4.38 Finding definitions of LaTeX control sequences?

use texdef -t latex <control word>

For example

>texdef -t latex input 
 
\input: 
macro:->\@ifnextchar \bgroup \@iinput \@@input 
 
> 
>texdef -t latex pi 
 
\pi: 
\mathchar"119 
 
 
\the\pi: 
281 
 
>texdef -t latex frac 
 
\frac: 
macro:#1#2->{\begingroup #1\endgroup \over #2}