fetch lines in a file with even intervals and window size

evint.sh

#!/bin/sh

# Function:
# print $output_num sections by order,
# each have $sec_num adjacent records
# from file $inp

inp=$1
output_num=$2
sec_num=$3

total_lines=$(<$inp wc -l)
intval=$(($total_lines / $output_num))

for idx in $(seq 1 $intval $total_lines); do
  nr_list=${nr_list}" NR >= $idx && NR < $idx + $sec_num;";
done

awk "$nr_list" $inp