所有的问号是做什么用的?向下滚动查看奇怪的输出。
$ ls -la /etc/apt/sources.list.d/
ls: cannot access '/etc/apt/sources.list.d/additional-repositories.list': Permission denied
ls: cannot access '/etc/apt/sources.list.d/.': Permission denied
ls: cannot access '/etc/apt/sources.list.d/official-package-repositories.list': Permission denied
ls: cannot access '/etc/apt/sources.list.d/..': Permission denied
ls: cannot access '/etc/apt/sources.list.d/opera-stable.list': Permission denied
total 0
d????????? ? ? ? ? ? .
d????????? ? ? ? ? ? ..
-????????? ? ? ? ? ? additional-repositories.list
-????????? ? ? ? ? ? official-package-repositories.list
-????????? ? ? ? ? ? opera-stable.list