write

Send a message to a user on a specific terminal

$ write [username] [terminal_id]

$ write [username] tty/5

$ write [username] pts/5

write user [ttyname]

write allows you to communicate with other users by copying lines from your terminal to theirs. When you run write, the user receives a message indicating who is writing. You can then type your message, line by line, which appears on their terminal.
Press Ctrl+C to end the message session. Use the who command to find terminal IDs of logged-in users.

The recipient must have their terminal writable (use mesg y). If the user is logged in on multiple terminals and ttyname is not specified, the first writable terminal is used. Only works for users on the same system.

mesg(1), wall(1), talk(1), who(1)