\documentclass{article}
\begin{document}
\section{Test input section}
Part of main document. Input \TeX\ style.
\input fragment1.tex
Back to main document
Part of main document. Input \LaTeX\ style.
\input{fragment1}
Back to main document
Part of main document. Input a jobname style.
\input \jobname.foo
Back to main document
\def\testa{Test A.}
\def\testb{Test B.}
\def\testatestb{Test A and B.}
Part of main document. 
\input fragment2.tex testb
Back to main document
\end{document}
%% TeXbook says input is expandable, expands to null and prepares to read from the file.
%% But, what does it do when expanded in `isolation' ?
%% => ``Missing number, treated as zero.''
%% Strange!
%% At any rate, that implies finding \input inside expand_tokens is allowed to fail!
%\edef\testx{\input fragment1.tex}
%Part of main document.
%\testx
%Back to main document
% This should also fail: couldn't find 2nd arg while scanning.
\def\testc#1#2{Test C (#1)(#2).}
Part of main document.
\input fragment3.tex {Y}
Back to main document
Testing inclusion inside a math environment.
\[  a + \input mathinc.tex
    z 
\]
\end{document}