One of the cool features of UNIX is that you don’t need to have any software installed to send message to users logged on to that machine. There is a handy command called “write” which enables you to do this. Here is the syntax of this command:
$write username tty
Now the question arises, how to know the tty of a particular user. The “who” command of UNIX comes in to play here. Just shoot “who” command on your terminal and see the magic. You will receive the list of all users logged on to that machine with their tty. Below is an example.
$who root :0 May 7 03:51 root pts/1 May 7 03:51 (:0.0) root pts/2 May 7 03:51 (:0.0) user5 pts/10 May 7 04:17 (dhcp-dwf1-4-10-176-162-74) user8 pts/13 May 7 04:12 (dhcp-dwf1-4-10-176-162-180) user9 pts/15 May 7 04:12 (10.176.146.212) user7 pts/14 May 7 04:23 (dhcp-dwf1-4-10-176-162-118) user12 pts/18 May 7 04:14 (dhcp-dwf1-4-10-176-162-119) user11 pts/23 May 7 04:14 (dhcp-dwf1-4-10-176-162-31) user1 pts/22 May 7 04:27 (dhcp-dwf1-4-10-176-162-125) user15 pts/24 May 7 04:14 (dhcp-dwf1-4-10-176-162-65) user6 pts/12 May 7 04:14 (dhcp-dwf1-4-10-176-162-120) user2 pts/28 May 7 04:14 (dhcp-dwf1-4-10-176-162-60) user7 pts/30 May 7 04:14 (dhcp-dwf1-4-10-176-162-118) user4 pts/31 May 7 04:14 (dhcp-dwf1-4-10-176-162-72) user10 pts/32 May 7 04:14 (dhcp-dwf1-4-10-176-162-68) user14 pts/33 May 7 04:14 (dhcp-dwf1-4-10-176-162-27)
If you want to send message to user8, issue following command at your terminal:
$write user8 pts/13
Now write any message you want. To end the message, press Ctrl+D. Your message would be sent.
(copied from) Reference: https://blogs.oracle.com/pranav/entry/how_to_send_message_to_users_l