Shell Script Loop statement--while loop

While loop
Suitable for environments where the proposition is true when the loop is executed
When a proposition is true, it enters a loop, and a proposition is a false exit loop.
Or when the condition is met, it goes into the loop.

Syntax format:
While CONDITION; Do
Loop body
Done

CONDITION: Cyclic control conditions
Entry condition: condition is true;
Exit Condition: Condition is false
Before entering the loop, make a judgment first, each time after the cycle will be judged again; the condition is "true", then a loop is executed, until the condition test state is "false" terminates the loop
Therefore: Condtion generally should have a cyclic control variable, and the value of this variable will be continuously corrected in the loop body

While loop application Example:

1, computes the and of all positive integers between 1-100

#!/bin/bash

#Author: Wangjun
#Contact qq:183530300
#Version: 1.0
#Create TIME:2016-08-17 16:59:54
#Description: 1-100 all positive integer ' s sum

Declare-i sum=0
Declare-i I=1

while [[$I-le]];d o
Let sum+= $I
Let i++
Done
echo "1-100 all positive integer ' sum: $sum"

2, automatic scanning 1-254 network segment of all hosts, statistics on the number of online hosts and the number of non-online hosts

#!/bin/bash

#Author: Wangjun
#Contact qq:183530300
#Version: 1.0
#Create TIME:2016-08-17 16:10:39
#Description:p ing all host and count the number

Declare-i online=0
Declare-i offline=0
Declare-i host=1

while [[$host-LT 255]];d o
Ping-c 1-w 1 10.1.250. $host &>/dev/null && echo "10.1.250. $host is Online" && let online= $online +1 | | Let offline= $offline +1
Let host= $host +1
Done
echo "Online hosts number: $online"
echo "Offline hosts number: $offline"

3, generate 10 random numbers and find the maximum random number and minimum random number

#!/bin/bash

#Author: Wangjun
#Contact qq:183530300
#Version: 1.0
#Create TIME:2016-08-17 20:22:58
#Description: Random number comparative

Minrandom= $RANDOM
Maxrandom= $minrandom
Declare-i r=1
Echo $minrandom

while [[$R-le 9]];d o
rd= $RANDOM
Echo $RD
if [[$RD-gt $maxrandom]];then
Maxrandom= $RD
elif [[$RD-lt $minrandom]];then
Minrandom= $RD
Fi
Let r++
Done
echo "Minrandom is: $minrandom"
echo "Maxrandom is: $maxrandom"

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4, print 99 multiplication table

#!/bin/bash

#Author: Wangjun
#Version: 1.0
#Contact qq:183530300
#Create time:2016-08-16 20:12:30
#Description: Multiplication table

Declare-i row=1
Declare-i Column=1

While ["$Row"-lt];d o
While ["$Column"-le "$Row"];d o
Echo-ne "${column}x${row}=$[$Column * $Row]\t"
Let column++
Done
Column=1
Let row++
Echo
Done

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5, every 3 seconds to query the system has logged on user information, if found hacker user login, log in log time and host records in/var/log/login.log, and prompts hacker users to exit the system

#!/bin/bash

#Author: Wangjun
#Contact qq:183530300
#Version: 1.0
#Create time:2016-08-18 08:14:07
#Description: Monitor Whether the hacker user has LoggedOn

While! (who | grep "^hacker\>");d O
Echo-e "' Date +%t ' \thacker user doesn ' T logged on"
Sleep 3
Done
Echo-e "' Date +%t ' \thacker user logged on" | Tee/var/log/login.log
echo "Please exit the current system" | Write hacker

This article is from the "Love Firewall" blog, be sure to keep this source http://183530300.blog.51cto.com/894387/1840655

Shell Script Loop statement--while loop