How can I send a message for all the users connected into the system at the same time? Let's say I need to reboot the server and I ask the users to save their jobs becasue the server will be rebooted?
man wall
regards
Thanks lijiajin,
Here is a bit more info for the command:
wall
This commands stands for: write all and is used to send messages to the currently logged users.
It is usually located in this directory:
/usr/bin/
This directory is not defined in the PATH so you will have to type in your shell the full path name to invoke it as follows:
/usr/bin/wall
For more information about this command and arguments to use, see:
man wall
i think you mean something like:
/usr/bin/wall
/etc ist for config files... (afaik)
Yep, you're right. 
Thanks!
Looking up for some information in the web I also find the command:
rwall
Which also sends messages to all users on the network, here are some variants of the command:
Send a message to a specified host:
/usr/sbin/rwall hostname
Send a message to a specified network:
/usr/sbin/rwall -n network-group
Send a message to a specified host on a network:
/usr/sbin/rwall -h hostname -n network-group
You can check for some information related with this command right here:Commands Reference, Volume 4 - rwall Command