How to compute sha256sum

[Tony Nelson]

On 09-11-19 00:11:27, Reg Clemens wrote:
> This may seem to be a stupid question, but .
> 
> I can download a iso image of Fedora, then do a sha256sum of the file
> and I get the result shown in the SHASUM file.
> 
> I can burn a DVD with k3b, ask for it to verify the checksum, and
> have it say the checksum is OK.
> 
> But if I just do
> 	sha256sum /dev/cdrom
> 
> I get a result which is NOT the published sum, even tho k3b said the
> sum was going to be OK.
> 
> So what am I doing wrong?  (and yes cdrom and dvd both point to sr0)

The kernel often copies a bit more data that was requested from a CD / 
DVD (I don't understand why, but I don't think dd is a workaround).  
What I do is just compare the CD / DVD with the original file with `cmp 
dev/dvd my.iso` and I usually get an error like "EOF on my.iso", which 
means that all the data in the original file matches the DVD.

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