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