Copy/Paste part of a file into another file using Terminal (or Shell)
if you know how many lines are in your source file (wc -l) you can do this .. assume 12000 lines and you want lines 2000 - 7000 in your new file (total of 5000 lines).
cat myfile | tail -10000 | head -5000 > newfile
Read the last 10k lines, then read the 1st 5k lines from that.
sed command should work fine, replace double quotes with single quotes.
sed -n '1000, 1000000p' path/first/file > path/second/file