Enabling double way for bibliography with \url: Both bibtex and biblatex (biber)

Question

I am trying to make a document to accept both bibtex and biblatex (during different compilations of course) and I have a problem because:

1. In bibtex I have to add \url command inside every url (really many entries)
2. But biblatex doesn't accept this (already there and so, it supposes it is a part of my url) and giving unwanted results.

Here is the code:

\documentclass{article}
\usepackage{filecontents}
% \UseBibLaTex is a variable that if is less than 1 the document 
% will have to compiled with `bibTeX`... else with biber
\def\UseBibLaTeX{0}
% ``url'' package is used in order to make LaTeX to break the urls 
% in places like the bibliography combined with the next (upcoming) 
% command
%\usepackage{breakurl}
\usepackage[hyphens]{url}
\Urlmuskip=0mu plus 1mu

\ifnum\UseBibLaTeX>0
\usepackage[backend=biber,style=authoryear]{biblatex}
\newcommand\myurl[1]{#1}
\DeclareFieldFormat{url}{{\footnotesize URL:}\space%\expandafter%\noexpand
\myurl{#1}}
\addbibresource{myBib.bib}
\else
% ``natbib'' package offers the bigger ammount possibilities in bibliography
% formating
\let\myurl\url
\usepackage{natbib}
\fi


\begin{filecontents*}{myBib.bib}
@Article{cite1,
title={One Article with a url that has to break both in bibtex and biblatex},
author={Someone NotMe and Someother Me},
year={2018},
month={10},
url={\url{http://A-long-URL-that-has-to-break-through-lines-and-biber-has-to-ignore-the-\textbackslash url-command}},
doi={http://dx.doi.org/10.1038/nphys1170}
}
\end{filecontents*}

\title{Double Functionality about Bibliography}
\author{Konstantinos Leledakis}
\date{October 2018}

\begin{document}

\maketitle

\nocite{*}
\ifnum\UseBibLaTeX>0
\printbibliography
\else
\bibliographystyle{agsm}
\begin{sloppypar}
  \bibliography{myBib}
\end{sloppypar}
\fi



\end{document}

The output of the above code, is as expected... but if I change the command \def\UseBibLaTeX{0} to \def\UseBibLaTeX{1} I get (an expected as a noob about biber but...) unwanted result:

Already tried some things like \noexpand or \expandafter that are commented in my code but could not make it work as I wanted.

Answer 1

biblatex expects the url field to contain the URL and the URL only. No additional markup like \url{...}, no escaping of special characters, nothing, just the verbose URL.

The problem you are seeing is that Biber applies percent encoding to the URL and this messes up your \url{...} wrapper. (There are ways to get rid of the percent encoding, see biblatex, using space in url doesnt work, How to disable percent-encoding in URLs?, Scandinavian letters in Biblatex URL, but that would not really help here.)

Luckily most .bst files that support a dedicated URL field also handle URLs in a way that does not require you to add \url{...} or additional commands; like biblatex they expect the raw URL.

The exact handling of the URL with BibTeX depends on the .bst style you use. agsm uses the command \harvardurl which you can redefine. Unfortunately, there is no general consensus amongst all .bst as to how URLs are handled. Many .bst files just use \url{...} directly (notably the standard natbib styles and urlbst), a few like agsm have a dedicated command.

edit: I originally understood the use of \myurl in the MWE as an attempt to uniformly change the URL format for both biblatex and BibTeX styles (possibly independent of other \urls displayed in the document). If that was indeed the aim of \myurl, you need to know how your .bst files handles URLs and redefine the commands involved accordingly. It would be nigh-impossible to pull this off in a style-agnostic fashion. If, on the other hand, you only intended to fix the undesired output of the biblatex URLs in the MWE and you generally want the URLs to be displayed "normally", then there is no need for \myurl at all and you can probably just use the styles as they are. See the edit history for the first version of the answer.

Either way, natbib's definition of agsm's \harvardurl should probably be tweaked a bit to allow for all kinds of URLs.

\documentclass{article}
\def\UseBibLaTeX{0}
\usepackage[hyphens]{url}


\ifnum\UseBibLaTeX>0
\usepackage[backend=biber,style=authoryear]{biblatex}
  \addbibresource{\jobname.bib}
\else
  \usepackage{natbib}
  \renewcommand\harvardurl{\textbf{URL:} \url}
\fi

\begin{filecontents*}{\jobname.bib}
@misc{cite1,
  title  = {One Article with a url that has to break both in bibtex and biblatex},
  author = {Someone NotMe and Someother Me},
  year   = {2018},
  url    = {http://A-long-URL-that-has-to-break-through-lines-and-biber-has-to-ignore-the-\textbackslash url-command},
}
\end{filecontents*}

\begin{document}
\nocite{*}
\ifnum\UseBibLaTeX>0
  \printbibliography
\else
  \bibliographystyle{agsm}
  \bibliography{\jobname}
\fi
\end{document}