I have a very basic question which concerns \renewcommand.
How dangerous it is to use it?
In other words, does the usage of such command can have effects in terms of the stability of the compilation of a file?
Thanks a lot for any feedback.
PS: I am not sure I explained myself.
Apart from the funny
\renewcommand{\fi}{Firenze} % a handy abbreviation
that will produce the amazing error message
! LaTeX Error: Missing \begin{document}.
See the LaTeX manual or LaTeX Companion for explanation.
Type H <return> for immediate help.
...
l.8 \begin{document}
or something like
\renewcommand{\box}[1]{\colorbox{red}{#1}}
that makes TeX suffer until giving up the ghost with
! TeX capacity exceeded, sorry [grouping levels=255].
\rlap #1->\hb@xt@ \z@
{#1\hss }
there are many other commands that should not be redefined under any circumstances. Note that \fi and \box are TeX primitives, so problems are expected if one changes their meaning. However also “handy” abbreviations such as
\renewcommand{\a}{\alpha}
\renewcommand{\b}{\beta}
\renewcommand{\c}{\gamma}
a colleague of mine was fond of are likely to produce mysterious effects. In a frantic email he asked me for help because his paper with a Turkish coauthor refused to typeset. Guess what? The coauthor's surname started with “Ş”, which is quite common a letter in Turkish. Typesetting “Ş” relies on \c being unchanged even if direct UTF-8 input is used. By the way, also \a and \b should not be redefined.
In some cases \renewcommand is however mandatory: if you want to change the interline skip in tables or arrays, you should indeed use
\renewcommand{\arraystretch}{<new factor>}
(the default definition assigns the value 1).
There are cases when \renewcommand is safe, I usually mention
\renewcommand{\phi}{\varphi}
\renewcommand{\epsilon}{\varepsilon}
in order to be sure to use the letterforms which are more common in continental Europe.
In other cases one must use the same care an elephant should in a glassware shop: move very carefully.
It depends what you use \renewcommand on! In general, LaTeX doesn’t provide any “safety” here; if you redefine a macro or even primitive that LaTeX relies on you could end up with either obvious errors or (probably worse) silent problems in the final outcome.
Having said that, in many cases it is expected that you use \renewcommand to alter behaviour, so I wouldn’t say that one should avoid using it.
A stupid sample document what could happen if \renewcommand is used without inspection of the previous code:
Usually \addcontentsline does have 3 arguments, so \addcontentsline{toc}{section}{Some text} will leave Some text in the ToC, formatted as section entry.
However, \renewcommand{\addcontentsline}{Oh my!} removes all three arguments of the macro, leaving their content in the input stream.
\documentclass{article}
\begin{document}
\tableofcontents
\section{Foo}
\renewcommand{\addcontentsline}{Oh my!}
\section{Foobar}
\end{document}
So, in the very end it is quite recommendable to look the definition of a macro up before trying to \renewcommand.
This can be done either by looking into class or package codes (or at latex.ltx, for example), reading the documentation or saying
\meaning\foo in the document body oder \show\foo (can be done in the preamble as well
You can always use \show to see the existing definition of a command (if it is defined). That can be done in Preamble or in the document body. The result will be displayed in your console (or terminal), then the program will halt.
For example, using LuaLaTeX (with utf-8 encoding, \show\a replies that it is a macro, defined as:
\expandafter \@changed@cmd \csname \string #1\endcsname \relax
So I look at that, and I ask myself, is that something I wish to redefine? Probably not! It looks essential, and I'm not quite sure what it does.
On the other hand, \show\i yields:
TU-cmd \i \TU\i
I happen to understand what that means, and it is something I am not likely to need in my own work. So, I can consider renewing that command, if \i would be a handy macro in my own code.
\a is only used in the tabbing environment, so not really essential. On the other hand, \i is part of LICR (LaTeX internal character representation), so it’s in the same class as \c that I mentioned in my answer.
\i.
\renewcommandor change the number of arguments of an existing macro, i.e. a former\newcommand{\foo}[1]{...}would become\renewcommand{\foo}[2]{...}-- this will potentially lead to problems as well as\renewcommand{\foo}{...}, leaving\foo{Some stuff}being\fooand then some textSome stuffthat at best is typeset, at worst it appears in the preamble, breaking compilation\renewcommand{\foo}you should know the old meaning of\foofirst, then act wisely ;-)\usepackage{color}\renewcommand{\box}[1]{\colorbox{red}{#1}}\renewcommand{\box}is a sledge-hammer in order to show the wrong use, isn't it? ;-)\ifdefinedor\ifcsnamebefore "renew" the command that could be just a "new" command (if a package haven't be loaded and we didn't thought of that possibility... etc)