Keep back straight and drink water remind by Ubuntu notification.

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Solutions:

1. Create file water.sh

eval "export $(egrep -z DBUS_SESSION_BUS_ADDRESS /proc/$(pgrep -u $LOGNAME gnome-session)/environ)";
DISPLAY=:0 notify-send "Hey, Tran Xuan Nam" "Keep your back straight and drink water!"

2. Make water.sh executable

sudo chmod +x /path/to/water.sh

3. Test notification

/path/to/water.sh

4. Create a job which runs every 15 minutes

crontab -e

5. Add this line to crontab config

*/15 * * * * /path/to/water.sh

6. Enjoy!