Why am I getting undefined references with bibtex?

Question

I am using texlive on ubuntu and getting

LaTeX Warning: Citation `texbook' on page 1 undefined on input line 9.

And in the output file no reference is showing. I have followed the examples from http://www.bibtex.org/Using/ and https://www.youtube.com/watch?v=KS9GvK7cvmo

My main.tex file:

\documentclass[a4paper]{article}
\title{ demo bibtex}
\author{kazi}
\begin{document}
\maketitle
some claim \cite{texbook}.
\bibliographystyle{plain}
\bibliography{refs}
\end{document}

And my refs.bib file:

@book{texbook,
  author = {Donald E. Knuth},
  year = {1986},
  title = {The {\TeX} Book},
  publisher = {Addison-Wesley Professional}
}
@book{latex:companion,
  author = {Frank Mittelbach and Michel Gossens
            and Johannes Braams and David Carlisle
            and Chris Rowley},
  year = {2004},
  title = {The {\LaTeX} Companion},
  publisher = {Addison-Wesley Professional},
  edition = {2}
}
@book{latex2e,
  author = {Leslie Lamport},
  year = {1994},
  title = {{\LaTeX}: a Document Preparation System},
  publisher = {Addison Wesley},
  address = {Massachusetts},
  edition = {2}
}