To send messages to user that log on a Linux System, you can use the “message of the day” function.
Just edit the file
/etc/motd and add there the message you want the users to see each time the log into the system.
sudo vim /etc/motd
Add there something like this:
Be sure to change your password at least once a month. Just enter the command passwd at the prompt, and enter twice the new password The sysadmin
Now when someone, logs into the console, or ssh the system will see that message.