Two versions of Python under Ubuntu toggle

Recently installed in Ubuntu16.04 python3.5 and the system comes with the python2.7.13, summarize the different versions of the switching problem.

• Alias: Aliases
• BASHRC: Personal Profile

First, modify the Python version (for users)

To modify a Python version for a particular user, simply create one (alias) in its home directory alias . Open the user's ~/.bashrc file and add the new alias information to modify the Python version that is used by default.

Alias python= '/usr/bin/python3.5 '

Once you have done this, log back in or reload .bashrc the file for the operation to take effect.

. ~/.bashrc

Check the current Python version.

Python--version

　　

Second, modify the Python version (for the entire system)

The Python version can be modified throughout the system level with the Update-alternatives directive. The premise is to log in as the root user.

• Go to root User:

  sudo-

  You will be prompted to enter password: Enter your password to log in.

• If this is the first time to use root, Ubuntu system is not activated by default, the root user needs to be activated manually, terminal input:

sudo passwd root

Prompt: Enter new UNIX password:, enter the password as required, so that the activation is completed, sudo again-, login completed.

• Su
  • SU//Switch to Super User
  • Su username//switch to username user
  • Su differs from sudo su: refer here.

After logging in to the root user, view all versions of Python:

Update-alternatives

If an error message appears:

Update-alternatives:error:no Alternatives for Python

This shows that Python alternatives is not update-alternatives recognized, and we need to update alternatives table to include Python 2.7 and python3.5.

update-alternatives:using/usr/bin/python2.7 to Provide/usr/bin/python (python) in Auto modeupdate-alternatives:using /usr/bin/python3.5 to Provide/usr/bin/python (python) in auto mode

This time the computer tip:syntax error near unexpected token ' (', which is not recognized (, need to be escaped:

update-alternatives:using/usr/bin/python2.7 to Provide/usr/bin/python \ (python\) in Auto modeupdate-alternatives: using/usr/bin/python3.5 to Provide/usr/bin/python \ (python\) in Auto mode

It also indicates an error:

No command ' update-alternatives: ' Found, did you mean:

Command ' update-alternatives ' from the package ' dpkg ' (main)

Update-alternatives:: Command not found

not resolved temporarily.

Reference:

• Https://linuxconfig.org/how-to-change-from-default-to-alternative-python-version-on-debian-linux

Two versions of Python under Ubuntu toggle