Да маны нынче читать уже не модно.
Note that sending a SIGUSR1 signal to a running ‘dd’ process makes it print to standard error the number of records read and written so far, then to resume copying.
$ dd if=/dev/zero of=/dev/null& pid=$!
$ kill -USR1 $pid; sleep 1; kill $pid
10899206+0 records in 10899206+0 records out