Too often, I see the recommendation on forums, IRC, and elsewhere across the internet to use improper tools for creating ISO images. For example, consider the following two commands, both of which are not the correct way to build a CD/DVD image:
$ dd if=/dev/scd0 of=cdimage.iso # NO!
Or worse yet:
$ cat /dev/scd0 > cdimage.iso # NO!
As you are probably thinking, the problem with the two commands above, is that they provide no error checking while building the image. So, in order to make sure you have all the bits, you need to use another tool, such as using the MD5 hashing algorithm:
$ md5sum /dev/scd0 cdimage.iso d642d524dd2187834a418710001bbf82 /dev/cdrom d642d524dd2187834a418710001bbf82 cdimage.iso
Thankfully, the hashes above match. But, what if they didn’t? Then, you get to redo your dd(1) command (or, shudder, cat(1)) from above, and then rerun md5sum(1) to make sure you got all the bits. Doesn’t sound like much fun to me. Thankfully, there is a better way, one which handles the checksum while doing the copy.
You want to use readom(1) (“read optical media”) from the wodim(1) (“write optical disk media”) package. Consider the following command:
$ readom dev=/dev/scd0 f=cdimage.iso # YES!
If readom(1) fails to get the bits during the copy, it will let you know that it’s struggling. If it got all the bits, you know you have them all, because of the error checking during the copy. Sure will save you a lot of time running manual hashes when finished.
Now, what about burning a copy of the ISO image? Surely you use dd(1), yes? Something like:
$ dd if=cdimage.iso of=/dev/scd0 # NO!
NO! Instead, use wodim(1) directly:
$ wodim -v -eject cdimage.iso # YES!
For the same reasons that you want to use readom(1) for creating ISO images from CD/DVD, you want to use wodim(1) for burning ISO images to CD/DVD. What happens if after using dd(1) to create your CD/DVD, the md5sum(1) hash doesn’t line up with the image? You didn’t get all the bits, and created a coaster. Use wodim(1) and should it succeed, you can rest assured that you have all the bits.
So, remember, readom(1) and wodim(1) are the tools you want when creating and/or burning ISO images from the command line. Any other tool, and you’re likely doing it wrong.