# Where do I find out how a command/environment is defined?

Say I want to redefine the itemize environment, or make a modified version of it. I want to know how it is currently defined. Where do I find this information? If it's some particular package command, I can just look at the .sty file for that package, but if it's something more basic, I don't know where to look...

Is there a reference manual of where all this stuff is explained? Or is there a file in the depths of my texmf tree I should poke around in?

• Jun 11, 2014 at 14:50
• You might want to look into the package enumitem, it'll provide everything you need for creating new itemize like environments or change the appearance of the old ones. Oct 21, 2015 at 6:55

LaTeX itself is documented in source2e.pdf (texdoc source2e) and the standard classes (article, book, report, etc) are documented in classes.pdf (texdoc classes).

• Isn't source2e more relevant for those of us who don't live on the bleeding edge? Oct 20, 2010 at 13:04
• Heh. Oops. Fixed :) Oct 20, 2010 at 13:35
• I don't know where to find source2e.pdf or the file texdoc. I am using MiKTeX. Jul 14, 2016 at 6:43
• texdoc is a program you use from the Command Prompt. You can easily google for source2e.pdf but be warned some of it is not for the faint-hearted. Jul 14, 2016 at 12:48

To know how a command is defined, you can use the \show command:

\documentclass{article}
\begin{document}
\show\itemize
\end{document}


The log wfile will show:

> \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 .
l.3 \show\itemize


To locate the file containing the definition the script texgrepis useful, which I posted answering this topic: Grepping through an entire texmf tree.

• \show seems to show the last definition, which might be a definition in terms of the same command (overriding). See comments on tex.stackexchange.com/a/185679/1362 for information about \ShowCommand, which can provide more information in these cases.
– Gus
May 30, 2021 at 17:47

There is now the (la)texdef script on CTAN which can be used to display (La)TeX definitions. The current version also support to display the package which defines the macro, however you need to state a list of packages to be loaded. For itemize the usage would be

latexdef -f itemize


or, if only texdef was installed by your distro (latexdef is only a symlink to texdef which activates LaTeX mode):

texdef -t latex -f itemize


which prints:

\itemize is defined by (La)TeX.

\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


To display the definition of any macro from any package use:

latexdef -p package macro


See latexdef --help for more information.

With v1.6 from 2012/05/02 you can also get the original source code for most macros using the -s/--source option:

$latexdef -f itemize -s -E % latex.ltx, line 4556: \def\itemize{% \ifnum \@itemdepth >\thr@@\@toodeep\else \advance\@itemdepth\@ne \edef\@itemitem{labelitem\romannumeral\the\@itemdepth}% \expandafter \list \csname\@itemitem\endcsname {\def\makelabel##1{\hss\llap{##1}}}% \fi} % latex.ltx, line 4565: \let\enditemize =\endlist % latex.ltx, line 4422: \def\endlist{% \global\advance\@listdepth\m@ne \endtrivlist}  Here -E tells that itemize is an environment. As a complement to Stefan's answer: If you don't want to peruse the logfile, but to typeset a command's definition directly in the document, use \meaning instead of \show: \documentclass{article} \usepackage[T1]{fontenc} \begin{document} \meaning\itemize \end{document}  Note: \usepackage[T1]{fontenc} is necessary for correctly typesetting backslashes and braces. None of the answers were as conveniently to use as I'd like, so I put together a small script that basically greps through the whole LaTeX (in my case, TeX Live, hence the name) installation and looks for a single command/environment name. It recognises some (not all) ways to define commands and environments (cf the comments). This sounds like an awful amount of work, but it's indeed blazingly fast. Which makes you wonder why LaTeX "IDE"s don't do something similar. For instance, on a full TeX Live installation, you'd get ~$ tlwhich todo
Files that define command todo:
easy-todo.sty
lilyglyphsStyle.sty
todonotes.sty
todo.sty
tudscrdoc.cls
udesoftec.cls

Files that define environment todo:
ed.sty

Files that provide a package with similar name:
easy-todo.sty
fixmetodonotes.sty
todonotes.sty
todo.sty


Regular expressions (as compatible with grep) are possible:

~tlwhich "align(\*|ed)?" Files that define command align(\*|ed)?: amstex.sty divers.sty dprogress.sty euro.sty Files that define environment align(\*|ed)?: amsmath.sty amstex.sty Files that provide a package with similar name: asyalign.sty  If it does not find something it should, please drop me a comment (or ticket on Github). Environments are nearly always defined using either pairs \def\env..., \def\endenv... (used mainly in the Latex base code) or \newenvironment{env} (used mainly in the classes), so grepping the dtx source files might narrow you in to the code you want that bit faster, e.g., on a unixlike with TEXMF the root of a Texlive install, fgrep -e "\\itemize"TEXMF/texmf-dist/source/latex/base/*.dtx shows the file and text where itemize is defined.

• My distribution does not have a base subfolder in \$TEXMF/texmf-dist/source/latex/... Jun 16, 2017 at 0:45

Reading source code is a great way to learn TeX programming but it's not for the weak of stomach. If you just want to modify an environment you can try the etoolbox package or some of the other answers on the UK TeX FAQ.

Oh, and one more thing: \show\cmd in a TeX file will interrupt processing to tell you the definition of \cmd.

• Reading TeX source makes my brain hurt. But it's good for me, I guess... Oct 21, 2010 at 16:25

## Figure out where the command comes from

### The easy way

Occasionally you can figure out which package contains the macro you're looking for directly with latexdef --find, find, grep, or simply searching on Google/Startpage (something I took a lot of time to figure out: DuckDuckGo is particularly bad at this, unfortunately)

### The hard way

You should know where the command (macro, environment, control sequence) is defined.

For quick reference:

### Addition: What are all these layers?

Also read What is the difference between TeX and LaTeX? and the first comment there which points to https://www.tug.org/levels.html .

• First, there's the core, TeX, which has some 300 primitives.

They're all documented in the TeXBook (you have to buy this one. See Is it actually illegal to TeX texbook.tex? for the legal details -- warning: users on this site are not necessarily lawyers!), and (mostly) TeX by Topic.

(I remember seeing some command in the TeXBook but not in TeX by Topic, but perhaps it's in Plain TeX instead of a primitive.)

• There's also an engine extension (e-TeX).

• Then, there are the TeX engines (tex, pdftex, xetex, luatex).

(analogy: gcc, clang, tcc, Microsoft Visual C++ compiler for C++; or CPython, PyPy, Jython for Python)

It's fairly rare to see an engine-specific command/syntax (such as \directlua or ^^^^), but if it's then reading the documentation of the engine works.

• Then, there's a TeX format (set of macros predefined). The (not necessarily common) formats are "virgin" TeX, Plain TeX, LaTeX, ConTeXt.

(analogy: legacy C headers/C++ standard library/Boost for C++. There isn't a good analogy, as most languages only have one "standard library")

Now that the default format for LaTeX is LaTeX3, both LaTeX2e macros and LaTeX3 macros are automatically loaded.

## Understanding how to use the command

Most of the time, you only need to know how to use the command.

For that, reading the documentation of the package is sufficient. See How to find the documentation for a package? .

Note: If you decide to be lazy and not read through the whole TeXBook/TeX by Topic first, you risk not understanding some concepts to make sense of the documentation. In that case search for the concepts recursively (for example, you can find those in The TeXBook: what is a token? What is category code (catcode)? What is a box's depth/height/width?)

## Patch the command

Also most of the time, you don't need to know how the command is defined to patch it.

You can do something like this

\let \oldcommand=\command
\def \command{something \oldcommand something}


If you really need to know how the command is defined, it's usually beneficial to read through the command's source documentation (the .dtx file compiled to a .pdf file) if available, (otherwise read the source code, instead of the .sty file, which is a stripped-comment version) which usually contains explanation on how particular things are defined.

For instance, for the unicode-math package, it can be accessed at "Documentation of the source code" link in CTAN page.

## Note

• How can I know how to use this command/macro/environment?
• Where can I look up LaTeX commands?
• How can I understand what this answer found on the site/snippet of code I found somewhere is doing?

Related questions:

• tl;dr: The difficulty I faced while learning TeX as a programming language, and what I hoped I found earlier. Instead of complaining how difficult it is to learn ☹️, I try to make it easier... Nov 6, 2021 at 16:26
• Nice "summary". Possibly helpful hint: to identify a TeX primitive, all of these are starred in the index of the TeXbook. Dec 18, 2021 at 17:02
• Sometimes ltluatex too: luatex - How to register callbacks in lualatex? - TeX - LaTeX Stack Exchange Feb 2 at 10:55
Just concerning itemize: This is defined in the basic latex file latex.ltx. It is redefined,e.g. in the enumitem package, i.e, in enumitem.sty.
• source2e.pdf has an annotated version of latex.ltx. Oct 21, 2010 at 3:12
i would say that merely looking at the output of \show isn't going to get you very far: itemize is a refinement of trivlist so you need to read that definition too.