10,000 lines is not too long. It sounds more like this is a DOS file that has different line terminators, so it looks like one very long line.
If this is so, convert with dos2unix.
Once that is done, you can do:
head -100 yourfile > A
sed -n '101,$p' yourfile > B
Last edited by Mr. C.; 07-07-2008 at 12:32 AM.