Linux command: While loop

while Introduction to Commands:

While loop: For scenarios where the number of cycles is unknown, there is an exit condition, otherwise it becomes a dead loop.

1 . Command format:

Grammar:

While CONDITION; Do

Statement

...

Done

2 . Command function:

While loop: For scenarios where the number of cycles is unknown, there is an exit condition, otherwise it becomes a dead loop.

3 . Command parameters:

While CONDITION; Do condition: conditions for entering the loop

Statement

...

Done

4 . Command instance:

1, calculate the integer within 100 and

[Email protected] test]# vim while100.sh

#!/bin/bash

#program:

#练习使用while循环计算1到100正整数的和

#history Donggen 2016-11-02-21:21

Path=/bin:/sbin:/usr/bin:/usr/sbin:/usr/local/bin:/usr/local/sbin:~/bash

Export PATH

Declare-i I=1

Declare-i sum=0

While [$I-le 100]; Do

Let sum+= $I

Let i++

Done

echo "$SUM"

[Email protected] test]# chmod +x while100.sh

[Email protected] test]#./while100.sh

5050 positive integers from 1 to 100 and 5050

[Email protected] test]#

2, if the user is prompted to enter characters, if not quit the lowercase letters all into uppercase,

Quit the script if it is quit.

[Email protected] test]# vim whilequit.sh

#!/bin/bash

#program:

#如果输入的不是quit则把小写字母全部换成大写字母, quit quit the loop

#history Donggen 2016-11-02-21:31

Path=/bin:/sbin:/usr/bin:/usr/sbin:/usr/local/bin:/usr/local/sbin:~/bash

Export PATH

Read-p "Input Something:" SCRIPT

while [$SCRIPT! = "Quit"]; Do

echo "$SCRIPT" | Tr ' A-Z ' A-Z

Read-p "Input Something:" SCRIPT

Done

[Email protected] test]# chmod +x whilequit.sh

[Email protected] test]#./whilequit.sh

Input SOMETHING:ABC

Abc

Input SOMETHING:EROQE

Eroqe

Input Something:quit

[Email protected] test]#

3, write a script to see if a user is logged in, if not login to continue to view, if the login

User has logged in and exited the loop.

[Email protected] test]# vim whiledonggen.sh

#!/bin/bash

#program:

#查看用户是否登录

#history Donggen 2016-11-02-21:45

Path=/bin:/sbin:/usr/bin:/usr/sbin:/usr/local/bin:/usr/local/sbin:~/bash

Export PATH

Usernum=donggen

W.H.O. | grep "$USERNUM" &>/dev/null

Num=$?

While [$NUM-ne 0]; Do

echo "$USERNUM is not login."

Sleep 3

W.H.O. | grep "$USERNUM" &>/dev/null

Num=$?

Done

echo "The $USERNUM is login."

[Email protected] test]# chmod +x whiledonggen.sh

[Email protected] test]#./whiledonggen.sh

Donggen is not login. Donggen users are not logged on without always prompting for no sign-in

Donggen is not login.

Donggen is not login.

Donggen is not login.

The Donggen is login. Donggen the user to log in and exit the loop after logging in.

[Email protected] test]#

This article is from the "Learn Linux history" blog, please be sure to keep this source http://woyaoxuelinux.blog.51cto.com/5663865/1868711

Linux command: While loop