Shell Script - If and elif and else statement with disk usage example

#!/bin/bash

disk=$(df / | awk 'NR==2 {print $5}' | sed 's/%//')

if [ $disk -lt 50 ]; then
echo "Disk usage is under control."
elif [ $disk -lt 80 ]; then
echo "Warning: Disk usage is moderate ($disk%)."
else
echo "CRITICAL: Disk usage is high ($disk%)! Take action!"
fi