Как отправлять сообщения по локальной сети, работающей в Windows и Ubuntu

Чтобы отправить одному пользователю, взгляните на команду write. На соответствующей странице руководства:

 The write utility allows you to communicate with other users, by copying
 lines from your terminal to theirs.

 When you run the write command, the user you are writing to gets a mes‐
 sage of the form:

       Message from yourname@yourhost on yourtty at hh:mm ...

 Any further lines you enter will be copied to the specified user's termi‐
 nal.  If the other user wants to reply, they must run write as well.

 When you are done, type an end-of-file or interrupt character.  The other
 user will see the message ‘EOF’ indicating that the conversation is over.

Чтобы отправить нескольким пользователям, посмотрите на команду wall:

 Wall displays the contents of file or, by default, its standard input, on
 the terminals of all currently logged in users. The command will cut over
 79 character long lines to new lines. Short lines are white space padded
 to have 79 characters. The command will always put carriage return and
 new line at the end of each line.

 Only the super-user can write on the terminals of users who have chosen
 to deny messages or are using a program which automatically denies mes‐
 sages.

[ 118] Также имеет значение команда mesg:

   Mesg  controls  the  access to your terminal by others.  It's typically
   used to allow or disallow other users to write to  your  terminal  (see
   write(1)).