I'm using the sed command to pull a line from a file and place it into another file:

sed -n '1p' students >tmp

this pulls the first line, I want to change the 1 to a variable which will be a counter to go through the rest of the lines but can't get it. I've tried:

sed -n '$cp students >tmp
sed -n $cp students >tmp

neither work. Any ideas? Thanks!