BASH Script: Tame Python 2 / Python 3 version conflicts on Linux systems – hackish, but works!

WARNING: This is not the right way, nor the best way, of working with different python versions on the same system! virtualenv is the correct tool for the job, no doubt. Before running this hackish script you must make certain that the paths match your system’s particular Python installation paths!

That being said, if you run a system that uses Python 2.x for most things and you just need to occasionally run a Python 3.x application, for example, this is a quick and dirty hack to help out! It is by no means a method which should be packaged with any apps you plan to distribute, or anything that will see something remotely resembling a ‘production’ environment. This is just a hack to use if you need to quickly switch back/forth between Python versions in a system wide manner!

The example below was designed to work on an Arch Linux system, where most everything uses Python 2, but in which I have installed a couple of programs from the AUR (Arch User Repository) which require Python 3. Even if you run Arch Linux DO NOT ASSUME THIS WILL JUST WORK FOR YOU! Check the paths and make sure! Arch is a very dynamic distro, and these things could change tomorrow for all I know! This is a highly hackish solution, and if you fubar your python installation because you didn’t check these file locations on your particular system, don’t blame me! :P Warnings aside, I have tried my best to put some checks in place to make sure it isn’t too dangerous, but I am no bash scripting guru, just a guy sharing a hack – so your mileage may very!

Just put this in a file named chpy.sh. Then run ‘chmod +x’ on it, to make it executable. And to be safe, first run it without any parameters to see a usage example and to verify the config variables. So long as you run it with no parameters it will attempt no changes to your system!

chpy.sh

#!/bin/bash
#
# chpy.sh // by Adam Gaskins : self@adamgaskins.com
#
# This script automatically swaps out Python 2/Python 3 symbolic
# links, effectively changing the currently active Python version
# on the system.
#
# You are free to do anything you want with this script; personal,
# commercial, or otherwise. No attribution required. If it does
# help you in some way I'd love to hear about it on LinuxLusers.com!

####################################################
# CONFIGURE THE FOLLOWING VARIABLES FOR YOUR SYSTEM
# Note that some or all of the files below may be
# symbolic-links on your particular system!

# Directory where Python's binaries & symbolic-links
# in the following variables reside, usually /usr/bin.
py_dir="/usr/bin/"

# python file-names (py_file should be a symlink)
py_file="python"
py2_file="python2"
py3_file="python3"

# python-config file-names (pycon_file should be a symlink)
pycon_file="python-config"
pycon2_file="python2-config"
pycon3_file="python3-config"

# pydoc file-names (pydoc_file should be a symlink)
pydoc_file="pydoc"
pydoc2_file="pydoc2"
pydoc3_file="pydoc3"

# END CONFIGURATION VARIABLES - DONT CHANGE BELOW!
###################################################

# FUNCTIONS
function check_file_exists {
    if [ -e $1$2 ]; then
        echo "* $2 exists in $1" 2>&1 | sed 's/^/\t/'
    else
        echo "ERROR: $2 not found in $1, exiting."
        exit 1
    fi
}
function verify_symlink {
    if [ -h $1$2 ]; then
        echo "* $1$2 is a symbolic-link" 2>&1 | sed 's/^/\t/'
    else
        echo "ERROR: $1$2 is not a symbolic-link, exiting."
        exit 1
    fi
}

# USAGE
echo
echo "Please specify either 'python2' or 'python3'."
echo "Usage: ./chpy.sh python2"

# PATH & FILE VERIFICATION
echo
echo "Checking that all necessary files exist..."
check_file_exists $py_dir $py_file
check_file_exists $py_dir $py2_file
check_file_exists $py_dir $py3_file
check_file_exists $py_dir $pycon_file
check_file_exists $py_dir $pycon2_file
check_file_exists $py_dir $pycon3_file
check_file_exists $py_dir $pydoc_file
check_file_exists $py_dir $pydoc2_file
check_file_exists $py_dir $pydoc3_file

echo
echo "Verifying symbolic-links..."
verify_symlink $py_dir $py_file
verify_symlink $py_dir $pycon_file
verify_symlink $py_dir $pydoc_file

# CHANGE THE SYMBOLIC-LINKS
if [ -n "$1" ]; then
    echo
    echo "Changing symbolic-links in $py_dir to use $1."
    echo "Note: This script uses 'sudo', so you may be asked for your password!"

    shopt -s nocasematch

    if [ $1 = "python2" ]; then
        echo "*" `sudo rm -rfv $py_dir$py_file` 2>&1 | sed 's/^/\t/'
        echo "* linking " `sudo ln -sv $py_dir$py2_file $py_dir$py_file` 2>&1 | sed 's/^/\t/'

        echo "*" `sudo rm -rfv $py_dir$pycon_file` 2>&1 | sed 's/^/\t/'
        echo "* linking " `sudo ln -sv $py_dir$pycon2_file $py_dir$pycon_file` 2>&1 | sed 's/^/\t/'

        echo "*" `sudo rm -rfv $py_dir$pydoc_file` 2>&1 | sed 's/^/\t/'
        echo "* linking " `sudo ln -sv $py_dir$pydoc2_file $py_dir$pydoc_file` 2>&1 | sed 's/^/\t/'

    elif [ $1 = "python3" ]; then
        echo "*" `sudo rm -rfv $py_dir$py_file` 2>&1 | sed 's/^/\t/'
        echo "* linking " `sudo ln -sv $py_dir$py3_file $py_dir$py_file` 2>&1 | sed 's/^/\t/'

        echo "*" `sudo rm -rfv $py_dir$pycon_file` 2>&1 | sed 's/^/\t/'
        echo "* linking " `sudo ln -sv $py_dir$pycon3_file $py_dir$pycon_file` 2>&1 | sed 's/^/\t/'

        echo "*" `sudo rm -rfv $py_dir$pydoc_file` 2>&1 | sed 's/^/\t/'
        echo "* linking " `sudo ln -sv $py_dir$pydoc3_file $py_dir$pydoc_file` 2>&1 | sed 's/^/\t/'

    else
        echo "ERROR: invalid argument, '$1'. Please specify either 'Python2' or 'Python3'. Exiting."
        exit 1
    fi
    shopt -u nocasematch

    echo
    py_vers=`python --version 2>&1 | sed 's/^/\t/'`
    echo "$py_vers now set as default on this system!"
else
    echo
    echo 'WARNING:'
    echo 'No parameter specified! No system changes have been made.'
fi

# AMOR FATI!
exit 0