The write and wall commands are provided by the sys-apps/util-linux package, but only if you have the tty-helpers use flag enabled.
echo 'sys-apps/util-linux tty-helpers' >> /etc/portage/package.use
Write
Send a message to a specified user.
me@pc ~ $ echo 'hello' | write steph
write: steph is logged in more than once; writing to pts/0
steph will see the following:
Message from me@pc on pts/4 at 20:40 ...
hello
EOF
Wall
Send a message to all logged-in users:
me@pc ~ $ echo 'hello' | wall
Broadcast message from me@pc (pts/4) (Wed Mar 25 20:40:57 2015):
hello
All users see the broadcast message.
