dd seek problem Post: 302230820

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Top Forums UNIX for Advanced & Expert Users dd seek problem Post 302230820 by silvermoon on Sunday 31st of August 2008 03:35:36 PM

08-31-2008

Registered User

Quote:

Originally Posted by era

Can you examine the resulting file? I assume you get the result from the first experiment in the first 281,881 bytes after the second operation, and its contents are 2 x 281,881 bytes (but of course, in the absence of a file system, it's hard to tell). What about when you copy it back, do you get padding on either end of the file, or the wrong number of bytes, or the wrong contents?

When it's copied back, I get the same size file. When I view the contents of the file, it has got the last few hundred bytes of the jpg. The rest of the file is zero's that were placed on the memory stick before the copy operation occurred.

I finally realized that default 550 obs was the issue when combined with skip, whereas seek used 281881.

The program I'm writing is in Python. My goal is to write a file at any location I want on any drive. It's part of an encryption idea, where the encryption decides where to store the file rather than the file management system. That's when I need to seek a number of bytes that isn't the ibs. I'm not sure the dd is going to help me with this. But I can't say that with full confidence, because I don't fully understand how it works yet.

silvermoon

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dd

DD(1)							      General Commands Manual							     DD(1)

NAME

       dd - convert and copy a file

SYNOPSIS

       dd [ option value ] ...

DESCRIPTION

       Dd  copies  the specified input file to the specified output with possible conversions.	The standard input and output are used by default.
       The input and output block size may be specified to take advantage of raw physical I/O.	The options are

       -if f   Open file f for input.

       -of f   Open file f for output.

       -ibs n  Set input block size to n bytes (default 512).

       -obs n  Set output block size (default 512).

       -bs n   Set both input and output block size, superseding ibs and obs.  If no conversion  is  specified,  preserve  the	input  block  size
	       instead of packing short blocks into the output buffer.	This is particularly efficient since no in-core copy need be done.

       -cbs n  Set conversion buffer size.

       -skip n Skip n input records before copying.

       -iseek n
	       Seek n records forward on input file before copying.

       -files n
	       Catenate n input files (useful only for magnetic tape or similar input device).

       -oseek n
	       Seek n records from beginning of output file before copying.

       -count n
	       Copy only n input records.

       -conv ascii    Convert EBCDIC to ASCII.

	    ebcdic   Convert ASCII to EBCDIC.

	    ibm      Like ebcdic but with a slightly different character map.

	    block    Convert variable length ASCII records to fixed length.

	    unblock  Convert fixed length ASCII records to variable length.

	    lcase    Map alphabetics to lower case.

	    ucase    Map alphabetics to upper case.

	    swab     Swap every pair of bytes.

	    noerror  Do not stop processing on an error.

	    sync     Pad every input record to ibs bytes.

       Where  sizes are specified, a number of bytes is expected.  A number may end with or to specify multiplication by 1024 or 512 respectively;
       a pair of numbers may be separated by to indicate a product.  Multiple conversions may be specified in the style:

       is used only if or conversion is specified.  In the first two cases, n characters are copied into  the  conversion  buffer,  any  specified
       character  mapping  is  done, trailing blanks are trimmed and new-line is added before sending the line to the output.  In the latter three
       cases, characters are read into the conversion buffer and blanks are added to make up an output record of size n.   If  is  unspecified	or
       zero,  the  and	options  convert the character set without changing the block structure of the input file; the and options become a simple
       file copy.

SOURCE

       /sys/src/cmd/dd.c

SEE ALSO

       cp(1)

DIAGNOSTICS

       Dd reports the number of full + partial input and output blocks handled.

																	     DD(1)