[系统运维]【Linux shell if条件判断、case语句、for循环、while循环、until循环】

一、if条件判断

1. 语法格式：
if
then
else
fi
2. 实例：
如果上条命令执行正确，返回commend execute successful
如果上条命令执行错误，返回commend execute failed

[root@localhost script]# vim if.sh
[root@localhost script]# chmod +x if.sh 
[root@localhost script]# cat if.sh 
#!/bin/bash
ls &>> /dev/null
if [ "$?"==0 ]
then
	echo "commend execute successful"
else
	echo "commend execute failed"
fi
[root@localhost script]# ./if.sh 
commend execute successful

#同上，只是把 $? 的引号去掉了：

[root@localhost script]# cat if.sh 
#!/bin/bash
ls &>> /dev/null
if [ $? == 0 ]
then
	echo "commend execute successful"
else
	echo "commend execute failed"
fi
[root@localhost script]# ./if.sh 
commend execute successful

[root@localhost script]# cat ifwrong.sh 
#!/bin/bash
Ls &>> /dev/null
if [ "$?" == 0 ]
then
	echo "commend execute successful"
else
	echo "commend execute failed"
fi
[root@localhost script]# ./ifwrong.sh 
commend execute failed

判断ip是否up

[root@localhost script]# cat ifping.sh 
#!/bin/bash
ping -c 1 $1 &>> /dev/null
if [ $? == 0 ]
then
	echo "$1 is up"
else
	echo "$1 is down"
fi
[root@localhost script]# chmod +x ifping.sh
[root@localhost script]# ./ifping.sh 10.10.10.10
10.10.10.10 is down
[root@localhost script]# ping 10.10.10.10
PING 10.10.10.10 (10.10.10.10) 56(84) bytes of data.

逻辑运算符： && ||

[root@localhost script]# ping -c 1 www.baidu.com &>> /dev/null && echo "up" || echo "down" 

&&：前面执行正确，就不执行后面
||：前面执行错误，才会执行后面（执行第一个正确的）

二、case语句

1. 语法格式：
case var in
var1)
;;
var2)
;;
*) 匹配不到结果，执行该条
;;
2. 实例

[root@localhost script]# vim case.sh
[root@localhost script]# chmod +x case.sh 
[root@localhost script]# cat case.sh 
#!/bin/bash
echo 'Input a number between 1 to 4'
echo 'Your number is:\c'
read aNum
case $aNum in
1) echo 'You select 1'
;;
2) echo 'You select 2'
;;
3) echo 'You select 3'
;;
4) echo 'You select 4'
;;
*) echo 'You do not select a number between 1 to 4'
;;
esac
[root@localhost script]# ./case.sh 
Input a number between 1 to 4
Your number is:\c
2
You select 2
[root@localhost script]# ./case.sh 
Input a number between 1 to 4
Your number is:\c
5
You do not select a number between 1 to 4

注：
read：表示从用户的输入获取变量

三、for循环

1. 语法格式
for var in ##
do
done

2.实例
在这里，总结了 in 后面的两种样式：

in后面样式一：

[root@localhost script]# vim for.sh
[root@localhost script]# cat for.sh 
#!/bin/bash
for i in {1..10}
do
	echo "The vaule is: $i"
done
[root@localhost script]# chmod +x for.sh 
[root@localhost script]# ./for.sh 
The vaule is: 1
The vaule is: 2
The vaule is: 3
The vaule is: 4
The vaule is: 5
The vaule is: 6
The vaule is: 7
The vaule is: 8
The vaule is: 9
The vaule is: 10

in后面样式二：

[root@localhost script]# cp for.sh for2.sh
[root@localhost script]# cat for2.sh 
#!/bin/bash
for i in 1 2 3 4 5
do
	echo "The vaule is: $i"
done
[root@localhost script]# ./for2.sh 
The vaule is: 1
The vaule is: 2
The vaule is: 3
The vaule is: 4
The vaule is: 5

实例三：
使用本机 ping 一下10.163.1.2-10网段的设备，检查哪些设备是 up ，哪些设备 down

[root@localhost script]# cat for2.sh 
#!/bin/bash
for i in {2..10}
do
	ping 10.163.1.$i -c 1 &>> /dev/null && echo "$i up" || echo "$i down" &
done
wait
[root@localhost script]# chmod +x for2.sh 
[root@localhost script]# ./for2.sh 
7 down
8 down
3 down
4 down
5 down
9 down
6 down
2 down
10 down

同理：使用本机 ping 一下10.163.1.*/24网段的设备，检查哪些设备是 up ，哪些设备 down

[root@localhost script]# cat for3.sh 
#!/bin/bash
for i in {1..254}
do
	ping 10.163.1.$i -c 1 &>> /dev/null && echo "$i up" || echo "$i down" &
done
wait

注：
&： 放到最后面，表示放到后台执行，2百多条都放到后台，营造一种并发的效果。前台执行只能一条执行完再执行下一条。
wait： 表示等待上面的命令执行完，再能执行其他命令。放到后台执行时，一定要写wait。

四、while循环

1.语法格式

while [条件] #当while后面的条件成立时，执行do里面的内容
do
done

2.几个“运算符”

ge： greater equal 大于等于
gt： grater than 大于
le： less equal 小于等于
lt： less than 小于
eq： equal 等于

3.实例

当COUNTER变量的值小于 5 时，输出 COUNTER + 1 的值

[root@localhost script]# cat while.sh 
#!/bin/bash
COUNTER=0
while [ "$COUNTER" -lt 5 ]
do
	COUNTER=`expr $COUNTER + 1`
	echo $COUNTER
done
[root@localhost script]# ./while.sh 
1
2
3
4
5

同上，只是将 $COUNTER 的引号去掉了，依然成立：

[root@localhost script]# vim while.sh 
[root@localhost script]# cat while.sh 
#!/bin/bash
COUNTER=0
while [ $COUNTER -lt 5 ]
do
	COUNTER=`expr $COUNTER + 1`
	echo $COUNTER
done
[root@localhost script]# chmod +x while.sh 
[root@localhost script]# ./while.sh 
1
2
3
4
5

五、until循环

1.语法格式

until [条件] #当until后面的条件成立时，退出循环
do
done

2.实例

当变量 a 的值不小于 10 时（即>=10时），退出循环

[root@localhost script]# cat until.sh 
#!/bin/bash
a=0
until [ ! $a -lt 10 ]
do
	echo $a
	a=`expr $a + 1`
done
[root@localhost script]# chmod +x until.sh 
[root@localhost script]# ./until.sh 
0
1
2
3
4
5
6
7
8
9

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