I'm sure you guys have answered this elsewhere but I can't seem to find where so here goes.

#!/bin/bash
n=120
a=$(sed '120q;d' energy.xvg)
while [ a != exit ];do
a=$(sed $n'q;d' energy.xvg)
echo "$a \n" > newfile
n=$(($n+100))
done
exit 0

that script should read the file energy.xvg, start at line 120, and copy every 100th line to newfile. The problem is that it only outputs to one line in the file and so every line overwrites the one before it. I want it to write to a new line each time.