1.for循环
2.while循环
3.until循环
1.for循环
for循环就是遍历，访问序列中的所有元素
语法一：
for i in 1 2 3 4
do
echo "hello world"#space键对齐
done
echo "loop done"
语法二：
for i in `seq 10`
do
echo "hello world"
done
echo "loop done"
语法三：
for i ((i=1,i<100,i++))
do
echo "my age is ：$i"
2.while循环
一、有限循环：
while 条件
do
执行语句
done
二、无限循环
while TRUE
do
执行语句
done
#!/bin/bash
ram=$(($RANDOM%1000+999))
guest=100
echo $ram
while [ $guest != $ram ]
do
if [ $guest -gt $ram ];then
echo "$guest is more than $ram"
else
echo "$guest is less than $ram"
fi
read -p "guest is :" guest
done
echo "number is $ram"