成功安装apt-get或pip后，软件包不可用

Question

我正试图在我的Ubuntu机器上安装一些库。以这个库为例

ubuntu@ip-172-31-44-172:~$ sudo apt install virtualenv
Reading package lists... Done
Building dependency tree... 50%
Building dependency tree       
Reading state information... Done
virtualenv is already the newest version (15.0.1+ds-3ubuntu1).
The following package was automatically installed and is no longer required:
  libllvm4.0
Use 'sudo apt autoremove' to remove it.
0 upgraded, 0 newly installed, 0 to remove and 0 not upgraded.
ubuntu@ip-172-31-44-172:~$ 

但现在当我尝试使用virtualenv时，我得到-

ubuntu@ip-172-31-44-172:~$ virtualenv
The program 'virtualenv' is currently not installed. You can install it by typing:
sudo apt install virtualenv

“pip3”库也一样

ubuntu@ip-172-31-44-172:~$ sudo apt install python3-pip
Reading package lists... Done
Building dependency tree       
Reading state information... Done
python3-pip is already the newest version (8.1.1-2ubuntu0.4).
The following package was automatically installed and is no longer required:
  libllvm4.0
Use 'sudo apt autoremove' to remove it.
0 upgraded, 0 newly installed, 0 to remove and 0 not upgraded.
ubuntu@ip-172-31-44-172:~$

ubuntu@ip-172-31-44-172:~$ pip3
The program 'pip3' is currently not installed. You can install it by typing:
sudo apt install python3-pip

这里有什么问题？

Answer 1

我通过卸载和重新安装上述所有库来修复这个问题。

sudo apt-get purge virtualenv
sudo apt-get purge python3-pip

然后我又重新安装了这些。我认为问题可能是-我将默认的python版本从2.7改为3.5。