while循环 

1) 

while CONDITION; do

statement

statement

<改变循环条件真假的语句>

done 

编写脚本，计算1---100的和 

#!/bin/bash

#

sum=0

i=1

while [ $i -le 100 ]; do

   let sum=$sum+$i

   let i=$i+1

done

echo $sum

2) 

while true; do 

statement

statement

done 

#!/bin/bash

#

while true; do

   read -p "请输入你的选择：" choice

   if [ $choice == "q" ]; then

     break

   fi

done

#!/bin/bash

#

while true; do

   uptime

   sleep 3

done

3)

while read line; do 

statement

statement

done < file 

#!/bin/bash

#

bash_num=0

nologin_num=0

while read line; do

   sh_name=$(echo $line | awk -F: '{print $7}')

   case $sh_name in

        /bin/bash)

           let bash_num=$bash_num+1

           ;;

        /sbin/nologin)

           let nologin_num=$nologin_num+1

           ;;

   esac

done < /etc/passwd

echo $bash_num

echo $nologin_num

util循环：

util CONDITION; do 

    statement

statement

done 

条件为假时，执行循环，条件为真时，结束循环