Tips from an RHCE: How can I make dd give me a progress report?

Contributed by Andrew C. Dingman

If you’ve been working with Linux very long, you’ve probably encountered dd, the deceptively simple utility for copying a stream of data from here to there. You may have used it to zero a disk before letting it leave the building, to benchmark io hardware by writing a certain number of bytes, to put a disk image on a floppy or usb drive, or even to back up an entire disk.

Like many commands, dd doesn’t generate much output as long as things are going well. This is great for scripting, but can be frustrating when you run it interactively. On large transfers, such as wiping or imaging a disk, it can be a total mystery how much longer you have to wait.

Also like many other commands, dd has some tricks up its sleeve that it will show off if it gets the right signal. In particular, dd will respond to the USR1 signal with a status report on TDERR. It is the same data you’d normally get at the very end of the transfer and will look something like this:

    $ dd if=/dev/zero of=/tmp/demo bs=1M count=1536
    33+0 records in
    33+0 records out
    34603008 bytes (35 MB) copied, 0.355191 seconds, 97.4 MB/s

So how do we send this signal? Using your choice of kill, killall, or pkill. All of these will, by default, send a TERM signal (15), which is not what we want at all. Instead, use the following to send the USR1 signal to all processes you own named dd.

    $ pkill -USR1 ^dd$

Kill and killall support the -USR1 option in the same manner.

If you want a periodic status report, you can extend the same idea with watch in another terminal. Since we’re just sending a signal, there won’t be any output in the display from watch, but it’s a nice shortcut for periodic execution.

    $ watch -n5 -- pkill -USR1 ^dd$

Will send signal USR1 to any dd process you own every five seconds, triggering dd to tell you where it is in the transfer and how fast it’s going. Remember, look at dd’s terminal for the output!

    $ dd if=/dev/zero of=/tmp/demo bs=1M count=1536
    137+0 records in
    137+0 records out
    143654912 bytes (144 MB) copied, 5.66717 seconds, 25.3 MB/s
    249+0 records in
    249+0 records out
    261095424 bytes (261 MB) copied, 11.5736 seconds, 22.6 MB/s
    388+0 records in
    387+0 records out
    405798912 bytes (406 MB) copied, 18.8116 seconds, 21.6 MB/s 

Red Hat’s acclaimed training programs offer a lifetime of experience in a week’s worth of class instruction. And a good portion of the credit for that success goes to the Red Hat Certified Instructors that make a Red Hat training course something special. They have the best tips, and the trickiest tricks. So we thought we’d ask ‘em to share. The information provided in this article is for your information only. The origin of this information may be internal or external to Red Hat. While Red Hat attempts to verify the validity of this information before it is posted, Red Hat makes no express or implied claims to its validity.

This entry was posted by the editorial team on Thursday, August 16th, 2007 at 1:11 pm and is filed under tips and tricks. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.