Writing a Message to Logged in Users through the Terminal

To write a message to logged in users, you can use a write command. The write utility allows you to communicate with other users, by copying lines from your terminal to theirs.

First of all you need to check who is currently logged in, and which terminal he is logged in to.
$ who
root pts/0 2012-04-25 12:57 (192.168.0.207)
john pts/1 2012-04-25 13:20 (192.168.0.101)

Now you can write a messages to the user John. For example :
$ write john pts/1
Hello!

When you hit 'Enter', your message will be sent to that terminal.
Use Ctrl+D to terminate write
Also you can cat a file and pipe it to the write command too :
$ cat file.txt | write stan pts/1

To broadcast your message to all logged in users you can use a wall command (wall = write to all) :
$ wall
Hey you people!

For wall, the message will be sent only after you hit Ctrl+D

Or you can cat a file and pipe it to the wall command :
$ cat announcement.txt | wall