Schedule a wake for up to 23 hours in the future, and then shut down

schedule-wake-then-shutdown.sh

#!/bin/bash
set -e

HOURS_FROM_NOW=${1,,}

if ! [[ $HOURS_FROM_NOW =~ ^-?[0-9]+$ ]] ; then
    echo 'Argument "hours from now" must be an integer'
    exit 1
fi

if ! [[ $HOURS_FROM_NOW < 24 ]] ; then
    echo 'Argument "hours from now" must be less than 24'
    exit 1
fi

echo "Clearing existing RTC wake alarms"
sh -c "echo 0 > /sys/class/rtc/rtc0/wakealarm"

echo "Setting wake alarm for $HOURS_FROM_NOW hours from now"
#sh -c "echo `date '+%s' -d '+ "$HOURS_FROM_NOW" hours'` > /sys/class/rtc/rtc0/wakealarm"

NOW=$(date +%s)
WAKE_TIMESTAMP=$((NOW+(HOURS_FROM_NOW*3600)))

sh -c "echo ${WAKE_TIMESTAMP} > /sys/class/rtc/rtc0/wakealarm"

echo "Shutting down now"
/sbin/shutdown -h now