Menu Calculator

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

operation="${1:-add}"
left="${2:-10}"
right="${3:-5}"

case "$operation" in
	add)
		result=$((left + right))
		;;
	sub)
		result=$((left - right))
		;;
	mul)
		result=$((left * right))
		;;
	div)
		if ((right == 0)); then
			printf 'Cannot divide by zero.\n' >&2
			exit 1
		fi
		result=$((left / right))
		;;
	*)
		printf 'Usage: %s {add|sub|mul|div} LEFT RIGHT\n' "$0" >&2
		exit 1
		;;
esac

printf '%s %s %s = %s\n' "$left" "$operation" "$right" "$result"