Python Script – How to Change Python Version a Script is Referring To

bashpythonpython3scriptubuntu 18.04

I have downloaded the script "install_esoreflex" (ftp://ftp.eso.org/pub/dfs/reflex/install_esoreflex) and then execute the following commands:

chmod u+x install_esoreflex

./install_esoreflex

I get the following warnings

WARNING: The following Python version is installed in your system Python 3.7.0
WARNING: Some of the available workflows use functionality
WARNING: only available in version Python 2.6.0 or greater
WARNING: and will not work properly with the installed version.
WARNING: Additionally, python 3.x is not yet supported.

Eventhough I have both Python 3.7.0 and 2.7. Python 2 is installed in /usr/bin/python2 and Python 3 is installed in /home/USER/miniconda3/bin/python3.
How can I make the script "know" I have python 2?

Best Answer

From Ubuntu version 18.04, python 3 is the default python version (link). This means that /usr/bin/python is symlinked to /usr/bin/python3. You can check this by running python —version.

To default to python 2 in scripts (as your install script requires), you need to install the python 2 version (apt-get install python2). And tell Ubuntu to use that python version.

It is not recommended to change the /usr/bin/python symlink, because it is part of the distribution maintained configuration. Instead you should use the alias command (link): alias python=python2. This (locally) points python to python2, which is found in /usr/bin. You could use this command once in the terminal before running your install script. To change it more permanently, add it to your .bash_profile.

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I think you need to set PYTHONPATH after changing the links. On a fresh install of Ubuntu 10.10 I just did, I have:

evadeflow(57)% echo $PYTHONPATH 
/home/evadeflow/lib/python2.6/site-packages:/usr/lib/python2.6/site-packages

You should set this in your environment so it points to the version you want to use (i.e., change the 2.6 to 2.7...)

Python /usr/bin command not found

As you mentioned in the comment, ls -l /usb/bin/python* shows:

 lrwxr-xr-x 1 root wheel 75 28 Jul 16:27 /usr/bin/python2.5 -> ../../System/Library/Frameworks/Python.framework/Versions/2.5/bin/python2.5 
 lrwxr-xr-x 1 root wheel 82 28 Jul 16:27 /usr/bin/python2.5-config -> ../../System/Library/Frameworks/Python.framework/Versions/2.5/bin/python2.5-conf??ig 
 lrwxr-xr-x 1 root wheel 75 28 Jul 16:27 /usr/bin/python2.6 -> ../../System/Library/Frameworks/Python.framework/Versions/2.6/bin/python2.6 
 lrwxr-xr-x 1 root wheel 82 28 Jul 16:27 /usr/bin/python2.6-config

Note that there is no /usr/bin/python.

As @Wouter mentioned, you'll want to create a symlink:

ln -s /usr/bin/python2.6 /usr/bin/python

That'll create a symlink (think of it as a shortcut in Windows), from /usr/bin/python to /usr/bin/python2.6

For a bit more detail, you mention the first line of /usr/bin/easy_install is :

#!/usr/bin/python

This is commonly known as the shebang line, and tells the script which interpretor to use - in this case it's trying to use /usr/bin/python, which, as we've seen from the ls command, does not exist, which is why we create a symlink

Best Answer

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