone [root@centos01 ~] # chmod + x useradd.sh [root@centos01 ~] #. / useradd.sh [root@centos01 ~] # tail-5 / etc/passwd mysql:x:1001:1001::/home/mysql : / sbin/nologinzhangsan:x:1002:1002::/home/zhangsan:/bin/bashlisi:x:1003:1003::/home/lisi:/bin/bashwangwu:x:1004:1004::/home/wangwu:/bin/bashzhaoliu:x:1005:1005::/home/zhaoliu:/bin/bash

To delete the user added by the useradd.sh script, simply refer to the above script code and change the sequence of commands to add the user in the body of the for loop to delete the user.

[root@centos01 ~] # vim deluser.sh #! / bin/bashuser=$ (cat / root/user.txt) for username in $userdouserdel-r $usernamedone [root@centos01 ~] # chmod + x deluser.sh [root@centos01 ~] #. / deluser.sh [root@centos01 ~] # tail-5 / etc/passwd postfix:x:89:89::/var/spool/postfix:/sbin/nologinsshd:x:74:74:Privilege-separated SSH:/var/empty/sshd:/sbin / nologintcpdump:x:72:72::/:/sbin/nologintest:x:1000:1000:test:/home/test:/bin/bashmysql:x:1001:1001::/home/mysql:/sbin/nologin II. Use while loop statements

For loop statements are ideal for situations where list objects are irregular and the source of the list is fixed (such as a list file). However, when it is required to control the number of cycles, the operands are numbered in numerical order, and repeated operations are performed according to specific conditions, another kind of loop-while statement is more suitable.

1. The structure of whie statement

When a while loop statement is applied, a sequence of commands can be executed repeatedly according to a specific condition until the condition is no longer satisfied. In the application of scripts, endless loops should be avoided, otherwise subsequent command operations will not be executed. Therefore, the sequence of commands in the loop should include statements that modify the test condition so that the test condition is no longer established at the appropriate time, thus ending the loop.

The syntax structure of the while loop statement is as follows:

While conditional Test Action do Command sequence done2, while statement execution flow

Firstly, the conditional test operation result after while is judged, and if the condition is true, the command sequence in the do...done loop body is executed; after returning to while, the conditional test result is judged again, if the condition is still true, the loop body is continued to execute, and after returning to while again, the conditional test result is judged. Such a loop, until the conditional test result after while is no longer valid, finally jumps to the done statement to indicate the end of the loop. As shown in the following figure:

When using while loop statements, there are two special conditional test operations, true (true) and false (false). When using true as a condition, it means that the condition is always true, and the sequence of commands in the loop will be executed indefinitely unless the script is forcibly terminated (or exited through an exit statement); conversely, if false is used as a condition, the loop body will not be executed. These two special conditions can also be used in conditional testing of if statements.

3. Application example of while statement 1) users who add regular numbers in batches

In some areas of technical training and learning, for the purpose of experiment or testing, it is necessary to add user accounts in batches. The names of these users contain fixed prefix strings and are numbered in numerical order, and the number of accounts is often fixed.

[root@centos01 ~] # vim useraddress.sh [#! / bin/bashprefix= "user" i=1while [$I-le 20] douseradd ${prefix} $I echo "pwd@123" | passwd-- stdin ${prefix} $I & > / dev/nulllet i++done [root@centos01 ~] # chmod + x useraddress.sh [root@centos01 ~] #. / useraddress.sh [root@centos01 ~] # grep "user" / etc/passwd | tail-20 user1:x:1002:1002::/home / user1:/bin/bashuser2:x:1003:1003::/home/user2:/bin/bashuser3:x:1004:1004::/home/user3:/bin/bashuser4:x:1005:1005::/home/user4:/bin/bashuser5:x:1006:1006::/home/user5:/bin/bashuser6:x:1007:1007::/home/user6:/bin/bashuser7:x:1008:1008::/home/user7:/bin/bashuser8:x:1009:1009::/home/ User8:/bin/bashuser9:x:1010:1010::/home/user9:/bin/bashuser10:x:1011:1011::/home/user10:/bin/bashuser11:x:1012:1012::/home/user11:/bin/bashuser12:x:1013:1013::/home/user12:/bin/bashuser13:x:1014:1014::/home/user13:/bin/bashuser14:x:1015:1015::/home/user14:/bin/bashuser15:x:1016:1016::/home/user15 : / bin/bashuser16:x:1017:1017::/home/user16:/bin/bashuser17:x:1018:1018::/home/user17:/bin/bashuser18:x:1019:1019::/home/user18:/bin/bashuser19:x:1020:1020::/home/user19:/bin/bashuser20:x:1021:1021::/home/user20:/bin/bash

In the above script code, the variable I is used to control the number of the user name, the initial assignment is 1, and the loop is terminated when the value is greater than 20:00. Inside the body of the loop, the value of the variable I is incremented by 1 through the statement "let I +" (equivalent to I = 'expr $I + 1'), so that the value of I becomes 2 after the first loop, 3 minutes after the second loop, and so on.

To delete the user added by the useraddress.sh ji script, simply refer to the above script code and change the sequence of commands to add the user in the body of the while loop to delete the user.

[root@centos01 ~] # vim deluseraddress.sh #! / bin/bashprefix= "user" i=1while [$I-le 20] douserdel-r ${prefix} $ilet i++done [root@centos01 ~] # chmod + x deluseraddress.sh [root@centos01 ~] #. / deluseraddress.sh [root@centos01 ~] # id user20 id: no such user III. Use case branch statement 1, structure of case statement

The case statement is mainly suitable for the following situations: there are multiple values for a variable, and a different command sequence needs to be executed for each of them. This situation is very similar to the branch if statement, except that the if statement needs to judge several different conditions, while the case statement simply determines the different values of a variable. The syntax structure of the case branch statement is as follows:

Case variable value in Mode 1) Command sequence 1; Mode 2) Command sequence 2;. *) default command sequence esac

In the above statement structure, the keyword case is followed by "variable value", that is, "$variable name", which needs to be distinguished from the structure of the for loop statement. The whole branch structure includes between case...esac, the middle mode 1, pattern 2,., corresponding to the different values of variables (the expected values of the program), in which as wildcards, can match any value.

2. The execution flow of case statement

First of all, use "variable value" to compare with mode 1, if the value is the same, execute the command sequence after mode 1 until you encounter a double semicolon ";" then jump to esac to indicate the end of the branch; if it does not match with mode 1, then continue to compare with mode 2; if the value is the same, execute the command sequence after mode 2 until the double semicolon is encountered "; then jump to esac to indicate the end of the branch. And so on, if no matching value is found, the command sequence after the default mode "*)" is executed until the end of the branch after the esac is encountered. As shown in the following figure:

When using case branch statements, several noteworthy features are as follows:

The case line must end with the word "in", and each pattern must end with a closing parenthesis ")". The double semicolon ";" indicates the end of the command sequence. In a pattern string, you can use square brackets to indicate a contiguous range, such as "[0-9]", or the vertical bar symbol "|". The last ")" represents the default mode, where it is equivalent to a wildcard. 3. Case statement application example 1) check the type of characters entered by the user

Prompt the user to input a character from the keyboard, determine whether the character is a letter, number or other control character through the case statement, and give the corresponding prompt information.

[root@centos01 ~] # vim hitkey.sh #! / bin/bash read-p "Please enter a character and press enter to confirm:" KEYcase "$KEY" in [Amurz] | [Amurz]) echo "you entered a letter."; [0-9]) echo "you entered a number." *) echo "you entered a space, function key or other control character." esac [root@centos01 ~] # chmod + x hitkey.sh [root@centos01 ~] #. / hitkey.sh Please enter a character and press enter to confirm: K you entered a letter. [root@centos01 ~] #. / hitkey.sh Please enter a character and press enter to confirm: 6 you entered a number. [root@centos01 ~] #. / hitkey.sh Please enter a character and press enter to confirm that you are entering a space, function key or other control character. 2) write a system service script

Write a system service script named myprog, which is used to start, stop and restart the process through the start, stop, and restart control parameters specified by the location variable $1.

[root@centos01 ~] # vim services.sh #! / bin/bash#chkconfig:35 90 21#Description:testcase "$1" instart) echo "is starting the Apache service. [OK] "; stop) echo" is stopping the Apache service... [OK] "; restart) echo" is restarting the Apache service. [OK] "; *) echo" usage: $0 {start | stop | restart} ";; esac [root@centos01 ~] # chmod + x services.sh [root@centos01 ~] #. / services.sh stop is stopping the Apache service. [OK] [root@centos01 ~] #. / services.sh start is starting the Apache service. [OK] [root@centos01 ~] #. / services.sh restart is restarting the Apache service. [OK] [root@centos01 ~] #. / services.sh reload usage:. / services.sh {start | stop | restart}

-this is the end of this article. Thank you for reading-

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