While & For 循环

While 循环

可以将 While 循环称为 "条件循环"

While 循环语法

# 条件为 true 就执行循环体代码，条件变为 false 循环结束
while 条件
do
    命令1
    命令2
    命令3
    ...
done

# 示例
#!/bin/bash

count=0

while [ $count -lt 5 ]
do
    echo $count
    let count++  # 5
done

Until 循环语法

# 条件为 false 就执行循环体代码，条件变为 true 循环结束
until 条件
do
    命令1
    命令2
    命令3
    ...
done

# 示例
#!/bin/bash

count=0

until [ $count -eq 5 ]
do
    echo $count
    let count++  # 5
done

While 循环关键字

Break：立即终止本层循环

Continue：终止本次循环

#!/bin/bash

count=0

while [ $count -lt 5 ]  # 1 < 5
do
    if [ $count -eq 3 ];then
        # break
        let count++
        continue
    fi
    echo $count
    let count++  # 3
done

总结：结束 while 循环的两种方案

①. — 在循环体代码中把某个值改掉，让 While 循环条件变为假

[root@zzzwqh wqh05]# cat test.sh
#!/bin/bash

count=0
while [ $count -lt 3 ]
do
    echo 1111
    count=100
    echo 2222
    echo 3333

done

# 此时循环体内代码会执行完，之后结束脚本
[root@zzzwqh wqh05]# ./test.sh
1111
2222
3333
[root@zzzwqh wqh05]#

②. — break，直接退出本层 While 循环

[root@zzzwqh wqh05]# cat test.sh
#!/bin/bash

count=0
while [ $count -lt 3 ]
do
    echo 1111
    #count=100
    break
    echo 2222
    echo 3333

done

# 此时循环体内代码不会执行完，break 后的代码不会执行
[root@zzzwqh wqh05]# ./test.sh
1111
[root@zzzwqh wqh05]#

③. — 延伸，多层嵌套时，设置相同 While 循环条件，退出循环时，一起退出

# 正常情况下，break 结束嵌套多层的循环，要写很多层
while 条件1
do
	while 条件2
    do
        while 条件3
        do
        	break
        done
        break
    done
    break
done

# 把条件改为 false
tag=true
while $tag
do
	while $tag
    do
        while $tag
        do
        	tag=false
        done
    done
done

While 循环示例

Wget 测试网络

[root@zzzwqh wqh05]# cat check_web.sh
#!/bin/bash

[ $# -ne 1 ] && echo "只能输入一个参数" && exit

url=$1
count=0

while true
do
    echo "=========>$count"
    if [ $count -eq 3 ];then
        echo "报警：$url is dead"
        break
    fi

    wget --timeout=1 --tries=3 $url -q
    if [ $? -eq 0 ];then
        echo "$url is ok"
        break
    else
	let count++
    fi
done
[root@zzzwqh wqh05]# 

[root@zzzwqh wqh05]# ./check_web.sh
只能输入一个参数
[root@zzzwqh wqh05]# ./check_web.sh www.baidu.com
=========>0
www.baidu.com is ok
[root@zzzwqh wqh05]# ./check_web.sh www.zzzwqh.com.cn
=========>0
=========>1
=========>2
=========>3
报警：www.zzzwqh.com.cn is dead

读取文件按行打印

[root@zzzwqh wqh05]# cat 6.sh
#!/bin/bash

while read line
do
    echo $line
done < a.txt
[root@zzzwqh wqh05]# ./6.sh
11111 1:1 1
22222
3333
4444
[root@zzzwqh wqh05]#

动态监测系统状态

[root@zzzwqh ~]# while :;do free;sleep 0.5;clear;done

For 循环

可以将 For 循环称为 "取值循环"

Shell 风格语法

for i in value1 value2 value3
do
    echo $i
done

# 示例
[root@zzzwqh wqh05]# cat example.sh
#!/bin/bash

for i in "aaa" 222 "ccc"
do
    echo $i
done

# 取范围
[root@zzzwqh wqh05]# example.sh
#!/bin/bash

for i in {5..1}
do
   echo $i
done

for i in {a..z}
do
   echo $i
done

C 语言风格语法

# 语法格式
#!/bin/bash
for ((i=0;i<3;i++))
do
    echo "命令1"
    echo "命令2"
    echo "命令3"
done

Break & Continue 适用于 For 循环

# A-Z 忽略 C
[root@zzzwqh wqh05]# cat 9.sh
#!/bin/bash

for i in {A..Z}
do
   if [ $i == "C" ];then
     # break
     continue
   fi
   echo $i
done
[root@zzzwqh wqh05]#

For 循环示例

统计指定目录下文件的类型

[root@k8s-master 828]# cat file_type_sum.sh
#!/bin/bash
path=$1
block_type=0
regular_type=0
symbolic_type=0
directory_type=0
character_type=0

[ $# -ne 1 ] && echo "需要指定一个目录" && exit

for i in `ls $path`
do
    [ -b /$path/$i ] && let block_type++
    [ -L /$path/$i ] && let symbolic_type++
    [ -d /$path/$i ] && let directory_type++
    [ -f /$path/$i ] && let regular_type++
    [ -c /$path/$i ] && let character_type++
done

echo "块文件：$block_type"
echo "普通文件：$regular_type"
echo "软链接文件：$symbolic_type"
echo "目录文件：$directory_type"
echo "字符设备文件：$character_type"