Please look at these commands:

$ notify-send SYNC TIME!

$ notify-send 'SYNC TIME!'

$ notify-send "SYNC TIME!"

bash: !": event not found

$

The first two commands produce a notification bubble as expected. The third gives the error shown.

and

$ echo SYNC TIME!

SYNC TIME!

$ echo 'SYNC TIME!'

SYNC TIME!

$ echo "SYNC TIME!"

bash: !": event not found

$

Here as well, the echo works for first two commands but not in the third.

More problems here (although I was not planning on using this): both notify-send "SYNC!TIME" and echo "SYNC!TIME" give bash: !TIME": event not found.

But both notify-send and echo work with "SYNC! TIME"

Can someone please explain why the bash: !": event not found error appears?