how to check which version of nltk, scikit learn installed?

import nltk is Python syntax, and as such won’t work in a shell script.

To test the version of nltk and scikit_learn, you can write a Python script and run it. Such a script may look like

import nltk
import sklearn

print('The nltk version is {}.'.format(nltk.__version__))
print('The scikit-learn version is {}.'.format(sklearn.__version__))

# The nltk version is 3.0.0.
# The scikit-learn version is 0.15.2.

Note that not all Python packages are guaranteed to have a __version__ attribute, so for some others it may fail, but for nltk and scikit-learn at least it will work.