ubunto查看tree时The program 'tree' is currently not installed. You can install it by typing: sudo apt i

执行tree时候出现如下错误：

一般apt-get出现这种情况：

Reading package lists… Done
Building dependency tree
Reading state information… Done
E: Unable to locate package ****
是没有更新APT库

执行如下代码更新一下就好了：

	udo apt-get update
	sudo apt-get upgrade

然后再apt-get install *** 就好了，解决。