message from yourname... 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 writes ‘EOT’ on the other terminal and exits. If you want to write to a user who is logged in more than once, the argument may be used to indicate the last character of the appropriate typewriter name. Permission to write may be denied or granted by use of the command. At the outset writing is allowed. Certain commands, in particular and disallow messages in order to prevent messy output. If the character ‘!’ is found at the beginning of a line, calls the shell to execute the rest of the line as a command. The following protocol is suggested for using 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 ( for ‘over’ is conventional) that the other may reply. (for ‘over and out’) is suggested when conversation is about to be terminated. /etc/utmp

to find user

/bin/sh

to execute ‘!’ mesg (I), who (I), mail (I)