How to avoid empty 'thebibliography' environment (bibtex) if there are no references?

Question

I have a general and central preamble for a lot of similar-style documents (several dozens) and most of the documents have a Bibliography at the beginning, so I call

\bibliographystyle{alphadin}
\bibliography{refs}

in the central preamble to avoid a lot of duplicate code and to be able to change the bibliography style centrally. But sadly, a minority of these documents has an empty bibliography. And these give me a warning about an empty thebibliography environment. But the bigger problem is: the bibliography headline still appears in the pdf output.

Is there a way to only get the bibliography headline and no warning when there is a non-empty bibliography? What would be the correct counters or booleans to address? Or is there an easy-to-use option?

Minmal example:

\documentclass{article}

\begin{document}
  \bibliographystyle{alphadin}
  \bibliography{test}
\end{document}

Run latex, bibtex, latex, latex. Get for test.bbl

LaTeX Warning: Empty `thebibliography' environment on input line 10.

Entering the "patch" doesn't solve the problem. What I want ist: If there's no \cite{} within the document, don't display the bibliography.

Content of bbl:

\begin{thebibliography}{}

% this bibliography is generated by alphadin.bst [8.2] from 2005-12-21

\providecommand{\url}[1]{\texttt{#1}}
\expandafter\ifx\csname urlstyle\endcsname\relax
  \providecommand{\doi}[1]{doi: #1}\else
  \providecommand{\doi}{doi: \begingroup \urlstyle{rm}\Url}\fi

\end{thebibliography}

Answer 1

A test if the argument of thebibliography is empty

\documentclass{article}
\let\myBib\thebibliography
\let\endmyBib\endthebibliography

\renewcommand\thebibliography[1]{\ifx\relax#1\relax\else\myBib{#1}\fi}
\begin{document}
foo
  \bibliographystyle{alphadin}
  \bibliography{test}
\end{document}

Answer 2

Method environment thebibliography that is really empty

This method catches the case (with or without empty line inbetween):

\begin{thebibliography}{...}

\end{thebibliography}

The patch for environment thebibliography looks for \end right after \begin{thebibliography}. Also an empty line before \end is supported. If \end is found, then the environment is ignored.

\documentclass{article}

\makeatletter
\@ifdefinable{org@thebibliography}{%
  \let\org@thebibliography\thebibliography
  \def\thebibliography#1{%
    \@ifnextchar\par{%
      \par@thebibliography{#1}%
    }{%
      \check@thebibliography{#1}{}%
    }%
  }%
  \newcommand{\par@thebibliography}[2]{%
    \check@thebibliography{#1}{\par}%
  }%
  \newcommand{\check@thebibliography}[2]{%
    \@ifnextchar\end{%
      \let\endthebibliography\relax
    }{%
      \org@thebibliography{#1}#2%
    }%
  }%
}
\makeatother

\begin{document}
  \bibliographystyle{alpha}
  \bibliography{test}
\end{document}

No warnings, no pages.

(Edit: matching the example in the question).

Method that checks the protocol file .blg of bibtex

If there are not any citations, bibtex complains with an error:

I found no \citation commands---while reading file test.aux

This error message is also written in the .blg file.

The patch looks for that error message and excludes the bibliography, if the error message can be found.

\documentclass{article}

\makeatletter
\newif\ifblg@empty
\IfFileExists{\jobname.blg}\@firstofone\@gobble{%
  \RequirePackage{xstring}%
  \begingroup
    \fullexpandarg
    % Method for reading the file is taken from `graphics.sty'.
    \newif\ifblg@read
    \blg@readtrue
    \openin\@inputcheck=\jobname.blg\relax
    \let\do\@makeother
    \dospecials
    \loop
      \read\@inputcheck to\@tempa
      \ifeof\@inputcheck
        \blg@readfalse
      \else
        \IfBeginWith*{\@tempa}
                    {I found no \@backslashchar citation commands}{%
          \global\blg@emptytrue
          \blg@readfalse
        }{}%
      \fi
      \ifblg@read
    \repeat
    \immediate\closein\@inputcheck
  \endgroup
}
\ifblg@empty
  \RequirePackage{version}%
  \excludeversion{thebibliography}%
\fi
\makeatother

\begin{document}
  \bibliographystyle{alphadin}
  \bibliography{test}
\end{document}

Method based on empty argument of thebibliography

@egreg suggested to check the argument of thebibliography. If it is empty, then it is assumed that the bibliography is empty (updated):

\documentclass{article}
\usepackage{version}
\makeatletter
\@ifdefinable{org@thebibliography}{%
  \let\org@thebibliography\thebibliography
  \def\thebibliography#1{%
    \def\@tempa{#1}%
    \ifx\@tempa\@empty
      \expandafter\@firstoftwo
    \else
      \expandafter\@secondoftwo
    \fi
    {%
      \let\endthebibliography\relax
      \end{thebibliography}%
      \excludeversion{thebibliography}%
      \begin{thebibliography}{}%
    }{%
      \org@thebibliography{#1}%
    }%
  }%
}
\makeatother

\begin{document}
  \bibliographystyle{alphadin}
  \bibliography{test}
\end{document}

Answer 3

No citation in the document will make BibTeX write

\begin{thebibliography}{}

in the .bbl file. So we can check whether the argument is empty, because a citation will put the widest entry in the argument.

\documentclass{article}

\let\origthebibliography\thebibliography
\let\origendthebibliography\endthebibliography
\newif\ifemptybibliography
\renewenvironment{thebibliography}[1]
 {\if\relax\detokenize{#1}\relax\emptybibliographytrue\else\origthebibliography{#1}\fi}
 {\ifemptybibliography\else\origendthebibliography\fi}


\begin{document}
x%\cite{HH}


  \bibliographystyle{alphadin}
  \bibliography{/usr/local/texlive/2012/texmf-dist/doc/plain/texbytopic/tex}
\end{document}

Check uncommenting the citation.