V3/man/man1/write.1

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NAME		write  --  write to another user
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SYNOPSIS	write_____ user
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DESCRIPTION	write_____ copies lines from your typewriter to that of
another user.  When first called, write_____ sends the message
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     message from yourname...
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The recipient of the message should write back at this point.
Communication continues until an end of file is
read from the typewriter or an interrupt is sent.
At that point write_____ writes "EOT" on the other terminal.
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Permission to write may be denied or granted by use of the mesg____
command.
At the outset writing is allowed.
Certain commands, in particular roff____ and pr__, disallow
messages in order to prevent messy output.

If the character "!" is found at the beginning of a line,
write_____ calls the mini-shell msh___ 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.

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FILES		/tmp/utmp	to find user
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/etc/msh	to execute !
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SEE ALSO	mesg(I), msh(VII)
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DIAGNOSTICS	"user not logged in"; "permission denied".
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BUGS		write_____
should check the mode of the other user's typewriter
and refuse to proceed unless non-user
write permission is given.
Currently it is possible to write to another
person with the same user-ID even though
he has forbidden messages.

write_____ should also allow specification of
the typewriter name of a user who
is logged in several times instead
of picking out the instance with the
lowest name.