When dd is done with copying data, it prints statistics that show the amount of copied data and the transfer rate such as in the following example:

$ dd if=/dev/zero of=/tmp/fileWithZeros bs=1M count=2
 2+0 records in
 2+0 records out
 2097152 bytes (2,1 MB) copied, 0,00179993 s, 1,2 GB/s

When dd receives the signal USR1 while it is running, it prints its current progress in the same format. For testing, start a process that copies data from /dev/zero to a new file (/tmp/fileWithZeros) indefinitely:

dd if=/dev/zero of=/tmp/fileWithZeros

Afterwards, open second terminal and execute

killall -USR1 dd

In the first terminal, dd prints transfer statistics to standard error. The process can be automatized with the following command (e.g. every 3 seconds)

watch --interval 3 killall -USR1 dd

References

• [1] Detailed description of dd in Wikipedia
• [2] Blog article with instructions how to send the USR1 signal to dd