What is the 'bs' option in dd?

Question

I've found conflicting information on the internet on whether the bs in a command like dd bs=512 if=/home/Downloads/ubuntu.iso of=/dev/sdb stands for "bytes per second" or "block size". I couldn't find anything in the man pages, as the only reference to bs is:

bs=BYTES

read and write up to BYTES bytes at a time

I know, that was just the first number that popped into my head while writing that — user8292439, Apr 03 '18 at 13:02

score 6 · Accepted Answer · answered Apr 03 '18 at 13:00

As with other GNU Coreutils utilities, the primary documentation is the info page.

From info coreutils dd invocation :

‘bs=BYTES’
     Set both input and output block sizes to BYTES.  This makes ‘dd’
     read and write BYTES per block, overriding any ‘ibs’ and ‘obs’
     settings.  In addition, if no data-transforming ‘conv’ option is
     specified, input is copied to the output as soon as it’s read, even
     if it is smaller than the block size.

There are corresponding parameters for the individual read, write and conversion block sizes:

‘ibs=BYTES’
     Set the input block size to BYTES.  This makes ‘dd’ read BYTES per
     block.  The default is 512 bytes.

‘obs=BYTES’
     Set the output block size to BYTES.  This makes ‘dd’ write BYTES
     per block.  The default is 512 bytes.

‘cbs=BYTES’
     Set the conversion block size to BYTES.  When converting
     variable-length records to fixed-length ones (‘conv=block’) or the
     reverse (‘conv=unblock’), use BYTES as the fixed record length.

What is the 'bs' option in dd?

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