The Cult of DD

You’ll often see instructions for creating and using disk images on Unix systems making use of the dd command. This is a strange program of obscure provenance that somehow, still manages to survive in the 21st century.

Actually, using dd is almost never necessary, and due to its highly nonstandard syntax is usually just an easy way to mess things up. For instance, you’ll see instructions like this asking you to run commands like:

# Obscure dd version
dd if=image.iso of=/dev/sdb bs=4M

Guess what? This is exactly equivalent to a regular shell pipeline using cat and shell redirection:

# Equivalent cat version
cat image.iso >/dev/sdb

That weird bs=4M argument in the dd version isn’t actually doing anything special—all it’s doing is instructing the dd command to use a 4 MB buffer size while copying. But who cares? Why not just let the command figure out the right buffer size automatically?

Another reason to prefer the cat variant is that it lets you actually string together a normal shell pipeline. For instance, if you want progress information with cat you can combine it with the pv command:

# Cat version with progress meter
cat image.iso | pv >/dev/sdb

There’s an obscure option to GNU dd to get it to display a progress meter as well. But why bother memorizing that? If you learn the pv trick once, you can use it with any program.

If you want to create a file of a certain size, you can do so using other standard programs like head. For instance, here are two ways to create a 100 MB file containing all zeroes:

# Obscure dd version
dd if=/dev/zero of=image.iso bs=4MB count=25

# Regular head version
head -c 100MB /dev/zero >image.iso

The head command is useful for lots of things, not just creating disk images. Therefore it’s a better investment of your time to learn head than it is to learn dd. In fact, you probably already know how to use it.

I will confess: there are some interesting options that dd has, which aren’t easily replicated with cat or head. For instance, you can use dd to convert a file between ASCII and EBCDIC encodings. So if you find yourself doing that a lot, I won’t blame you for reaching for dd. But otherwise, try to stick to more standard Unix tools.