By Shell Script applications (2) learn to use if conditions such as conditional statements. Shell as a scripting programming language, also containing other process control loop structure, branch, etc., making it easy to complete more complex and powerful. We come to know for, while, the specific application case statement today.
A, for loop
In practice, often encounter cases a task needs to be performed several times, but each time the target is just not the same treatment, the same as other commands. For example: create a list of account numbers according to the system for a contact.
When faced with a variety of repeat list tasks, use an if statement can not meet the need, you need to use the for statement.
1.for sentence structure
When using a for loop, you need to specify a list of possible variables and values, repeat the same sequence of commands for different values, know the value of the variable exhausted exit the loop.
1) for circulating the grammatical structure of the sentence:
for 变量名 in 取值列表 do 命令序列 done
Sentence structure, for the operation of the sentences for the user to specify the name of the variable, right, and by keyword in this tag is preset to a value separated by spaces list multiple values. Located between commands do ...... done called loop sequence, wherein the variable to execute statements need to be applied to accomplish the task.
2) for loop statement flowchart
3) for loop usage scenarios
1. 循环次数未知； 2.完整遍历整个取值列表。
2.for statement Application Examples
要求： 1.根据姓名列表批量创建用户； 2.用户的初始密码是“123456” 实施步骤： [root@localhost ~]# vim /root/users.txt //新建一个取值列表 xiaozhang xiaoli xiaowang xiaosun [root@localhost ~]# vim useraddfor.sh //创建批量创建用户的脚本 #!/bin/bash ULIST=$(cat /root/users.txt) for UNAME in $ULIST do useradd $UNAME echo "123456" | passwd --stdin $UNAME &> /dev/null done [root@localhost ~]# sh useraddfor.sh //执行脚本 [root@localhost ~]# tail -4 /etc/passwd //验证效果 xiaozhang:x:1001:1001::/home/xiaozhang:/bin/bash xiaoli:x:1002:1002::/home/xiaoli:/bin/bash xiaowang:x:1003:1003::/home/xiaowang:/bin/bash xiaosun:x:1004:1004::/home/xiaosun:/bin/bash
Note: if statements, for statements and various other shell scripts can be nested statements are used.
Two, while loop
loop suitable for very irregular objects list, and the case where the source list has been fixed, and for controlling the number of cycles required, the operation target numbers in numerical order, the case of performing repeated operations in a particular condition, it is more suitable for use cycle --while statements.
Structure 1.while statement
Use while loop is a sequence of commands can be repeatedly performed in accordance with specific conditions, this condition is not satisfied so far known. In the script application, you should try to avoid the emergence of an infinite loop, or behind the command operation can not be performed. Thus, the command sequence in the loop body should include statements modified test conditions, test conditions so that the appropriate time is no longer established, thereby ending the cycle.
1) grammatical structure while loop statement
while 条件测试操作 do 命令序列 done
2) while the flowchart loop
When using the while loop, there are two special test operation conditions. That is true (true) and false (false). Use as a true condition, the condition is always represented established body of the loop will be infinite sequence of commands to perform it, unless forced to terminate script (or use the exit statement exits the script); on the contrary, if the use of false As a condition, the loop will not be executed.
3) while loop usage scenarios
1.循环次数已知； 2.必须有一个可以控制循环变量的语句。
2.while statement Application Examples
要求： 批量创建有规律编号的用户 实施步骤： [root@localhost ~]# vim useraddwhile.sh #!/bin/bash PREFIX="stu" i=1 while [ $i -le 10 ] do useradd ${PREFIX}$i echo "123456" | passwd --stdin ${PREFIX}$i &> /dev/null let i++ done [root@localhost ~]# sh useraddwhile.sh [root@localhost ~]# tail /etc/passwd stu1:x:1005:1005::/home/stu1:/bin/bash stu2:x:1006:1006::/home/stu2:/bin/bash stu3:x:1007:1007::/home/stu3:/bin/bash stu4:x:1008:1008::/home/stu4:/bin/bash stu5:x:1009:1009::/home/stu5:/bin/bash stu6:x:1010:1010::/home/stu6:/bin/bash stu7:x:1011:1011::/home/stu7:/bin/bash stu8:x:1012:1012::/home/stu8:/bin/bash stu9:x:1013:1013::/home/stu9:/bin/bash stu10:x:1014:1014::/home/stu10:/bin/bash
Circulating inside of the body, by the statement "let i ++" (i = equivalent expr $i + 1) to the value of the variable i by 1 until cycle until the condition is satisfied.
Three, case branching statements
Structure 1.case statement
1) the grammatical structure of the sentence case
case 变量值 in 模式1) 命令序列1 ;; 模式2) 命令序列2 ;; …… * ) 默认命令序列 esac
2) case statement flowchart
Use case branching statements, several noteworthy features:
3) case statement usage scenarios
There are many values of a variable, the need performed separately for each different value of the command sequence which, in this case, the multi-branch if statement is very similar, except that if statement is necessary to determine a plurality of different conditions, and case statement just judge the different values of a variable.
Application examples 2.case statement
[root@localhost ~]# vim hitkey.sh #!/bin/bash read -p "请输入一个字符，并按Enter键确认：" KEY case "$KEY" in [a-z]|[A-Z]) echo "你输入的是字母 $KEY" ;; [0-9]) echo "你输入的是数字 $KEY" ;; *) echo "你输入的是非法字符 $KEY" esac [root@localhost ~]# sh hitkey.sh 请输入一个字符，并按Enter键确认：1 你输入的是数字 1 [root@localhost ~]# sh hitkey.sh 请输入一个字符，并按Enter键确认：w 你输入的是字母 w [root@localhost ~]# sh hitkey.sh 请输入一个字符，并按Enter键确认：@ 你输入的是非法字符 @