Brief

(You’ll use a linux terminal/console)

Use head -n NUMBEROFLINES file.csv to get the first NUMBEROFLINES of lines. Write it into another file using shell redirection (>) like this:

head -n NUMBEROFLINES file.csv > mynewfile.csv

Note that this will totally recreate mynewfile.csv, if it had any content before it is now deleted forever(-ish).

If you ever happen to want the opposite (last x lines), use tail.

Both tools come with man and info pages (man head or info head – get used to man, though) and a --help flag (head --help actually shows me more or less the man page).

Full example

head -n 10 data.csv >> /tmp/first_and_last.csv # Note the ">>"
tail -n 10 data.csv >> /tmp/first_and_last.csv # Note the ">>"

This would open the file /tmp/first_and_last.csv and attach (>>, > would recreate/delete the file!) the first and the last 10 lines of data.csv at the “end” of /tmp/first_and_last.csv.

Mac OS X:
According to the internet ™ these commands are available in (Unix-based) Mac OS as well (you have to start the Terminal via Finder).

More speaking examples

-n is short for --lines=, so you could also use:

tail --lines=10 data.csv >> addtothisfile.txt
head --lines=10 data.csv >> addtothisfile.txt