1. test if the command notify-send is installed
notify-send "Hello"
If the command is not found, install the required package following the tips
2. vi notify.sh and input the below content into it.
#!/bin/bash
DISPLAY=:0.0 /usr/bin/notify-send -t 30000 "<b>Take a Break!</b>"
3. chmod +x notify.sh
4. crontab -e
5. add the below content as a new line in the opened file. and then save it
*/45 * * * * /home/jimmy/notify.sh
6. Done!