I read The UNIX Programming Environment by Brian W. Kernighan, Rob Pike and see in section the file system the following example:

...
$ date > x
$ ls -i
15768 junk
15274 recipes
15852 x
$ od -c .
0000000 4 ; . \0 \0 \0 \0 \0 \0 \0 \0 \0 \0 \0 \0 \0
0000020 273 ( . . \0 \0 \0 \0 \0 \0 \0 \0 \0 \0 \0 \0
0000040 252 ; r e c i p e s \0 \0 \0 \0 \0 \0 \0
0000060 230 = j u n k \0 \0 \0 \0 \0 \0 \0 \0 \0 \0
0000100 354 = x \0 \0 \0 \0 \0 \0 \0 \0 \0 \0 \0 \0 \0
0000120

When I have entered this example I see an error message

How to see the internal data structure of the ext3 filesystem?

UNIX != Linux

Linux is a clone of UNIX rather than a direct variant. While many of the same commands exist with many of the same options they are not derived from the same source code and there is no guarantee the author of the Linux code made it do everything the equivalent UNIX code did. Often the Linux command will have additional options not found in the UNIX command.

Even in "UNIX" there are variants (e.g. HP-UX, Solaris, AIX, SCO) and though original source code might have been based on System V UNIX or BSD UNIX they often have their own take on what should be there. What keeps them more or less the same are POSIX standards but that deals with what must be included - it doesn't preclude adding what you think is helfpul.

The issue isn't the ext3 filesystem but the od command itself and it clearly says "." is a directory which is a hint that the Linux od doesn't accept "directories" as "files" as the UNIX od might. (e.g. on my HP-UX server od -c . works just fine).

If really interested you can get the source for the Linux od itself and see what error checking it does that spawns the message about directory. You could then modify the code to make your own od command that acts on directories. Personally I think it would be a lot easier just to say "od -c ./*" - it will show you information for each file just as the "od -c ." would have if it had worked.

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