View source for Linux Sending message from one terminal user to another user

;Send a broadcast message to all logged in terminal users
 $ sudo wall -n hi
 Remote broadcast message (Fri Aug  8 13:49:18 2016):
 hi

;Send a message to a specific terminal
 $ w
  13:54:26 up 2 days, 36 min,  4 users,  load average: 4.09, 4.20, 3.73
 USER     TTY      FROM              LOGIN@   IDLE   JCPU   PCPU WHAT
 saml     tty1     :0               Wed13    2days  3:55m  0.04s pam: gdm-password
 saml     pts/0    :0.0             Wed13   24:16m  0.35s  0.35s bash
 saml     pts/1    :0.0             Wed20    0.00s  3.71s  0.00s w
 saml     pts/4    :0.0             01:20   12:33m  0.36s  0.05s man rsync

Send the message
 $ sudo echo "Let's go have lunch... ok?" > /dev/pts/4

;Send a message to a specific user
 $ mesg     #check if you can receive messages 
 is n       #y -Allow write access to your terminal; n -Disallow write access to your terminal.
 
 $ mesg y   #enables

 $ write saml     #sends message to ''saml'' user

Determine which port to use for writing. The '+' or '-' character indicates whether the terminal is writable
 [piotr@linux ~]$ who -T
 piotr     ? :0           2016-07-27 15:18 (:0)
 piotr     + pts/0        2016-07-27 19:58 (:0)
 piotr     - pts/1        2016-07-27 21:24 (:0)


Preview default setting of write buffer set it up in the system
 cat /etc/default/devpts