make python3 default linux code example

Example 1: linux set python 3 as default

1. Check current version:
$ python --version
2. get root permissions:
$ sudo su
3. set python3 as default:
$ update-alternatives --install /usr/bin/python python /usr/bin/python3 1
4. Done. (Check by $ python --version)

Example 2: set python 3 as default ubuntu

sudo update-alternatives --install /usr/bin/python python /usr/bin/python3.4 1
sudo update-alternatives --install /usr/bin/python python /usr/bin/python3.6 2

Example 3: set python 3 as default ubuntu

sudo update-alternatives --config python