Is there a way to show notification from bash script in Ubuntu?

If you're using the new notification system in Jaunty, you want the notify-send command

notify-send - a program to send desktop notifications

SYNOPSIS

With notify-send you can sends desktop notifications to the user via
a notification daemon from the command line.  These notifications can be
used to inform the user about an event or display some form of information
without getting in the user's way.

OPTIONS

-u, --urgency=LEVEL
Specifies the urgency level (low, normal, critical).

-t, --expire-time=TIME
    Specifies the timeout in milliseconds at which to expire the notification.
-i, --icon=ICON[,ICON...]
    Specifies an icon filename or stock icon to display.
-c, --category=TYPE[,TYPE...]
    Specifies the notification category.

Found another way, through Zenity

echo 'message:hi' | zenity --notification --listen

or like this:

zenity --notification --text "System update necessary!" 

(This also has the benefit of already being installed on Ubuntu.)

Tested on Ubuntu 14.04, 16.04, 18.04, 20.04. Screenshots from Ubuntu 20.04.

1. [WORKS WELL] Popup notification that auto-closes after 4~10 seconds (somehow tied to your OS settings?):

   notify-send "Hello world"
   

   
   Source: https://superuser.com/a/31919/425838

2. Popup window with buttons to click:

   1. Window does NOT get auto-focus: Source: myself; note: for older Unity-based versions of Ubuntu, such as 16.04, -t is ignored for all values except 0--how stupid. :(. For newer Gnome-based versions of Ubuntu, such as 18.04 or 20.04, -t is ignored entirely. Therefore, on older Unity-based versions of Ubuntu, such as 16.04, using -t 0 causes buttons to show up, but on newer Gnome-based versions, it does NOT. That means that for the Ubuntu 20.04 screenshot shown below, the behavior and look of notify-send -t 0 "Hello world" is exactly identical to notify-send "Hello world" above.

      notify-send -t 0 "Hello world"
      

      

      On Ubuntu 18.04 or 20.04 or later, just add -u critical to the command instead to get it to stay open indefinitely until you click anywhere on it!:

      notify-send -u critical "Hello world"
      

      
      Source: @lucidbrot's comment below this answer, plus my own testing.

   2. OR Window DOES get auto-focus:

      zenity --info --title "Hello" --text "World"
      

      Note: the window will NOT close until you click the OK button.
      
      Source: https://askubuntu.com/a/804475/327339

3. [MY FAVORITE] The window auto-closes after the specified --timeout in seconds, OR after you click the "OK" button!

   zenity --info --title "Hello" --text "World" --timeout=2
   

   Note: the window WILL automatically close after the specified timeout above, in seconds!
   
   Source: myself reading the man pages: man zenity

4. [super ugly-looking]

   xmessage 'hello world'
   

   Note: the window will NOT close until you click the okay button.
   
   Source: http://www.linux-commands-examples.com/xmessage

Play sounds too

1. If you want to play sounds too, along with the window popup, to signify the completion of a command or something, see my other answer here: AskUbuntu.com: How to make a sound once a process is complete?