How to make \meaningbody say the command is undefined?

Question

This command come from the question Equivalent of \show to display the LaTeX code in the document, the standard \meaning correctly says a command is undefined,

\documentclass{article}

\makeatletter
\newcommand\meaningbody[1]{%
  {\ttfamily
    \expandafter\strip@prefix\meaning#1}%
}
\makeatother

\begin{document}

\meaningbody\sectio

\end{document}

But that \meaningbody throw the errors when the command is undefined:

! Argument of \strip@prefix has an extra }.
<inserted text> 
                \par 
l.12 \meaningbody\sectio

Then I tried this to fix:

\documentclass{article}

\makeatletter
\newcommand{\meaningbody}[1]
{%
    {\ttfamily
    \ifdefined#1
        \expandafter\strip@prefix\meaning#1
    \else
        Command #1 undefined.
    \fi}%
}
\makeatother

\begin{document}

\meaningbody\sectio

\end{document}

But still do errors:

! Undefined control sequence.
<argument> \sectio 

l.17 \meaningbody\sectio

---

Related questions:

1. What is the meaning of \meaning?
2. How to undo a \def (i.e., Need a \undef capability)

Answer 1

You are putting the undefined command into the message, use \string to render it safe.

\documentclass{article}

\makeatletter

\newcommand{\meaningbody}[1]
{%
    {\ttfamily
    \ifdefined#1%
        \expandafter\strip@prefix\meaning#1%
    \else
        Command \string#1 undefined.%
    \fi}%
}
\makeatother

\begin{document}

\meaningbody\sectio

\end{document}

---

A more complete version, that copes with primitives as well as macros and undefined commands so @egreg can finally find the meaning of \box

\documentclass{article}

\makeatletter
\def\mystripprefix#1>#2{%
  \ifx\relax#2%
    #1 (not macro)%
   \else
    \expandafter\zaplessrelax\expandafter#2%
   \fi}
\def\zaplessrelax#1>\relax{#1}

\newcommand{\meaningbody}[1]
{%
    {\ttfamily
    \ifdefined#1%
        \expandafter\mystripprefix\meaning#1>\relax
    \else
        Command \string#1 undefined.%
    \fi}%
}
\makeatother

\begin{document}

A \meaningbody\sectio


B \meaningbody\section

C \meaningbody\box

\end{document}

Answer 2

A fairly general version that shows more information about macros and doesn't choke with other control sequences. A test whether the argument is a single control sequence could be added.

\documentclass{article}
\usepackage{xparse}

\ExplSyntaxOn
\NewDocumentCommand{\meaningbody}{m}
 {
  \texttt
   {
    \egreg_meaning_body:N #1
   }
 }

\cs_new:Nn \egreg_meaning_body:N
 {
  \cs_if_exist:NTF #1
   {
    \__egreg_meaning_body:N #1
   }
   {
    Command~\token_to_str:N #1~undefined
   }
 }

\cs_new:Nn \__egreg_meaning_body:N
 {
  \token_if_macro:NTF #1
   {
    Command~\token_to_str:N #1~is~a~macro;~
    \str_if_eq_x:nnTF { \token_get_prefix_spec:N #1 } { }
     {
      no prefix;~
     }
     {
      prefix:~\token_get_prefix_spec:N #1;~
     }
    parameter~text:~\token_get_arg_spec:N #1;~
    replacement~text:~\token_get_replacement_spec:N #1.
   }
   {
    Command~\token_to_str:N #1~means~\token_to_meaning:N #1
   }
 }
\ExplSyntaxOff

\begin{document}

\raggedright

\meaningbody\sectio

\meaningbody\mbox

\newcommand*\foo[1]{Whatever #1}
\meaningbody\foo

\meaningbody\box

\meaningbody\alpha

\end{document}