Shell Programming and Scripting

Hi

I have to write a shell script called lines
$./lines nameofile thefirstline thelastline
This script should output the lines the user has requested, for example:

For example, .lines myfile 6 11 should output lines 6 to 11 of the file called myfile.
I also need to make sure I show the usage of lines if any wrong arguments are to be used.

Below is my code it shows certain lines of a file but not the lines I had requested, Can you tell me what the problem is with the code below. Thanks.
if [ $# -eq 0 ]
then
echo "$0:Error command arguments missing!"
echo "Usage: $0 start_line uptoline filename"
echo "Where start_line is line number from which you would like to print file"
echo "uptoline is line number upto which would like to print"
echo "For eg. $0 5 5 myfile"
echo "Here from myfile total 5 lines printed starting from line no. 5 to"
echo "line no 10."
exit 1
fi

#
# Look for sufficent arg's
#

if [ $# -eq 3 ]; then
if [ -e $3 ]; then
tail +$1 $3 | head -n$2
else
echo "$0: Error opening file $3"
exit 2
fi
else
echo "Missing arguments!"
fi

This has to be homework - please repost in the special homework forum