Break a file into chunks

chunk.sh

#!/bin/bash

if [ $# -lt 3 ] ;
then
    echo "Usage: $0 inputfile output_prefix lines_per_chunk [header=1]"
    exit
fi

header=1
if [ $# -eq 4 ] ;
then
  header=$4
fi

lines=$(wc -l $1|awk '{print($1)}')
numchunks=$(($lines/$3))+1

START=1
END=numchunks
for ((i=$START; i<=$END; i++))
do
  if [ $header -eq 0 ];
  then
    startchunk=$((($i-1)*$3+1))
  else
    startchunk=$((($i-1)*$3+2))
  fi
  endchunk="$(($startchunk + $3 - 1))p"
  if [ $header -eq 0 ];
  then
    sed -n $startchunk,$endchunk $1 > "$2_chunk$i"
  else
    sed -n -e $startchunk,$endchunk -e 1p $1 > "$2_chunk$i"
  fi
done