The definitions of LaTeX commands

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.

Using a TeX executable of some sort, the simple answer is to try \show, in a run that is taking commands from the terminal:

*\makeatletter
*\show\protected@edef
> \protected@edef=macro:
->\let \@@protect \protect
  \let \protect \@unexpandable@protect
  \afterassignment \restore@protect \edef .

(I've rearranged the output there, from the rather confused version $\TeX{}$ itself produces.)

So, what about \@unexpandable@protect?:

*\show\@unexpandable@protect
> \@unexpandable@protect=macro:
->\noexpand \protect \noexpand .

and we're starting to see how one part of the \protection mechanism works (one can probably fairly safely guess what \restore@protect does).

Many kernel commands are declared robust:

*\show\texttt
> \texttt=macro:
->\protect \texttt  .

so that \show isn't much help. Define a command \pshow as shown below, and simply execute the command to find its definition:

*\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.)

The command texdef (or latexdef — the same command with a different name) will do all that for you and return the results slightly more tidily than LaTeX itself manages. For example:

$ latexdef texttt

gives:

macro:->\protect \texttt  
 
\texttt :
#1->\ifmmode \nfss@text {\ttfamily #1}%
    \else \hmode@bgroup \text@command {#1}%
          \ttfamily \check@icl #1\check@icr
    \expandafter \egroup \fi .

(again, the output has been sanitised — but we see that latexdef has useful “intelligence” in it, as it has spotted and dealt with the \protect.)

With the -s switch, latexdef will give you a source location:

$ latexdef -s texttt
% latex.ltx, line 3736:
\DeclareTextFontCommand{\texttt}{\ttfamily}

though one should note that it doesn't give you the detail of the actual coding, merely the definition's location.

Environments also surrender their details to latexdef:

$ latexdef -E itemize
 
\itemize:
macro:->\ifnum \@itemdepth >\thr@@ \@toodeep 
  \else \advance \@itemdepth \@ne
    \edef \@itemitem {labelitem\romannumeral \the \@itemdepth}%
    \expandafter \list \csname \@itemitem \endcsname
      {\def \makelabel ##1{\hss \llap {##1}}}%
  \fi 
 
\enditemize:
macro:->\global \advance \@listdepth \m@ne \endtrivlist

(Yet again, this is a sanitised version of the macro definition output, which appears as a single very wide line for each definition.)

If one has a malleable text editor, the same investigation may 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 with %%% From File: that line tells you which dtx file contains the definition you are interested in. Doing this for \protected@edef, we find:

%%% From File: ltdefns.dtx

When we come to look at it, ltdefns.dtx proves to contain quite a dissertation on the methods of handling \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:

%% This is file `article.cls',
%% generated with the docstrip utility.
%%
%% The original source files were:
%%
%% classes.dtx  (with options: `article')

so we need to format classes.dtx to see the definition in context.

All these .dtx files are on CTAN as part of the main LaTeX distribution.

Source: The definitions of LaTeX commands