文章目录

流程控制之for循环

一 、语法

#======~~__~~=====》Shell风格语法
for 变量名 [ in 取值列表 ]
do
  循环体
done

#===========》C语言风格语法
for ((初值;条件;步长))
do  
  循环体
done

1、shell风格的for，常用in列表方式

for i in 1 2 3
for i in {1,2,3}
for i in {1..9}
for i in {9..1}
for i in {a..z}
for i in {A..Z}
for i in {X..Z}
for i in $(命令)  # 例如：for i in $(head -10 /etc/passwd);do echo $i|cut -d: -f1,2;done
for i in $(find ...)

2、continue与for

continue：默认退出本次循环

break：默认退出本层循环

二、 案例

案例1：shell风格的for

for i in {1..10}
do
    echo $i
done

[root@openvpn day5]# cat for.sh 
#! /bin/bash

for i in `touch {1..10}.txt`
do
   echo $i
done

案例2：c语言风格的for

for ((i=1;i<=6;i++))
do
    echo $i
done

案例3：检查内网存活的ip，使用&符号提升脚本的运行效率

[root@openvpn day5]# cat ping.sh 
#!/bin/bash

for i in {1..254}
do
    (ping -c1 -t1  192.168.15.$i &>/dev/null

    if [ $? -eq 0 ];then
        echo "ping 192.168.15.$i up"
    else
        echo "ping 192.168.15.$i down"
    fi)&
done
#### 案例4：编写文件类型测试脚本，支持多个参数
[root@egon /]# cat file.sh 
#!/bin/bash

for i in $@
do
    if [[ -d $i ]];then
        echo "$i is directory."
    elif [[ -b $i  ]];then
        echo "$i is block device."
    elif [[ -f $i ]];then
        echo "$i is a regular file."
    else
        echo "unknow."
    fi
done
[root@egon /]# chmod +x file.sh 
[root@egon /]# 
[root@egon /]# ./file.sh a.txt /etc /root /abc
a.txt is a regular file.
/etc is directory.
/root is directory.
unknow.

案例5：可以直接在命令行编写for循环

[root@egon /]#  for i in {1..10};do [ $i -eq 5 ] && continue || echo $i;done
[root@egon /]#  for i in {1..10};do [ $i -eq 5 ] && break || echo $i;done

[root@openvpn day5]# cat for2.sh 
 #! /bin/bash


 for i in {1..6}
 do
     if [ $i -eq 2 ];then
      continue
     fi 
     echo $i
done
[root@openvpn day5]# ./for2.sh 
1
3
4
5
6

案例6：统计etc下每种文件类型的数量

[root@openvpn day5]# cat dir_root.sh 
#! /bin/bash

DIR='/root/'
for i in `ls $DIR`
do
    if [ -f $DIR$i ];then
        ((file++))
    elif [ -d $DIR$i ];then
        ((directory++))
    else
        (unkown++)
    fi
done
echo 'regular file' $file
echo 'directory' $directory
echo 'unkown' $unkown
[root@openvpn day5]# ./dir_root.sh 
regular file 7
directory 1
unkown

[root@openvpn day5]# cat dir_etc.sh 
#!/bin/bash
dir='/etc'
block=0
file=0
directory=0
other=0
for i in `ls $dir`
do
    if [ -b $dir/$i ];then
        ((block++))  # 或者 let block++,下同
    elif [ -f $dir/$i ];then
        ((file++))
         
    elif [ -d $dir/$i ];then
        ((directory++))
    else
        ((other++))
    fi
done

echo 'block:' $block
echo 'regular file:' $file
echo 'directory:' $directory
echo  'other:' $other
[root@openvpn day5]# ./dir_etc.sh 
block: 0
regular file: 108
directory: 96
other: 0

案例7：向脚本传递一个用户名,验证这个用户是否存在.

[root@egon ~]# cat testuser.sh 
#!/bin/bash
id $1 &> /dev/null
if [ $? -eq 0 ];then
    echo "用户$1存在"
else
    echo "用户$1不存在"
fi
[root@egon ~]# ./testuser.sh root
用户root存在

案例8：添加30个用户,再将它们删除

for i in {1..10};
do
    useradd user$i&&echo "user$i create successful"
done


for i in {1..10};
do
    userdel -r user$i&&echo "user$i delete successful"
done

案例9：输入账号信息，输入个数，批量创建用户user01、user02、user03…，密码默认123

[root@openvpn day5]# cat user.sh 
#!/bin/bash

read -p "请输入创建的用户名信息: " name
read -p "请输入创建的用户数量: " count
for i in `seq -w $count`
do
    echo $name$i
    useradd $name$i &>/dev/null
    echo 123 | passwd --stdin $name$i &>/dev/null
    id $name$i &>/dev/null
    if [ $? -eq 0 ];then

        echo "$name$i create is ok" 
    else
        echo "$name$i create is failed"
    fi
done

案例10：嵌套多层for循环，结合break与continue，（了解即可）

#1、使用break：
break 默认参数是 1 
所以写 break 等于 break 1
意义:退出当前循环层
break 2 则向上退出2层循环 当前循环也计算在退出层次里

# 示例
for i in {0..3}
do
    echo -e "第一层循环：loop$i"
    for j in {0..3} 
    do
        echo -e "\t第二层循环：loop$j"
        for n in {0..3}
        do
            echo -e "\t\t第三层循环：loop$n：$i$j$n"
            if ((n==2));then
                break 3
            fi
        done
    done
done


#2、使用continue
continue = continue 1
在当次循环中忽略continue后续的代码
就是:立即结束当前循环中的当次循环,而转入当前循环的下一次循环

continue 2 等同于 break 1
continue 3 等同于 break 2
总结：continue n 等同于 break n-1

for i in {0..3}
do
    echo -e "第一层循环：loop$i"
    for j in {0..3} 
    do
        echo -e "\t第二层循环：loop$j"
        for n in {0..3}
        do
            echo -e "\t\t第三层循环：loop$n：$i$j$n"
            if ((n==2));then
                continue 3
            fi
        done
    done
done