Shell循环：while until

shell循环：while untile

循环次数不一定是固定的

  可以固定

  可以不固定

  对文件进行处理优先使用while循环

 

语法结构

一、while语法结构
while 条件测试  #当条件测试成立(条件测试为真，执行循环体)
do
    循环体
done

二、until语法结构
until 条件测试 #当条件测试成立(条件测试为假)，执行循环体
do
    循环体
done

示例

实现批量用户创建

#!/bin/bash
# for 是按照空格和tab键分隔，while是按照行处理。
while read line
do
    if [ ${#line} -eq 0 ];then
	echo "----------continue---------"
	continue  #跳过
        #break
        #exit 2
    fi
    user=`echo $line|awk '{print $1}'`
    pass=`echo $line|awk '{print $2}'`
    id $user &>/dev/null
    if [ $? -eq 0 ];then
	echo "user $user already exists"
    else
	useradd $user
        echo "$pass" | passwd --stdin $user &>/dev/null
        if [ $? -eq 0 ];then
	    echo "$user is created."
	fi
    fi
done < user.list
echo "ok..."

实现批量ssh处理

#!/bin/bash

#for ip in `cat ip.txt`
whlie read line
do
    {
	ping -c1 $ip &>/dev/null
        if [ $? -eq 0 ];then
	    ssh $ip "sed -i '/^#UseDNS/c UseDNS no' /etc/ssh/sshd_config"
	    ssh $ip "sed -i '/^GSSAPIAuthentication/c GSSAPIAuthentication no' /etc/ssh/sshd_config"
	    ssh $ip "sed -i '/^SELINUX=/c SELINUX=enforcing' /etc/selinux/config"
            ssh $ip "systemctl stop firewalld && systemctl disable firewalld"
            ssh $ip "setenforce 0"

	fi
    }&
done
wait
echo "all ok ..."

测试远程主机连接

#!/bin/bash
IP=192.168.128.200
while ping -c1 ${IP} &>/dev/null
do
    sleep 1
done
echo "${IP} is down.."

until ping -c1 ${IP} &>/dev/null
do
    sleep 1
done
echo "${IP} is up.."

测试远程主机连接

#!/bin/bash

#for i in {2..254}
i=2
#while [ $i -le 254 ]
until [ $i -gt 254 ]
do
    {
	ip=192.168.128.$i
	ping -c1 $ip &>/dev/null
	if [ $? -eq 0 ];then
	    echo "$ip is up."
	fi
    }&
    let i++
done
wait
echo "finish..."

数值运算

#!/bin/bash

i=1
#for i in {1..100}
#while [ $i -le 100 ]
until [ $i -gt 100 ]
do
    #let sum=sum+$i
    let sum+=$i
    #echo $sum
    let i++
done
echo $sum