There is a utility called "wall" to send messages to all the users who are logged in.

I hv. a setup of 250 thin clients having mozilla browser in build. Through this mozilla browser the users are accessing the application developed under LAMP on a server from these thin clients.

While shutting down the server I didn't came to know that somebody is working i.e. using the server's application or not. Hence I want to know the method by which i can pass a message to all such users.

Secondly on my another server nearabout 50 users are logged in via X session. I want to send message to all these at a time.

Is there any similar utility which will send message to all the gui users under Red Hat Linux?

When you use the shutdown command you get an option to notify all users on the system and to give them time to log off.

Thats what i do when i need to reboot a hpux box anyway, but i'm sure i have seen the same command in linux.

No doubt that shutdown sends message while shutting down the server and it waits until the grace period is over. But after that specific grace period the server gets shutdown.

In case if any users informs me that he is working then i will have to kill the shutdown process manually. Instead of that if there is any utility then i can wait for 5-10 minutes for the user call,if not replied by him then i can directly shutdown the machine.

shutdown -k
Don't really shutdown but just send the message?