wall [-n] [ message ]
Wall sends a message to everybody logged in with their mesg(1) permis-
sion set to yes. The message can be given as an argument to wall, or
it can be sent to wall’s standard input. When using the standard input
from a terminal, the message should be terminated with the EOF key
(usually Control-D).
The length of the message is limited to 20 lines. For every invocation
of wall a notification will be written to syslog, with facility
LOG_USER and level LOG_INFO.