由一道bash jail题引出的琐事@_@

关键词：Terminal devices、shell、stdio

题目入口：

（需要注册）

root@kali:~# ssh level1@24.37.41.154 -p 1016
level1@24.37.41.154's password: 

RingZer0 Team Online CTF

BASH Jail Level 1:
Current user is uid=1000(level1) gid=1000(level1) groups=1000(level1)

Flag is located at /home/level1/flag.txt

Challenge bash code:
-----------------------------

while :
do
	echo "Your input:"
	read input
	output=`$input`
done 

-----------------------------
Your input:

先说几个关键点和概念问题，然后再分析writeup，请耐着性子看完吧