====== Send messages to other users ======
<code bash>
# Findout who is logged on
$ who
su       pts/0        2019-05-14 08:15 (172.16.11.44)
jack     pts/1        2019-05-14 09:47 (172.16.11.44)


# Broadcast a message
$ wall "System will go down in a minute!"

Broadcast message from su@hweb (pts/0) (Tue May 14 09:48:18 2019):

System will go down in a minute!


# Broadcast a message, but do not display the banner (available only to root)
wall -n "System will go down in a minute!" 

System will go down in a minute!


# Send message to a specific user
$ write jack pts/1
</code>