Set a timed reminder from the terminal

Question

How can I issue a shell command in my terminal that will alert me with a notification once a given time has passed?

To make it a bit more specific, some things I am looking for are:

• I only need one off alerts, not recurring. For example, systemd timers have such a functionality, but having to create a whole systemd unit and timer every time (and then having to delete it) would be too impractical.
• A simple CLI, so I can do something like remindme "3 hours" "see if thing x has finished yet" and not have to do too much complicated shell magic. Failing that, if it's at least something that can be wrapped in a script, that would be preferable.
• It's enough if the notification is just a string meant for me to read.
• Cross-platform: It should at least work on Linux and Mac.
• The notifications should not require me to leave my shell open.
• If the notification happens through a GUI, it should at least support Xorg and Macs. I assume this will be something like a balloon popup or dialog box; this should support different DEs and not force me to use Gnome for example.
• If the notification happens through a terminal, it would be nice if it worked both in the TTY (without Xorg) and in a terminal emulator under X.
• The notification doesn't have to be right on the dot, for example if I set it to 1 hour, and it actually ends up notifying after 63 mins, that's okay.
• I shouldn't have to run a command to check notifications, since I will inevitably forget to do so. So for example taskwarrior supports various timed tasks, but I wouldn't see those until I run task next.
• It shouldn't overshoot my time delay by too much. For example, if I tried to add a checkreminders command to my bashrc it might actually be a long time before I launch a new shell and see the notifications. The same goes for things like alias ls=checkreminders && ls.
• It should not depend on an external service (eg. webapp or my gmail account). For example, Slack has a reminder bot but I don't want to run Slack all the time on every computer just so I can use the notifications.
• I would prefer if the dependencies were minimal, and I didn't have to install a whole bunch of gigantic programs on every computer where I wish to use this. Smaller CLI programs that are widely available in package managers are okay, if they come with most Linux distros by default, even better.

Answer 1

Usage: remindme -t "3m 1s" -m "I have to do homework"
Time can be simply number meaning second, or have suffix s, m, h or d. See man page for GNU sleep. For macOS you need to convert time into seconds or write a function to do so.

Solution 1: Using wall. Works on Linux, graphically and non-graphically, should work on macOS, atleast non-graphically.

#!/bin/bash
while getopts "t:m:" optname; do
case $optname in
t)
time=${OPTARG}
;;
m)
message=${OPTARG}
;;
esac
done
coproc (sleep $time && wall $message)

Solution 2.1: Desktop notifiaction for Linux and Windows, require notify-send provided by libnotify

#!/bin/bash
while getopts "t:m:" optname; do
case $optname in
t)
time=${OPTARG}
;;
m)
message=${OPTARG}
;;
esac
done
coproc (sleep $time && notify-send -a Reminder "$message")

Solution 2.2: Desktop notification for macOS. Not tested.

#!/bin/bash
while getopts "t:m:" optname; do
case $optname in
t)
time=${OPTARG}
;;
m)
message=${OPTARG}
;;
esac
done
coproc (sleep $time && osascript -e 'display notification "$message" with title "Remainder"')