How do I use \renewcommand to get BACK my greek letters?

Question

I'm a LaTeX newbie, but I've been doing my homework, and now I have a question that I can't seem to find the answer to. I create the definition of an equation, let's just say it's this one:

The potential is characterized by a length $\sigma$ and an energy $\epsilon$.

In reality, this equation is more complex, which is why I wanted to try a shortcut. If my equation were this simplistic, I wouldn't try my substitution technique. I use the \renewcommand to save me some time:

\renewcommand{\sigma}{1}

And this works fabulously and will replace all instances of sigma with 1. Unfortunately though, since \sigma has a global scope, I need to reset it. I tried a couple different ways:
Attempt 1: -deadlock due to circular reference?

\newcommand{\holdsigma}{\sigma}
\renewcommand{\sigma}{1}
The potential is characterized by a length $\sigma$ and an energy $\epsilon$.
\renewcommand{\sigma}{\holdsigma}

I would think to reset the command, it should look something like this:

\renewcommand{\sigma}{\greek{\sigma}}

but that obviously hasn't worked out for me.

Any idea about how the greek letters are originally defined in the language?

Answer 1

I have to admit that I don't understand why you want to do what you're asking, but this should work:

\documentclass{article}
\begin{document}

Before redefinition, \verb|\sigma| looks like $\sigma$.

% Copy the current definition of \sigma to \oldsigma
\let\oldsigma\sigma

% Redefine \sigma to be '1'
\renewcommand{\sigma}{1}

After redefinition, \verb|\sigma| looks like $\sigma$.

You can still use \verb|\oldsigma| if you want to use the original definition $\oldsigma$.

% Restore the original definition of \sigma
\let\sigma\oldsigma

Now \verb|\sigma| is back to its normal appearance $\sigma$.

\end{document}