Limit scope of `newcommand` (without using `renewcommand`)? - latex

Question

I'm combining a few latex documents and would like to limit the scope of my macros. I would like something like the following to work:
{
\input{macros1.tex}
\input{body1.tex}
}
{
\input{macros2.tex}
\input{body2.tex}
}
Where body1.tex uses macros defined via newcommand in macros1.tex, and macros2 and body2 are similar. However, macros2.tex might redefine commands found in macros1.tex. I thought that the braces might limit the scope of the macros, but it seems like that is not the case (I get "command already defined errors). Is there a way to solve this without replacing everything in macros2 with renewcommand?

Answer 1

Macros defined inside groups get local scope. However other things, such as length names or custom counters, still become global at least by default as in this answer. While it is possible via packages, author of those files would have to prepare code for this case.
If you uncomment lines (re)defining counters and names in this code below, you get error "<something> already exists"
\begin{filecontents*}[overwrite]{fileA.tex}
\newcommand\xxx{A}
\newlength\lenx
\setlength\lenx{15pt}
\newcounter{cx}
\setcounter{cx}{3}
\xxx, \the\lenx, \thecx
\end{filecontents*}
\begin{filecontents*}[overwrite]{fileB.tex}
\newcommand\xxx{B}
% \newlength\lenx % !!! Error
\setlength\lenx{30pt}
% \newcounter{cx} % !!! Error
\setcounter{cx}{6}
\xxx, \the\lenx, \thecx
\end{filecontents*}
\begin{filecontents*}[overwrite]{fileC.tex}
\newcommand\xxx{C}
% \newlength\lenx % !!! Error
\setlength\lenx{45pt}
% \newcounter{cx} 5 !!! Error
\setcounter{cx}{9}
\xxx, \the\lenx, \thecx
\end{filecontents*}
%%%
\documentclass{article}
\begin{document}
%%% Group A
\begingroup
\input{fileA.tex}
\endgroup
%%% Group B
\begingroup
\input{fileB.tex}
\endgroup
%%% Group C
\begingroup
\input{fileC.tex}
\endgroup
\end{document}