Draws squares in Bash

square.sh

#!/bin/bash
function internal_rtfm {
	echo "Draw squares in bash, with different sides and corners"
	echo "Usage: $0 x y z"
	echo " x - size of the square to draw"
	echo " y - character for sides"
	echo " z - character for corners"
	exit
}

function fill_middle {
	for((Y=2; Y < SIDE_LENGTH; Y++)); do
		printf "$1"
	done
}

function internal_draw {
	printf "$1"
	fill_middle "$2"
	echo "$1"
}

function draw_middle {
	for((X=2; X < SIDE_LENGTH; X++)); do
		internal_draw "$SIDE_SIGN" " "
	done
}

if [ "$#" -lt "3" ]; then
	internal_rtfm
fi

SIDE_LENGTH=$1
SIDE_SIGN=$2
CORNER_SIGN=$3

if [ "$SIDE_LENGTH" -le "0" ]; then
	echo "Side length must be a natural number"
	internal_rtfm
elif [ "$SIDE_LENGTH" -eq "1" ]; then
	echo $CORNER_SIGN
	exit
fi

internal_draw "$CORNER_SIGN" "$SIDE_SIGN"
draw_middle
internal_draw "$CORNER_SIGN" "$SIDE_SIGN"