dd

The dd command without any options simply reads stdin and writes stdout; it's a copier like cat ("dd" stands for datadumper). If that were the end of it, there would be no reason for it to exist.

However, it can do much more, such as converting ASCII to EBCDIC and vice versa. The need for that has almost (but not quite) disappeared, yet dd still has use. It can block and unblock records (converting vast sequences of bytes to line oriented data and vice versa), programatically skip or seek past input or output (useful for reading or writing X bytes into a file), and more.

Given a file "t" containng "123456790",

 dd if=t bs=1 skip=3
 

produces "4567890", and

echo "abc" | dd of=t seek=3 count=2 bs=1 conv=notrunc
 

leaves "t" as "123ab67890".

If "t" is instead:

123467890
123467890
123467890
123467890
 

Then "dd if=t conv=block cbs=10 " will produce exactly 40 bytes of "1234567890123456789012345678901234567890". Given those 40 bytes, "dd if=u conv=unblock cbs=5" will give you back 8 lines:

12345
67890
12345
67890
12345
67890
12345
67890
 

At the end of any transfer, dd always reports the number of full and partial blocks it read and wrote. Given our same file "t"

echo "1234567890" > t
dd if=t ibs=3 obs=4 
1234567890
3+1 records in
2+1 records out
dd if=t ibs=3 obs=1
1234567890
3+1 records in
11+0 records out
 

Those partial blocks can be important - see dd to and from tape; using diff to compare.

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