-Z, --random
Play files in pseudo-random order forever.

ogg123 -Z  ~/path/to/loud-song.ogg 

The obvious thing to note if using as an alarm is make sure you only have loud files in the directory.

Incidentally I was woken at 7;30 AM this morning with this blasting in my ears

https://derryth.com/s/free-software-song-best-version

Hence, I can recommend reading the man page at a sane time of day! If using the at command you can check the queue with

atq 

and clear items in it with atrm queue number

atrm 1 2 4 8

:-)

#!/bin/sh

echo "Hello $USER, please enter a countdown time
Add m for minutes and h for hours, seconds is assumed
For example 1h 1m 1 would set the time to 1 hour, 1 minute and 1 second"

read -r time

# shellcheck disable=2086 # word splitting is needed here
sleep $time

while aplay /path/to/sound/file ; do
   :
done

Or

sleep 300 ; while mplayer loud-song.ogg ; do : ; done

Both versions will play the sound file until it ends (or until the script/command is interrupted with <ctrl>+c).

I'm sorry, I wasn't clear. What I want is for it to repeat the sound file until I tell it to stop repeating.

For Kelsoo's simple command: sleep 300 ; mplayer loud-song.ogg

is there a way to set it so that mplayer continues to play the sound file, until I tell it to stop?

For HoaS's script, same question.

#!/bin/sh

echo "Hello $USER, please enter a countdown time
Add m for minutes and h for hours, seconds is assumed
For example 1h 1m 1 would set the time to 1 hour, 1 minute and 1 second"

read -r time

# shellcheck disable=2086 # word splitting is needed here
sleep $time

aplay /path/to/sound/file

^ Save that as timer then

chmod +x timer
sudo mv timer /usr/local/bin/

Then make a menu entry for it:

[Desktop Entry]
Type=Application
Name=Timer
Generic=Timer
Comment=HoaS' simple timer script
Exec=timer
Terminal=true
Categories=Utility;ConsoleOnly;
Keywords=utility;timer

^ Save that as timer.desktop then

sudo mkdir -p /usr/local/share/applications
sudo mv timer.desktop /usr/local/share/applications

Now log out and back in again and the "Timer" application should be available under "Utilities".

EDIT: the alsa-utils package supplies the aplay command so you'll need to install that along with any required CODECs for the sound file of choice.

at -t ccyymmddhhmm.ss # cc=century yy=year  mm=month dd=day hh=hour mm=minute .ss=seconds (note the dot!) or just don't bother with .ss

Set the time to run

at -t 202201192359

press return

Command to run  e.g.   

ogg123 ~/Music/Rock/loud-song.ogg

press return
press ctrl D

That's it. see "man at" for more options like

Boil an egg

at now + 3 minutes

press "return"

ogg123 ~/path/to/loud-song.ogg

press "return" then press "ctrl D"

I believe  the return after ogg123 ~/path/to/loud-song.ogg is not strictly necessary but I've found it more intuitive and it doesn't hurt. In other words muscle memory. I keep doing it and it works fine! :-$

To switch off the alarm

killall ogg123

Morning alarm clock

at 7:30 AM tomorrow

press "return"

ogg123 ~/Music/Rock/loud-song.ogg

press "return" then press "ctrl D"

Note make sure your volume is up!

I just want a simple timer

KTeaTime is in Devuan 3 repos: https://apps.kde.org/kteatime/

p.s. not using alarm clocks, so i don't have an alternative. still boil the water away occasionally

sleep 300 ; mplayer loud-song.ogg

  works for me

On a related question, I see one potential replacement, gnome-clocks, however it needs avahi-daemon installed. On my list of "things to do" after a fresh OS install is to delete avahi-daemon. I don't have why, just delete it. Is there any reason why having this avahi-daemon is a bad idea? There must have been a reason why I wanted/needed it deleted.