NAME
write - write to another user

SYNOPSIS
write user [ ttyname ]

DESCRIPTION
Write copies lines from your terminal to that of another user.
When first called, it sends the message

Message from yourname@yoursystem on yourttyname at time...

The recipient of the message should write back at this point.
Communication continues until an end of file is read from the
terminal or an interrupt is sent. At that point write writes ‘EOT’
on the other terminal and exits.

If you want to write to a user who is logged in more than once, the
ttyname argument may be used to indicate the appropriate terminal
name.

Permission to write may be denied or granted by use of the mesg(1)*
command. At the outset writing is allowed. Certain commands, in
particular nroff(1) and pr(1) disallow messages in order to prevent
messy output.

If the character ‘!’ is found at the beginning of a line, write
calls the shell to execute the rest of the line as a command.

The following protocol is suggested for using write: when you first
write to another user, wait for him to write back before starting
to send. Each party should end each message with a distinctive
signal-(o) for ‘over’ is conventional-that the other may reply.
(oo) for ‘over and out’ is suggested when conversation is about to
be terminated.

FILES
/etc/utmp to find user
/bin/sh to execute ‘!’