Here’s a script for creating simple delayed alerts from Terminal. It doesn’t use system notifications, Calendar, or Reminders, just the sleep, afplay, and say commands (making it Mac-only), and optionally LaunchBar large text display.

To use it, just save the script as dontforget in your path and make it executable. Then run a command such as dontforget take a break and stand up in 30m to create a reminder to stretch in half an hour. Your computer will ding and say “take a break and stand up” when the timer runs out. If you set DF_LAUNCHBAR to true, it will also provide a visual alert using LaunchBar’s large text display. In your bash profile, add the line export DF_LAUNCHBAR=true.

It parses a certain amount of natural language, so you can run dontforget to let my dog inside in 10m and after 10 minutes it will say “let your dog inside.”

You can cancel the last added reminder with dontforget cancel. Running dontforget with no arguments will list the upcoming alerts.

Note that this just uses the Unix sleep command to background the reminder for a set interval. If you quit the Terminal session, you won’t get your reminder. It’s designed for short term use while you’re working, not for important, farther-future dates.

Grab the script from this gist.

Update: Via Nick in the comments, a version for Linux using ogg123 and espeak.