ubuntu下先是安装了python3.6和python2.7，现在由于其他需要又装了python3.5，但现在终端中输入python3 --version 命令显示默认加载python3.6，那么如何让它默认python3.5启动呢：
- 使用以下命令，更改python默认为python3.5：
sudo update-alternatives --install /usr/bin/python python /usr/bin/python2 100
sudo update-alternatives --install /usr/bin/python python /usr/bin/python3 150
sudo update-alternatives --install /usr/bin/python python /usr/bin/python3.5 200
- 取消原本的 Python 3.6 ，并将 Python3 链接到最新的python3.5 上：
sudo mv /usr/bin/python3 /usr/bin/python3-old
sudo ln -s /usr/bin/python3.5 /usr/bin/python3
- 成功默认从python3.5启动：
nuc@xy:/usr/lib/x86_64-linux-gnu$ python
Python 3.5.2 (default, Apr 16 2020, 17:47:17)
[GCC 5.4.0 20160609] on linux
Type "help", "copyright", "credits" or "license" for more information.
- 切换回来链接文件：
sudo rm /usr/bin/python3
sudo mv /usr/bin/python3-old /usr/bin/python3
- 现在发现直接在终端中输入python及具体版本号，即可加载你需要的python环境：
nuc@xy:/usr/lib/x86_64-linux-gnu$ python2
Python 2.7.12 (default, Oct 8 2019, 14:14:10)
[GCC 5.4.0 20160609] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>>
nuc@xy:/usr/lib/x86_64-linux-gnu$ python3
Python 3.5.2 (default, Apr 16 2020, 17:47:17)
[GCC 5.4.0 20160609] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>>
nuc@xy:/usr/lib/x86_64-linux-gnu$ python3.6
Python 3.6.10 (default, Dec 19 2019, 23:04:32)
[GCC 5.4.0 20160609] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>>