Broadcast Message from source . . .
The command is used to warn all users, typically before shutting down the system. When the -g option is specified, the command uses the /etc/group file to determine which logged in users should receive the message. When the -g option is specified, the message is sent only to members of the specified group, instead of to all users.
The sender must be a privileged user to override any protections the users may have invoked [see mesg(1)].
wall runs setgid [see setuid(2)] to the group ID tty, to ensure write permissions on other user's terminals.
The wall command processes supplementary code set characters according to the locale specified in the LC_CTYPE environment variable [see LANG on environ(5)]. It detects non-printable characters before sending them to a user's terminal. Control characters appear as a ``^'' followed by the appropriate ASCII character; for example, ``\003'' is displayed as ``^C''.
UX:wall:ERROR:Cannot send to . . .