UK TeX FAQ -- question 238

### 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.

The simple answer is to try `\show`, in a run of LaTeX that is taking commands from the terminal:

    *\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.) We may perhaps, now, wonder about `\@unexpandable@protect`:

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

and we're starting to see how one part of the `\protect`ion 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 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 | the which `.dtx` file contains the definition you are interested in. Doing this for `\protected@edef`, we find:

    %%% From File: ltdefns.dtx

It proves that ltdefns.dtx contains quite a dissertation on the methods of handling `\protect`ion; it also contains some of the 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.

`LaTeX distribution`
  macros/latex/base.tar.gz