Simple uptime in pure macOS shell

uptime.sh

#!/bin/bash

set -eu -o pipefail

function machine_uptime_seconds() {
    boottime=$(sysctl -n kern.boottime | perl -n -e'/^.* sec = (\d+)?\,.*$/; print $1')
    now=$(date +%s)
    uptime=$((now - boottime))
    echo $uptime
}

machine_uptime_seconds