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Re: Trying to understand man page for dd

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On 05/28/2017 07:40 AM, Thomas Schmitt wrote:
> Hi,
>
> Richard Owlett wrote:
>> It says in part:
>>>     $ dd if=/dev/zero of=/dev/null& pid=$!
>>>     $ kill -USR1 $pid; sleep 1; kill $pid
>> I think it is trying to tell me what I need to know.
>
> dd is started as background process, busily copying bytes from nowhere
> to nowhere:
>
>   dd if=/dev/zero of=/dev/null &
>
> Because of the ampersand "&", the shell prompt comes back immediately
> after the command line is interpreted and performed. This line contains
> the additional assignment of the new process id number to variable "pid":
>
>   pid=$!
>
> The first command in the second line then sends signal USR1 to the
> process that is busily copying bytes in background:
>
>   kill -USR1 $pid
>
> This causes the output lines shown in the example
>
>   18335302+0  records  in  18335302+0 records out 9387674624 bytes
>   (9.4 GB) copied, 34.6279 seconds, 271 MB/s
>
> The second command simply waits a second before it ends:
>
>   sleep 1
>
> The third command sends signal TERM to the dd process:
>
>   kill $pid
>
> which causes it to end without message.
>
>
> In the full documentation at
>   https://www.gnu.org/software/coreutils/manual/html_node/dd-invocation.html#dd-invocation
> a paragraph near the page end gives a more detailed example:
>   "Sending an ‘INFO’ signal (or ‘USR1’ signal where that is unavailable)
>    to a running dd process makes it print I/O statistics to standard error
>    ...
>    # Output stats every second.
>    ...
>    "
>
> Have a nice day :)
>
> Thomas
>
>

*THANK YOU* !!!
I was closer to understanding it than I thought.
I've done a first read of the link you gave.
Not so much for this particular question, but in combination with your 
explanation it will lead to "grokking" some other things.
[For non-R.A.Heinlein readers, see 
https://www.merriam-webster.com/dictionary/grok ;]

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Trying to understand man page for dd Richard Owlett <rowlett@cloud85.net> - 2017-05-28 14:00 +0200
  Re: Trying to understand man page for dd JPlews <jw@plews.org.uk> - 2017-05-28 14:10 +0200
    Re: Trying to understand man page for dd Richard Owlett <rowlett@cloud85.net> - 2017-05-28 14:30 +0200
    Re: Trying to understand man page for dd Ulf Volmer <u.volmer@u-v.de> - 2017-06-02 00:00 +0200
      Re: Trying to understand man page for dd Curt <curty@free.fr> - 2017-06-03 14:00 +0200
  Re: Trying to understand man page for dd The Wanderer <wanderer@fastmail.fm> - 2017-05-28 14:10 +0200
    Re: Trying to understand man page for dd Richard Owlett <rowlett@cloud85.net> - 2017-05-28 14:30 +0200
  Re: Trying to understand man page for dd "Thomas Schmitt" <scdbackup@gmx.net> - 2017-05-28 14:50 +0200
    Re: Trying to understand man page for dd Richard Owlett <rowlett@cloud85.net> - 2017-05-28 15:00 +0200
  Re: Trying to understand man page for dd David <bouncingcats@gmail.com> - 2017-05-29 02:50 +0200
    Re: Trying to understand man page for dd "Thomas Schmitt" <scdbackup@gmx.net> - 2017-05-29 08:30 +0200
      Re: Trying to understand man page for dd Richard Owlett <rowlett@cloud85.net> - 2017-05-29 14:10 +0200
        Re: Trying to understand man page for dd David Wright <deblis@lionunicorn.co.uk> - 2017-05-29 15:40 +0200
          Re: Trying to understand man page for dd Richard Owlett <rowlett@cloud85.net> - 2017-05-29 16:20 +0200
            Re: Trying to understand man page for dd "Thomas Schmitt" <scdbackup@gmx.net> - 2017-05-29 17:20 +0200
            Re: Trying to understand man page for dd David Wright <deblis@lionunicorn.co.uk> - 2017-05-29 17:20 +0200
        Re: Trying to understand man page for dd David <bouncingcats@gmail.com> - 2017-05-30 04:30 +0200
    Re: Trying to understand man page for dd mbroe@oakland.edu - 2017-05-31 15:20 +0200

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