wall [file] wall [-n] [message]
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, for example, if file is specified, wall reads input from that file rather than from standard input, and only the superuser can write to a terminal if the user has disallowed messages. With the second syntax, distributed by Red Hat, 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 superuser, and only if wall was installed set-group-id.
Send the message contained in the file message.txt to all users:
$ wall < message.txt