Two versions of python on linux. how to make 2.7 the default
Add /usr/local/bin to your PATH environment variable, earlier in the list than /usr/bin.
Generally this is done in your shell's rc file, e.g. for bash, you'd put this in .bashrc:
export PATH="/usr/local/bin:$PATH"
This will cause your shell to look first for a python in /usr/local/bin, before it goes with the one in /usr/bin.
(Of course, this means you also need to have /usr/local/bin/python point to python2.7 - if it doesn't already, you'll need to symlink it.)
Enter the command
which python
//output:
/usr/bin/python
cd /usr/bin
ls -l
Here you can see something like this
lrwxrwxrwx 1 root root 9 Mar 7 17:04 python -> python2.7
your default python2.7 is soft linked to the text 'python'
So remove the softlink python
sudo rm -r python
then retry the above command
ls -l
you can see the softlink is removed
-rwxr-xr-x 1 root root 3670448 Nov 12 20:01 python2.7
Then create a new softlink for python3.6
ln -s /usr/bin/python3.6 python
Then try the command python in terminal
//output:
Python 3.6.7 (default, Oct 22 2018, 11:32:17)
[GCC 8.2.0] on linux
Type help, copyright, credits or license for more information.
You probably don't actually want to change your default Python.
Your distro installed a standard system Python in /usr/bin, and may have scripts that depend on this being present, and selected by #! /usr/bin/env python. You can usually get away with running Python 2.6 scripts in 2.7, but do you want to risk it?
On top of that, monkeying with /usr/bin can break your package manager's ability to manage packages. And changing the order of directories in your PATH will affect a lot of other things besides Python. (In fact, it's more common to have /usr/local/bin ahead of /usr/bin, and it may be what you actually want—but if you have it the other way around, presumably there's a good reason for that.)
But you don't need to change your default Python to get the system to run 2.7 when you type python.
First, you can set up a shell alias:
alias python=/usr/local/bin/python2.7
Type that at a prompt, or put it in your ~/.bashrc if you want the change to be persistent, and now when you type python it runs your chosen 2.7, but when some program on your system tries to run a script with /usr/bin/env python it runs the standard 2.6.
Alternatively, just create a virtual environment out of your 2.7 (or separate venvs for different projects), and do your work inside the venv.