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

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.