How to locate a path

Yes, you can use xargs for this.

For example a simple:

$ locate commands.cfg | xargs grep check_dns

(When grep sees multiple files it searches in each one and enables filename printing along matches.)

Or you can explicitly enable filename printing via:

$ locate commands.cfg | xargs grep -H check_dns

(Just in case one grep is called only with 1 argument by xargs)

For programs that only accept one filename argument (unlike grep) you can restrict the number of supplied arguments like this:

$ locate commands.cfg | xargs -n1 grep check_dns

That does not print the names of files where matched lines are from.

The result is equivalent to:

$ locate commands.cfg | xargs grep -h check_dns

With a modern locate/xargs you can also protect against whitespace issues:

$ locate -0 commands.cfg | xargs -0 grep -H check_dns

(By default whitespace separates input of xargs - which is of course a problem when your filenames contain whitespace ...)

There's no option for that in updatedb.conf. You'll have to arrange to pass options to updatedb manually.

With updatedb from GNU findutils, pass --localpaths.

updatedb --localpaths '/ /media/win_c/somewhere/Music /media/win_c/somewhere/Photos'

With updatedb from mlocate, there doesn't appear a way to specify multiple roots or exclude a directory from pruning, so I think you're stuck with one database per directory. Set the environment variable LOCATE_PATH to the list of databases:

updatedb --output ~/.media.mlocate.db --database-root /media/win_c/somewhere --prunepaths '/media/win_c/somewhere/Videos'

export LOCATE_PATH="$LOCATE_PATH:$HOME/.media.mlocate.db"