Question about a ' dd ' command.

[elmo]

Peter Garrett wrote:
> On Fri, 07 Mar 2008 17:20:03 -0500
> elmo <elmo at ne.rr.com> wrote:
>
>   
>> I read somewhere that these two commands are special application of  the
>> basic 'dd' command that will copy and display the transfer action.
>>     
>
> Yes - but these two lines are just an illustration that lasts slightly
> more than a second, and does nothing harmful. It just sends a lot of zeros
> to nowhere  -  /dev/null is a useful black hole ;-)
>   
>>  
>> $ dd if=/dev/zero of=/dev/null& pid=$!
>> $ kill -USR1 $pid; sleep 1; kill $pid
>>
>> I don't understand the null& and pid=$! in the first line 
>>     
>
> Attempted explanation follows... The lines you quote are just intended as
> an example, as you will see.
>
> $ dd if=/dev/zero of=/dev/null
>
> "Send zeros to the bit bucket" ( that is, to nowhere, if you prefer)
>
> &  : Put the previous command in the background.
>
> pid=$!   : Store the process number identifying the "dd" process in a
> variable named "pid" so we can use it later to send signals to the process.
>
> In this case, the signal " kill -USR1 $pid" tells "dd" to output some
> information about what it is doing. See "man signal" ( "kill" doesn't
> always mean destruction, in Bash !)
>
> ;  semicolon divides one command from the next on the command line.
>
> sleep 1  ; Do nothing for one second
>
> kill $pid  : Kill the actual process.
>
> Output here looks like this:
>
> $ dd if=/dev/zero of=/dev/null& pid=$!
> [1] 16690
> $ kill -USR1 $pid ; sleep 1 ; kill $pid
> 42209510+0 records in
> 42209510+0 records out
> 21611269120 bytes (22 GB) copied, 56.7523 seconds, 381 MB/s
>
> As you see, I let it run for nearly a minute.
> The alarming-looking 22 GB actually went nowhere, as explained above.
>
>
>   
>> and I don't
>> understand any of the second line.  There doesn't seem to be any
>> explanation of these anywhere.  It appears to me that the writer used
>> some notations that can be confusing to other guys like me,
>>     
>
> Yes, true that it is confusing if you don't already know.
> Have a look for example at
>
> http://tldp.org/LDP/abs/html/
>
> For more information.
>
>   
>> Is there  a  more complete  discussion of  these  commands somewhere?
>>
>> Could someone please show me what the commands
>> would look like if the simple basic version of ' dd 'is:
>>
>>       dd if =/dev/sda2 of=/dev/sdc3  bs=4096
>>     
>
> If you wanted to see periodic information during the process, you could
> append "&" without the quotes, to that line, as in the example, and assign
> the pid with pid=$!
>
> dd if =/dev/sda2 of=/dev/sdc3  bs=4096 & pid=$!
>
> Then every now and then run
>
> kill -USR1 $pid
>
> The final "kill" after the "sleep" would not be good move, unless you
> wanted to stop the dd process, which you probably don't want to do. You
> could also stop the "kill -USR $pid" command for example with ctrl+c - the
> dd command is by then already in the background ( & ) , so it continues to
> completion unless specifically killed with " kill $pid"
>
> dd is powerful and low level, so experiment at your own risk! That's why
> the commands used /dev/zero as input and sent them to /dev/null - that's
> harmless. If you make a slight error using dd, though, disastrous things
> can happen silently ( say, overwriting your whole partition with zeros
> unintentionally, and so on. )
>
> Peter
>
>   
Thanks all..  I'm beginning to get the picture and feel more comfortable 
about digging into the references you
mentioned.  Now why couldn't the author have taken a few seconds to 
supply just a little more clarification?
elmo