Find out which package enables a given environment or command

Question

How do I find out the package that enable the compilation of a tex file containing a given \begin{X}...\end{X} environment?

Answer 1

I came up with this macro \findpackagebycommand:

\makeatletter
% patched command of loading a package
\def\find@load#1[#2]#3[#4]{%
    % load the package
    \find@fileswith@pti@ns{#1}[#2]{#3}[#4]%
    % check if commands exists now
    \ifcsname\find@command\endcsname
        \typeout{Package #3 introduces command \find@command.}%
        % command has been found, revert to original version without checks
        \let\@fileswith@pti@ns\find@fileswith@pti@ns
    \else
        % somehow, \@fileswith@pti@ns is restored after loading a package
        % thus, patch it again
        \let\@fileswith@pti@ns\find@load
    \fi
}%

\newcommand*{\findpackagebycommand}[1]{%
    % using this multiple times - esp. when the package has not been found yet -
    % will break things. Thus, check first that \find@command has never been defined before
    \ifx\find@command\undefined
        \def\find@command{#1}%
        % first, check if this command is already defined
        \ifcsname\find@command\endcsname
            % in this case, just issue a warning and do nothing
            \@latex@warning@no@line{Command \find@command \space is already defined}%
        \else
            % overwrite the internal \@fileswith@pti@ns command, which does the actual loading
            % \@fileswith@pti@ns is used internally by \usepackage and \RequirePackage
            \let\find@fileswith@pti@ns\@fileswith@pti@ns
            \let\@fileswith@pti@ns\find@load
        \fi
    \else
        % used multiple times - prevent and give a warning
        \@latex@warning@no@line{You can use \protect\findpackagebycommand \space only once}%
        \errmessage{Invalid use of command findpackagebycommand.}%
    \fi
}

\makeatother

It overwrites an internal command used by \usepackage with a simple test if the name of the command/environment is defined after loading the package.

This is not perfect and has some drawbacks:

• it does not separate the packages if you use \usepackage{foo,bar,baz}. It simply shows you that this command is loaded by one of these packages.
• It cannot be used multiple times. Some more logic will be required to enable searching for multiple commands at once.

Use it like this, before loading any other packages (and the document class, if you wish):

\makeatletter
\newcommand*{\findpackagebycommand}[1]{
    … like above …
}
\makeatother

\findpackagebycommand{lstlisting}

\documentclass{article}

\usepackage[utf8]{inputenc}
\usepackage{amsmath}
\usepackage{listings}
\usepackage[T1]{fontenc}

\begin{document}

Test

\end{document}

The name of the package will be shown in the latex output, line "Package listings introduces command lstlisting.":

This is pdfTeX, Version 3.14159265-2.6-1.40.17 (TeX Live 2016/Arch Linux) (preloaded format=pdflatex)
 restricted \write18 enabled.
entering extended mode
(./packagetest.tex
[…]
(/usr/share/texmf-dist/tex/latex/base/inputenc.sty […])
(/usr/share/texmf-dist/tex/latex/amsmath/amsmath.sty […])
(/usr/share/texmf-dist/tex/latex/listings/listings.sty
(/usr/share/texmf-dist/tex/latex/graphics/keyval.sty)
(/usr/share/texmf-dist/tex/latex/listings/lstmisc.sty)
(/usr/share/texmf-dist/tex/latex/listings/listings.cfg))
Package listings introduces command lstlisting.
(/usr/share/texmf-dist/tex/latex/base/fontenc.sty
(/usr/share/texmf-dist/tex/latex/base/t1enc.def))
[…]

(When loaded by an indirect package loading (e.g. \define@key defined by some package loading keyval), it prints the chain of loaded packages backwards, but at different lines. This can be improved.)