import nltk
nltk.download("popular")

import sys
sys.version
sys.version_info 2_Pythonsys.version_info(major=2, minor=7, micro=6, releaselevel='final', serial=0)Installation of pyhton on Ubuntu 14.04Ubuntu system has pyhton 2.7.X inbuilt no need to download and install it To confirm whether python has been properly install or not do following steps.Step 1: Open system terminal
Step 2: 
$ python -V
Python 2.7.6Install pip and setup tools On Linux ubuntu 14.04What is pip?
pip is a package management system used to install and manage software packages written in PythonOpen your system's terminal and execute following commands$ sudo apt-get install -y python-pip
$ sudo pip install -U pip
$ sudo pip install setuptools==33.1.1Confirm pip has been installed successfully.$ pip -V
pip 9.0.1 from /usr/local/lib/python2.7/dist-packages (python 2.7)Next step is to install nltk package and download nltk dataInstallation steps for NLTK on terminal execute following command
$ sudo pip install nltk
$ pythonAfter this now you are inside the python shell and check your nltk package install properly or not$ python
Python 2.7.6 (default, Oct 26 2016, 20:30:19) 
[GCC 4.8.4] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>>import nltk
>>>nltk.download()Choose the path where you wnat to download nltk data and click "Download" option.Download process may take long time.
you can refer the diagram 1.4 given in chapter no 1OR If you don't want to go inside python shell you can open your system's terminal and execute the following command
Command : sudo python -m nltk.downloader -d PATH_OF_NLTK_DATA all
Actual command : sudo python -m nltk.downloader -d /usr/local/share/nltk_data all