file] wall [-n] [
Write to all users. Depending on your Linux distribution, wall uses one of the two syntaxes shown. In both versions, the default is for wall to read a message from standard input and send the message to all users currently logged in, preceded by “Broadcast Message from . . . .” With the first syntax, which comes with Debian-based systems, for example, if file is specified, wall reads input from that file rather than from standard input, and only the super-user can write to a terminal if the user has disallowed messages. With the second syntax, distributed with Red Hat-based systems, for example, the text of the message can be included on the command line, and the message is limited to 20 lines. In this form, if -n is specified, the default banner message is replaced with “Remote broadcast message.” -n can only be specified by the super-user, and only if wall was installed set-group-id.
Send the message contained in the file message.txt to all users:
wall < message.txt