Cite references in the order that they appear in another file

Question

Suppose I have file A.tex that is the main document and has a list of references, say

[1] Reference 1
[2] Reference 2
[3] Reference 3
[4] Reference 4

There is also B.tex which is like a letter and I want to be able to cite some references from A in B, keeping the order as in A. For example, I would like to cite [2] and [4] in B and also have the reference list

[2] Reference 2
[4] Reference 4

at the end of B. Is this possible and how?

Answer 1

Here is a solution using the docmute package.

The idea is to import the other document in a savebox but not actually typeset it. In order to be able to print only those entries cited in our actual main document we will have to use a refsegment. To ensure that all defernumbers features work as expected we also always print the bibliography from the the other document to make sure the numbering is fine.

\makeatletter
\defbibenvironment{plainimport}{}{}{}
\newsavebox{\importbox}
\newcommand{\importcites}[1]{%
  \sbox\importbox{\vbox{%
    \citetrackerfalse
    \begin{refsegment}
      \input{#1}
      \let\blx@anchor\@empty
      \printbibliography[env=plainimport,segment=\therefsegment]
    \end{refsegment}}}}
\makeatother

We then have a command \importcites which takes the file name of the .tex document we want to import the citations from. Use it directly after \begin{document} of your main file.

In the example setup \jobname.tex corresponds to your file B, and \jobname-ext.tex is file A. The technical machinery needs to reside in file B, file A is only ever mentioned in \importcites. You will have to call \printbibliography in file B with segment=\therefsegment (or segment=0) to work properly. If you omit the segment argument you will get to see the references of file A as well.

MWE

\documentclass[british]{scrartcl}  
\usepackage{babel}
\usepackage{csquotes}
\usepackage[style=numeric,backend=biber,sorting=none,backref=true,defernumbers=true]{biblatex}
\addbibresource{biblatex-examples.bib}
\usepackage{docmute}

\usepackage{filecontents}
\begin{filecontents*}{\jobname-ext.tex}
\documentclass[british]{scrartcl}  
\usepackage{babel}
\usepackage{csquotes}
\usepackage[style=numeric,backend=biber,sorting=none]{biblatex}
\addbibresource{biblatex-examples.bib}

\begin{document}
\cite{sigfridsson} and \cite{geer} and \cite{worman} and \cite{cicero}
\end{document}
\end{filecontents*}

\makeatletter
\defbibenvironment{plainimport}{}{}{}
\newsavebox{\importbox}
\newcommand{\importcites}[1]{%
  \sbox\importbox{\vbox{%
    \citetrackerfalse
    \begin{refsegment}
      \input{#1}
      \let\blx@anchor\@empty
      \printbibliography[env=plainimport,segment=\therefsegment]
    \end{refsegment}}}}
\makeatother

\begin{document}
\importcites{\jobname-ext}
\cite{geer} and \cite{cicero}
\printbibliography[segment=\therefsegment]
\end{document}