Linux: One time message on login

I am looking for a command similar to wall but will deliver a message to users currently sshed in and if they are not logged in will display upon their next ssh login( but only once ). Context is that I may( not guaranteed ) have a network outage but not everyone on the machine(s) will have gotten the maintenance email and I would rather forward the email to all the users of the machine(s). Not looking to put a message in /etc/motd since that will put up the message upon every login.

There's a standard way of doing this: news. In olden times this was a standard command, but these days it's a bit crusty and has to be installed manually. The Debian/Ubuntu package you want is sysnews.

To use it,

• create a file called something like /var/lib/sysnews/login-message containing your message
• add news to /etc/profile (or equivalent for your users' shell)

When your users log in, they'll see the message the first time, but then .news_time is created in their home directory, and they'll only see news items that have been modified since the timestamp of that file.

Do you have a global /etc/profile that you could do this from? Won't catch everyone and their weird shell of the day, but basically

if [ ! -e "$HOME/.saw-the-message" ];
   echo "The system will go down tomorrow, but I will never tell you again!"
   touch $HOME/.saw-the-message
fi

Problems:

• you'll have to clean up the lock files afterwards
• misses some people who use $weird_shell