Virtualenv

Virtualenv package isolate python package system and corresponding binaries into a user defined folder.

First install virtualenv package by using default pythons package installation tool or you can use pip.

#easy_install virtualenv

To create new virtual environment,

$virtualenv python_project

This command will create new folder 'python_project', inside this folder we have three other folders 'bin' for python and other binaries , 'include' for python header files and finally 'lib' folder holds all python standard packages. When we will install new python packages under this virtual environment those files also been comes under this lib folder.

To activate and use this virtual environment,

$cd python_project
$source ./bin/activate

Above command will change the current shell session by updating the system PATH and it also change the shell prompt, to get a notion of we are in the virtual environment.

(virtual)host@name$ which python
PATH-to-DIR/python_project/bin/python
(virtual)host@name$ which easy_install
PATH-to-DIR/python_project/bin/easy_install

Now you can see that python and easy_install commands were from our virtual environment. When creating a virtual environment the basic python interpreter and package installation tools (easy_install and pip) were created under the bin directory so you can directly use them to install new packages under virtual environment easily that never going to affect the python packages installed in the default system path(/usr/lib/python-2.x/).

(virtual)host@name$ deactivate

This will deactivate our virtual environment and the control will be return to system shell prompt, after removing all changes in the system PATH variable.

So like this you can create any number of virtual environments with different package installed in it depending on your project requirements.

UPDATE

There is another package 'virtualenvwrapper' to organize and use the multiple virtual environments by a single set of shell commands. Here is the brief description about virtualenvwrapper.

To install virtualenvwrapper, as usual you can use easy_install or pip

#pip install virtualenvwrapper

After the installation append the following two lines to .bashrc or .profile file.

export WORKON_HOME="~/.virtualenvs"
source /usr/local/bin/virtualenvwrapper.sh

Now from next shell session onwards following commands would be available to us, which helps to manage the multiple virtual environments.

Create a new virtual environment and enter into it.

$mkvirtualenv test_proj

To deactivate
$deactivate

If we created multiple virtual environments, to list it by using,
$workon

To Activate a particular virtual environment from the above list.
$workon

To remove the virtual environment
$rmvirtualenv

This package also provide PRE and POST hooks for all of its commands , we can use those hooks to inject our codes while running the virtualenvwrapper commands.

This virtualenvwrapper is an additional package that really help both administrators and programmers to manage multiple project environments with different versions of python packages were installed.

Now take a look at how we can do the similar thing in Ruby ,

Ruby Version Manager - RVM

RVM tool handle multiple ruby versions (ruby-1.8.2, ruby-1.9.1, ruby-1.9.2 etc..) in our system and help us change the version of ruby in our system without affecting the ruby installed in our system default path. for eg; by default our system have ruby-1.8.2 installed, but we want ruby-1.9.2 for our project. So after installing the ruby-1.9.2 using rvm we can change system default ruby version to ruby-1.9.2, we can revert this back to system ruby when we require.

So in this manner we can install multiple ruby versions and switch between them, once we switch to a particular version of ruby then that ruby is available for that system user. This is the brief description how RVM works in our system. Let's move to the setup part.

You can install RVM from root (Multi user mode)user privilege or from a user privilege(Single user mode). The single user mode were recommended, because the RVM only available to that user only, but in Multi-user mode of RVM installation make it available to all users in the system.Here we will install RVM in single user mode.

To install RVM you can use git if it available in your system,

$bash < <(curl -s https://rvm.beginrescueend.com/install/rvm)

OR (If we don't have git installed )

Fetch installer script and run it yourself.
$ curl -s https://rvm.beginrescueend.com/install/rvm -o rvm-installer

$chmod +x rvm-installer
$./rvm-installer --version latest

After installation you can see that a folder (.rvm) were created in your home directory. This folder holds all RVM related files. Then to activate the rvm command for this user, you have to take one more step.

Add following line to your .bashrc file, so new shell session onwards the rvm commands were available to this user. OR you can just run bellow code in your current shell to activate it for this user session only.

[[ -s "$HOME/.rvm/scripts/rvm" ]] && source "$HOME/.rvm/scripts/rvm" #
   This loads RVM into a shell session

Open a new shell session,

$ ruby --version
ruby 1.8.7 (2010-08-16 patchlevel 302) [i486-linux]

Current system ruby version
$ rvm install 1.9.2

This will install new ruby-1.9.2
$ rvm install 1.9.1

This will install new ruby-1.9.1
$ rvm list
rvm rubies
ruby-1.9.1-p431 [ i386 ]
ruby-1.9.2-p290 [ i386 ]

Now you can see that we have three versions of ruby in our system, ruby-1.9.1 and ruby-1.9.2 were installed via RVM and ruby-1.8.7 from our system. To switch between these versions,

To use ruby-1.9.1 in one shell session only, try below commands.

$rvm use 1.9.1
$ruby --version
ruby 1.9.1p431 (2011-02-18 revision 30908) [i686-linux]
[/shell]
To make this version change permanent for all user shell sessions use,
[shell]
$rvm --default use 1.9.1
$rvm default list
rvm rubies
=> ruby-1.9.1-p431 [ i386 ]
ruby-1.9.2-p290 [ i386 ]

This change will available to current and all new current user
   sessions.
To get back to system ruby version
$rvm reset

$ruby --version
ruby 1.8.7 (2010-08-16 patchlevel 302) [i486-linux]

Using RVM we can now set your suitable version of ruby, after that you can install ruby gem packages using gem command of current ruby version.

$ ruby --version
ruby 1.8.7 (2010-08-16 patchlevel 302) [i486-linux]
$ gem --version
1.3.7
$ rvm --default use 1.9.2
Using /home/haridas/.rvm/gems/ruby-1.9.2-p290
$ gem --version
1.8.6

Check the gem versions while we switching between different version of ruby.

Ok, that's it. Try out these tools.... have a happy hacking....:)