记录 shell学习过程（6）while 以及 while的嵌套 以及 until

while中的5种条件

1.数学比较

 read -p "Num :" num1
 while [ $num1 -gt  ]
   do
       echo 'greater'
       sleep 

 done

2.字符串比较

 read -p "enter the password:" pw
 while [ $pw != password ]
   do
       read -p "enter the password:" pw

 done
 echo 'welcome'

3.检测文件是否存在

 while [ ! -e /tmp/testfolder  ]
   do
       echo '/tmp/testfolder not exist'
       sleep
 done

4.多个条件判断

 read -p "money:" money
 read -p "car:" car
 read -p "house:" house

 while [ $money -lt  ] || [ $car -lt  ] || [ $house -lt  ]
   do
         echo 'no way!'
         read -p "money:" money
         read -p "car:" car
         read -p "house:" house
 done
 echo 'Ok!'

5.赋值判断

 read -p "char:" ch

 while [ $ch != 'q' ]
   do
         echo "char is : $ch"
         read -p "char:" ch
 done
 echo 'you just press q'

6.类似于for的写法

 i=
 while [ $i -lt  ]
   do
       echo $i
       i=$((i+))
 done

7.while使用break还有continue的时候需要注意累加的顺序

 i=
 while [ $i -lt  ]
   do
       echo $i
       i=$(( i+ ))
       if [ $i -eq  ];then
          continue
       fi
       #i=$(( i+ )) continue时先累加，break时后累加
 done

8.九九乘法表

 n=
 while [ $n -lt  ]
 do
       for (( m=;m<=$n;m++ )) #此处 m<=$n 如果写成 m<10 输出的样式会有很大的不同
          do
          echo -n -e "$m x $n = $((n*m))\t"
       done
       echo
       n=$((n+))
 done

如果将上面的for换成while 即双重while的嵌套

 n=
 while [ $n -lt  ]
 do
       #for (( m=;m<=$n;m++ ))
       # do
       m=
       while [ $m -lt $n ];do

          echo -n -e "$m x $n = $((n*m))\t"

       m=$(($m+))
       done
       echo
       n=$((n+))
 done

9.读取 passwd中的内容

 while read i;do
   echo $i
 done < /etc/passwd

10.读取 passwd中的内容 仅显示第一列 第三列 第五列

 IFS=$":"
 while read f1 f2 f3 f4 f5 f6 f7;do
     echo "$f1 $f3 $f5"

 done < /etc/passwd

11.until

　until和while的区别就是 until是条件为假才执行

打印 10-20

 i=
 until [ $i -gt  ];do
   echo $i
   i=$((i+))
 done