Linux上的两个版本的python。如何使2.7成为默认值

I’ve got two versions of python on my linuxbox:

$python
Python 2.6.6 (r266:84292, Jul 10 2013, 22:48:45) 
[GCC 4.4.7 20120313 (Red Hat 4.4.7-3)] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> 


$ /usr/local/bin/python2.7
Python 2.7.3 (default, Oct  8 2013, 15:53:09) 
[GCC 4.4.7 20120313 (Red Hat 4.4.7-3)] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> 

$ which python
/usr/bin/python
$ ls -al /usr/bin/python
-rwxr-xr-x. 2 root root 4864 Jul 10 22:49 /usr/bin/python

How can I make 2.7 be the default version so when I type python it puts me in 2.7?

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.

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.)

Verify current version of python by:

$ python --version

then check python is symbolic link to which file.

  $ ll /usr/bin/python

Output Ex:

 lrwxrwxrwx 1 root root 9 Jun 16  2014 /usr/bin/python -> python2.7*

Check other available versions of python:

$ ls /usr/bin/python*

Output Ex:

/usr/bin/python     /usr/bin/python2.7-config  /usr/bin/python3.4         /usr/bin/python3.4m-config  /usr/bin/python3.6m         /usr/bin/python3m
/usr/bin/python2    /usr/bin/python2-config    /usr/bin/python3.4-config  /usr/bin/python3.6          /usr/bin/python3.6m-config  /usr/bin/python3m-config
/usr/bin/python2.7  /usr/bin/python3           /usr/bin/python3.4m        /usr/bin/python3.6-config   /usr/bin/python3-config     /usr/bin/python-config

If want to change current version of python to 3.6 version edit file ~/.bashrc:

vim ~/.bashrc

add below line in the end of file and save:

alias python=/usr/local/bin/python3.6

To install pip for python 3.6

$ sudo apt-get install python3.6 python3.6-dev
$ sudo curl https://bootstrap.pypa.io/ez_setup.py -o - | sudo python3.6
$ sudo easy_install pip

On Success, check current version of pip:

$ pip3 -V

Output Ex:

pip 1.5.4 from /usr/lib/python3/dist-packages (python 3.6)

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.

All OS comes with a default version of python and it resides in /usr/bin. All scripts that come with the OS (e.g. yum) point this version of python residing in /usr/bin. When you want to install a new version of python you do not want to break the existing scripts which may not work with new version of python.

The right way of doing this is to install the python as an alternate version.

e.g.
wget http://www.python.org/ftp/python/2.7.3/Python-2.7.3.tar.bz2 
tar xf Python-2.7.3.tar.bz2
cd Python-2.7.3
./configure --prefix=/usr/local/
make && make altinstall

Now by doing this the existing scripts like yum still work with /usr/bin/python. and your default python version would be the one installed in /usr/local/bin. i.e. when you type python you would get 2.7.3

This happens because. $PATH variable has /usr/local/bin before usr/bin.

/usr/lib64/qt-3.3/bin:/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin:/root/bin

If python2.7 still does not take effect as the default python version you would need to do

export PATH="/usr/lib64/qt-3.3/bin:/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin:/root/bin"

I guess you have installed the 2.7 version manually, while 2.6 comes from a package?

The simple answer is: uninstall python package.

The more complex one is: do not install manually in /usr/local. Build a package with 2.7 version and then upgrade.

Package handling depends on what distribution you use.