From ProgSoc Wiki
write is a unix command for talking to other users on the system. To use write to talk to other progsoc users:
1. SSH to <username>@ssh.progsoc.org and login with your ProgSoc username and password.
2. Issue the following command to find out who is logged onto the system:
3. Issue the following command to talk to a specific user:
$ write <username>
4. Enter the message you'd like to send the user, followed by Ctrl+C to send.