#!/bin/sh
set -eu

usage() {
	cat <<-'EOF'
		Usage:
		  lines START [END]
		  lines -h
	EOF
}

help() {
	cat <<-'EOF'


		Options:
		  -h, --help    show this message

		  START         the nth line number to start showing
		  END           the nth line number to stop
		                showing (default: START + 1)


		Print the range START-END of lines of the content of STDIN.


		Examples:

		  Print 3rd line:

		    $ seq 10 | lines 3
		    3


		  Print lines 5~8:

		    $ lines 5 8 < file.txt
	EOF
}


for flag in "$@"; do
	case "$flag" in
		(--)
			break
			;;
		(--help)
			usage
			help
			exit
			;;
		(*)
			;;
	esac
done

while getopts 'h' flag; do
	case "$flag" in
		(h)
			usage
			help
			exit
			;;
		(*)
			usage >&2
			exit 2
			;;
	esac
done
shift $((OPTIND - 1))

START="${1:-}"

if [ -z "${2:-}" ]; then
	END=1
else
	END=$(($2 - START + 1))
fi

eval "$(assert-arg -- "$START" 'START')"

tail -n +"$START" | head -n "$END"