Progress with Your Image

Learn a few different ways to get a progress bar for your dd command.

The dd tool has been a critical component on the Linux (and UNIX) command line for ages. You know a command-line tool is important if it has only two letters, and dd is no exception. What I love about it in particular is that it truly embodies the sense of a powerful tool with no safety features, as described in Neal Stephenson's In the Beginning was the Command Line. The dd command does something simple: it takes input from one file and outputs it to another file, and since in UNIX "everything is a file", that means dd doesn't care if the output file is another file on your disk, a partition or even your active hard drive, it happily will overwrite it! Because of this, dd fits in that immortal category of sysadmin tools that I type out and then pause for five to ten seconds, examining the command, before I press Enter.

Unfortunately, dd has fallen out of favor lately, and some distributions even will advise using tools like cp or a graphical tool to image drives. This is largely out of the concern that dd doesn't wait for the disk to sync before it exits, so even if it thinks it's done writing, that doesn't mean all of the data is on the output file, particularly if it's over slow I/O like in the case of USB flash storage. The other reason people have tended to use other imaging tools is that traditionally dd doesn't output any progress. You type the command, and then if the image is large, you just wait, wait and then wait some more, wondering if dd will ever complete.

But, it turns out that there are quite a few different ways to get progress output from dd, so I cover a few popular ones here, all based on the following dd command to image an ISO file to a disk:

$ sudo dd if=/some/file.iso of=/dev/sdX bs=1M

Option 1: Use pv

Like many command-line tools, dd can accept input from a pipe and output to a pipe. This means if you had a tool that could measure the data flowing over a pipe, you could sandwich it in between two different dd commands and get live progress output. The pv (pipe viewer) command-line tool is just such a tool, so one approach is to install pv using your distribution's packaging tool and then create a pv and dd sandwich:

$ sudo dd if=/some/file.iso bs=1M | pv | dd of=/dev/sdX

In this command, I'm imaging my ISO image to a disk. Notice that the first dd command lists not only the if argument to specify the input file, I also added the bs argument to this side. In general, you will want to add all of your dd arguments to the first dd command.

Option 2: Use kill

The dd command has an often-forgotten feature buried within its man pages. If you send a running dd command a kill -USR1 signal, it will output its current progress. So run the initial dd command in this example, and then in a different terminal, identify its process ID so you can send it the USR1 signal:

$ sudo kill -USR1 <pidofddcommand>

You can use a bit of a shell shortcut if you don't want to identify the PID command independently and put this all in one line:

$ sudo kill -USR1 $(pgrep ^dd)

Option 3: Use dd's Embedded Progress Bar

Many people are unaware that relatively recently, dd added its own live progress option! For the longest time, I was using the USR1 trick until someone told me about dd's new status=progress option added in GNU coreutils 8.24. So now, you just have to type:

$ sudo dd if=/some/file.iso of=/dev/sdX bs=1M status=progress

And, dd will output its progress periodically while it's running!

Kyle Rankin is a Tech Editor and columnist at Linux Journal and the Chief Security Officer at Purism. He is the author of Linux Hardening in Hostile Networks, DevOps Troubleshooting, The Official Ubuntu Server Book, Knoppix Hacks, Knoppix Pocket Reference, Linux Multimedia Hacks and Ubuntu Hacks, and also a contributor to a number of other O'Reilly books. Rankin speaks frequently on security and open-source software including at BsidesLV, O'Reilly Security Conference, OSCON, SCALE, CactusCon, Linux World Expo and Penguicon. You can follow him at @kylerankin.

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