Trouble Using dd with cblock

Environment

Problem

   dd if=[filename] of=[filename] bs=512 cbs=512 conv=unblock 
conv=sync

Solution

• Altering the command in the following manner removes the extra record.

     dd if=[filename] of=[filename] bs=512 conv=sync
  

  Removing the cbs and conv settings shows that both the input and output files to have 20 lines each.

• The cbs setting gives the dd command the expected size of the record. If the actual image exceeds that size, it inserts a blank line at the bottom of the image file. To view the last line of an image instead of the carriage return, the command tail -2 | head -1 may be used.

• When conv=unblock is specified with the dd command, a new line (/n) is inserted into the output after every cbs number of bytes. Take the following data for example:

     123456789012345678
  

  The following command:

     dd if=file cbs=6 conv=unblock
  

  produces this output

     123456\n789012\n345678\n
  

  The dd command inserted a new line at byte intervals specified by cbs. In this case, the value is 6. If, for example, all of the 8 characters in the sample data are new lines, the output would be

     23456\n7\n9012\n34567\n\n
  

  With conv=unblock specified, new lines already in the data are treated like any other character.