$ cat text
first
second
third
fourth
fifth
sixth
seventh
eighth
$ cat nums
1
4
6
7
$ ed -s text < nums
first
fourth
sixth
seventh

##</code><code>##

perl -we 'open N, "<", shift or die; $p = 0; while(defined($n = <N>)) { $s = <> for $p + 1 .. $n + 0; print $s; $p = $n; }' nums text