while CONDITION; do
循环体
done
CONDITION：循环控制条件；进入循环之前，先做一次判断；每一次循环之后会再次做判断。

条件为“true”，则执行一次循环；直到条件测试状态为“false”终止循环。

因此CONDTION一般应该有循环控制变量；而此变量的值会在循环体不断地被修正。

进入条件：CONDITION为true

退出条件：CONDITION为false

[root@centos73 ~]# type  while

while is a shell keyword

[root@centos73 ~]# help  while

while: while COMMANDS; do COMMANDS; done

    Execute commands as long as a test succeeds.

    Expand and execute COMMANDS as long as the final command in the

    `while' COMMANDS has an exit status of zero.

    Exit Status:

    Returns the status of the last command executed.