what does this "od" i.e octal dump exactly mean??

rs_vijay · 12-15-2006, 06:58 AM

I have just started studying UNIX frm Kernighan & Pike.Things are better graspable until I encountered this "od" command which he says is for octal dump.

I tried to understand this from the man page but in vain.Again thinking that experimenting comes to the rescue,i just wanted to match standard input with the output ,
so first I tried
"aaaa
^d " n the output was
0000000 060541 060541 000012
0000005 something as five strings against 4 chars

next "aaaaaaaaa
^d " n this time
0000000 060541 060541 060541 060541 005141
0000012
7 strings against 9 chars

next " aaaaa
aaaa
aaa
^d " n this time
0000000 060541 060541 005141 060541 060541 060412 060541 000012
0000017 10 strings against 12+3 chars

so there i struck,found two things in common the first one in each case is " 0000000 " of these three trails and second is each pattern consists of either 6 or 7 digits..
culdn't make anything more.....

hope i am pretty clear abt my problem so can u plz help me....
thnx for the same!!

crabboy · 12-15-2006, 08:37 AM

The key is to the type modifier that od is using. By default od uses a parameter something like this:

Code:

   od -t oS

Looking at the man page, it tells us:

Quote:

o[SIZE]
octal, SIZE bytes per integer

SIZE is a number. For TYPE in doux, SIZE may also be C for sizeof(char), S for sizeof(short),
<snip>

Since the SIZE is a short, each two groups of characters in your example are being interpreted as a single octal number in the output, since the sizeof(short) on your Linux system is 2. If you'd like to see a 1-1 mapping between the output choose the size to be a char.

Code:

# echo aaaa | od -t oS
0000000 060541 060541 000012
0000005

# echo aaaa | od -t oC
0000000 141 141 141 141 012
0000005

The examples above show the output you were seeing on the top and one with char size input on the bottom. The 000000 and 0000005 are not part of your input, they are the starting address and the size of the input.

If you'd like to see the input interpreted as a 1:1 character output or as decimals choose these options respectively:

Code:

# echo aaaa | od -t a
0000000   a   a   a   a  nl
0000005
# echo aaaa | od -t dC
0000000   97   97   97   97   10
0000005

Hope this helps.