There are several reasons to want to know the definitions of LaTeX commands: from the simplest "idle curiosity", to the pressing need to patch something to make it "work the way you want it". None of these are pure motives, but knowledge and expertise seldom arrive through the purest of motives.
The simple answer is to try \show, in a run of LaTeX that is
taking commands from the terminal:
(I've rearranged the output there, from the rather confused version TeX itself produces.) We may perhaps, now, wonder about*\show\protected@edef > \protected@edef=macro: ->\let \@@protect \protect \let \protect \@unexpandable@protect \afterassignment \restore@protect \edef .
\@unexpandable@protect:
and we're starting to see how one part of the*\show\@unexpandable@protect > \@unexpandable@protect=macro: ->\noexpand \protect \noexpand .
\protection
mechanism works (one can probably fairly safely guess what
\restore@protect does).
Many kernel commands are declared robust:
so that*\show\texttt > \texttt=macro: ->\protect \texttt .
\show isn't much help. Define a command \pshow as
shown below, and use that instead:
*\def\pshow#1{{\let\protect\show #1}}
*\pshow\texttt
> \texttt =\long macro:
#1->\ifmmode \nfss@text {\ttfamily #1}%
\else \hmode@bgroup \text@command {#1}%
\ttfamily \check@icl #1\check@icr
\expandafter \egroup \fi .
Note that the command name that is protected is the 'base' command,
with a space appended. This is cryptically visible, in a couple of
places above. (Again, the output has been sanitised.)
If one has a malleable text editor, the same investigation may more comfortably be conducted by examining the file latex.ltx (which is usually to be found, in a TDS system, in directory tex/latex/base).
In fact, latex.ltx is the product of a docstrip
process on a large number of .dtx files, and
you can refer to those instead. The LaTeX distribution includes a file
source2e.tex, and most systems retain it, again in
tex/latex/base. Source2e.tex may be processed to
provide a complete source listing of the LaTeX kernel (in fact the
process isn't entirely straightforward, but the file produces messages
advising you what to do). The result is a huge document, with a
line-number index of control sequences the entire kernel and a
separate index of changes recorded in each of the files since the
LaTeX team took over.
The printed kernel is a nice thing to have, but it's unwieldy and sits on my shelves, seldom used. One problem is that the comments are patchy: the different modules range from well and lucidly documented, through modules documented only through an automatic process that converted the documentation of the source of LaTeX 2.09, to modules that hardly had any useful documentation even in the LaTeX 2.09 original.
In fact, each kernel module .dtx file will process separately
through LaTeX, so you don't have to work with the whole of
source2e. You can easily determine which module defines the
macro you're interested in: use your "malleable text editor" to find
the definition in latex.ltx; then search backwards from that
point for a line that starts
%%% From File: - that line
tells you which .dtx file contains the definition you are interested
in. Doing this for \protected@edef, we find:
When we come to look at it, ltdefns.dtx proves to contain quite a dissertation on the methods of handling%%% From File: ltdefns.dtx
\protection; it
also contains some automatically-converted LaTeX 2.09 documentation.
And of course, the kernel isn't all of LaTeX: your command may be
defined in one of LaTeX's class or package files. For example, we
find a definition of \thebibliography in article, but
there's no article.dtx. Some such files are generated from
parts of the kernel, some from other files in the distribution. You
find which by looking at the start of the file: in article.cls,
we find:
so we need to format classes.dtx to see the definition in context.%% This is file `article.cls', %% generated with the docstrip utility. %% %% The original source files were: %% %% classes.dtx (with options: `article')
All these .dtx files are on CTAN as part of the main LaTeX distribution.
This question on the Web: http://www.tex.ac.uk/cgi-bin/texfaq2html?label=ltxcmds