Including only a part of a TEX file

Question

I am writing a report and want to include another TEX file in it, which I update separately. I use the input command

\documentclass[11pt,a4paper]{report}
\usepackage{amsmath,amssymb}
\usepackage{pdfpages}
\usepackage{float}
\begin{document}
...
\input{extra.tex}
...
\end{document}

In this extra.tex file I need only a certain part of the document, the body of the document between BEGIN and END or a certain section. In particular, I don't need to include the file's header.

\documentclass[11pt,a4paper]{report}
% header
\usepackage{amsmath,amssymb}
\usepackage{pdfpages}
\usepackage{float}
\begin{document}
...
% include only
I want to include only this, $E = mc^2$.
% end include
...
I don't want to include this.
...
\end{document}

How do I include only a certain part of this extra.tex file in the main report report.tex?

Answer 1

This is really two questions in one. Let's have one main.tex where you want to include extra.tex:

---

Answer to question 1:

I don't need to include the file's header.

Sharelatex has a nice overview of how to work with multiple files. You can find it here. Here I will use the standalone package in your mainfile, which should do as you want:

\documentclass{article}
\usepackage[subpreambles=true]{standalone}
\begin{document}
  \input{extra}
\end{document}

---

Answer to question 2

How do I include only a certain part of this extra.tex file in the main report report.tex?

I think the best way would be to wrap the parts that you'd want to toggle into environments you could control. Here I suggest the comments package:

% extra.tex
\documentclass{article}
\usepackage{comment}
% Make new environments `hide' and `keep'.
% You could make more of these for the parts you want to toggle!
\excludecomment{hide}
\includecomment{keep}
\begin{document}
  \begin{keep}
      This text will be shown $E = mc^2$.
  \end{keep}
  \begin{hide}
      This text will be hidden.
  \end{hide}
\end{document}

Answer 2

Divide the extra.tex in two files, one with the preamble and commands \begin{document} and \end{document}, and the second with the contents. This way you can input the contents into the new document without problems.

And if you want to selectively include only fragments of your text, use one the conditional compilation packages to select/mark the desired fragments.