Shell Programming (iii)-for loop, while loop, Break,continue,exit

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One, for loop

Repeated execution of a series of commands is common in programming. Usually you need to repeat a set of commands until you reach a certain condition, such as working with all the files in a directory, all users on the system, or all the lines in a text file.

There are two common loops that are commonly used in scripts.

For loop
While loop

Syntax: for variable name in condition; Do ...; Done

for var in listdo  commandsdone在list参数中，提供了迭代中要用的一系列值

Example 1: Use a For loop to write a sum of 1-100.

Ideas:

• [] First you need to put 1-100 loops
• [] The loop is added once and assigned to sum
• [] outputs the value of sum
• Seq This is the traversal of 1-100 of these numbers.

[[email protected] ~]# !vimvim sum01.sh#!/bin/bashsum=0for i in `seq 1 100`do   sum=$[ $sum+$i ]   echo $idoneecho $sum

Example 2: Traversing directories or files of a directory

#!/bin/bashcd /etc/            //脚本中如果查询的目录在多级目录下，首先要进入到目录，或者，直接跟绝对路径for a in `ls /etc/`     //遍历/etc目录do    if [ -d $a ]        //一个一个的去判断是否为目录    then    ls -d $a    //如果是目录，就查看下目录内的文件+子目录    fidone

[[email protected] ~]# !vimvim for02.sh#!/bin/bashcd /etc/for a in `ls /etc/`do   [ -d $a ] && ls $a # 判断是否是目录，并列出其下文件和子目录   if [ -d $a ]   then       echo $a       ls $a   fidone~             

Special for loop Example: When a list loop, a space or carriage return is used as a delimiter

[[email protected] ~]# mkdir xavi[[email protected] ~]# cd xavi/[[email protected] xavi]# ls[[email protected] xavi]# touch 1 2[[email protected] xavi]# touch 3\ 4.txt[[email protected] xavi]# ls1  2  3 4.txt[[email protected] xavi]# for i in `ls ./`; do echo $i ; done1234.txt //把3 4.txt一个文件拆分成两个了

Second, while loop

Grammar:

while test commanddo  other commands  done

Case 1: Write a script to monitor the load on the system

#!/bin/bashwhile :         //：为死循环的意思,也相当于ture,do    load=`w|head -1|awk -F ‘load average: ‘ ‘{print $2}‘|cut -d. -f1`if [ $load -gt 10 ]then    top|mail -s "load is high: $load" [email protected]fisleep 30// 负载有时候不可能一秒钟就起来了，所以每隔30秒判断一次即可。我们使用sleep 去暂停30秒钟。然后再次执行。。。done

Case 2: Get a value and judge by interacting with the user.

The digital output of the user input.

• [] First of all, if the user does not input what to do?
• [] What happens if the user does not enter a pure number?
• [] Based on the number entered by the user, and output.

#!/bin/bashwhile :do    read -p "Please input a number: " n    if [ -z "$n" ]    then        echo "you need input sth."        continue    fi    n1=`echo $n|sed ‘s/[0-9]//g‘`    if [ -n "$n1" ]  // -n = ! -z ,记得加双引号    then        echo "you just only input numbers."        continue    fi    breakdoneecho $n

• [] Continue: When the user does not enter the time, the first prompt, and then continue to let the user input. Until the input is a real number, jump out of the IF statement.

• [] Break: If it is a true number, jump out of the entire while statement.

Third, the use of break

Example:

#! /bin/bashfor i in `seq 1 5`do     echo $iif [ $i == 3 ]then    breakfiecho $idoneecho aaa

The following steps are performed:

[[email protected] xavi]# sh -x break.sh++ seq 1 5+ for i in ‘`seq 1 5`‘+ echo 11+ ‘[‘ 1 == 3 ‘]‘+ echo 11+ for i in ‘`seq 1 5`‘+ echo 22+ ‘[‘ 2 == 3 ‘]‘+ echo 22+ for i in ‘`seq 1 5`‘+ echo 33+ ‘[‘ 3 == 3 ‘]‘+ break+ echo aaaaaaaa

In other words, once a break is encountered, it jumps out of the current loop and makes the next step.

Iv. usage of continue:

Ignore the code under continue, and proceed to the next loop directly.

#! /bin/bashfor i in `seq 1 5`do     echo $iif [ $i == 3 ]then    continuefiecho $idoneecho aaa

The results of the implementation are as follows:

112234455aaa

That is, when executed to continue, any of the following statements are ignored directly: The next for loop is made directly.

V. Exit exits the entire script

#! /bin/bashfor i in `seq 1 5`do     echo $iif [ $i == 3 ]then    exitfiecho $idoneecho aaa

The following steps are performed:

++ seq 1 5+ for i in ‘`seq 1 5`‘+ echo 11+ ‘[‘ 1 == 3 ‘]‘+ echo 11+ for i in ‘`seq 1 5`‘+ echo 22+ ‘[‘ 2 == 3 ‘]‘+ echo 22+ for i in ‘`seq 1 5`‘+ echo 33+ ‘[‘ 3 == 3 ‘]‘+ exit

• [] When the line is straight to 3, exit the script directly.

Shell Programming (iii)-for loop, while loop, Break,continue,exit