How can I send a custom desktop notification?

There are a bunch of other cool features with notify-send

We can run a command and make it display in the notification:

notify-send <title> <`command`>
notify-send Date "`date`"
notify-send Disk "`df / -H`"

We can use icons with the notifications

notify-send -i <icon> <Message>
notify-send -i face-wink "Hello! January"

Really annoying pop up

notify-send  -t 0 "Bringing down the system"

and

notify-send <title> <message>
notify-send "who am i" "I am January"

For more options check here

Just to add to the other answers, when running the command locally from cron, I use

DISPLAY=:0.0 /usr/bin/notify-send "TITLE" "MESSAGE"

I stumbled upon that one by chance. Answer: use the program notify-send:

notify-send "Hello world!"

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