Compare Time and Dates in Bash. And the Simplest Alarm on Bash

Using bash it is possible to compare time easily in epoch format. If you need to compare date like 14:23 with 14:17 you need to transform it to seconds since epoch and compare as two integers. Here it is my function in ~/.bashrc and examples

# compare_time time1 time2 - say which is greater: 1 or 2; if they are equal say 1.
#  Arguments should fit for utility date.
compare_time() {
        greater='1'
        s1=$(date -d"$1" +%s)
        s2=$(date -d"$2" +%s)
        [ $s2 -gt $s1 ] && greater='2'
        echo $greater
}

After adding this to ~/.bashrc need to source it to terminal for use new function compare_time:

source ~/.bashrc

Let's try with some dates:

compare_time 10:34 02:00
1

compare_time 10:34 22:00
2

compare_time 10:34:55 10:34:41
1

compare_time "2022-08-04" "2022-08-15"
2

compare_time "2022-08-04 15:15" "2022-08-04 11:45"
1

The simplest alarm on Bash

Check time and play music if current time is after necessary date&time point:

# clo until - simplest alarm playing music when date and time reach 'until'
#   argument should fit for utility date.
clo() {
        until="$1"
        while [ $(compare_time "$(date)" "$until") = "2" ]; do
                echo `date +%H:%M` wait until $until
                sleep 10
        done
        echo `date` it\'s time now
        mplayer -really-quiet -volume 40 /home/user/music/alarm_sound.mp3;
}

clo 15:05
15:03 wait until 15:05
..
Fri 12 Aug 2022 03:05:10 PM UTC it's time now
..Playing /home/user/music/alarm_sound.mp3.

Here we use mplayer, that may be you need to install `apt install mplayer` or use another like audacious, vlc, and so on.

Also there is utility to compare date points dateutils.ddiff, it is more complicated.