Temperature Converter

#!/usr/bin/env bash
set -euo pipefail

celsius="${1:-25}"

if [[ ! "$celsius" =~ ^-?[0-9]+$ ]]; then
	printf 'Please pass a whole Celsius number.\n' >&2
	exit 1
fi

fahrenheit=$((celsius * 9 / 5 + 32))

printf '%s C = %s F\n' "$celsius" "$fahrenheit"