Ubuntu向我展示了两个不同版本的python。我打电话给我用的是什么？
$ python --version
2.7.6
$ python2.7 --version
2.7.14
我从tarball安装了2.7.14。它是我想要使用的版本，并希望python命令引用它。我怎么能这样做？
$ whereis python
python: /usr/src/Python-2.7.14/python /usr/bin/python2.7 /usr/bin/python /usr/bin/python3.5-config /usr/bin/python3.6m /usr/bin/python3.4-config /usr/bin/python3.6 /usr/bin/python3.4m /usr/bin/python3.5m-config /usr/bin/python3.5m /usr/bin/python3.5 /usr/bin/python3.4 /usr/bin/python3.6-config /usr/bin/python3.6m-config /usr/bin/python3.4m-config /etc/python2.7 /etc/python /etc/python3.6 /etc/python3.5 /etc/python3.4 /usr/lib/python2.7 /usr/lib/python3.6 /usr/lib/python3.5 /usr/lib/python3.4 /usr/bin/X11/python2.7 /usr/bin/X11/python /usr/bin/X11/python3.5-config /usr/bin/X11/python3.6m /usr/bin/X11/python3.4-config /usr/bin/X11/python3.6 /usr/bin/X11/python3.4m /usr/bin/X11/python3.5m-config /usr/bin/X11/python3.5m /usr/bin/X11/python3.5 /usr/bin/X11/python3.4 /usr/bin/X11/python3.6-config /usr/bin/X11/python3.6m-config /usr/bin/X11/python3.4m-config /usr/local/bin/python2.7 /usr/local/bin/python2.7-config /usr/local/lib/python2.7 /usr/local/lib/python3.6 /usr/local/lib/python3.5 /usr/local/lib/python3.4 /usr/include/python3.6m /usr/include/python3.6 /usr/include/python3.4m /usr/include/python3.5m /usr/include/python3.5 /usr/include/python3.4 /usr/share/python /usr/share/man/man1/python.1.gz
$ which python
/usr/bin/python
$ which python2.7
/usr/local/bin/python2.7