ls ( -i option )

UNIX Command

$ls
bp  cfg.c  cfg.h  CVS  gtk  Makefile  sample.cfg  simpleproxy.c
$ls -i
5448322 bp     7569479 cfg.h  5448162 gtk	 925726 sample.cfg
7569478 cfg.c  5448150 CVS    8489600 Makefile	5448260 simpleproxy.c
$

UNIX Explanation

-i, --inode
              print the index number of each file

Theory Drop

In  computing, an  index-node  (inode)  is a  data  structure on  a
traditional Unix-style file system such as UFS. An inode stores all
the  information about  a regular  file, directory,  or  other file
system object, except its data and name.[1]

source : http://en.wikipedia.org/wiki/Inode

Follow Up : readdir ( A Prototype Of dirent.h )