I am trying to use ddrescue to recover data off a hard drive and I
have the map file of what it was able to recover.  I can get ddrescue
to focus on a single area by having what is essentially a single line
map file


<base byte> <byte length> +

yes, this is sort of the opposite of what one normally uses the domain
map file for (i.e. this is the good content from the source that you
should read, I'm trying to use it to focus the reads and repeat on a
constrained area)

however, if I try to put in multiple lines like that

<base1> <span length 1> +
<base 2> <span length 2> +

it fails as this isn't a considered a valid map file (I guess all
spans have to be contagious).

I think this could be a useful addition, if not abusing the domain map
file as I am, but as perhaps a "focus map file" that accepts just
"valid spans" to enable you to focus your recovery on.

why might one want to do this?

If one is trying to recover a file system and there are specific files
that one cares about, one can have a utility that outputs the "focus"
file and you can then let ddrescue run for a long time and do the best
it can do, without it trying to read data that is unimportant to you
at the moment.

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