错误信息:usr/bin/pip: No such file or directory
解决办法(一共四步，按照下面的步骤来从1到4，最后你会发现问题都解决了):
1.which pip
/usr/local/bin/pip 2.pip
-su: /usr/bin/pip: No such file or directory 3.type pip
pip is hashed (/usr/bin/pip)
So pip is definintely in /usr/local/bin/pip but it is been cached as in /usr/bin/pip, thanks to the * question, the solution is very simple: 4.hash -r
When the cache is clear, pip is working again.