A dumb timer-notifier!

alert.sh

#!/bin/bash

# Alerts a message using `notify-send` at the time specified.
# The time specified should be understood by `date -d` option.

set -e

[ -z "$2" ] && {
    echo "Usage: $0 <time-desc> <message>"
    echo
    echo "Examples:"
    echo "\$ $0 +30mins 'Hello, World!'"
    echo "\$ $0 22:30 'Good night!'"
    exit
}

future_secs=$(date -d "$1" +%s)
now_secs=$(date +%s)
secs_till_future=$((future_secs - now_secs))

echo "Sleeping for $((secs_till_future/60)) minutes $((secs_till_future%60)) seconds"
sleep $secs_till_future && notify-send --urgency=critical "$2"