Trouble Using dd with cblock

OS Level: AIX Version 4.1.4
Type/Model: Unspecified

In a given file, 200 records are expected to be read in and out with the following command:

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

However, the output produces one extra record. The input file has 20 lines with 638 characters and the output file has 22 lines with 640 characters.

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.

[ Doc Ref: 95634928522000 Publish Date: Apr. 28, 2000]