menuentry ... {
....
....
}

....

menuentry ... {
....
....
}

cat $file | sed '/^menuentry.*{/!d;x;s/^/x/;/x{1}/!{x;d};x;:a;n;/^}/!ba;q'

numEntries=$( egrep -c "^menuentry.*{" $file )
for i in $( seq 1 $numEntries); do

    i=$( echo $i | tr -d 'n' ) # A google search indicated sed  encounters problems
                                # when a substituted variable has a trailing return char

    # Get the nth entry block of text with the sed statement from before,
    # but replace with variable $i
    entry=$( cat $file | sed '/^menuentry.*{/!d;x;s/^/x/;/x{$i}/!{x;d};x;:a;n;/^}/!ba;q')

    # Do some stuff with $entry #

done